The plasma volume (PV) begins to rise slowly above nonpregnant levels around the 6th week of pregnancy (Fig. 1). At 16 weeks, it is approximately 10% above normal and rises rapidly until approximately 26 weeks to levels greater than 50% above baseline at which time a constant plateau is maintained until near term.
Fig. 1. Changes in blood volume (in milliliters) during antepartum, intrapartum, and postpartum portions of pregnancy.
The mechanism of increase in PV appears to be hormonal, resulting in retention of sodium and water as pregnancy progresses. Increased levels of estrogen, aldosterone, cortisol, prolactin, and human placental lactogen (HPL), which are known to cause sodium retention, are present as early as the 6th week of pregnancy. In addition, the control of vasopressin (i.e., antidiuretic hormone) appears to be altered during pregnancy, resulting in a positive water balance relative to sodium. Although production of some of these hormones wanes in late pregnancy, the increasing sensitivity of the maternal vascular system to certain agents, particularly aldosterone and prolactin, may continue the hydremia of pregnancy until term.
In the early postpartum period, PV decreases, only to increase again 2–5 days after delivery (see Fig. 1). The increase may be related to the rise in aldosterone observed at approximately 72 h postpartum. This elevation usually abates by 6 days postpartum, and the PV begins its final decrease to normal levels. By 3 weeks postpartum, the PV is still elevated 10–15% above nonpregnant levels, but when measured at 6 weeks, has usually returned to normal.
There are several advantages to the physiologic increase in PV during pregnancy. The apparent decrease in plasma cellular volume (PCV) reduces viscosity, decreasing the risk of thrombosis. Lowering of peripheral resistance and expansion of the blood volume appear to promote more oxygen exchange at the tissue level for a given cardiac output, reducing cardiovascular work. Also, there is moderate impairment of venous return late in pregnancy, and, since arteriolar sympathetic sensitivity is reduced in the normal pregnancy, an increase in PV may prevent syncope during positional change. It also may afford protection against acute blood loss, allow a graduated response to the hemoconcentration characteristic of such maternal disorders as pregnancy-induced hypertension, and enable the parturient a margin of tolerance for various anesthetic procedures. Finally, the failure of the dilution of hemoglobin to occur in early pregnancy has been related to stillbirth and intrauterine growth restriction (IUGR).19 The only disadvantage appears to be related to the timing of maximal PV increase (i.e., during the second trimester and at delivery) in patients with cardiac or renal disorders who may experience decompensation if their clinical disease is severe.
The increase in RBC mass does not begin until approximately 20 weeks, but then increases more rapidly than the PV until 28 weeks (see Fig. 1). From 28 weeks to term, the RBC mass rises only slightly, but the slope of erythrocyte increase begins to exceed that of the PV (a situation opposite that found earlier in pregnancy). The RBC mass is approximately 30% higher than its maximum in the nonpregnant state. In the early postpartum period, the RBC mass remains approximately 10% above nonpregnant levels for 1–2 weeks but returns to normal by 6 weeks. The decrease is principally related to blood loss at delivery and a decline in erythrocyte production. There is no evidence of increased RBC destruction during the puerperium. Bone marrow erythropoiesis assumes a normal level of RBC production by the end of the postpartum period.
The increase of RBC mass during pregnancy is accomplished by a complex interaction of several hormonal and physiologic factors, but in general it follows the erythropoietin production. In normal pregnancy, the erythropoietin level begins to rise slowly at 15 weeks, but the effects of this stimulation on RBC mass are first documented at 18–20 weeks. The maximal activity for erythropoietin occurs between 20 and 29 weeks, corresponding with the maximal increase in uterine blood flow and basal oxygen consumption. The level of erythropoietin begins to decrease slowly after birth in spite of blood loss at delivery. Studies in mice, but not humans, have shown that the RBC mass is increased in lactating animals compared with nonlactating controls. Thus, in some species at least, prolactin may continue its erythropoietic activity in the puerperium. Increased erythropoietin levels are also prominent during hypoxia, phlebotomy, polycythemia, some anemias (those due to iron, vitamin B12, or FAD but not those due to starvation, infection, most malignancies, and chronic renal disease), and hypernephroma or other erythropoietin-secreting tumors. It is decreased by hyperoxygenation, ordinary transfusion, uremia, and malnutrition.
Erythropoietin production and the subsequent size of the erythrocyte mass are directly related to increased basal oxygen consumption, an event associated with pregnancy. Other factors, such as elevation of cardiac output, decreases in peripheral resistance, reduction in viscosity, and increased erythrocyte content of 2, 3-diphosphoglycerate, are also related to the increased need of maternal and fetal tissues for oxygenation. This oxygen requirement stimulates the kidneys as well as other organs to produce a renal erythropoietic factor. This precursor transforms a dormant circulating pre-hormone into erythropoietin, a glycoprotein with a molecular weight of 34,000 Da, that is found in the plasma and urine. It stimulates the genetically predetermined precursor stem cells in the bone marrow to differentiate by way of the erythroid cell line into erythrocytes (Fig. 2). Prolactin also appears to enhance the effect of the erythropoietin already produced. HPL, by its general anabolic action, may support endogenous erythropoietin. The site of HPL action appears to be at the level of the stem cell, whereas prolactin seems to act on the late erythroid precursors.
Fig. 2. Time sequence of the maturation process of erythrocytic cell lines (erythropoiesis), revealing metabolic determinants and cellular appearance at each stage from immature stem cell in the marrow to destruction of mature red blood cells of the reticuloendothelial system.
Therefore, during normal pregnancy, the PV rises early and the RBC mass rises to its maximal level during the third trimester, and in iron-sufficient women the difference in PCV and the concentration at term is minimal. The total blood volume, as shown in Fig. 1, is composed of and follows the increase in PV and erythrocyte production.1 The significance of all the changes previously discussed are related to the diagnostic and therapeutic considerations of the specific anemias later in the chapter. It must be remembered, however, that failure to take these physiologic changes into account may confuse the clinician in cases of absolute anemias.
As in most diseases, the diagnosis of anemia rests on a thorough history, physical examination, and laboratory assessment.20 Most of the symptomatology and physical signs of anemia can be attributed to a reduction in the oxygen-carrying capacity of the blood. Tissue hypoxia usually does not occur in the patient with anemia except in the most severe cases, although relative hypoxia can be observed as a result of increased oxygen consumption during pregnancy. In most cases, however, a compensatory expansion of RBC mass occurs to offset this process. The decrease in peripheral resistance, modest elevation of cardiac output, increased tissue perfusion, and hypocapnia seen during pregnancy may ameliorate anemia during pregnancy. Obviously, precursors such as iron, vitamins, and folic acid must be present or the anemia may proceed uncompensated, It should be remembered that severe, primary hematologic disease usually occurs infrequently during the childbearing years; however, hematologic manifestations secondary to other diseases occur as frequently in the pregnant patient as in the non-gravid patient.
The history should include general symptoms (e.g., evaluation of the performance status of the patient), which may be helpful in establishing the magnitude of the anemia as well as in delineating the effect of therapy. Symptoms of mild anemia, such as easy fatigability and malaise, are common in normal pregnancy. Patients with anemia during pregnancy of mild-to-moderate degree may have no additional manifestations. Patients presenting with the classic symptoms of tachycardia, exertional dyspnea, pallor, and palpitations should be carefully evaluated. Not only are these manifestations of moderate-to-severe anemia but these symptoms may also herald a rare underlying hematologic disorder such as leukemia or cardiorespiratory disease. Adolescent pregnancy, short interconceptual spacing, nutritional deficiencies, and concurrent medical diseases may contribute to an anemia process. A history of reaction or exposure to various drugs and chemicals may be an important factor in the diagnosis of hemolytic anemias. Finally, a family history may be helpful, particularly as it relates to hemoglobinopathies and other inherited hematologic disorders.
On the physical examination, a central feature of anemia appears to be the pallor caused by the reduced hemoglobin level. This is most helpful in Caucasians and Eurasians, but examination of the mucous membranes may be used for similar purposes in blacks. During pregnancy, however, examination of the skin and mucous membranes in any race may be misleading owing to the hyperemia of these areas. Palmar creases, which usually are white in the fully open hand if the hemoglobin level is less than 10 g/dL, may appear pink in anemic pregnant patients because of hyperemic effects of HPL and progesterone. Pallor of the nail beds, however, is a reliable indicator of anemia during pregnancy in any racial group. Nails that are ridged longitudinally and flattened (koilonychia) rather than convex are present in chronic IDA. Other observations that may be helpful include cyanosis (congenital methemoglobinemia) and jaundice (hemoglobinopathies and hemolytic processes). An enlarged, smooth tongue is associated with pernicious anemia, but is quite rare in the United States. Glossitis related to IDA is more common; the tongue is coated, enlarged, and painful. With severe IDA, the lips may reveal cracks, particularly at the edges (cheilosis). Neurologic examination may be needed if vitamin B12
deficiency or IDA is suspected, since both may give rise to peripheral neuropathies. With the exception of pallor, these aforementioned signs, although not altered by pregnancy, are usually not present unless the anemia is severe (hemoglobin <6–8 g/dL). It is also important that the skin, liver, spleen, and lymph nodes be evaluated for enlargement, excoriation, or other abnormalities that may indicate primary hematologic disease or a secondary response to other disease states.
Since anemias are so common in women of reproductive age and since most women with mild-to-moderate anemias during pregnancy are asymptomatic, it is recommended that all patients be assessed for anemia during their initial prenatal visit, late in pregnancy (34–36 weeks), and at the postpartum visit. Laboratory assessment includes a complete blood count (red blood cell indices, hematocrit, hemoglobin concentration, white blood cell count, and platelet count). A peripheral smear and a reticulocyte count should be considered if the initial CBC is abnormal. The laboratory should examine carefully the RBC morphology in all anemic prenatal patients, because it may reveal several hematologic abnormalities. Because there is a demonstrated association between maternal anemia and unfavorable pregnancy outcome, these assessment tests for anemia appear to be cost effective and beneficial for the parturient and fetus. Normal screening and specific values for the pregnant and the nonpregnant state are shown in Table 1.
Table 1. Laboratory norms for the nonpregnant and pregnant patient
|General assessment|| |
|Hemoglobin||13–15 g/dL||11.5–12.5 g/dL|
|Packed cell volume||37–47%||33–38%|
|RBC count||4.2–5.4 million/mm3||3.8–4.4 million/mm3|
|Mean corpuscular volume||80–100 µm3/cell||70–90 µm3/cell|
|Mean corpuscular hemoglobin||27–34 pg/cell||23–31 pg/cell|
|Mean corpuscular hemoglobin concentration||31–36 g/dL||Unchanged|
|Specific diagnostic tests|| |
|Serum iron||50–110 µg/dL||40–100 µg/dL|
|Unsaturated iron binding capacity||250–300 µg/dL||280–400 µg/dL|
|Serum ferritin||75–100 µg/L||55–70 µg/L|
|Free erythrocyte protoporphyrin||25 µg/L||35 µg/L|
|Estimated sedimentation rate||0–15 mm/h||40–50 mm/h|
|Serum folate (fasting)||6.5–19.6 ng/ml||5–10 ng/ml|
|Serum B12||150–450 pg/ml||Unchanged|
The laboratory assessment of anemia is more difficult during pregnancy. In general, iron-sufficient, disease-free women with relative anemia during pregnancy have a hemoglobin level above 11 g/dL and a PCV (or hematocrit) above 35%. The average hemoglobin levels during pregnancy are between 11.5 and 12.5 g/dL instead of the normal 13–15 g/dL found in the nonpregnant state (see Table 1). The hemoglobin content during pregnancy tends to be further reduced, since the derivative actually measured by most laboratory techniques (cyanmethemoglobin) is slightly lower during gestation. The PCV or hematocrit likewise is lower in pregnancy, averaging 33–38% compared with the 37–47% range associated with the normal nonpregnant female. Since the relationship of hemoglobin level to PCV is more erratic in pregnancy, the measurements of RBC mass and other indices (i.e., mean corpuscular volume, MCV; mean corpuscular hemoglobin, MCH; and mean corpuscular hemoglobin concentration, MCHC) may not be as helpful as during the nonpregnant state. Of these, the MCV appears to be a good discriminator of the various types of hypoproliferative anemias (Fig. 3). All indices reflect average cell values and do not detect abnormalities in mixed cell populations. A patient with an IDA and concomitant megaloblastic process may reveal normal indices rather than the classic presentation of either disease.
Fig. 3. Anemias and related laboratory findings.
If the initial assessment of the patient's hematologic state indicates an anemia, the classification by mechanism as outlined in Fig. 3 can allow the physician to arrive at a diagnosis by an orderly method of laboratory analysis. The reticulocyte count (RC) divides anemias into categories in which the marrow is proliferative or hypoproliferative. Methylene blue is used to stain the reticulocytes (young, nonnucleated RBCs) on the peripheral smear. Usually if the RC in the presence of anemia is below 3%, the mechanism is one of diminished erythropoiesis. If it is above 3%, excessive hemolysis or acute blood loss is usually the cause. Therefore, a normal RC of 1–2% during pregnancy in a patient with anemia indicates a hypoproliferative process in which the patient cannot respond with new RBC production. Examination of the bone marrow is usually not performed during pregnancy owing to hypervascularity and subsequent maternal risk. However, if serious hematologic disease is suspected, then this diagnostic technique is usually required. Specific laboratory methods used during the diagnosis of anemia are discussed under the particular anemia processes. A flow sheet as shown in Fig. 3 allows the orderly procurement of essential laboratory tests in the diagnosis of anemia.
DECREASED ERYTHROCYTE PRODUCTION
Disturbance in maturation: hemoglobin synthesis
IRON DEFICIENCY ANEMIA
Iron deficiency anemia (IDA) afflicts 6% of the 60 million women of reproductive age in the United States and disproportionately represented are those who are multiparous, socioeconomically disadvantaged as well as those who are of African-American or Hispanic descent.21 IDA accounts for up to 75% of all the anemias diagnosed during pregnancy and, along with acute blood loss, comprises most of the anemias during the puerperal period.22 IDA complicates over 10% of all postpartum women up to 6 months postpartum.23 It is known that 1 in 8 American women have iron deficiency up to 12 months after delivery, and even when extended to the second year of postpartum, 1 in 12 women have IDA; statistics that have prompted a call for screening of recently pregnant women for up to 1–2 years after birth.24 As previously noted, particularly IDA, is associated with maternal adverse affects such as dizziness, fatigue, infections, lactation problems, and a need for prolonged hospitalization.14 In addition, cognitive function as well as postpartum depression and a reduced sense of well-being are co-morbidities associated with IDA.15, 16 The infants of women with IDA have also been found to have smaller placental weights, preterm birth, growth restriction, lower hemoglobin values, lower birth weights, and lower Apgar scores.25, 26 Understandably, given these adverse maternal–fetal effects of IDA, it is incumbent upon the providlowerer to understand iron absorption, metabolism and replacement.27
Not only is iron necessary for RBC formation but it is also intimately involved with proteins integral to intermediary metabolism as nearly half of the enzymes and co-factors in the Krebs cycle either contain iron or require its presence. The basic compartments of iron distribution include hemoglobin iron, storage iron, myoglobin iron, labile pool iron, other tissue iron, and transport iron. Table 2 illustrates the significant differences in the total body iron distribution between the adult male and the female during the reproductive years. Pregnancy normally increases the amount of iron but not the percentage distribution to each compartment. However, socioeconomic status, nutritional status, and concurrent disease processes can modify the dispersion of iron in each compartment.
Table 2. Iron compartments*
Iron content (mg)
Total body iron (%)
|Other tissue iron|
* These values represent estimates for an iron-sufficient person.
Assessments with radioactive tracers have shown that hemoglobin iron makes up 65–70% of total body iron and averages 1700 mg in a normal adult woman. Iron deficiency is precipitated during pregnancy because of expansion of the red blood cell mass during the second trimester. IDA is compounded by the preferential fetal utilization of iron from maternal storage compartments. Iron loss during late pregnancy and after delivery further contributes to postpartum IDA. Approximately 1000 mg of elemental iron are needed during pregnancy with 300 mg being used for the fetus and placenta, while 700 mg is added in the expansion of maternal hemoglobin.27 In addition, approximately 200 mg is lost in bleeding during and after delivery, but some of the 500 mg of iron from metabolized maternal RBC is returned to the iron storage postpartum, resulting in at least 500–600 mg of iron debt. Thus, this amount of iron reserve is considered the minimum for pregnant women. However, a recent report indicates that only 20% of women of reproductive age have such a reserve of iron, 40% had iron storage between 100 and 500 mg, with 40% having virtually no iron storage.28 As IDA in the postpartum period may impair the woman’s ability to participate in child care, household tasks and social activities, it also diminishes their productivity in regard to physical and intellectual work.29 These changes are worrisome because they have been shown to lead to disturbed maternal–infant interactions as shown by one study comparing infants of IDA women to non-anemic parturient controls.30 These infants when examined at 10 weeks of age were developmentally delayed and these delays persisted long after correction of the maternal iron status which, at least in this study, raises the possibility that postpartum maternal IDA may irreversibly impair childhood development.30
Storage iron exists in two forms: ferritin and hemosiderin. In healthy, iron-sufficient women, storage iron totals approximately 600–800 mg and makes up approximately 27–30% of total body iron. Depletion of this storage compartment occurs when iron loss or use exceeds iron absorption; it is usually decreased during pregnancy even in iron-sufficient women.
Ferritin contains approximately half the storage iron and is found in plasma as well as in most of the cells in the body; several varieties of ferritin have been demonstrated by electrophoresis and isoelectric focusing. Ferritins indigenous to many organs and to reticuloendothelial cells have been identified, supporting the hypothesis that these iron proteins are organ specific and are products of different genotypes. Ferritin has been identified by electron microscopy as a water-soluble complex with a molecular weight of approximately 450,000 Daltons, which is 20% iron. Stereochemically, it exists as a complex of ferric hydroxide and the protein carrier apo-ferritin. The role of ferritin in iron absorption is direct; a low concentration of this compound in the intestinal mucosal cell enhances the biosynthesis of additional apo-ferritin, which results in more iron absorption. The life span of apo-ferritin is only a few days; the degeneration and re-synthesis provides an available intracellular iron pool. The measurement of serum ferritin has become one method of delineating iron stores without having to resort to bone marrow sampling.
Hemosiderin is found only in cells of the reticuloendothelial system, such as those of bone marrow, liver, and spleen (about one-third in each organ). Immunologic studies have shown that the protein components of both ferritin and hemosiderin are identical; hemosiderin may represent a partially denatured ferritin. Hemosiderin is approximately 25–30% iron by weight, is water soluble, and, in contrast to ferritin, can be seen with the light microscope. It represents about 50% of the storage iron and about 12–15% of the total body iron. Hemosiderin represents that quantity of iron demonstrated by Prussian blue stains of bone marrow aspirates. This storage iron is attached to a substrate called apohemosiderin, which is amorphous, thus lacking the firm, crystal like molecular arrangement of ferritin. The mechanism of mobilization, use, and contribution of storage iron in an anemia process is discussed in the sections on iron absorption and homeostasis. While the measurement of ferritin is involved with an assessment of iron absorption capabilities, the assessment of hemosiderin is a measure of iron balance; until hemosiderin becomes depleted, no signs of iron deficiency develop (Fig. 4). Therefore, as long as storage iron is present and released normally, no change in the peripheral blood smear is seen.
Fig. 4. Relationships of known iron compartments to various stages of clinical iron depletions as depicted in the cellular appearance of the RBC.
During pregnancy, the increase in RBC mass gives an indirect measurement of iron stores. The principal cause for a failure of the expansion of the hematocrit is the absence of bone marrow hemosiderin. This finding indicates exhaustion of storage iron, its absence being the earliest sign of iron deficiency. If IDA is clinically evident by a decreased hemoglobin concentration, by a characteristic microcytic, hypochromic blood smear; and by altered RBC indices, the iron stores will be nonexistent. On the other hand, bone marrow stores decrease to minimal levels during pregnancy, even in iron-sufficient women, in the absence of iron supplementation. Since a bone marrow examination is not practical during pregnancy unless severe hematologic disease is suspected, reductions in serum ferritin, hemoglobin concentration, and RBC indices are the first laboratory indications of inadequate iron stores. Iron stores are reduced in IDA, blood loss, and nutritional anemias; however, hemolytic processes, hereditary anemias, and ineffective iron utilization during infection or inflammation may be associated with normal to increased storage iron levels and concomitant reductions in hemoglobin levels, and RBC indices.
Myoglobin iron accounts for approximately 3–4% of total body iron (130 mg) and is relatively constant between men and women, even in pregnancy. Each myoglobin molecule consists of a heme moiety surrounded by a helical chain of 150 amino acids. The myoglobin is less than 1% iron and has a molecular weight of approximately 17,000 Da. It is found in most muscle cells and appears to serve as an oxygen reservoir when cellular damage from hypoxia occurs. Function of this iron component in pregnancy has not been reported.
Labile iron pool
The labile iron pool represents approximately 80 mg of iron constantly in exchange between the plasma, interstitial, and intracellular compartments. An intracellular protein produced by the liver appears to be responsible for short-term binding and release of the labile iron. This protein has been termed acetate-extractable ferroprotein and has a molecular weight of approximately 12,000 Da. It has been isolated from cells in the lung, liver, intestinal mucosa, spleen, and kidneys, as well as from RBCs. Iron kinetic studies have shown that approximately 80% of this transitional protein eventually is reincorporated into hemoglobin and that the remaining 20% is associated with storage compounds. The labile iron pool offers a method of studying the clearance rate, incorporation data, and daily iron turnover; ferrokinetic studies indicate that in IDA the incorporation of iron into hemoglobin and subsequently into normal RBCs is almost 100%. In contrast, in hemolytic or megaloblastic anemia, the incorporation of iron is rapid and complete, but there is ineffective erythropoiesis and early destruction of the erythroblasts. Patients with aplastic anemia or thalassemia also demonstrate an incorporation rate that is markedly reduced in the presence of normal erythrocytes.
Tissue iron is constant during pregnancy and represents approximately 6–8 mg of iron, or between 0.2% and 0.5% of total body iron. Although extremely small, this compartment is very important because it includes enzymes such as the cytochrome peroxidase catalase dehydrogenase, as well as acyl-CoA and various other oxidases. Changes in enzymatic iron are reflected in decreased efficiency of the mitochondrial systems; thus, severe anemias may affect fetal growth and development by such a mechanism. In addition, maternal–fetal steroidogenesis and host response to various disease states may be modified by functional changes in these critical enzyme systems. As shown in Fig. 4, the amount of parenchymal iron is altered only in severe IDA, and it is one of the last compartments to become depleted.
Transport iron represents less than 0.1% of the total iron (3 mg) in both men and women, and yet, kinetically, it is the most active compartment, being replaced approximately every 2.5 h. The metal in this category represents the interchange between all compartments previously mentioned and is not altered during pregnancy. Transport iron is loosely bound to a specific protein (i.e., transferrin), as shown in Fig. 5. Transferrin migrates electrophoretically with the β-globulins and has a molecular weight of approximately 80,000 Daltons; it is called apotransferrin when not bound to iron. Although the kinetic properties of each apotransferrin are similar, cellular specificity is evident, since 19 genetically distinct molecular variants have been described. It is synthesized principally by the liver but also by other tissues of the reticuloendothelial system and is the only iron form that shows diurnal variation; the highest values are obtained in the morning.
Fig. 5. Transportation sequence of iron involving intestinal absorption, marrow hemoglobin incorporation, RBC destruction, return to labile iron pool, and storage deposition.
A trivalent iron atom may be bound at both ends of the polypeptide chain (see Fig. 5), and binding affinity is increased when one site is occupied. Transferrin receives absorbed iron and transports it to the immature marrow normoblasts. The transferrin in the plasma in iron-replete subjects is one-third saturated, leaving two-thirds unoccupied; this is measured as the unsaturated iron binding capacity (UIBC). When the serum iron is decreased, the UIBC is increased and vice versa.
Iron absorption depends on many factors (Table 3), reflecting the diet, the status of the bowel lumen, and mucosal abnormalities, as well as systemic factors. Iron is principally absorbed in the duodenum and proximal small intestine. The iron presented to the gastrointestinal tract is usually in one of three forms: the ferrous form (from elemental iron), hemoglobin (from animal protein sources), and the trivalent or ferric form (from vegetable complexes) (Fig. 6). The ferrous salts are best, since they need no conversion to be absorbed; the ferric iron in vegetable protein complexes must be reduced to the divalent state before it can pass into the mucosal cell. Hemoglobin iron is readily absorbed after being hydrolyzed in the gut lumen into heme and globin. The globin, although degraded by intestinal enzymes into small peptides, remains an integral factor in absorption, since it continues to stabilize the heme iron in the ferrous state.
Table 3. Conditions affecting iron absorption
|Increased absorption||Decreased absorption|
|Iron content|| || |
|Form of iron||Heme iron|
Adequate ferrous salt
|Intraluminal factors|| || |
|Intestinal secretions||Hydrochloric acid|
|Stomach contents||Ascorbic and other acids, cysteine||Oxalates, phytates, phosphorus, carbonate|
|Chelators|| ||EDTA, desferrioxamine|
|Metallic cations|| ||Cobalt, nickel|
|Mucosal factors|| || |
|Disease states||Intermittent outlet obstruction|
Chronic diarrhea (sprue)
|Cellular||Decreased mucosal iron||Increased mucosal iron|
|Systemic factors|| || |
|Erythropoiesis||Acute blood loss|
Transfusion, chronic infection
|Iron requirements||Pregnancy, growth|
|Weight loss, thalassemia|
Fig. 6. Mechanism of iron absorption from the intestinal lumen through the mucosal epithelium and into various iron compartments.
Gastric acid degrades the organic iron complexes (vegetable protein) and reduces the trivalent mineral to ferrous iron. Moreover, hydrochloric acid serves to prevent alkaline dietary constituents from forming insoluble complexes with ferrous iron, to prevent various agents from oxidizing iron to the ferric state, and to prevent iron precipitating as insoluble ferric hydroxide. While gastric secretions may facilitate elemental iron absorption, it does not appear to have any effect on hemoglobin iron absorption. The alkaline milieu of the duodenum and upper small bowel favors absorption, since the ferrous iron has greater solubility in these surroundings; this is true for all three sources of iron, but it is particularly necessary for hemoglobin iron. Therefore, agents that form alkaline-soluble complexes promote iron absorption; these include the reducing agents ascorbic acid, fructose succinic acid, lactate, pyruvate, and sorbitol and certain amino acids such as cysteine. Certain drugs such as hydroquinone and alcohol may also increase iron absorption by increasing gastric acid production, while medications that decrease intestinal motility, such as atropine or reserpine, also enhance iron absorption. Bile may have a limited role in facilitating iron absorption, but pancreatic secretions are thought to be noncontributory.
In contrast, oxidants and certain medications reduce iron absorption by increasing the concentration of the trivalent form. Other substances form insoluble complexes with the ferrous iron; chelators have a similar mode of action. Metallic cation competitors decrease absorption by competing for binding sites, while other agents reduce absorption by reducing gastric acidity. Cathartics and disease processes reduce iron absorption by increasing intestinal motility, reducing the time of exposure of mucosal cells to iron. Surgical disruption of the gastric (i.e. gastric bypass surgery) or duodenal mucosa limits the surface area for absorption.
Finally, another factor, pica, has long been associated with IDA. Pica is the ingestion of various substances that have no dietary value; pagophagia (ice), geophagia (clay), and amylophagia (starch) are examples. In some ethnic groups, it is a common presenting symptom in patients with proven IDA. The “cravings” these patients experience are not always for nonfoods, but may be for vegetables (carrots or celery) or commercial items such as “fast foods” (invariably not a good source of iron). Formerly all forms of pica were thought to delay iron absorption. Now it is clear that pica is a sign of IDA rather than a cause. A history of pica in an adult suggests that the patient has IDA, since the abnormal craving usually disappears in a few days after the institution of iron supplementation. Although not diagnostic, certain food cravings during pregnancy may represent pica due to the increased iron demands of pregnancy. This hypothesis is supported by the observation that most pica occurs after the 20th week, a time during which the iron demands increase most rapidly. The relationship of pica to IDA is not completely understood; however, it seems to be related to a decrease of iron-containing enzymes in the oral mucosal cells of these patients.
The regulatory mechanism of iron uptake is closely related to the plasma iron turnover rate (see Fig. 5). It appears that transferrin from the labile pool is deposited into the developing mucosal cells destined to form the basal layer of the crypts of Lieberkühn in the duodenum. This protein controls the amount of iron absorbed by trapping the metal entering the cell. The conversion of the absorbed ferrous iron into ferritin is a metabolic process that requires a chelator, such as ascorbic acid, an energy source (probably adenosine triphosphate), and, finally, oxidation of the divalent iron to the ferric iron, which requires ceruloplasmin (see Fig. 6). This sequence is adequate for elemental iron and vegetable protein iron absorbed as the simple ferrous cation. However, hemoglobin or myoglobin iron must be further degraded by a microsomal enzyme called heme oxygenase. Once the mucosal cells are saturated with ferritin iron, absorption is halted. If the iron stores are reduced, very little ferritin is present in the mucosal lining and iron is absorbed rapidly through the cell, by transferrin, into the bloodstream. In contrast, when iron levels are sufficient, increased amounts of cellular ferritin are present and the iron is either stored as ferritin or is not absorbed. Ferritin is lost as the duodenal mucosal cell matures and works its way from the base of the crypt to the luminal border. The mucosal cell control of absorption is not, however, absolute; it can be overcome if the concentration of iron at the intestinal lumen increases sufficiently, as in toxic overdoses. If large amounts of iron are presented chronically, hemochromatosis may eventually result.
The transport of iron from the mucosal cell to the utilization pool is performed by transferrin (see Fig. 6). When iron is required by the marrow, transferrin obtains trivalent iron from the ferritin in the mucosal cells. The ferric iron binds to one of the glycoprotein moieties of transferrin, is transported to the marrow, and is used for hemoglobin synthesis. The transferrin binds briefly on the normoblast membrane, where it delivers one or both iron atoms. During the binding process, transferrin may partially enter the cell by a process of endomitosis and actually participate in hemoglobin formation. The release of iron is an active process that can be abolished by enzyme inhibitors. Although direct transfer of transferrin to the normoblast is thought to be the major pathway, rhophenocytosis (accepting of transferrin by reticuloendothelial cells) may represent a minor pathway. Once the iron–transferrin complex is attached to the receptor site inside the normoblast, it is conjugated to a protein molecule with a molecular weight of approximately 20,000 Da and transported to the mitochondria, where heme synthesis occurs; this process takes 8–10 min. As the erythrocyte matures, it retains its capacity to take up iron and synthesize heme until after the reticulocyte stage, in which the mitochondria are lost (see Fig. 2). After the erythroid cells mature, they are extruded from the bone marrow into the peripheral circulation and have a normal life span of 120 days. The life cycle is complete when the red cells are destroyed and hemoglobin degradation takes place within the reticuloendothelial system, chiefly the liver and the spleen (see Fig. 5). Digestion of the red cells occurs within a few hours, and the plasma iron is redistributed, 80% into new hemoglobin and 20% as storage iron. The mechanism by which the reticuloendothelial cells transfer the iron to transferrin appears to involve ceruloplasmin (ferroxidase), which is also involved in the binding of iron within the mucosal cells.
The conservation of iron in humans is tenacious, with only 0.1% of the total amount of body iron lost each day. This amount is easily replaced in the nonpregnant adult if the dietary source is adequate. The average amount of iron excreted by the adult averages 0.9 mg/day, with most being lost in the intestinal tract as desquamated gastrointestinal cells, blood, and bile. Additionally, epidermal cell loss and sweat produce a daily iron loss of 0.2 mg. In areas of high temperature and humidity, an additional 0.5 mg/day may be lost, but this loss rarely produces IDA. Finally, a small amount (0.1 mg) of iron is excreted daily in the urine. Both sexes lose a similar amount of iron through these mechanisms; the basal loss is approximately 14 µg/kg/day. In women during their reproductive years, the iron losses are compounded by menses. Although the blood loss is relatively constant in successive periods, the individual variation between women is large. Most normal, iron-sufficient women lose an average of 25–45 ml of blood through menses each month, which approximates to 0.7–1.4 mg/day in terms of iron loss. The blood loss is lower in the younger age groups, in women taking oral contraceptives, in women with good nutrition, and in higher socioeconomic groups. Blood loss is exaggerated in women over 40 years old, those with intrauterine contraceptive devices, patients after tubal sterilization, females of high parity, women of lower socioeconomic groups, and those with poor nutrition. A menstrual blood loss of 50–60 ml seems to be the upper limit of normal, since women whose losses have exceeded this amount eventually develop IDA. Therefore, the total basal iron loss of a woman in the reproductive years would appear to average 1 mg for menstrual loss in addition to the 1 mg obligatory loss experienced by men and women alike.
Many factors, both physiologic and those related to disease states, can cause a healthy woman to become iron deficient. Pregnancy has a marked effect on iron homeostasis (Table 4). In healthy menstruating women, the loss of 2 mg/day can be overcome by a daily food intake of 1800–2200 calories, which contain 11–13 mg of iron. However, even in an iron-sufficient state, large amounts of iron must be borrowed from iron stores to complete a pregnancy (Table 4). During the first half of pregnancy, iron requirements are not increased; in the absence of menses, an intake of 11–13 mg/day is adequate. After the 20th week, however, the RBC mass begins to expand and the fetus requires more iron. Even with increased absorption, the amount of dietary iron is not adequate to prevent a reduction in iron stores. Obviously, if dietary iron does not meet the requirements, then the storage iron must supply the needs. Since the average North American diet provides about 6 mg of iron per 1000 calories and the pregnant woman of ideal weight should consume 1800–2200 calories each day, the average pregnant woman should receive between 10 and 12 mg of elemental iron daily. Even though the absorption rate increases from 10% in the first trimester to 30% during the latter half of pregnancy, the iron acquired from the diet alone during this time ranges from 450 to 500 mg; thus, approximately 400–500 mg must be supplied by storage iron during pregnancy. In women who are deficient in storage iron prior to pregnancy, this further requirement may lead to overt IDA. It should also be noted that if the storage iron is insufficient at the beginning of pregnancy, the maternal hemoglobin mass will not be expanded until the fetal demands are met. The lack of expansion of the RBC mass thus may be an indication of inadequate iron stores. Postpartum, the amount of iron in the expanded hemoglobin mass not lost at delivery can be returned to the iron stores and the anemia can be partially balanced. However, in most women this is not sufficient replacement, and almost all women will remain deficient in storage iron unless they receive supplementation during gestation.
Table 4. Iron homeostasis in pregnancy,*expressed in milligrams of elemental Iron
|500 (RBC mass expansion)||490 (diet)†|| |
|300 (fetal/placental)||270 (returned to storage after delivery)|| |
|190 (basal loss)|| || |
|230 (loss at delivery)|| || |
|TOTAL: 1220||TOTAL: 760||460|
* Lasts 20 weeks.
† Average of 3–4 mg/day actual absorption.
The laboratory diagnosis of IDA depends on the severity of iron depletion. In the mildest stage, iron deficiency is manifested by a decrease in serum ferritin, but serum iron, hematocrit, and hemoglobin values are usually normal (see Fig. 4). Iron deficiency without anemia is the next stage; absence of storage iron, manifested by reduced serum ferritin, low serum iron, and decreased transferrin saturation without anemia are characteristic. IDA is first reflected by a reduced RBC mass, then reductions in hematocrit and hemoglobin levels following hypochromasia and microcytosis (see Fig. 4).
Iron assessment tests must be altered during pregnancy. A relative decrease in serum iron occurs from approximately 12 weeks through 32–34 weeks owing to the increase in PV. However, as the RBC mass approaches the increase in PV, the serum iron rises to normal nonpregnant levels. During the early postpartum period, serum iron again rapidly decreases over the first 4–5 days before returning to normal at the end of the first week. This is probably related to ineffective release of storage iron owing to the change in hormonal milieu. Transferrin begins to increase from 12 weeks through 34–36 weeks, but a slight decrease occurs toward term. During the first 7 days after delivery, the transferrin concentration increases before it returns to normal levels approximately 10 days postpartum. These changes are also thought to be hormonally mediated, since similar changes have been observed in women taking oral contraceptives. These observations are present only in iron sufficient pregnant women and are not significant enough to affect the diagnosis of IDA.
The most consistent findings in the blood of a patient with IDA are a decrease in the hematocrit and hemoglobin concentration, with concomitant hypochromia and microcytosis observed on peripheral smear. Serum iron, serum ferritin, total iron binding capacity (TIBC), and transferrin saturation may be used to confirm IDA, although they are not routinely obtained during antepartum screening.20 IDA is usually suspected if the serum iron is below 60 µg/dL, the serum ferritin is below 30 µg/L, the transferrin saturation is below 20%, and the TIBC is above 350 µg/dL; the normal values are shown in Table 1. Values of less than 30 µg/dL, less than 10 µg/L, less than 10%, and greater than 400 µg/dL in the serum iron, ferritin, transferrin saturation, and TIBC, respectively, are diagnostic of IDA. Some authors have advocated further evaluation in an attempt to detect early iron-deficient states; however, these have yielded limited clinical information.31
In iron-deficient states, zinc may replace iron in the protoporphyrin ring; thus, the measurement of RBC zinc protoporphyrin (RBC ZP) may be an accurate predictor of IDA. This test is relatively rapid and is not costly, although it is not specific for IDA. In addition, the measurement of iron chelating agents such as desferrioxamine, followed by measurement of mobilized iron stores excreted in the urine, has been of value in some cases. The measurement of iron stores by nuclear magnetic resonance may also be helpful in cases of early IDA. In mild cases of IDA, the measurement of free erythrocyte protoporphyrin (FEP) is increased fivefold. Measurement of FEP has been advocated as a screening test for IDA, but the results are not related to the severity of the anemia, nor is it specific for IDA.
Although iron depletion or IDA can be easily treated in most cases, it is important to rule out more severe hematologic or systemic diseases. Hypochromic anemia, caused by the following conditions, may be confused with IDA: (1) chemical toxicity related to the intake of chloramphenicol, lead, alcohol, or isoniazid; (2) inflammatory processes; (3) malignancy; (4) pyridoxine-responsive anemia; and (5) hemoglobinopathies.
Drug effects may be toxic (lead, alcohol) or idiosyncratic (isoniazid). Maternal encephalopathy and basophilic stippling of the normal red cells may be observed with lead poisoning. On the other hand, toxic dosages of alcohol or reactions to drugs (chloramphenicol) usually cause a deficiency in porphyrin synthesis, which leads to sideroblastic anemia. On the peripheral smear, the red cells are hypochromic, but the presence of target cells and ring sideroblasts helps differentiate this process from IDA. Characteristically, in toxic anemias the serum iron is elevated and the transferrin saturation is increased; the opposite is found in IDA. Examination of the bone marrow, if performed, reveals the presence of ring sideroblasts, along with increased iron stores common in porphyrin synthesis deficiencies.
Chronic diseases, such as malignancies, and inflammatory processes, including arthritis and the collagen-vascular diseases, may cause anemia, but usually the peripheral smear reveals normochromic and normocytic cells. Unfortunately, the symptoms from these processes are protean, and they may be difficult to differentiate from those of IDA if the peripheral blood picture is similar. As with IDA, patients with chronic disorders will characteristically have low serum iron levels. Elevated-to-normal serum ferritins are seen in these disorders, while patients with IDA exhibit decreased ferritin values. In those affected with inflammatory processes, the transferrin saturation is decreased and TIBC is increased, as seen in IDA. On the other hand, patients with malignancies exhibit an increased transferrin saturation and decreased TIBC (see Fig. 3). Another screening device to differentiate IDA from acute and chronic inflammation is the erythrocyte sedimentation rate. It almost always is increased in neoplasia or acute and chronic inflammation over that found during normal pregnancy, whereas in severe IDA it is normal. Erythrocyte survival times are slightly decreased in IDA and may also be shortened in chronic diseases. Infestation (e.g., hookworms), although rare in the United States, must be included in the differential diagnosis in developing countries, in world travelers, or in areas of poor sanitation. Other causes of excess intestinal iron loss, such as diverticulitis, intestinal cancer, and peptic ulcer disease, are rare during the menstrual years and even more infrequent during pregnancy.
Pyridoxine (vitamin B6)-responsive anemias are characterized by hypochromic, microcytic erythrocytes with increased ring sideroblasts and prominent target cells. Unlike IDA, serum iron levels, transferrin saturations, and serum ferritins are increased in patients with pyridoxine-responsive anemia. The TIBC is characteristically decreased.
In hemoglobin synthesis abnormalities, such as heterozygous thalassemia, a microcytic, hypochromic pattern may also be demonstrated, and these abnormalities may be confused with IDA. RBC morphology usually shows more basophilic stippling and target cells in thalassemia patients; these are not common in patients with IDA. In a person of African or Mediterranean origin, thalassemia should be ruled out by hemoglobin electrophoresis. Other aids in diagnosis include the erythrocyte count, the MCV, and the reticulocyte count (RC). Almost 85% of patients with heterozygous thalassemia have an RBC count greater than 5 million/mm3 despite a reduced hemoglobin concentration. In contrast, only 3% of adults with IDA have RBC counts over 5 million/mm3 The MCV is reduced in heterozygous thalassemia; values of 55–70 µm3/cell are the rule, whereas values below 70 µm3/cell are very uncommon in IDA. Reticulocytosis of greater than 5% is much more commonly noted in thalassemia patients than in those with IDA. In addition, serum iron concentration and transferrin are not usually altered in the hereditary anemias; however, serum ferritin levels are characteristically normal to increased.
In the final analysis, the response to iron therapy may prove to be diagnostic. This method is commonly used in the low-risk patient and can be harmful only in patients who have allergic reactions or in those who have iron overload due to other hematologic diseases. In any case, in “therapeutic” diagnosis of IDA, the patient should be followed carefully to detect iron-unresponsive anemia. If the patient with IDA receives sufficient medication, an intense reticulocytosis should occur between the 7th and 10th day after the initiation of therapy. A significant increase in hemoglobin values should be evident in 3–4 weeks, and the hemoglobin concentration should approach normal values within 2 months. This may not occur during pregnancy, however, since the increase in RBC mass and transfer of iron to the fetus may continue the iron-depletion process. In this case, if other factors such as inflammatory diseases, chronic infections, or hemoglobinopathies are not suspected, the iron therapy should be continued until the end of the pregnancy, since eventually there will be a response.
There are basically three methods by which iron can be administered to correct IDA. Traditionally, the most common approach to the treatment of IDA has been oral iron supplementation as the elemental salt, parenteral iron therapy or in severe cases particularly those associated with blood loss, direct infusion of blood components. The most common and least expensive method of supplementation is oral iron supplementation in the form of ferrous sulfate, ferrous gluconate or carbonyl iron. The latter is more expensive but better tolerated from a gastrointestinal standpoint. Some iron preparations have been combined with vitamins but are no more effective than simple elemental iron and are more expensive. In addition, they are usually given once per day, a dosage not likely to eradicate IDA. Sustained-released capsules of iron given in an effort to avoid upper gastrointestinal side-effects such as nausea and vomiting have a reduced absorption rate (80%) compared to elemental iron. These preparations do not appear to be associated with any less symptomatology than carbonyl or fumarate elemental iron. Most oral agents contain 200–300 mg of the iron complex yielding approximately 40–60 mg of elemental iron.32, 33 This ideal dosage (40–60 mg) of elemental iron per day should be given in three divided doses to take advantage of the absorption rate which is maximal about 4–6 h after each dose. It should be administered with meals during pregnancy as this may reduce the incidence of gastric irritation, nausea, vomiting, and diarrhea. The frequency and timing of the dosage does not appear to affect the side-effect of constipation is observed in 10–12% of pregnant women. One study observed common adverse gastrointestinal side-effects including abdominal pain, dyspepsia, and constipation when a ferrous iron agent was administered daily for up to 12 weeks.34 In this study, 33% of women given oral iron, gastrointestinal side-effects resulted in their inability to take the iron as prescribed. Similarly, Wulff and Ekstrom35 noted that the recommendations for iron supplementation amongst their pregnant women were adhered to less than 40% of the time. In another investigation, using several types of elemental iron preparations, up to 10% of patients were nonadherent to therapy after just 2 weeks, 25% after 1 month and 32% after 2 months of therapy.36 Obviously in some cases, the treatment is worse than the disease as it interferes with the woman’s quality of life. Finally, one of the most common causes of fatality from poisoning in young children includes elemental iron preparations.37 Indeed, this has led many to suggest that food fortification, particularly in developing countries may enhance the ingestion of enough iron to prevent IDA.38 In any case, if oral iron therapy is well tolerated, it should be continued throughout the pregnancy and for up to 3 months postpartum.39 During the therapy in the patient with IDA progress should be monitored and if the hemoglobin level remains unchanged after 6 weeks of therapy other hematologic problems should be considered.
Prophylactic iron supplementation, even in low income women who are not anemic, has been suggested to be associated with higher mean birth weights and a decreased incidence of preterm deliveries.40 On the other hand, a meta-analysis of iron supplementation in such women did not show any significant benefits to this approach,41 while a more recent study has shown a higher rate of small for gestational age births and more maternal hypertensive disorders among supplemented women.42
The second alternative for treatment of anemia is blood transfusion, which is the most rapid therapy but this modality is rarely used unless rapid blood loss is noted at delivery or surgery as well as in those with placental abnormalities. Among obstetric populations, transfusions are administered in 0.3–1.7% of vaginal births and between 0.7 and 6.8% of cesarean sections.6 In addition, blood transfusions carry the risk of febrile reactions, hemolytic reactions, anaphylaxis, graft versus host disease, and significant infectious risk.43 Therefore, transfusions are almost exclusively reserved for patients with blood loss as is indicated in the later parts of this chapter and not to treat IDA or other forms of abnormal blood formation.
A third option, is intravenous iron therapy which has the potential to both quickly correct the anemia by providing sufficient iron to replenish storage and also holds promise of averting or minimizing blood transfusion in the postpartum anemic patient.44, 45 However, using dextran iron products, minor side-effects such as skin staining, discomfort at the site of injection, malaise and a metallic taste occur in 8–10% of patients. Moderate reactions such as lymphadenopathy, phlebitis, severe headaches, hyperpyrexia, and leukemoid reactions occur in as many as 1–2% of patients treated with dextran iron parenterally. Severe systemic adverse reactions such as anaphylaxis, renal toxicity, and bronchial spasm occur in 1 in 200 patients and therefore, earlier preparations such as the dextran-containing IV iron agents, are rarely used and have a black box warning in the PDR. The nondextran-containing IV iron agents such as sucrose or gluconate parenteral products are limited by the FDA approved dosing guidelines. Iron gluconate is only approved by the FDA for use in hemodialysis patients (maximum dose 125 mg IV over 10 min). Sucrose iron has been used and when compared in a randomized open-label study, it restored iron stores faster and more effectively than oral iron.46 However, iron sucrose products have a maximum dose limited to 100 mg intravenously. These agents, while they have a better safety profile, are inconvenient as they require 5–20 IV doses to be administered to replace iron stores.
On the other hand, over 5600 women have been treated with ferric carboxymatose (FCM) intravenously in an open-label fashion and over 2000 women have received this agent in 12 randomized trials using oral iron as a control. In these studies women achieved 12 g/dL hemoglobin levels (91 vs. 67%), a rise of ≥3 g/dL hemoglobin (91 vs. 65%) and had less postpartum fatigue, depression and child care issues.6 Therefore, for patients who cannot take oral iron or women who are nonadherent, intravenous FCM offers a reasonable alternative.
In summary, patients with iron depletion or deficiency will rarely be detected prior to changes in the hemoglobin or hematocrit level. Supplemental iron therapy is recommended in most pregnant women because (1) iron depletion is so common, (2) therapy is safe and inexpensive, (3) lack of response to supplements is an indication for further testing, (4) other testing is avoided, in most cases, and (5) the effects of an undetected common disorder like IDA on the mother and fetus are not fully known. Therefore, unless more severe hematologic disease is suspected, a therapeutic trial of iron should be given, at least to those at risk for iron depletion, if not to all pregnant women.
Another disorder in hemoglobin production is thalassemia, which is caused by an alteration in the rate of synthesis of the α- or β-chain. The hemoglobinopathy is classified based on which chain is affected. Clinical symptoms may also be used to classify thalassemia as thalassemia major, intermedia, and minor. In general, only the homozygous forms are in the major category, while the heterozygotes (α, β, α-β, δ) demonstrate variable degrees of symptomatology.47
α-Thalassemia major (hemoglobin Bart's disease) appears to be the most severe, with fetal death occurring in most cases. The poor outcome is due to the increased production of α-chains, which combine to form Bart's hemoglobin, a high-oxygen-affinity hemoglobin that functions poorly for oxygen transport. Electrophoresis on fetal blood shows 85–100% Bart's hemoglobin, with only small amounts of hemoglobin A and hemoglobin F. The infant develops severe anemia and hydrops similar to that noted in those with Rh alloimmunization. Homozygous α-thalassemia is the most common form of hydrops fetalis in Southeast Asia, but is rare in the United States. Homozygous β-thalassemia (Cooley's anemia) is characterized by severe anemia, usually leading to death in early childhood. On electrophoresis, the hemoglobin is usually 40–70%; hemoglobin A2 is mildly increased, while hemoglobin A is reduced and varies with the severity of the anemia.48 Treatment involves repeated transfusions, and pregnancy is infrequent.
Patients heterozygous for thalassemia rarely manifest signs or symptoms of the disease. Heterozygous β-thalassemia is the most common form; patients usually exhibit only mild hypochromic, microcytic anemia, with elevation of the hemoglobin A2 (above 3.5%) and a mildly increased hemoglobin F percentage (2–5%). Heterozygous α-thalassemia is difficult to detect by laboratory means because the affected globin chain is common to all types of hemoglobin. Therefore, there is a reduction in the amount of hemoglobin A, F, and A2, while the percentage of these compounds remains the same as in normal persons. This finding, combined with the fact that anemia is usually mild, also complicates the detection process. Although family pedigrees or intricate hematologic studies are usually required for a firm diagnosis in adults, α-thalassemia is easier to delineate in the neonate, since Bart's hemoglobin and small amounts of hemoglobin H (four β-chains) may be present. Concomitant pregnancy reveals few adverse maternal or fetal effects, except for a slight increase in spontaneous abortions. In general, the maternal outcome is related to the severity of the anemia; those more severely affected have an increased maternal morbidity, although fertility does not appear to be affected. The double heterozygote (α-β-thalassemia) is also difficult to diagnose and may be more common than previously suspected. Pregnancy statistics are sparse, although most authors suggest that the anemia and other symptoms are more severe than with α-thalassemia or β-thalassemia trait alone.
The prenatal diagnosis of thalassemia syndromes should be offered to any couple with a prior affected child or those at risk because of ethnic origin or pedigree. Prospective parents not having an affected child previously need verification of carrier status based on MCV, hemoglobin electrophoresis, and pedigree analysis before fetal assessment is undertaken. A sample of fetal deoxyribonucleic acid (DNA) can be obtained by chorionic villus sampling, amniocentesis, or percutaneous umbilical (cord) blood sampling. DNA analysis of that portion of the α-globin or β-globin gene containing the deletion or mutation can be identified using restriction fragment length polymorphisms and Southern blotting techniques. Direct DNA sequence analysis is available by performing polymerase chain reaction amplification of fetal DNA. Using this method of detection, prenatal diagnosis can usually be confirmed within 7 days of fetal sampling.49
Management of the thalassemia traits during pregnancy is much the same as for the patient with normal hemoglobin. Proper diagnosis is difficult, since the mild microcytic, hypochromic anemia can be easily confused with IDA, a condition that frequently coexists. Folic acid supplementation is recommended because of the increased utilization and high erythrocyte turnover. Blood transfusion or other invasive therapy should be avoided, if possible, unless severe anemia or other problems intercede.50
Disturbance in maturation: DNA synthesis (megaloblastic anemia)
Although not specific, the term megaloblastic anemia is used by most hematologists to describe a group of hypoproliferative disorders that have a characteristic morphologic appearance, ineffective erythropoiesis, and a moderate hemolysis of the circulating RBCs. Pernicious anemia (lack of vitamin B12
) and folate deficiencies are the prototypes of this disorder. The underlying biochemical defect is impaired thymidylate formation, an essential rate-limiting initial step in the DNA synthesis of the body's cells, which require tetrahydrofolic acid as a coenzyme.
The megaloblastic changes occur as a result of a maturation defect in the marrow affecting erythrocytic, leukocytic, and thrombocytic cell lines. The lack of vitamin B12 or folate slows DNA synthesis and delays the nuclear maturation of chromatin of the immature cell into the dense cyanotic figure associated with the normoblast during normal erythropoiesis (see Fig. 2). Although the nucleus remains large and immature, the cytoplasmic mass decreases normally as it does during maturation. Therefore, the large macrocytes found in the peripheral blood of those with megaloblastic anemia represent a delay in nuclear maturation in the erythrocytic series and are diagnostic by their abnormal nuclear cytoplasmic appearance. Likewise, in the leukocytic series, the abnormal granulocyte is referred to as a giant metamyelocyte because of the condensation failure in the large nucleus. Due to granulocyte marrow turnover, leukopenia develops and the cells found in the peripheral blood are hypersegmented, with the nucleus having six or more lobes rather than the normal three to five. Finally, inadequate thrombopoiesis occurs, which results in an increase in bone marrow megakaryocyte mass with fewer platelets in the peripheral blood. The platelets that are present may function poorly, and a bleeding diathesis may be present if the vitamin B12 or folate deficiency is severe.
Folic acid deficiency appears to be the most common cause of megaloblastic anemia. However, vitamin B12 appears to catalyze the conversion of 5-methyltetrahydrofolate to its active form, as well as to affect the storage and transport of folic acid within the body.51 Therefore, a deficiency in vitamin B12 may lead to megaloblastic anemia singularly or by its effect on folate metabolism.52 Causes of folate or vitamin B12 deficiency include decreased intake and conditions in which requirements are increased (e.g., pregnancy, hyperthyroidism, hyperparathyroidism, and malignancies). Causes that are not related to vitamin B12 or folate therapy include the purine and pyrimidine synthesis inhibitors (cancer chemotherapy), pyridoxine-responsive megaloblastic anemia, and Di Guglielmo's syndrome (erythremic myelosis), in which platelet production is more severely affected than erythrocyte or leukocyte maturation. Finally, in various inborn errors of metabolism, such as Lesch–Nyhan syndrome and hereditary orotic acid deficiency, either a lack of enzymatic catalases or metabolic blockage to folate incorporation can cause megaloblastic anemia. All the infrequent causes are difficult to diagnose, and megaloblastic anemia due to these causes is unresponsive to folate therapy. Fortunately, vitamin B12 deficiency and FAD are the most common etiologies of megaloblastic anemia, accounting for 98% of reported cases. FAD is a much more common cause during pregnancy (99%), and it causes 92% of all cases in any age group.53 Vitamin B12 deficiency is extremely uncommon during pregnancy (1:8000 pregnancies); the lack of intrinsic factor almost always occurs in persons beyond the childbearing age unless significant gastric surgery has been performed.54
The major clinical signs and symptoms of megaloblastic anemia are variable and are usually not detectable until the anemia is severe. As in other anemia processes, the symptoms increase in severity with the progression of anemia, from pallor, through weakness, malaise, dizziness, and shortness of breath, and finally to congestive heart failure. Patients with megaloblastic anemias may appear jaundiced more frequently than those with IDA because of the rapid cell turnover and their increased propensity for bleeding diathesis. If anemia is present, the appearance of the peripheral smear is usually diagnostic, demonstrating several hypersegmented granulocytes as well as RBC inclusions (i.e., stippling, Howell–Jolly bodies, Cabot's rings, and nonhemoglobin iron). The most sensitive indicator of early folate deficiency appears to be the hypersegmentation of neutrophilic nuclear material (average low value >3.27 or more than 4% with more than five lobes in 100 consecutive polymorphonuclear neutrophils). Serum folate values in pregnant women are usually lower than in the nonpregnant patient, and these decrease progressively toward term. In the absence of IDA, however, a low fasting serum folate (<3 ng/ml by radioimmunoassay) is virtually diagnostic of folate deficiency. In addition, a low erythrocyte folate activity (<20 ng/ml) is probably the best biochemical index of FAD. Although the anemia is macrocytic, it is usually normochromic with normal MCH and MCHC. The MCV, however, is strikingly elevated in contrast to IDA and may exceed 150 µm3/cell (see Table 1). The high MCV is helpful because it is in contrast to that found during pregnancy, IDA, and other microcytic processes. In general, the more severe the anemia, the more bizarre the erythrocytic changes, with nucleated RBCs appearing in the peripheral blood smear when the hematocrit is less than 20%. Unfortunately, if folate is deficient, there is usually a concomitant IDA, which may confuse the diagnosis.55 However, the RC is lower in a megaloblastic anemia compared with the level in IDA.
INCREASED ERYTHROCYTE LOSS
Anemias due to increased loss of erythrocytes can be divided into acute and chronic types. The chronic types exist in patients with intestinal disease, such as parasite infestation, significant hemorrhoids, or peptic ulcer disease. Since the loss of blood is chronic, the actual number of RBCs lost is probably not as important as the amount of iron lost in excess of what is absorbed. This, coupled with the increasing demand of the fetus and expansion of the maternal blood volume, can convert mild iron depletion to IDA during pregnancy. Although these causes are relatively rare in the reproductive age group in the United States, careful assessment of iron unresponsive anemias in women from developing countries or with histories suggestive of intestinal disorders should be undertaken. Diagnosis and treatment are directed toward the specific disorder.
Anemias resulting from acute blood loss during pregnancy usually have evident etiologies, since external blood loss usually occurs and symptoms are sudden. These disorders can include multiple trauma and spontaneous splenic rupture, as well as disorders of the gastrointestinal, pulmonary, or urinary tract, which may or may not be related to obstetric conditions. Obstetric disorders may occur during early pregnancy (e.g., abortion, ectopic pregnancy, molar pregnancy) or in late pregnancy (abruptio placentae and placenta previa).
Blood loss at delivery may also account for significant losses of iron. Estimation of these losses has been measured with dye and isotope techniques, but large errors are common. Factors that lead to an underestimation of external blood loss at delivery include failure to take placental blood volume into account, incomplete measurement of hemoglobin in solution, incomplete extraction of hemoglobin from clots, and hidden bleeding. The average loss, determined by Cr techniques, is between 150 and 250 ml, depending on the patient's parity and the use of episiotomy, while an additional 30 ml is usually sequestered in the placenta. Therefore, approximately 200 ml of packed RBCs, equivalent to 500 ml of whole blood, is lost at the time of delivery. Approximately 230 mg of the 500 mg of iron used to expand blood volume is lost, while the remaining 270 mg is returned to the iron stores.
The placental “loss” of iron is variable, averaging 25 mg per pregnancy. The transfer of iron appears to take place by the same iron–transferrin complex used in the mother. The placenta, however, appears to function independently, since it stores iron as ferritin and hemosiderin even in the absence of a fetus and after fetal death. The transport of iron across the placenta is unidirectional and preferential, with the fetus accepting iron even when maternal levels are low. This maternal–fetal transfer involves an active transport mechanism, since the saturation of transferrin and the plasma iron level are considerably higher in the fetus. Most of the iron transferred across the placenta (70–80%) goes to the fetal liver, which serves in a storage capacity as well as a hematopoietic organ. The placenta also functions as a barrier to iron overload; thus, iron toxicity in the fetus by maternal overdose does not occur. Study of the incorporation of placental iron into fetal hemoglobin using radioactive material cannot be undertaken because in early investigations, use of labeled iron was associated with infant malignancies. However, indirect evidence shows heme incorporation into fetal RBCs to be similar to maternal utilization. Diseases in the mother (e.g., infection or inflammation) may decrease the amount of iron transferred, owing to poor storage mobilization and transferrin release. On the other hand, anemia due to iron loss during lactation, FAD, or chronic blood loss does not affect transfer of iron across the placenta.
The iron loss in lactation is small and usually averages 1 mg/day. Breast milk contains an iron binding protein similar to transferrin, called lactoferrin. This protein is synthesized in the mammary gland and stored in the ductal lining cells. The secretion of lactoferrin into the breast milk accounts for the iron loss noted in iron sufficient women during lactation. This compound is also bacteriostatic, and thus assists the infant in combating early infection and possibly prevents maternal breast infections. Moreover, since many lactating women do not have menses, the excretion of 1 mg of iron per day is offset by the avoidance of menstrual loss. Therefore, dietary iron supplementation as prescribed during and after pregnancy is usually sufficient to replenish iron stores during lactation.
During the postpartum period, acute bleeding problems may also occur owing to uterine atony, genital lacerations, or retained placenta. The acute causes, both obstetric and nonobstetric, are usually best treated with blood transfusion and removal of the offending agent. Obviously, the diagnosis depends on the specific disorder, as does the proper therapy. After the correct diagnosis is made and acute therapy is started, treatment with oral iron is usually indicated. Patients with acute blood loss usually show a normochromic, normocytic cell pattern with normal blood indices, hemoglobin levels, and PCV until after equilibration occurs, a process that may take from 12 to 24 h. The iron studies are normal. Chronic blood loss, on the other hand, may be subtle enough to reveal only a loss in iron content if the loss in blood cells did not exceed the capacity of the bone marrow to produce the RBCs. In these cases, the chronic disorders would involve the same laboratory findings as IDA.
INCREASED ERYTHROCYTE DESTRUCTION
Diseases caused by increased destruction of erythrocytes present a hematologic profile characterized by persistent reticulocytosis, a finding that would indicate that there is increased erythrocyte destruction in excess of compensatory erythropoiesis by the bone marrow.56 The etiology of these disorders can be (1) extrinsic hemolytic anemia in which a mechanical, drug, infectious, or immune factor is responsible for the decreased life of the red cell; (2) intrinsic hemolytic anemia in which the red cell loss is due to an inherited defect in the blood cell or membrane; or (3) unknown.57, 58
Extrinsic hemolytic anemia
Extrinsic hemolytic anemia is a proliferative disorder that involves erythrocytes that are structurally and metabolically normal but that, owing to environmental factors, suffer early death. Most frequent causes are hypersplenism, Coombs-positive hemolytic anemia, and microangiopathic anemia.
Enlargement of the spleen usually leads to excessive RBC sequestration and destruction, and is most common in its severe and fatal form during childhood. In adulthood, the degree of anemia is usually mild, with an RC rarely greater than 6%. The anemia is usually normochromic and normocytic, and the overall prognosis is dependent on the underlying cause. In some cases, corticosteroids are helpful, and removal of the spleen, particularly during pregnancy, is used only as a last resort.
MICROANGIOPATHIC HEMOLYTIC ANEMIA
In microangiopathic hemolytic anemia (MHA), the products of destroyed red cells such as schistocytes are seen in the peripheral smear. Autoimmune diseases such as lupus erythematosus and polyarthritis have been associated with this disorder, as has pregnancy-induced hypertension, acute glomerular nephritis, and idiopathic thrombocytopenia purpura. In addition, other disorders associated with disseminated intravascular coagulation (DIC), such as fetal death, amniotic fluid embolus, and severe infection, may reveal these signs of MHA. In most of these diseases, small-vessel damage or abnormal coagulation leads to a network of fibrin strands that partially occlude the microcirculation. It is this basic pathophysiologic change that causes the fragmentation and lysis of the RBC. Therapy for those patients with MHA involves diagnosis and treatment of the underlying disorder and principally the reestablishment of the integrity of the microcirculation. The prognosis for both mother and fetus depends upon the promptness with which the underlying disorder can be overcome.
In some patients, a positive direct Coombs test is found in conjunction with hemolytic anemia. In these patients, usually a complement of the IgG antibody variety is irreversibly fixed to the erythrocyte membrane. This leads to premature destruction of the RBCs. These types of disorder usually are categorized as having “warm” or “cold” antibody hemolytic anemia. Those with warm antibody hemolytic anemias are usually associated with an IgG antibody and noted in many collagen-vascular diseases, patients with lymphomas, or drug reactions (particularly α-methyldopa). Those with cold autoantibodies are usually of the IgM class and are most commonly noted after a viral or mycoplasmal infection. The IgM antibody does not affix to the RBC membrane but rather stimulates complement, which is responsible for the positive antibody test. If the underlying disorder is treated or removed, the hemolytic process is usually resolved in 2–3 weeks, although the Coombs test may remain positive for up to a year. Transfusion is not used unless absolutely necessary because the transfused cells likewise undergo fragmentation. Frequently, corticosteroids are used in those with warm antibody or IgG-mediated disorder. The prognosis for mother and fetus is dependent on the prognosis of the underlying disease process.
Intrinsic hemolytic anemia
Intrinsic hemolytic anemias are those in which there are inherited disorders leading to premature destruction of the RBCs. Included in these types of anemias are hereditary spherocytosis, various RBC enzyme deficiencies, and the hemoglobinopathies.
This anemia is inherited as an autosomal dominant trait, and the RBC membrane is abnormally permeable to sodium. This leads to cellular disruption and also to premature cell death in the spleen. The diagnosis of hereditary spherocytosis is confirmed with the osmotic fragility test, and the treatment is usually transfusions or splenectomy. The prognosis for mother and fetus is usually excellent unless splenectomy must be performed during pregnancy. In most cases, treatment of this disorder with splenectomy has already been carried out prior to conception.
Various enzymatic defects inherited in RBCs can also lead to premature destruction of the erythrocyte. The two most common disorders involve pyruvate kinase deficiency and glucose-6-phosphate dehydrogenase (G6PD) deficiency. The pyruvate kinase deficiency is inherited as an autosomal recessive trait and is usually diagnosed during childhood because of persistent low-grade anemia and jaundice. The fluorescent spot test is the diagnostic test of choice. This is usually a mild disorder and does not disturb maternal or fetal homeostasis. G6PD deficiency, on the other hand, is inherited as an X-linked disorder and is more commonly found in blacks, with 13% of American black males being affected. The use of oxidizing drugs, such as nitrofurantoin and primaquine, stimulates acute hemolysis of red cells with accompanying hemoglobinuria. In addition, viral or certain bacterial infections may also stimulate this hemolytic event. The methemoglobin reduction test is the diagnostic assessment technique of choice, and treatment is usually removal of the offending agent. It is rare in today's society that this disorder would adversely affect the mother or developing fetus. Nevertheless, G6PD deficiency should be kept in mind prior to prescribing drugs to black parturients, since 25% of American black women are carriers of this trait.
Most of the disorders regarding hemoglobin usually involve the abnormal globin structure of the RBC. Fortunately, most of these abnormal hemoglobins, called hemoglobinopathies, are very rare; of the more than 300 types reported, only a few are clinically very common. Most of these disorders involve a single amino acid substitution in one of the globin chains, although some involve deletions, additions, or fusions in the α- or β-chains. All of these hemoglobinopathies, when clinically significant, lead to the production of erythrocytes that either are structurally unable to perform duties, such as oxygen transport, or are prematurely destroyed. All are associated with an increased reticulocytosis, indicating the body's ability to attempt compensation. To understand these, familiarity with hemoglobin synthesis and structure is necessary.
The erythrocyte is extremely complex; its membrane is composed of lipids and proteins, while the interior of the cell contains primarily hemoglobin, which is intimately associated with oxygen transport. The erythrocyte develops from the multipotential stem cell or myeloblast, which differentiates along the erythroid line in the bone marrow (see Fig. 2). During maturation, it loses most of the metabolic and biosynthetic capabilities inherent in most other cells. The capacity to synthesize nucleoproteins, such as DNA, is lost at the basophilic normoblast stage, and stainable DNA itself is removed before reticulocyte release, as shown in Fig. 2. Ribonucleic acid (RNA) synthesis is lost as the peripheral reticulocyte matures, and loss of electron transport mechanisms prevents the use of phosphate bonds as an energy vehicle, requiring the mature erythrocyte to employ anaerobic glycolysis by the hexose and penrose phosphate shunt pathways. Thus, the RBC's only metabolic function is the process of gaseous and ionic transfer, using the remaining pathways for purine and pyrimidine metabolism. Although these critical functions are performed well in the mature RBC, the lack of RNA and DNA is extremely important, since it renders the cell incapable of repair or repetition. Therefore, in certain disease processes that alter this maturation sequence (see Fig. 2), the production rate and functioning of the cell lines are impaired.
The structure of hemoglobin is shown in Fig. 7 and consists of heme, which is composed of a divalent iron atom connected to four pyrrole rings through their substituent nitrogen atoms.59 This polyvalent compound is surrounded by two pairs of dissimilar polypeptide chains, each linked to one of the pyrrole rings. Each pair of polypeptide chains has different sequences of amino acids and is stereochemically arranged in four helical configurations. The physical properties of heme, as well as its association with the iron atom, classify it as a metalloporphyrin, a structure similar to chlorophyll or vitamin B12. The conjugated double bonds of heme readily absorb visible light and are responsible for the red color of hemoglobin. The intense spectrophotometric absorption peak of hemoglobin at 412 nm, the Soret band, may be clinically important because it may overlap the peak of amniotic fluid bilirubin, measurement of which is used to assess the severity of Rh isoimmunization. Functionally, it is the iron atom that carries the oxygen; in its ferrous state (Fe2+ ), six binding sites are present. Four are attached to the heme, while another is attached to a histidine residue of a globin chain. The other coordination position of iron is unoccupied in deoxyhemoglobin under proper physiologic conditions. This final binding point is protected from oxidation by the globin chains surrounding the heme moiety. Separation of heme from the globin during destruction results in the oxidation of the iron atom and the formation of hematin. If hemoglobin iron is oxidized to its ferric state (Fe3+), methemoglobin is formed; this cannot serve as the oxygen carrier since the final coordination site is bound.
Fig. 7. Biosynthesis of hemoglobin
Heme synthesis takes place in the particulate and soluble fraction of the erythrocyte (see Fig. 7). Initially, δ-aminolevulinic acid (ALA) is synthesized from succinyl-CoA and glycine in the particulate fraction of the cell.60
Next, two moles of ALA combine to form porphobilinogen, and subsequently, four of these moieties converge to form one mole of uroporphyrinogen III. Through several intermediate steps, this compound is converted into coproporphyrinogen III. By successive decarboxylations, it is converted into the heme precursor protoporphyrin IX, a reaction catalyzed by a mitochondrial enzyme called ferrochelatase or heme synthetase. A ferrous iron moiety is inserted into the protoporphyrin as a final step in the pathway of formation of heme. Current data suggest that heme biosynthesis is unidirectional and the control mechanisms for the pathway are necessarily located in the first enzymatic step (ALA synthesis). Positive feedback inhibition of the ALA pathway by certain metabolic byproducts such as hematin or hemin occurs, while other degradatory products such as billverdin and bilirubin do not appear to affect heme synthesis. Finally, the lack of precursors or cofactors, such as Fe2+
, folic acid, vitamin B6
, and vitamin B12
, may impair heme production at various reaction sites during hemoglobin synthesis.
Globin chain synthesis
The sequence of amino acids in the globin chains is determined by genetic control. Once initiated, the sequence of the nucleotide bases in DNA is transcribed to messenger RNA (mRNA). The mRNA is transported from the nucleus to the ribosomal RNA (rRNA), which is the actual site of protein synthesis. The mRNA, by collation with transfer RNA (tRNA) on the rRNA template, translates the message from a language of nucleotide bases into that of amino acids in the proper sequence. The initiation of the translation always begins with the same code word (AUG) from mRNA regardless of the globin chain synthesized; therefore, the first amino acid is always methionine. The initial phase, called transcription, proceeds through approximately 40 amino acid subunits. The elongation phase extends nearly to the termination point and is catalyzed by a protein called elongation factor I, which is glucose triphosphate (GTP) and tRNA dependent. Finally, the parent mRNA gives the signal to terminate transcription and the newly formed globin chain is released, while the ribosome searches for the initial phase of another mRNA. The three nucleotide codons that signal for the chain termination of all proteins are UAA, UAG, and UGA. These are decoded by tRNA and by protein releasing factors in the particulate cell fraction. Once complete, each polypeptide chain contains 141–146 amino acids. After synthesis, two pairs of globin chains interact with the four pyrrole portions of the heme and the assembly of the hemoglobin moiety is complete (see Fig. 7).
The polypeptide chains in normal adult hemoglobin are termed alpha (α), beta (β), gamma (γ), and delta (δ). The nomenclature is based on various alterations in the globin chain and is summarized in Table 5. The α-chains are found in all normal adult hemoglobin and contain 141 amino acids. They combine with the β-, γ-, or δ-chains, each containing 146 residues, to form the two pairs of globin chains found in normal hemoglobin. For instance, when two β-chains accompany two α-chains (α2Aβ2A) hemoglobin A (HbA) is formed; this accounts for 95% of the total hemoglobin in the normal adult. Hemoglobin A2 (HbA2) is formed by two δ-chains accompanying the α-chains (a2Aδ2A) and accounts for 2–3.5% of normal hemoglobin. Fetal hemoglobin (HbF) is formed by two γ-chains coupled with two α-chains (α2A γ2F) and accounts for the remainder of hemoglobin found in the adult.
Table 5. Nomenclature of normal and abnormal hemoglobins
|Adult hemoglobin*|| ||2α|
|Adult hemoglobin*|| ||2α||HbA2||α2Aδ2A|
| || ||2δ|| || |
|Adult hemoglobin*|| ||2α|
|Embryonic fetal hemoglobin*|| ||2α|
|Sickle hemoglobion||β-chain substitution||2α|
|HbS||α2Aβ2S(6 glu ® val)|
|HbC||α2Aβ2C(6 glu ® lys)|
|HbMBoston||α2M(58 his ® tyr) β2A|
|Increased oxygen affinity†||α-chain substitution†||2α|
|HbChesapeake||α2Ches(92 arg ® leu) β2A|
|Unstable hemoglobin†||β-chain substitution‡||2α|
|HbSaki||α2Aβ2Saki(14 leu ® pro)|
|β-Thalassemia||None (decreased synthesis)||2α|
* Structurally normal variants.
† Structural abnormalities include substitution, inversion, or deletions of amino acids in either chain.
‡ Other abnormalities may affect either chain.
An amino acid deletion, inversion, or substitution structurally alters the globin chains to cause most of the abnormal hemoglobins. The location of an amino acid abnormality (if known) is designated by a superscript notation (in parentheses) next to the involved globin chain: α2Aβ2S(6 glu ® val).26 Abnormal hemoglobins may also involve the α-chains and are called α-hemoglobinopathies. If several abnormal hemoglobins are similar in their biochemical properties, a subscript is used to indicate the geographic location where the hemoglobinopathy was first described (e.g., HbMBoston, HbMMilwaukee). The changes in the amino acid sequence of the globin chains may evoke a wide spectrum of clinical symptoms. Abnormal hemoglobins may also be classified according to the changes they elicit in oxygen carrying capacity, stability, erythrocyte longevity, or synthesis rate (see Table 5).
The structure and function of the globin chains are determined by polygenetic inheritance, whereas there appears to be genetic control for heme synthesis common to all hemopoietic tissue. The four globin chains contain hundreds of amino acids as possible mutagenic sites, and since there may be at least four mutations per position by nucleotide sequencing, the number of changes in hemoglobin structure by genetic malfunction is enormous. Four separate genetic loci have been identified for the a, β, γ and δ polypeptide chains. The use of marker chromosomes in cytogenetic studies indicates that the genetic loci for the α-chain is on chromosome 16, while the β-, γ-, δ-chain loci are closely associated on chromosome 11. The structural changes in the amino acid sequencing characterizing most abnormal hemoglobins appear to be related to genetic mutation. In contrast, thalassemia seems to occur as a result of defective mRNA.
The cause of the various mutations responsible for the globin chain sequence for most hemoglobinopathies has not been determined; however, the mutation in hemoglobin S (HbS) may be a genetic response to malaria.
Sickle hemoglobin is rather common in this country among African-Americans. HbS-S (sickle cell anemia) and its more severe variants, HbS-C and HbS-Thal, comprise a group of hemoglobinopathies called sickle cell disease (SCD). It is these patients who are most severely affected clinically during pregnancy or in the nonpregnant state. Those affected with sickle cell trait and mild sickle cell anemia may be asymptomatic during gestation.
Hemoglobin S represents a structural defect caused by a genetic mutation, possibly as an adaptive response to falciparum malaria. Heterozygosity (HbA-S) gives a protective effect, while those persons who are HbA-A or HbS-S are more likely to die during childhood in areas endemic for malaria. The substitution of the amino acid valine for glutamic acid at the sixth position from the N-terminal of both β-chains gives S hemoglobin its unique sickling characteristic compared with normal adult HbA. Likewise, the substitution of lysine at the identical position results in hemoglobin C (HbC). Otherwise, these hemoglobins are exactly the same as normal adult hemoglobin, and this one change is responsible for all of the symptomatology. The amino acid substitution also causes the differences in electrophoretic migration of HbS and HbC, HbA, HbF, and HbA2. Changes caused by amino acid substitutions, deletions, or inversions in either the α- or β-chain give rise to other structurally abnormal variants, and although some of these hemoglobins have an adverse effect on pregnancy outcome, they are fortunately quite rare. Therefore, hemoglobinopathies listed in the SCD category are the only ones that are clinically important. Mild forms of the disease are important only when the patient is under severe stress or for genetic counseling purposes, since the clinical presentation is usually benign.
The terminology of sickle hemoglobinopathies has undergone many revisions since 1960. The currently accepted classifications include: (1) sickling disorder, any red cell that undergoes in vitro/in vivo sickling in the deoxygenated state; (2) sickle cell trait (HbA-S), the heterozygous form of HbS infrequently associated with clinical symptoms; (3) sickle cell anemia (HbS-S), the homozygous form of HbS frequently associated with severe clinical symptoms; (4) sickle cell disease (HbS-S, HbS-C, and HbS-Thal), disorders in which HbS is all or part of the abnormal hemoglobin composition and that are usually associated with severe symptoms; and (5) mild sickle cell states (HbS-D, HbS-E, HbS-Memphis), those in which HbS is present but clinical symptomatology is usually mild. Numerically, HbS, both homozygous and in combination with other hemoglobins, is the most common abnormal structural defect; HbC is the second most common hemoglobinopathy in the USA, and HbE is next in frequency. Hemoglobin D is similar in its other properties to HbS but does not sickle. Both HbE and HbD can combine with HbS to give mild clinical symptomatology.
Fig. 8. The sickle cell crisis cycle. (Martin JN Jr, Morrison JC. Managing the parturient with sickle cell crisis. In Huddleston JF (ed). Clinical Obstetrics and Gynecology: Sickle Cell in the Gravida Woman. Philadelphia, JB Lippincott, 1984)
Anemia is the most constant feature of subjects with severe hemoglobinopathy. Generally, patients with a hematocrit less than 20% or a hemoglobin level below 6 g/dL have homozygous β-thalassemia or HbS-S. Patients with a hematocrit of 20–25% may have a HbS-S, HbS-C, or HbS-Thal hemoglobinopathy, while subjects with a hematocrit of 25–30% may have any hemoglobinopathy except homozygous β-thalassemia and HbS-S. In most of the mild forms (HbS-D, HbS-E, HbA-S), the hematocrit is 30–35%. When the hematocrit is 25–35%, diagnosis is particularly difficult because many disorders are included in the differential diagnosis. The RBCs are usually normochromic, normocytic, but concomitant IDA may confuse the issue. A reticulocytosis of up to 20% and thrombocytosis are not uncommon in HbS-S patients during crisis. The serum bilirubin is usually mildly elevated in the asymptomatic patient with HbS-S but may reach 15–20% during crisis. The urinalysis usually shows hyposthenuria (with specific gravity <1.005), hematuria, and bile and direct bilirubin. Asymptomatic bacteriuria is more common in these patients, and pyuria usually leads to active symptomatic infection due to poor opsonization of bacteria. The liver enzymes are usually elevated and reach high levels during crisis; abnormalities of coagulation, such as a decrease in the platelet count and fibrinogen with presence of fibrin split products, are more common during a crisis but may be present at any time. The prothrombin, partial thromboplastin, and clotting times are normal in patients with sickle hemoglobinopathies.
Several commercial kits are available for diagnosis of HbS (Sickledex, Ortho Diagnostics, for example) and are based on turbidimetric assessment or color change. Although these tests have few false negative reactions, they do have a 5–8% incidence of false positive reactions; the standard deoxygenation assay using a reducing agent (sodium metabisulfate) is employed on positive screening samples. This microscopic test is very accurate but does not differentiate the various hemoglobinopathies. Electrophoresis and chromatography with starch gel, cellulose acetate, and agar gel are used to quantitate each hemoglobin and to aid in the diagnosis of the specific hemoglobinopathy. By electrophoresis, HbA-S usually has 25–35% HbS, 60% HbA, normal HbA2, and increased HbF; HbS-S demonstrates 90–95% HbS, with small concentrations of HbA2 and normal to increased amounts of HbF; HbS-C usually evidences normal percentages of HbA2 and HbF, with an equal division of the remainder between HbS and HbC (45–48%). Hemoglobin S-Thal is very difficult to diagnose even by electrophoresis but appears to be much like HbA-S with slightly more sickle hemoglobin.
Hemoglobin S-S, HbS-C, and HbS-Thal can be usually differentiated from the benign hemoglobinopathies, such as HbA-S, by a history of recurrent painful vaso-occlusive crises in the former. These crises usually last 2–6 days, may be associated with fever as well as leukocytosis, and may lead to unnecessary surgical procedures unless the hematologic diagnosis is suspected. These attacks are more common in HbS-S patients, usually average one to four a year, and are commonly associated with fever, pregnancy, trauma, or physical stress. The crisis is rarely truly “hemolytic” and is more commonly termed vaso-occlusive, although other types of crises are possible (see Fig. 8). Finally, cerebral manifestations are not extremely common but are ominous, since they may be associated with severe morbidity and death. The sudden deaths with SCD have been reported to involve both coronary and cerebral vessel occlusions. Coma, convulsions, and death due to cerebral infarction are most common in these patients. The sudden onset of symptoms after a benign history is characteristic of patients with HbS-C. Up to 20% of patients with severe sickle hemoglobinopathies die by age 10, and many more die by the age of 40.
SICKLE CELL DISEASE IN PREGNANCY
The hemoglobinopathies comprising SCD are more common than they appeared to be in the past. Hemoglobin S-S was said to occur in 1 in 600 Americans of African descent; however, the incidence is closer to 0.5–0.7% in certain areas in this country. Hemoglobin S-C is present in about 1 in 800 persons of African descent.61 The incidence of HbS-Thal is said to be 1 in 1250, but this is probably underreported, since the anemia is mild. The fertility of patients with SCD was assumed to be reduced because there are few pregnancies, but newer evidence shows that the incidence of pregnancy in these subjects is the same as in the population at large.62
The prevailing attitude of most perinatologists is that the patient with SCD appears to be at significantly greater risk for crisis and other dire complications when pregnant.63 Only infection, trauma, hypoxia, and acidosis approach pregnancy as a frequent cause of crisis and severe morbidity/mortality in patients with SCD. Factors associated with pregnancy that complicate the clinical picture include the “hypercoagulable state” of pregnancy, the increased susceptibility to infections, the increased vascular stasis in the pelvis and lower extremities, the increased metabolic and hematologic demands of pregnancy, and the stress of parturition. Because of these factors, the status of mother and fetus was very poor in many series.
Several large review series prior to 1969 show a morbidity rate of 50–80% and maternal mortality figures ranging from 2–25% for SCD patients during pregnancy.61, 63 Painful crises were among the most common symptoms, being reported by all investigators. In addition, most patients had a reduction in their hematologic values during gestation. Other major complications such as pneumonia (3–15%), pyelonephritis (5–12%), and endometritis (72–10%) are frequent causes of crisis. Pulmonary embolus (1–9%) and congestive heart failure (1–5%) are also common causes of severe morbidity, as is pregnancy induced hypertension. Since 1970, the maternal death rate has dropped considerably (i.e., 0–1% in most large series), as has the morbidity rate in this country. These changes probably reflect better antepartum scrutiny as well as new management techniques.
Although some authors feel that the clinical course of SCD in pregnancy tends to be similar to its course before pregnancy, many previously asymptomatic patients may suffer a serious, sometimes fatal, crisis during pregnancy. Also, several investigators have associated an increased level of hemoglobin F with a reduction in the number of complications. The hypothesis is supported by those rare persons with persistent hereditary fetal hemoglobin who have no crises. However, with persistent hereditary fetal hemoglobin, the hemoglobin F is homogeneously distributed in each cell, whereas in patients with SCD, the hemoglobin F, although present in larger quantities, is unequally distributed in the cell. Several studies have shown that the level of hemoglobin F varies greatly in both asymptomatic patients and those in crisis. Therefore, it is unwise to rely on a negative history or absolute hemoglobin F percentage to guide therapeutic judgment in pregnant patients with SCD.
Although hemoglobin C is more stable than hemoglobin S, the combination of hemoglobin C with hemoglobin S appears to facilitate sickling more than does a combination containing hemoglobin A. This is particularly true in late pregnancy, when venous stysis and maternal hypercarbia are usually present. It has been shown that the number of sickled cells remains constant for a given P O2 if hemoglobin S is present alone. However, if both hemoglobin C and hemoglobin S are present, the number of distorted cells increases to a greater degree under similar conditions. The red cells appear to have a shorter survival time in HbS-C patients due to the crystallization of hemoglobin C within the red cell. Symptomatically, patients with HbS-C appear to be healthier and have fewer crises than do those with HbS-S, but complications resulting from stress, such as pregnancy, infection, or trauma, are more severe and sudden (e.g., serious eye lesions, renal papillary necrosis, aseptic bone necrosis, acute pulmonary thrombosis, marrow emboli, and true hemolytic episodes). In general, pregnancy results in a similar risk for patients with HbS-C and HbS-S with regard to morbidity and mortality, while patients with HbS-Thal have a clinical course intermediate between those with HbS-S and HbA-S.
Perinatal salvage rates in patients with SCD have been very low in most series. The incidence of abortion is high (20–35%) and has been related to the “chronic disease state” or malnourished condition of the patient, sickling in the infundibulopelvic vessels (decreased progesterone), and distorted cells in the arcuate arteries of the uterus (decreased placental blood flow). These changes may also lead to intrauterine growth restriction and premature labor, which is also increased (10–55%) in gravid patients with SCD. The decrease in mean birth weight of these infants (250–500 g) has also been related to the high incidence of maternal infection and premature labor. Stillbirths are also increased (5–13%) and appear to be related to severe maternal crises with evidence of placental sickling. Although fetal deaths occur in asymptomatic subjects, there have been no reported stillbirths due to sickling in the HbS-S fetus. The overall neonatal mortality rates are not increased for progeny of women with SCD, provided asphyxia and growth related complications do not occur; actually, the incidence and severity of respiratory distress syndrome are decreased in these offspring.
The perinatal salvage rates are decreased in patients with HbS-C, ranging from 40 to 85%. There is an increase in miscarriages and fetal deaths. Both prematurity and small-for-gestational-age infants are more common, but not to the extent found in HbS-S patients. Hemoglobin S-Thal patients show even greater reduction in miscarriages and stillbirths compared with HbS-S and HbS-C patients.
The percentage of small-for-gestational-age and premature infants is increased in HbS-Thal and near average for HbS-C patients.
Intensive antepartum management of SCD patients, with visits at bi-monthly intervals for the first 20 weeks and then weekly until delivery, appears to be the best method of preventing decompensation and crisis. During each visit a thorough search should be made for infection and historical and physical evidence of sickling; also, a nutritional survey should be made and diet counseling should be provided. Vigilance for crisis is important, and the patient should be hospitalized if pain, fever, infection, or other signs of sickling are noted. Hematocrit and hemoglobin determinations should be performed on each visit, since rapid reductions in these indices may occur even in the asymptomatic patient. The RC, blood film, and serum bilirubin determination may also be important in assessing the patient's hematologic risk for crisis. Methods for assessment of fetal well being, such as ultrasonography, oxytocin challenge test, nonstress test, and biophysical profile, are also used to detect fetal jeopardy in these patients.64
Folic acid, 4 mg daily, is recommended in pregnant SCD patients. Iron therapy, in contrast, was thought to be ineffective and etiologically related to hemochromatosis if administered continually. However, IDA is common in these patients and was found in as many as 65% of pregnant HbS-S patients.65 Because the duration of pregnancy is short, many investigators administer 60 mg of elemental iron three times a day with meals during pregnancy if the blood smear or other hematologic assessment techniques show evidence of IDA in patients with SCD.
Therapy of patients with SCD has been largely symptomatic. Analgesics, hydration, vasodilatation, oxygen, bed rest, and anticoagulation have all been used with questionable success. Tolazine, cobaltous chloride, carbon monoxide, and carbonic anhydrase inhibitors have not been found useful. Hyperbaric oxygen, although effective, is cumbersome and most often unavailable. In general, only oxygen and mild analgesics are still used clinically in the treatment of crisis.
Antitumor drugs such as hydroxyurea increase production of hemoglobin F in primates and in patients with SCD.66, 67, 68 The drug, decitabine has shown efficacy in short- and long-term trials in increasing HbF levels and total hemoglobin levels in patients failing to respond to hydroxyurea.69, 70, 71, 72 However, animal studies have demonstrated teratogenicity, embryotoxicity, stillbirths and intrauterine growth restriction at doses lower than the usual human dose and therefore it is advised to avoid becoming pregnant during treatment with any of these agents. On the other hand, the use of erythropoietin during pregnancy is considered safe. In nonpregnant sickle cell patients, initial reports suggested that recombinant human erythropoietin (rhEpo) stimulated HbF production however, subsequent clinical studies with both rhEpo alone and in combination with standard doses of hydroxyurea have produced conflicting results.73, 74, 75, 76 Because of its documented safety, and owing to good, personal clinical experience with this medication during pregnancy, these authors continue to use erythropoietin in pregnant patients with significant anemia during pregnancy.
Red blood cell dehydration plays a key role in the SCD process due to the excess activity of potassium chloride and calcium activated potassium transport channels.77, 78
Research has focused on medications that inhibit the activity of these channels including oral supplementation with magnesium which has demonstrated some promise in regard to effectively treating sickle cell patients.79
Nicosan is an herbal plant extract that has been successfully used in Nigeria to prevent painful crises associated with SCD, but the safety of this medication during pregnancy is not known.80, 81, 82
Other promising areas of investigation include hematopoietic cell transplantation and gene therapy, neither of which would likely be utilized in the pregnant patient population.83
Controversy exists regarding the role of prophylactic blood transfusion in the management of SCD in pregnancy.84, 85, 86 In the only randomized controlled trial published to date, prophylactic transfusion was associated with a decreased risk for painful crisis and severe anemia, but no difference was observed for pregnancy outcome.84 By limiting transfusion to situations in which it is clinically indicated, patients are not subjected to the increased risk for blood-borne infections, iron overload, and alloimmunization. The rate of alloimmunization in sickle cell patients is estimated to be between 18% and 36%.87, 88 It is estimated that six to eight deaths per year in the United States occur as a result of delayed transfusion reactions.89 It appears from the available evidence that the reduction in morbidity and mortality of SCD in pregnancy may be attributable to improvements in general management of pregnancy rather than prophylactic transfusion per se.90 However, prophylactic transfusions may be indicated in patients who have particularly severe disease manifestations or for symptomatic patients who are unresponsive to conservative management.91 Major complications (e.g., worsening anemia; intrapartum complications such as hemorrhage, septicemia, and cesarean delivery; painful crisis; chest syndrome) may require intervention with an exchange transfusion. There is no consensus regarding the exact hematocrit value below which transfusion should be considered. However, when a transfusion is clinically indicated in the patient with SCD, the objective is to lower the percentage of HbS to approximately 40% while simultaneously raising the total hemoglobin concentration to about 10 g/dL.91 Exchange transfusion, also known as erythrocytapheresis, allows the HbS-containing cells and irreversibly sickled cells to be removed by extracorporeal differential centrifugation. It affords the simultaneous return by venous access in the other arm of the patient’s own plasma, leukocytes, platelets, and clotting factors along with donor, leukocyte-poor washed HbA-containing red cells. This type of transfusion occurs via a machine that allows for automated continuous erythrocytapheresis. In addition, this method maintains isovolemia at all times and allows the provider accurately to monitor the patient’s hematologic indices such as hemoglobin levels and hematocrit. It also has the advantage of administration on an outpatient basis. If such a device is not available, the manual “push–pull” mechanism can be considered.92 Generally, six units of donor packed red cells are exchanged by this process and there is rapid resolution of crisis symptomatology and ongoing sickling. In addition to being more isovolemic, automated continuous erythrocytapheresis requires less transfusion time than the manual method.93 Obviously, most of the complications associated with this approach are related to the risk of the blood products. Blood products given to these patients should be from cytomegalovirus-seronegative donors or leukoreduced. Careful cross-matching to minimize minor blood incompatibilities and isoimmunization is critical in avoiding problems later for these patients who may need blood products at various points during their life. The use of blood from family members or friends matched for recipient and donor red cell antigens are clinically helpful in reducing the number of posttransfusion crises. Such episodes are known as delayed hemolytic transfusion reactions. The exact mechanism of hemolytic transfusion reaction is not well understood, and the pathophysiology seems to be far more complex because it involves the destruction of both patient and donor RBCs. A subcategory of this problem is known as a delayed transfusion reaction, which is classically characterized by a clinical triad of fever, hyperbilirubinemia, and anemia occurring 3–10 days after transfusion. Laboratory tests that aid diagnosis include increased reticulocyte count, free hemoglobin in the urine, fragmented RBCs on peripheral smear, decreased posttransfusion HbA on electrophoresis, or discovery of previously undetected alloantibodies.94
During labor and delivery, segmental epidural block appears to be the best method of analgesia if performed by the obstetric anesthesiologist or skilled perinatologist; hypotension and resultant hypoxia greatly increase the risk of sickling. Alternatively, small amounts of meperidine or other analgesic agents may be used with pudendal or spinal anesthesia for delivery. Oxygen is administered at 4 L/min with the patient laboring in the semi-Fowler, lateral recumbent position. Patients may be followed by blood smears to detect sickling, and the fetus is assessed by continuous electronic monitoring. Delivery is usually spontaneous at term, and cesarean sections are performed only for obstetric indications. Oxytocin has not been found harmful in the patient with SCD and is used when indicated.
Postpartum SCD patients are scrutinized for excess blood loss, infection, or thrombophlebitis. Antithrombotic stockings, if properly applied, and early ambulation are encouraged; prophylactic antibiotics are usually not employed. Frequent visits (bi-weekly) to the clinic assure normality through the 6-week postpartum adjustment phase. The infant should be tested using cord blood for electrophoresis to detect heterozygotes and to identify those with SCD so that counseling and education of the parents can effect a more favorable outlook for the child.
In the recent past, there has been much controversy in regard to safe contraceptive methods in patients with SCD. In general, patients with this disease should be considered to have a condition that exposes them to increased risk in the event of an unintended pregnancy and therefore the benefits of most available contraceptive options is considered to outweigh potential risks. In 2010, the Centers for Disease Control modified the World Health Organization recommendations for medical eligibility criteria for contraceptive use.95 The use of combined oral, hormonal or vaginal ring contraception, or those utilizing the copper intrauterine device, should be considered methods that generally outweigh the theoretical or proven risks. Additionally, use of contraceptive options such as progestin only pills, depot medroxyprogesterone acetate, levonorgestrel releasing implants or intrauterine devices should be considered safe and used without restriction in this category of patients. The barrier contraceptives such as condoms, diaphragms, and foams are extremely safe but are not very effective; therefore, if they are to be used, intensive patient education is mandatory to ensure compliance.
Much has been written about patients with severe hemoglobinopathies concerning surgical sterilization or abortion. Based on earlier data, some have recommended sterilization prior to pregnancy, while others have urged abortion and sterilization or sterilization after one pregnancy. More recent statistics, however, indicate that the danger to the patient from pregnancy is less than in earlier years. For this reason, the patient should be able to make a decision based on her own desires as to family size, taking into account the genetic disability of the infant, the risk of the therapy during pregnancy, and her potentially decreased longevity. Although all the maternal and fetal statistics are improved, it is still much more dangerous for SCD patients to endure pregnancy than it is for those subjects without a hemoglobinopathy.
Sickle cell trait (HbA-S)
Sickle cell trait is a common hematologic abnormality and occurs in 5–14% of African Americans in the US. The patient is usually not anemic unless a concomitant IDA is present. The benign clinical course and inadequate screening lead to underdiagnosis of the HbA-S. The fertility of these patients is thought to be normal. Sickle trait has been associated with an increased susceptibility to renal problems, such as hematuria, frequent infections, and hyposthenuria. Splenic infarction has been reported, but actual crisis is rare unless hypoxia or infection is present. Studies in the literature concerning pregnant patients with HbA-S are less well documented and less numerous than those involving SCD and are conflicted.96, 97, 98 These patients appear to have an increased incidence of pyelonephritis, asymptomatic bacteriuria, and chronic renal disease when studied during pregnancy. The renal problems may be related to sickled cells lodging in the hypertonic portions of the renal medulla, leading to stasis, ischemia, and structural damage, followed by frequent infections. The persistent hyposthenuria may involve a similar mechanism, but these theories are unproven. Pneumonia and preeclampsia may be more common in pregnant HbA-S patients, but most data do not support this contention. Therefore, the maternal morbidity is much less than in patients with SCD and near that for similarly matched patients with HbA-A. The effect of sickle trait on the neonate is minimal, with most studies demonstrating that the mean birth weight, Apgar scores, prematurity rate, and abortions are identical to a matched control population of the hemoglobin A subjects.97, 98
Antepartum management of these patients should be directed toward screening programs for detection, close scrutiny for positive urine cultures, and avoidance of intrapartum complications such as hypotension or blood loss, which may lead to hypoxia. Routinely, iron and vitamin supplementation are given to these women as to the normal parturient, since nutritional IDA may exist. If asymptomatic bacteriuria is present and persists after treatment or if a single episode of cystitis or pyelonephritis occurs, continuous antibiotic administration is recommended during the entire pregnancy.
Other sickling hemoglobinopathies
Hereditary persistence of fetal hemoglobin (HPFH) occurs in 0.1% of African Americans in the United States. The inheritance pattern for HPFH is considered autosomal codominant, with allelic genes for hemoglobin A, S, and C. Whether pregnant or nonpregnant, persons with HPFH and hemoglobin S have a benign clinical course despite high concentrations of sickle hemoglobin. Persons with HPFH should be distinguished from other subjects with SCD who may have a high hemoglobin F. In HPFH-HbS the electrophoretic pattern is hemoglobin F, 25–30%; hemoglobin S, 70–75%; and hemoglobin A2, 2–3%. Persons with HbS-S rarely have over 20% hemoglobin F. More importantly, the hemoglobin F is heterogeneously distributed in routine hemoglobin S patients, with some erythrocytes having no hemoglobin F and others having 30–40%. HPFH patients, in contrast, have the hemoglobin F evenly distributed (approximately 30%) in each RBC, accounting for the lack of clinical symptoms. Hemoglobin S patients with HPFH should be reassured concerning their longevity and the benign clinical course.
Hemoglobin Lepore-S and hemoglobin O Arab-S are caused by a structural defect in the β-chain. They resemble HbS-S or HbS-C in symptomatology but their laboratory assessment mimics homozygous β-thalassemia. The therapy during pregnancy is similar to that for SCD because of the clinical severity.
Structural hemoglobinopathies without hemoglobin S
Other clinically significant hemoglobin variants include unstable hemoglobins, the M or cyanotic hemoglobins, as well as those with abnormal oxygen affinity.99
Clinical management is dependent o100n the clinical manifestations as well as the severity of the disease.
Diagnosis of the unstable hemoglobins can be assisted by analysis of a peripheral blood smear. Normocytic, hypochromic cells with some poikilocytosis and basophilic stippling are seen. Inclusion bodies may be difficult to distinguish prior to splenectomy but appear as finely distributed dots in their early stages. Characteristically, the RC and serum iron values are high. Electrophoretic separation can confirm the diagnosis. When the degree of hemolysis is severe and an acute hemolytic crisis occurs, transfusions and splenectomy may be performed.
Congenital methemoglobinemia, or hemoglobin M, can be linked to two etiologies. A single amino acid substitution in one of the globin chains of hemoglobin is the more common cause. Five varieties of this abnormal hemoglobin have been identified. Substitutions in the α-chain have been identified in two varieties leading to cyanosis in infants at birth. The remaining three variants involve substitutions in the β-chains; these induce cyanosis by the third or fourth month of life. However, chemically induced or congenital deficiency of the enzyme methemoglobin reductase has also been linked to this disorder. Normally, oxygen dissociates from the hemoglobin molecule, leaving the iron with the hemoglobin in the ferrous state. When oxidation to the ferric form results, methemoglobin is produced and an ineffective molecule for oxygen binding results. When the level of methemoglobin exceeds 2 g/dL, a state of cyanosis occurs.
Methemoglobinemia, resulting from amino acid substitution in a globin chain, has autosomal dominant transmission, whereas methemoglobin reductase deficiency is the result of autosomal recessive error. Although those affected are cyanotic, they remain asymptomatic. Diagnosis of hemoglobin M can be made by spectroscopy; the abnormal hemoglobins should first be identified by electrophoresis. More specific differentiation of the normal and abnormal hemoglobins can then be made using electron paramagnetic resonance to identify the amino acid substitution. Those affected with methemoglobin reductase deficiency reveal normal spectral analysis. When hemolysis occurs, hemoglobin M can induce slight jaundice. As with the other hemolytic anemias, sulfonamides can exacerbate the hemolysis. Methylene blue and ascorbic acid have been useful in treating those with the enzyme deficiency; however, all modes of therapy are ineffective in treating those with the structural defect. Because the disease is benign and lacks symptomatology, pregnancy is not affected by these abnormal hemoglobins.
Abnormalities of oxygen affinity have been reported, with almost 50 variants already identified. High-affinity disorders account for three-quarters of the variations, with 32 having been identified. Because of the increased affinity of the hemoglobin molecule to oxygen, smaller amounts of oxygen are released to the tissues. A compensatory polycythemia results in response to increased levels of erythropoietin. These variants are transmitted by autosomal recessive means.
Hemoglobin functions quite well in the oxyhemoglobin form with respect to oxygen affinity, heme interaction, Bohr effect, and reactivity with 2,3-diphosphoglycerate (2,3-DPG). In the deoxygenated state, the abnormal valine forms hydrophobic bonds with adjacent amino acids. This aggregation results in the formation of tetramers, which are relatively insoluble and coalesce to form microcables. These structures “gel” with similar aggregates to form a cable that distorts the cell into the classic sickled erythrocyte. As this process continues, the cell membrane is affected by loss of cations and phosphorylation capability; thus, the cell becomes irreversibly sickled. The age of the cell, concentration of S and F hemoglobin, extent of oxygenation, status of the cell membrane, temperature, pH, and osmotic pressure of the milieu in the microcirculation all play a role in determining whether a given cell will sickle. The initiation of the crisis may be the formation of sickled cells, with further deoxygenation in the microvasculature, creating a vicious cycle (Fig. 8). Continued sequestration of the cells may lead to the acute and chronic morphologic changes found in the organs of patients with sickle cell anemia. Clinically, this sequence of events causes the pain, leg ulcers, hepatomegaly, autosplenectomy, and chronic anemia found in these subjects. As a rule, hemoglobinopathies not associated with HbS, unless homozygous, usually present fewer management problems to the clinician than do those in the SCD category. Many of these (HbA-C, HbC-C, HbA-D, heterozygous thalassemia) are usually diagnosed only during the workup of a patient with mild, iron-unresponsive anemia.