CHAPTER 3: Overview of the Anemias

As stated in Chapter 1, the primary function of the red blood cell is to transport oxygen to respiring cells and tissues. Anemia is defined as a significant deficit in the mass of circulating red blood cells. As a result, the capacity of the blood to deliver oxygen is compromised. Clinicians document the presence of anemia by measurement of either the concentration of hemoglobin in the blood or the hematocrit, which is the ratio of the volume of red cells to the total volume of a blood sample. A patient is anemic if the hemoglobin or hematocrit value is more than two standard deviations below normal. As shown in Figure 3-1, the lower limits of normal vary with the age of the individual and, in adults, with gender. Occasionally, the documentation of anemia is confounded by a concurrent alteration in the plasma volume. For example, if a patient with a low mass of circulating red blood cells is also hypovolemic, owing to a concurrent loss of plasma volume from dehydration, the blood hemoglobin and hematocrit levels will be falsely elevated and may even be in the normal range. Another important example, discussed in more detail later in this chapter, is acute hemorrhage, in which there is concomitant loss of both red blood cells and plasma.

Oxygen transport to a given region in the body, as expressed in the Fick equation, is a product of three independent variables: blood flow, hemoglobin concentration, and the fraction of hemoglobin that has unloaded oxygen during transit from artery to vein. In anemic patients, the oxygen-carrying capacity of the blood is, by definition, low. As shown in Figure 3-2 and explained below, the two other components in the Fick equation undergo compensatory changes.

BLOOD FLOW

In all anemic individuals, there is enhanced shunting of blood to vital organs, including the heart, brain, liver, and kidneys, at the expense of non-vital organs. Anemic patients are pale because blood is shunted away from the skin to preserve oxygenation of these crucial organs. In patients with mild or moderate anemia, the cardiac output is normal at rest but, with exercise, increases more than that of a normal individual. In severe anemia, the resting cardiac output is increased, putting patients at risk of developing high-output cardiac failure, particularly those with coronary artery disease or other forms of preexistent cardiovascular disease.

OXYGEN UNLOADING

The variable (Asat-Vsat) in the Fick equation is the difference in percent oxygenation between the arterial (A) and venous (V) blood. This AV difference in oxygen saturation depends on the oxygen-binding curve. Figure 3-3 compares curves from an anemic patient and a normal individual. As the top panel shows, the anemic curve is shifted to the right. At any given oxygen tension (pO₂), the oxygen saturation is lower in the anemic individual. Thus, red cells of anemic patients have decreased oxygen affinity. This change is due entirely to elevated levels of 2,3-diphosphoglycerate (2,3-DPG) in red cells. 2,3-DPG and pH are the primary determinants of oxygen affinity in human red cells. Arterial blood normally has a pO₂ of about 95 Torr and is nearly 100% saturated with oxygen. As red cells from a normal individual pass from an artery through its capillary bed to its vein, oxygen is released to respiring cells. At a normal venous pO₂ of about 40 Torr, the oxygen saturation is about 80%. Thus, as shown on the left in Figure 3-3, 20% of the oxygen in the blood is unloaded. In contrast, in patients with anemia and elevated levels of red cell 2,3-DPG, the lower oxygen affinity of the blood enables a much higher fraction of the oxygen (about 35%) to be unloaded. This is a critically important way in which anemic patients compensate for their major deficit in red cell mass.

This benefit is shown quantitatively in the bottom panel of Figure 3-3. Here, the oxygen-binding curves are depicted with the volume fraction of oxygen plotted on the Y-axis. One gram of hemoglobin binds up to 1.34 mL of oxygen under standard conditions of temperature and pressure. Thus, in a normal individual with a hemoglobin of 15 g/dL, the oxygen-carrying capacity of the blood will be 15 × 1.34 or 20 mL O₂/dL. In traversing the earlier-mentioned capillary bed, 20% of this oxygen will be unloaded, that is, 4 mL per 100 mL of blood. In contrast, an anemic patient with a hemoglobin of 7.5 g/dL has an oxygen-binding capacity that is half normal, that is, 10 mL O₂/dL. If this patient had red cells with normal oxygen affinity, 20% or only 2 mL of oxygen would be unloaded per 100 mL of blood. However, as shown on the right in Figure 3-3, because the patient's red cells have a lower affinity for oxygen, 3.5 mL is unloaded, nearly as much as normal.

STIMULATION OF ERYTHROPOIESIS BY ERYTHROPOIETIN

There are two ways in which anemia impacts the middle component of the Fick equation, depicted in Figure 3-2. As mentioned earlier, by definition, hemoglobin concentration is low in anemic patients. The reduction in the oxygen-carrying capacity of the blood results in cellular hypoxia. A molecular sensor in all cells of the body detects even mild degrees of hypoxia and induces a hypoxia-inducible transcription factor called HIF. In the kidney, and to a lesser extent in the liver, HIF up-regulates the expression of the hormone erythropoietin, which stimulates red blood cell production. The physiology and cell biology of erythropoietin is discussed in more detail in Chapter 2. The regulated production of erythropoietin enables normal individuals to maintain nearly constant levels of circulating red cells. In anemic patients, the hypoxic signal in the kidneys and liver results in a marked enhancement in erythropoietin production. As shown in Figure 3-4, plasma erythropoietin levels are inversely proportional to the hematocrit and, in severely anemic patients, can be 1000-fold higher than normal. The erythroid progenitors in patients with anemia due to impaired red cell production are unresponsive to such high levels of plasma erythropoietin. In contrast, in patients whose anemia is due to blood loss or a short red cell life span, this regulatory loop maximizes red cell production and constitutes a third important mode of compensation.

Patients with mild or moderate anemia are often asymptomatic. Many note breathlessness and/or fatigue only upon strenuous exercise. In severe anemia, dyspnea and fatigue are common complaints. These symptoms reflect limitations in the earlier-mentioned compensations for the tissue hypoxia imposed by a low red blood cell mass.

Physical findings also depend on the severity of the anemia and are best understood in terms of the Fick equation depicted in Figure 3-2. As mentioned earlier, pallor reflects a compensatory shunting of blood away from the skin to ensure adequate flow to vital organs. In individuals with deeply pigmented skin, pallor can be detected in nail beds as well as in membranes such as conjunctivae and the buccal mucosa. Those with severe anemia may have tachycardia at rest, owing to a compensatory increase in basal cardiac output. The hyperdynamic circulation in such patients often gives rise to a systolic "flow" murmur that is transmitted into the neck. In individuals with lesser degrees of anemia, the heart rate is normal at rest but, on exercise, increases more than normally. Anemic patients may have many other informative physical findings that depend on specific underlying pathophysiology. For example, those with hemolytic anemia often have splenomegaly, owing to trapping of defective or damaged red blood cells in the spleen, and jaundice, reflecting increased plasma bilirubin levels due to rapid destruction of red blood cells.

The level of circulating red cells in the blood can be likened to the balance of cash in one's personal bank account. The balance is a function of how many dollars or cells are coming in each week versus how many are spent or lost. Thus, the anemias can be divided into three broad categories: decreased red cell production, increased red cell destruction, and blood loss. Often, the patient's history and physical examination provide information as to which process is going on. For example, the presence of blood loss is usually apparent from the history.

As mentioned earlier, physical findings such as jaundice and splenomegaly suggest hemolysis. Among the available laboratory tests, the reticulocyte count is the simplest and most reliable way to distinguish among the three major categories of anemia. This laboratory test is a measurement of the fraction of young red cells in the blood (<2.5 days old). In patients with impaired red cell production, the reticulocyte count will be inappropriately low. Despite elevated levels of plasma erythropoietin (Figures 3-2 and 3-4), the bone marrow is unable to respond to produce adequate numbers of new red cells. In contrast, the reticulocyte count is generally elevated in both hemolytic anemia and in acute blood loss.

ANEMIA DUE TO BLOOD LOSS

The loss of enough blood to cause significant anemia is usually due either to trauma, gastrointestinal hemorrhage, or uterine bleeding. The clinical presentation depends on both the rate and amount of blood loss. Relatively small amounts of hemorrhage are well-tolerated, owing to the compensatory mechanisms depicted in Figure 3-2. Donations at Red Cross or hospital blood banks are routinely 500 mL or 10% of the blood volume. Otherwise healthy individuals can lose up to 20% of their blood volume acutely without significant symptoms, owing to reflex vasospasm and redistribution of blood flow as described earlier. With greater blood loss, the patient develops the signs and symptoms of hypovolemia. Compensatory mechanisms such as redistribution of blood flow are no longer sufficient to maintain blood pressure. Postural hypotension, or a drop in systolic blood pressure when the patient goes from a supine to a sitting position, is a sign of significant hypovolemia. Even more extensive hemorrhage produces hypovolemic shock, which is denoted by altered mental status, cool clammy skin, tachycardia, hyperventilation, and hypotension in the supine position. Importantly, in patients with severe acute hemorrhage, the hematocrit and hemoglobin levels may initially be close to normal, owing to the concomitant loss of both red cells and plasma, such that the extent of the red cell loss becomes apparent only when the patient's plasma volume is restored, either spontaneously or by administration of intravenous fluids.

In patients with more gradual blood loss, the onset of the anemia is frequently insidious. Many patients develop non-specific symptoms and signs of anemia as described earlier in this chapter. These individuals are very likely to be or to become iron deficient.

The treatment of blood loss anemia depends on the extent and time frame of the hemorrhage. Patients in hypovolemic shock require both fluid and red cell replacement. Those with less severe blood loss may not require red cell transfusion. The rate of regeneration of new red blood cells may be enhanced by the administration of recombinant erythropoietin but is sometimes limited by lack of adequate iron stores. Thus, all patients with significant blood loss should receive enough therapeutic iron to replace the deficit from red cell loss. It is equally important to adequately monitor the patient to be sure blood loss is not continuing. Those with gastrointestinal blood loss need to have follow-up testing of stool specimens for the presence of blood.

ANEMIA DUE TO DECREASED RED CELL PRODUCTION

A wide variety of disorders result in anemia owing to inadequate production of red blood cells. However, these types of anemia have a number of features in common. The onset of symptoms is usually insidious. In individuals with a normal red cell life span of 120 days, even if the production of red cells abruptly ceases, the drop in hemoglobin and hematocrit levels is quite gradual, and compensatory mechanisms may stave off the onset of symptoms until the anemia is quite severe. As mentioned earlier in this chapter, the reticulocyte count is inappropriately low. As shown in Figures 3-2 and 3-4, plasma erythropoietin levels rise quite markedly as patients become increasingly anemic. However, this highly specific and potent stimulus to erythropoiesis is ineffective in disorders in which there is a defect in red cell production. Some clinicians use the reticulocyte index as a more informative measure of red cell production. It is defined as the reticulocyte percentage times the hematocrit divided by the normal hematocrit (0.45). Normal individuals have a reticulocyte count of 1-2%, and, therefore, a reticulocyte index of 1 to 2. By comparison, a patient with a hematocrit of 0.15 and a reticulocyte count of 3% has a reticulocyte index of 1. Even though the reticulocyte count is slightly higher than normal, the reticulocyte index falls in the low normal range. Thus the patient's red cell production is stalled at a normal level even though the severe anemia is expected to induce a very high plasma erythropoietin level that, in a normal bone marrow, should enhance red cell production as much as 8-fold. This example illustrates the inappropriately low reticulocyte count in underproduction anemias.

The red blood cell indices are particularly helpful in determining the cause of anemia due to impaired red cell production. As mentioned in Chapter 1, these indices provide quantitative information about the average or mean red cell size or volume (MCV), mean red cell hemoglobin, and mean hemoglobin concentration within red cells. As shown in Table 3-1, the MCV is particularly useful in the classification of the underproduction anemias. Anemias associated with a low MCV are designated microcytic, and those with a high MCV are designated macrocytic. These two groups of anemias both involve defects in the maturation of erythroid cells in the bone marrow. The anemia-induced elevation of plasma erythropoietin produces a reactive hyperplasia of erythroid progenitors and precursors in the bone marrow, a feature that is readily detectable by microscopic examination of the bone marrow. However, impaired erythroid maturation suppresses the production of fully differentiated red cells to varying degrees, depending on the nature and severity of the underlying defect. As a result, patients with microcytic or macrocytic underproduction anemias have a disparate combination of erythroid hyperplasia in the marrow and a relatively low reticulocyte count in the peripheral blood. In fact, many erythroid precursors are so defective that they undergo programmed cell death (apoptosis) in the marrow and therefore cannot produce reticulocytes, a phenomenon referred to as ineffective erythropoiesis.

Ineffective erythropoiesis is a critically important component of the pathophysiology of a number of important underproduction anemias, including beta thalassemia major and intermedia, iron deficiency, sideroblastic and other myelodysplastic anemias, and the megaloblastic anemias. These entities are covered in detail in subsequent chapters.

Decreased red cell production is due either to a deficient number of erythroid progenitors or ineffective erythropoiesis. The dynamics of erythropoiesis, red cell survival, and red cell destruction in different types of anemias are depicted in Figure 3-5. Because the bone marrow is the site of erythropoiesis, microscopic examination of the bone marrow is particularly informative in the diagnosis of anemias of underproduction. Bone marrow biopsies are used to establish the overall cellularity of the marrow, whereas aspirate smears reveal the relative proportion of erythroid versus non-erythroid precursor cells as well as more subtle morphological features that often point to specific defects in erythroid maturation. Because these two procedures yield complementary information, both usually are performed at the time of marrow examination.

In microcytic anemias the small red cell volume is accompanied by a decrease in the mean red cell hemoglobin. As shown in Figure 3-6, microcytic anemia is due to a defect involving one of the three components of hemoglobin: iron, porphyrin (into which iron is inserted to make heme), or globin. The MCV is low when iron stores are depressed sufficiently to cause anemia and also in thalassemias, inherited mutations that impair the synthesis of either alpha or beta globin. In sideroblastic anemias caused by acquired or inherited defects in porphyrin synthesis, there is typically a much broader distribution of red cell sizes, but a prominent population of microcytic red cells is usually present.

Macrocytic red cells are encountered in a number of diverse causes of anemia. If the MCV exceeds 120 fL, the anemia is nearly always megaloblastic, a morphological appearance that reflects a defect in maturation due to impaired DNA synthesis. Megaloblastic anemias are most often caused by deficiency in vitamin B₁₂ (cobalamin) or folate or by chemotherapy agents that block DNA synthesis, such as methotrexate. Lesser degrees of macrocytosis are encountered in hemolytic anemias, bone marrow failure, myelodysplastic syndromes, liver disease, alcoholism, and hypothyroidism.

Normocytic underproduction anemias also encompass a wide variety of disorders. Severe anemia associated with a normal MCV usually points to a primary marrow problem, either aplasia or marrow replacement by leukemia, metastatic tumor, fibrosis, or granulomas. The patient may have pancytopenia, defined as anemia associated with a concomitant reduction in the white blood cell count and the platelet count. Whenever the marrow is involved by infiltrative processes (e.g., tumor) or distorted by fibrosis, nucleated red cells and red cells with a tear-drop shape (Fig. 3-7) may appear in the peripheral blood. A bone marrow biopsy is essential to establish the diagnosis in such instances, because the marrow often cannot be aspirated, a clinical problem referred to as a "dry tap."

More commonly, normocytic anemias are secondary to an underlying systemic illness. Patients with cancer, long-standing infections, or connective tissue disorders such as rheumatoid arthritis have mild to moderate anemia, owing to chronic inflammation, which triggers abnormalities in iron homeostasis that impair erythropoiesis. The anemia of chronic inflammation is described in more detail in Chapter 7. Chronic liver disease and disorders of endocrine hypofunction also are associated with mild to moderate normocytic anemia. The anemia of renal insufficiency is the only secondary anemia that is commonly severe. These disorders are also covered in Chapter 7.

ANEMIA DUE TO INCREASED RED CELL DESTRUCTION (HEMOLYSIS)

A wide range of structural, metabolic, immunologic, and mechanical defects can result in premature destruction of circulating red cells. However, irrespective of etiology, uncomplicated hemolytic anemias have a number of features in common. The capacity for efficient erythropoiesis is preserved, and indeed, in response to hypoxia-induced erythropoietin production (Fig. 3-1), red cell production is often markedly increased, an adaptive response that is reflected by an elevation of the reticulocyte count. Anemia accompanied by reticulocytosis of 5% or greater strongly suggests the presence of hemolysis. However, elevated reticulocyte counts also can be seen in nutritional anemias during the first two weeks of replacement therapy with iron, cobalamin (vitamin B₁₂), or folic acid. Acute hypoxia can also cause a transient elevation of the reticulocyte count. Finally, infiltrative bone marrow disorders such as metastatic cancer can also induce a modest sustained elevation of the reticulocyte count due to early release.

Microscopic examination of a carefully spread and well-stained blood smear is an important part of the evaluation of any unexplained anemia, but it is particularly informative in identifying the cause of hemolysis. A fascinating variety of red cell abnormalities can be observed in the different types of hemolytic anemia, many of which are shown in Figure 3-7. More details on the features and pathogenesis of these abnormal cells are provided in Chapters 9, 10, and 11, which cover specific types of hemolytic anemia.

In contrast to the value of examining the peripheral blood film, microscopic examination of the bone marrow is rarely helpful in identifying the cause of hemolytic anemia. The result that is nearly always obtained, erythroid hyperplasia, is predictable from the elevated reticulocyte count. A marrow examination is helpful only in exceptional instances such as the coexistence of lymphoma and immune hemolytic anemia and the presence of hypoplasia in paroxysmal nocturnal hemoglobinuria. These unusual but interesting exceptions to the rule are discussed in Chapter 11.

The presence of hemolysis can be further confirmed by readily available serum assays. These tests are based on the physiologic steps involved in the breakdown of red cells, as depicted in Figure 3-8. At the end of their life spans, both normal and abnormal red cells are engulfed by macrophages in the spleen, liver, and bone marrow. Here, globin, the protein in hemoglobin, undergoes proteolytic degradation. The heme is catabolized by heme oxygenase to biliverdin (a straight-chain tetrapyrole), carbon monoxide, and free iron. The biliverdin is then reduced to bilirubin, which exits the macrophage and binds to albumin in the plasma. The albumin-bound bilirubin is taken up by the liver, where it is conjugated to glucuronide, rendering it soluble in water. Conjugated bilirubin is excreted into the bile, enters the duodenum, and traverses the intestines. Bacteria in the gut mediate the conversion of conjugated bilirubin to urobilinogen, which in turn is converted to stercobilin, which causes the stool to be brown. In patients with hemolytic anemia, enhanced red cell breakdown and catabolism of heme result in an increase in the level of non-conjugated or "indirect" bilirubin in the plasma/serum. Patients with moderate or severe hemolytic anemia are often jaundiced (icteric). Chronic hemolysis puts patients at risk of developing bilirubin gall stones. If patients with hemolytic anemia have concomitant liver and biliary disease, serum levels of conjugated bilirubin will also be increased, and the total of non-conjugated and conjugated bilirubin may reach very high levels, resulting in much deeper jaundice.

Excess destruction of red cells, whether intravascular (within the blood) or extravascular (within macrophages), results in the leakage of hemoglobin and red cell enzymes into the circulation. Lactate dehydrogenase (LDH) is one easily measured enzyme, the levels of which increase above normal in proportion to the rate of hemolysis. It must be remembered, however, that even higher levels of LDH are seen in conditions such as megaloblastic anemia that are associated with severe, ineffective erythropoiesis. Free hemoglobin in the plasma binds with high affinity and specificity to haptoglobin, an abundant plasma protein, forming a hemoglobin-haptoglobin complex that is rapidly cleared from the circulation. As a result, haptoglobin is often completely absent from the plasma of patients with significant degrees of hemolysis.

Most commonly, hemolysis occurs extravascularly within phagocytic macrophages. In patients with severe intravascular hemolysis, the amount of hemoglobin released from the lysed red cells exceeds the binding capacity of plasma haptoglobin. Free hemoglobin tetramers in the circulation (α₂β₂) readily dissociate into αβ dimers that are cleared by the renal glomeruli and reabsorbed by proximal tubule epithelial cells. Here, the hemoglobin is catabolized, and the heme iron is transferred to ferritin and hemosiderin (Chapter 5). Shedding of these cells results in the loss of iron, such that long-standing intravascular hemolysis can lead to iron deficiency. If the amount of filtered hemoglobin exceeds the reabsorptive capacity of the renal tubules, hemoglobin passes into the urine to produce a red-brown color (hemoglobinuria).

Once the presence of hemolysis is established, specific laboratory tests are available that, when used rationally in conjunction with relevant information from the history and physical examination, are very effective at homing in on specific causes. In planning the diagnostic workup, it is useful to draw on a coherent classification of the hemolytic anemias. Two different approaches, summarized in Table 3-2, are useful in considering these disorders. One approach groups hemolytic anemias according to whether the causative insult is an environmental factor versus a defect involving the cell membrane or the inside of the cell (hemoglobin or enzymes). A second approach distinguishes between inherited and acquired forms. These principles should be of value in understanding the different hemolytic anemias described in Chapters 9, 10, and 11.