Chapter 34: Clinical Manifestations and Classification of Erythrocyte Disorders

PATHOPHYSIOLOGY AND MANIFESTATIONS

Effects on Oxygen Transport

The clinical manifestations of anemia are a function of the degree of tissue hypoxia and the etiology and pathogenesis of the specific anemia (e.g., splenomegaly characteristic of hereditary spherocytosis, neurologic degeneration, or gastric atrophy of pernicious anemia). Decreased oxygen-carrying capacity mobilizes compensatory mechanisms designed to prevent or ameliorate tissue hypoxia. Red cells also transport carbon dioxide from tissues to the lungs and help distribute nitric oxide throughout the body (Chap. 50), but transport of these gases does not appear to be dependent on the concentration of red cells in the blood and is normal in anemic patients. Tissue hypoxia occurs when the pressure of oxygen in the capillaries is too low to provide cells with enough oxygen for cell metabolic needs. In an average person, the red cell mass must provide the total body tissues with approximately 250 mL/min of oxygen to support life. The oxygen-carrying capacity of normal blood is 1.34 mL per gram of hemoglobin (approximately 200 mL/L of normal blood) and cardiac output is approximately 5000 mL/min; thus, 1000 mL/min of oxygen is available at the tissue level. Extraction of one-fourth of this amount reduces the oxygen tension of 100 torr in the arterial end of the capillary to 40 torr in the venous end. This partial extraction ensures the presence of sufficient diffusion pressure throughout the capillaries to provide all cells with enough oxygen for the cell’s metabolic needs (Fig. 34–1). In anemia, extraction of the same amount of oxygen leads to greater hemoglobin desaturation and lower oxygen tension at the venous end of the capillary. The resulting hypoxia in the immediate vicinity initiates a number of compensatory, and frequently symptomatic, adjustments in the supply of blood and oxygen.

Hypoxia-Inducible Transcription Factors

Hypoxia-inducible transcription factor (HIF)-1 and its homologue with tissue-restricted expression, HIF-2, play a central role in the body’s response to hypoxia (Chaps. 32 and 57). HIF-1 was first identified as a factor regulating the transcriptional activity of the erythropoietin (EPO) gene (Chap. 32).¹ The essential role of this transcriptional factor in global regulation of protection against hypoxia soon became clear. Its actions include respiratory control, transcriptional regulation of glycolytic enzyme genes, angiogenesis, and energy metabolism.^2,3,4 The prediction that degradation of the hypoxia-regulated subunit of HIF-1 (HIF-1α) is controlled by an enzyme sensitive to the presence or absence of oxygen⁵ proved to be prescient. Thus, HIF’s downregulation is mediated by two principal negative regulators: von Hippel-Lindau tumor suppressor (VHL) and prolyl hydroxylase domain-containing protein 2 (PHD2). Chapter 32 describes the current knowledge of hypoxia sensing in greater detail; however, it is now clear that HIF-2, not HIF-1, is the major regulator of EPO production (Chap. 32). Tissue-specific factors are responsible for tissue-specific mobilization of the compensatory mechanisms listed below that permit survival under hypoxic conditions. Figure 34–2 outlines the regulation of some physiologic processes by hypoxia.

Decreased Oxygen Consumption

Energy metabolism at the optimal oxygen supply is sustained by energy-efficient oxidative phosphorylation. In hypoxia, energy is produced by less-efficient glycolysis accomplished by upregulation of transcription of glycolytic enzyme genes⁴ and increased glucose transport, a process known as the Pasteur effect. The Pasteur effect and its exception in the metabolism observed in malignant tissue, referred to as the Warburg effect, are both explained at the molecular level by changes in HIF-1 levels.^4,6,7,8

Decreased Oxygen Affinity

Efficient increase in tissue oxygen delivery is accomplished by decreasing the affinity of hemoglobin for oxygen (right-shifted hemoglobin oxygen dissociation curve). This action permits increased oxygen extraction from the same amount of hemoglobin (Chap. 49).⁹ Acutely, a very small shift in pH produces a large effect on the dissociation curve because of the Bohr effect (described by Danish physician Christian Bohr in 1904: “hemoglobin’s oxygen binding affinity is inversely related both to acidity and to the concentration of carbon dioxide”).¹⁰ In chronic anemia, increased oxygen tissue delivery is accomplished by increased amounts of 2,3-bisphosphoglycerate (Chap. 47).⁹ The increased synthesis of 2,3-bisphosphoglycerate in anemia is accomplished by increasing the intracellular pH of red cells (Chap. 47) by respiratory alkalosis resulting from increased respiration. This effect is clearly demonstrated in individuals with high-altitude hypoxemia.¹¹

Increased Tissue Perfusion

The effect of decreased oxygen-carrying capacity on the tissue tension of oxygen can be compensated acutely by increasing tissue perfusion locally via changing vasomotor activity and, in the long run, by enhanced tissue angiogenesis.² Because in chronic anemia the blood volume is not changed (Fig. 34–3),¹² increased tissue perfusion is organ selective, accomplished by shunting the blood from nonvital donor-tissue areas to oxygen-sensitive essential recipient organs. In acute anemia, the major donor areas for redistribution of blood are the mesenteric and iliac beds.¹³ In chronic anemia in humans, the donor areas are the cutaneous tissue¹⁴ and the kidneys.¹⁵ Vasoconstriction and oxygen deprivation in the skin cause the characteristic pallor of anemia. In the kidneys, the oxygen supply under normal conditions exceeds oxygen demands. The arteriovenous oxygen difference in the kidney is as low as 1.4 mL/dL (compared with the myocardium, where the difference can be as high as 20 mL/dL), indicating that even a severe reduction in kidney blood perfusion can be tolerated. Nevertheless, enough renal hypoxia must be present to activate HIF-2 and stimulate increased EPO production and erythropoiesis (Chap. 32). The effect on renal excretory mechanisms is slight because the reduction in renal blood flow is offset by a high plasmacrit. Even in severe anemia in which renal blood flow is reduced by almost 50 percent, the total renal plasma flow is only moderately reduced. Thus, organs with the most pressing need for oxygen, such as the myocardium and brain, are largely unimpeded by a moderate reduction in oxygen-carrying capacity, whereas in other tissues severe anemia leads to tissue hypoxia, with some tissue-specific consequences such as retinal hemorrhages.¹⁶

Increased Cardiac Output

Increased cardiac output is a metabolically expensive compensatory device.¹⁷ It decreases the fraction of oxygen that must be extracted during each circulation, thereby maintaining higher oxygen pressure. Because the viscosity of blood in anemia is decreased and selective vascular dilatation decreases peripheral resistance, high cardiac output can be maintained without any increase in blood pressure.¹⁸ In an otherwise healthy person, a measurable increase in resting cardiac output does not occur until hemoglobin concentration is less than 7 g/dL, and clinical signs of cardiac hyperactivity usually are not present until hemoglobin concentration reaches even lower levels.¹⁹

Signs of cardiac hyperactivity include tachycardia, increased arterial and capillary pulsation, and hemodynamic “flow” murmurs.²⁰ Murmurs usually are heard during systole. Murmurs and bruits have been described in many regions, such as over the jugular vein, the closed eye, and the parietal region of the skull, and may be sensed by the patient as roaring in the ears (tinnitus), especially at night. They disappear promptly after the hemoglobin concentration is restored to normal.²⁰ The myocardium tolerates a prolonged period of sustained hyperactivity. However, angina pectoris and high-output failure may supervene if anemia is so severe that it exceeds myocardial oxygen demands or if the patient has coronary artery disease. Cardiomegaly, pulmonary congestion, ascites, and edema have been observed, and they require prompt treatment with oxygen and transfusion of packed red cells.

Increased Pulmonary Function

Significant anemia leads to a compensatory increase in respiratory rate that decreases the oxygen gradient from ambient air to alveolar air and increases the amount of oxygen available to oxygenate a greater than normal cardiac output. Consequently, exertional dyspnea and orthopnea are characteristic clinical manifestations of moderate to severe anemia.^19,20,21,22

Increased Red Cell Production

The most appropriate response to anemia is a compensatory increase of red cell production, which may increase about twofold to threefold acutely and fourfold to sixfold chronically, and 10-fold in the most extreme case. The increase is mediated by increased production of EPO. The rate of EPO synthesis is inversely and logarithmically related to hemoglobin concentration (Chap. 32). EPO concentration can increase from approximately 10 mU/mL at normal hemoglobin concentrations to 10,000 mU/mL in severe anemia (Fig. 34–4).^23,24 The change in EPO levels ensures that red cell production increases in response to hemolytic and other anemias or subacute blood loss. If the former is mild, the anemia may be compensated and, if iron is available, the blood loss will be repaired after it ceases. Augmented erythroid activity expands marrow space, which, if intense, can cause sternal tenderness and diffuse bone pains. The proportion and number of reticulocytes increase. Because erythroid transit time through the marrow is shortened, “stress reticulocytes” have increased cell volume and surface area (see Chap. 32, Fig. 32–2). They develop characteristic surface folds as a result of the increased surface-area-to-volume ratio that can be identified in the blood film. Nucleated red cells may be observed in the blood in severe anemia.²⁵

Administration of human recombinant EPO augments or replaces endogenous synthesis. In pharmacologic amounts, the effect on hemoglobin concentration is most noticeable if endogenous production is subnormal as a result of renal failure or systemic illnesses (Chap. 37). In severe anemia where endogenous EPO production (providing production is not impaired) has already increased red cell production maximally, administration of EPO rarely helps, and the patients require transfusion.²⁴

Uncorrected Tissue Hypoxia

A certain residual degree of tissue hypoxia remains despite mobilization of compensatory mechanisms. Hypoxia is essential for initiation of adequate cardiovascular and erythropoietic compensation mechanisms, but severe tissue hypoxia can cause the following symptoms: dyspnea on exertion or even at rest; angina; intermittent claudication; muscle cramps, typically at night; headache; light-headedness; and fatigue. A number of diffuse gastrointestinal symptoms are associated with anemia (e.g., abdominal cramps, nausea), but whether the symptoms should be attributed to tissue hypoxia, compensatory redistribution of blood, or the underlying cause of anemia is uncertain.

CLASSIFICATION

Based on determination of the red cell mass, anemia can be classified as either relative or absolute. Relative anemia is characterized by a normal total red cell mass in an increased plasma volume, resulting in a dilution anemia, a disturbance in plasma volume regulation. However, dilution anemia is of clinical and differential diagnostic importance for the hematologist.

Classification of the absolute anemias with decreased red cell mass is difficult because the classification has to consider kinetic, morphologic, and pathophysiologic interacting criteria. Anemia of acute hemorrhage is not a diagnostic problem and is usually a genitourinary or gastrointestinal matter, not a hematologic consideration. Initially, all anemias should be divided into anemias caused by decreased production and anemias caused by increased destruction of red cells. The differentiation is based largely on the reticulocyte count. Subsequent diagnostic breakdown can be based on either morphologic or pathophysiologic criteria.

Morphologic classification subdivides anemia into (1) macrocytic anemia, (2) normocytic anemia, and (3) microcytic hypochromic anemia. The main advantages of this classification are that the classification is simple, is based on readily available red cell indices, for example, mean corpuscular volume (MCV) and mean corpuscular hemoglobin concentration (MCHC), and forces the physician to consider the most important types of curable anemia: vitamin B₁₂, folic acid, and iron-deficiency anemias. Such practical considerations have led to wide acceptance of this classification.

Pathophysiologic classification (Table 34–1) is best suited for relating disease processes to potential treatment. In addition, anemia resulting from vitamin or iron-deficiency states occurs in a significant proportion of patients with normal red cell indices.

This chapter presents a classification based on our present concepts of normal red cell production and red cell destruction. Figure 34–5 outlines the cascade of proliferation, differentiation, and maturation underlying the transformation of a multipotential stem cell, first to erythroid progenitor cells, then to erythroid precursor cells, and finally to mature red cells. Each of these steps can become impaired and cause anemia. Therapeutic intervention depends on identifying the defective step and instituting the specific therapy. The limitation of such a classification is that, in most anemias, the pathogenesis involves several steps. For example, a decreased rate of production most often results in production of defective red cells with a shortened life span. Thus, the outline provided is a conceptual guide to our present understanding of the processes underlying the production and destruction of red cells.

PATHOPHYSIOLOGY

The production and presence of an increased number of red cells are associated with general and specific effects generated by changes in blood viscosity and blood volume.

The viscosity of blood increases logarithmically with an increase in hematocrit (Fig. 34–6). At hematocrits above the normal range, the increase in blood viscosity impairs blood flow and increases cardiac workload. The resulting decrease in blood flow reduces the transport of oxygen, with average optimal values at hematocrit readings between 40 and 45 percent.^26,27 In a study of red cells from a number of animal species, the optimal value of oxygen transport corresponded closely to their normal hematocrits,²⁸ which may explain the evolutionary determination of optimal hematocrit levels.²⁹ However, before concluding that polycythemia always is a suboptimal condition, consider that it may be inappropriate to correlate viscosity readings, derived from blood tested in a rigid glass viscometer (Ostwald) or even in a cone-plate viscometer, with those in flowing blood through tiny distensible vessels in vivo.³⁰ First, the flow through these narrow channels is rapid (high shear rate), which in a nonnewtonian fluid such as blood causes a marked decrease in viscosity. Second, blood flowing through narrow channels in vivo is axial, with a central core of packed red cells sliding over a peripheral layer of lubricating low-viscosity plasma. Finally, and most importantly, absolute polycythemia is not normovolemic but is accompanied by increased blood volume, which, in turn, enlarges the vascular bed and decreases peripheral resistance. Because blood pressure remains stable, the increased blood volume must be associated with increased cardiac output and increased oxygen transport (cardiac output times hemoglobin concentration). Using measurements of cardiac output in dogs³¹ and tissue oxygen tension in rats and mice,³⁰ construction of curves (Fig. 34–7) that relate oxygen transport to hematocrit in normovolemic and hypervolemic states is possible. These curves show that hypervolemia per se increases oxygen transport and that the optimum oxygen transport in these conditions occurs at higher hematocrit values than in normovolemic states. Consequently, despite the increased viscosity, a moderate increase in hematocrit is beneficial. The same may not be true of a more pronounced increase in hematocrit. Observations in humans³² and experimental animals³¹ indicate that high viscosity causes reduced blood flow to most tissues and may be responsible for the cerebral and cardiovascular impairment experienced occasionally by high-altitude dwellers,³³ patients with severe polycythemia,^34,35 and athletes self-administering overdoses of EPO (Chap. 57).

MANIFESTATIONS

The rate of red cell production is increased in true polycythemias, but changes in erythroid marrow cellularity can be difficult to assess by microscopy means, although the marrow is hypercellular in a typical patient with polycythemia vera. Under normal conditions, the rate of red cell production is adjusted to maintain the red cell mass at approximately 30 mL per kilogram of body weight. Because the life span of red cells in polycythemia is normal, a doubling of the daily rate of red cell production is adequate to maintain a polycythemic red cell mass of 60 mL/kg. Consequently, the morphology and volume of the marrow are only moderately altered in polycythemia compared with the changes observed in some types of hemolytic anemia, in which the rate of red cell production can be four to six times normal. In erythrocytosis, the number of red cells destroyed daily merely causes a slight increase in bilirubin levels. The presence of secondary gout and splenomegaly are usually signs of a myeloproliferative neoplasm rather than of erythrocytosis alone. Although considerable homology exists between EPO and thrombopoietin,³⁶ there is no evidence that the two molecules crossreact at the level of their respective receptors.³⁷ EPO-driven erythrocytosis is generally not associated with increased platelet production.

The increased viscosity and expansion of vascular space are responsible for many of the signs and symptoms of polycythemia. The characteristic rubor in patients with polycythemia vera is caused by excessive deoxygenation of blood flowing sluggishly through dilated cutaneous vessels. Nonspecific symptoms such as headaches, dizziness, tinnitus, and a reported feeling of fullness of the face and head probably are caused by a combination of increased viscosity and vascular dilatation. In extreme polycythemia and some specific types of polycythemia (e.g., methemoglobinemia; Chap. 50), cyanosis can result from greater than 4 g/dL of deoxygenated hemoglobin (accomplished more easily at higher hemoglobin concentrations [see “blue bloaters” and “pink puffers” in Chap. 57]) or greater than 1.5 g/dL of methemoglobin.

Hemorrhage from the nose or stomach in patients with normal platelets and coagulation proteins can be attributed to capillary distention; however, circulatory stagnation causing ischemia and necrosis may contribute. Thrombosis are common in polycythemia vera, but are not seen at similar frequencies in other types of polycythemias (Chaps. 57 and 84). Coronary blood flow is decreased in polycythemia,³⁴ so the risk of coronary thrombosis in patients with a high hematocrit is assumed to be increased; however, statistical analyses have yielded equivocal evidence of such a relationship.^35,38,39 Polycythemia reportedly does not pose a risk in surgical patients.⁴⁰ Although cerebral blood flow is materially reduced in patients with moderately elevated hematocrit,^32,41 such reductions may have little practical significance. In polycythemia vera, however, it has been advocated that normalization of red cell mass should be accomplished before surgery; again, firm data supporting this practice are lacking (Chap. 84).

CLASSIFICATION

Polycythemia, or erythrocytosis, is a condition in which the hematocrit percentage is above the upper limits of normal: greater than 51 percent in men and greater than 48 percent in women. Polycythemia can be classified as: (1) relative, in which the red cell mass is normal but the plasma volume is decreased, or (2) absolute, in which the red cell mass is increased above normal (Chap. 57). Table 34–2 outlines the polycythemic states.

Differentiation of absolute from relative polycythemia can be difficult at hematocrits of less than 60 percent. Designation of a measured red cell mass as normal is imprecise because the red cell mass depends on the patient’s age, sex, weight, height, and body frame, and because only increases above the mean of greater than 25 percent are considered abnormal.

Primary Polycythemias

Primary or secondary polycythemias are caused by either acquired (polycythemia vera) or inherited mutations (such as gain-of-function erythropoietin receptor [EPOR] causing primary familial and congenital polycythemia [PFCP]) expressed within hematopoietic progenitors, leading to increased production of red cells.

Secondary Polycythemias

Secondary polycythemias are caused by augmentation of erythropoiesis by circulating stimulatory factors such as EPO (polycythemia of high altitude), cobalt, or insulin-like growth factor 1 (Chap. 57).

Disorders of Hypoxia Sensing

Polycythemias from VHL Gene Mutation

Chuvash polycythemia and some other disorders of hypoxia sensing, including several recessively inherited mutations of the VHL gene, have elevated or inappropriately normal EPO levels in relation to elevated hematocrit. This phenotype is different from other, more common VHL mutations encompassing VHL tumor predisposition syndrome, wherein dominantly inherited germline VHL mutations precede the acquired somatic mutations that eventually result in tumor genesis (hemangioblastoma, renal cell carcinoma, pheochromocytoma, pancreatic endocrine tumors, and endolymphatic sac tumors). Following the description of Chuvash polycythemia, other homozygous and compound heterozygous inherited VHL mutations causing polycythemia, but not tumors, have been described (Chap. 57). Some patients with tumor predisposition VHL syndrome also develop acquired polycythemia as a consequence of EPO production in tumor tissue.⁴²

Clinically, Chuvash polycythemia patients are prone to develop thrombosis, they have elevated pulmonary pressure and have increased mortality independent of the increase in hematocrit.⁴³

Polycythemias from HIF2α and PHD2 Mutations

Dominantly inherited gain-of-function HIF2α (encoded by EPAS1) gene mutations have elevated or inappropriately normal EPO levels in relation to elevated hematocrits. All reported polycythemic heterozygotes for loss-of-function PHD2 variants (encoded by EGLN1 gene) have normal EPO levels.

Because of the rarity of HIF2a and PHD2 mutations, their nonerythroid phenotype is not well defined; however, some HIF2a mutations, possibly associated with genetic mosaicism, are associated with congenital polycythemia and later development of pheochromocytoma/paraganglioma and somatistatinoma tumors. In these instances, in most patients, the HIF2a mutation is present in the tumors, which frequently recur, but not in leukocytes.^44,45