Chapter 57: Primary and Secondary Erythrocytoses

The term polycythemia, denoting an increased amount of blood cells, has traditionally been applied to those conditions in which the mass of erythrocytes is increased. Erythrocytosis is an alternative term that has also been applied to circumstances in which the increased red cell mass is the singular finding, distinguishing it from polycythemia vera in which, classically, there is an increase in red cells, granulocytes, and platelets. Although this usage has much to recommend it, no consensus about terminology has been reached and the term polycythemia is used interchangeably with erythrocytosis by many physicians. In some instances, time-honored terms such as post–renal transplant erythrocytosis or Chuvash polycythemia will be used. A classification of the polycythemias is presented in Chap. 34 in Table 34–2.

PRIMARY POLYCYTHEMIAS

Polycythemia vera (Chap. 84) and primary familial and congenital polycythemia (PFCP) are primary polycythemic disorders, which have erythroid progenitors that are hypersensitive to erythropoietin.^1,2,3 These are caused by somatic (polycythemia vera) or germline (PFCP) mutations wherein erythroid progenitors are intrinsically hyperproliferative by mechanisms other than the presence of increased levels of erythropoietin and have, as shown by in vitro assays, an augmented response to erythropoietin. Some congenital polycythemias, as best described in Chuvash polycythemia, have erythroid progenitors that are hypersensitive to erythropoietin, but also may have normal or even increased erythropoietin levels despite the increased red cell volume.^4,5 Thus, some of these rare inherited polycythemias share features of both primary and secondary polycythemias.⁶

SECONDARY POLYCYTHEMIAS/ERYTHROCYTOSES

The term secondary polycythemia, more appropriately secondary erythrocytosis, which refers to those conditions in which only erythrocytes are increased in number and volume, describes a group of disorders characterized by an increased red cell mass brought about by enhanced stimulation of red cell production by circulating physiologic mediators, most commonly erythropoietin. Secondary polycythemia may be subdivided into appropriate polycythemia, that is, responding normally to tissue hypoxia such as in pulmonary disease, Eisenmenger complex, high-altitude polycythemia and hemoglobins with increased affinity for oxygen (Chaps. 49 and 50), and inappropriate polycythemia in which erythropoiesis is being stimulated by the aberrant production of erythropoietin, as in erythropoietin-secreting tumors, by high levels of insulin growth factor 1, by cobalt toxicity, by self-administration of erythropoietin, androgens, or adrenocorticotropic hormone, or by other stimulators of erythropoiesis (as in post–renal transplant erythrocytosis).⁷

In his important monograph on barometric pressure published in 1878, Paul Bert showed that physiologic impairment observed at high altitude was caused by a reduction in the oxygen content of the air.⁸ A few years earlier, his friend and mentor, Dennis Jourdanet, had observed an increase in the number of red corpuscles in blood of the highlanders in Mexico,⁹ and Bert recognized that such an increase would tend to ameliorate the effect of atmospheric hypoxia. However, neither Bert nor Jourdanet suspected a cause-and-effect relationship. It was not until 1890, when Viault¹⁰ observed a prompt increase in the number of his own red corpuscles after having traveled from Lima, Peru, at sea level, to Morococha, at 4570 m (15,000 ft) above sea level, that altitude erythrocytosis was accepted as a compensatory adaptation to hypoxia.¹¹ At about the same time, it was observed that many patients with cyanosis were also polycythemic. Both cardiacos negros,¹² with severe pulmonary failure and arterial oxygen desaturation, and children with morbus caeruleus, or right-to-left shunt through a congenital cardiac malformation, were found to have increased red cell counts.¹³ Mechanical or neurogenic hypoventilation as a cause of cyanosis and polycythemia was first popularized in 1956 with the classic description of the pickwickian syndrome by Burwell and colleagues.¹⁴ Polycythemia associated with carboxyhemoglobinemia resulting in hypoxemia as a result of smoking and with tissue hypoxia because of inherited abnormal hemoglobins with high-affinity oxygen binding to hemoglobin was recognized subsequently (Chaps. 49 and 50).¹⁵ Erythrocytosis associated with abnormal hemoglobins with an increased affinity for oxygen also represents an appropriate response to hypoxia first noted by Charache and colleagues¹⁵ in 1966 when they described hemoglobin Chesapeake.

Relative erythrocytosis is the term used to depict enhanced red cell count or blood hemoglobin values resulting from reduced plasma volume, not increased red cell mass. The disorder is, therefore, not a true polycythemia and is designated apparent, spurious, or relative polycythemia. The cause of the reduced plasma volume and hence relative erythrocytosis is often known, that is, diuretic use, dehydration from excessive sweating, etc. However, there are some patients with mild erythrocytosis in which neither the cause nor the clinical significance is clear. In 1905, Gaisbock reported that a number of hypertensive patients had plethora and an elevated red cell count but no splenomegaly, a condition he termed polycythemia hypertonica, sometimes called Gaisbock syndrome.¹⁶ In 1952, direct measurement of blood volume in patients with polycythemia led Lawrence and Berlin to identify a subgroup of patients with a normal red cell volume but reduced plasma volume. Although some members of this group were hypertensive, the authors were more impressed by their tense and anxious behavior and coined the term stress polycythemia.¹⁷

PRIMARY POLYCYTHEMIAS

Primary Familial and Congenital Polycythemia

This autosomal dominant disorder (designated PFCP) is uncommon but more prevalent than is generally appreciated, as many affected subjects are initially misdiagnosed as having polycythemia vera. Its prevalence is similar to congenital polycythemias because of high oxygen-affinity hemoglobin mutants, and far more common than 2,3-bisphosphoglycerate (2,3-BPG) deficiency.¹⁸

SECONDARY POLYCYTHEMIAS

Pulmonary Disease with Hypoxia

In one study of 2524 patients with severe chronic obstructive pulmonary disease (COPD), 8.4 percent had a hematocrit higher than 55 percent. In this study, hematocrit was an independent predictor of longer survival, decreased hospital admission rate, and decreased cumulative duration of hospitalization.¹⁹ In another, smaller study of 309 subjects with COPD and chronic respiratory failure, 67 percent had normal hemoglobin levels, 20 percent had anemia, and 18 percent had polycythemia.²⁰

Obstructive Sleep Apnea

Although the evidence is largely anecdotal,²¹ secondary polycythemia is a widely recognized complication of longstanding obstructive sleep apnea (OSA), being found in 5 to 10 percent of those with nocturnal apnea and hypopnea.²² The published studies remain controversial; in a study of 263 patients (189 men and 74 women), patients with severe sleep apnea had significantly higher hematocrit values than did patients with mild to moderate sleep apnea or nonapneic controls (p <0.01).²³ In contrast, in other studies, there were no significant differences in hemoglobin levels or hematocrit between subjects with and without OSA.^24,25 The total number of hours of hypoxia per day may dictate whether the stimulus to erythropoietin production is sufficient to cause erythrocytosis.

Polycythemia Associated with Smoking

Smoking clearly increases hematocrit in COPD compared to controls of comparable pulmonary function. In a study of 2524 patients with severe COPD, 10.2 percent of patients reported as current smokers had significantly higher hematocrit values than did ex-smokers or nonsmokers of comparable pulmonary impairment of both genders, with p <0.02.²⁰

Even young male smokers without pulmonary impairment have higher hematocrits. In one study, 1169 subjects (age range: 18.6 to 22.8 years; mean: 19.4 years) were recruited and 25 percent were smokers. Predictably, carboxyhemoglobin was much higher in smokers than in nonsmokers (r = 0.958, p <0.001) and both hemoglobin and hematocrits were also markedly higher in smokers (hemoglobin (p = 0.001), hematocrit (p = 0.004).²⁶

High Oxygen Affinity and Hemoglobins

These disorders are reviewed in detail in Chaps. 49 and 50. High oxygen-affinity hemoglobins deliver less oxygen to tissues, which is appropriately compensated by increased erythropoiesis and a higher steady-state hemoglobin concentration. While considered rare, high oxygen affinity has been found, according to one report, in approximately 20 percent of 70 unrelated subjects with otherwise idiopathic polycythemia.²⁷

Polycythemia of Eisenmenger Complex

Eisenmenger syndrome, characterized by elevated pulmonary vascular resistance and right-to-left shunting of blood, is usually accompanied by polycythemia.²⁸ Most patients with the syndrome survive for 20 to 30 years.

Polycythemia of Endocrine Disorders and from Iatrogenic or Self-Administration of Androgens

Erythrocytosis has been reported in Cushing syndrome,²⁹ primary aldosteronism,³⁰ and Bartter syndrome (Chap. 38).³¹

In a prospective trial of testosterone use in older men, erythrocytosis (defined as a hematocrit greater than 50 or 52 percent) was three times more likely to occur in the testosterone-treated group compared to placebo.³² (Refer to Chap. 38 for more details.)

Inappropriate Tissue Elaboration of Erythropoietin

The prevalence of various types of secondary polycythemia is a function of underlying causes, such as geographical location of the patient or presence of a causative neoplasm. Approximately 1 to 3 percent of all patients with pheochromocytoma or paraganglioma have erythrocytosis.³³ Rare patients with congenital erythrocytosis will develop pheochromocytoma or paraganglioma.³⁴ Uterine leiomyomas in premenopausal women are very common, estimated at 20 to 40 percent, and the occurrence of erythrocytosis ranges from 0.02 to 0.5 percent of cases.³⁵ Isolated instances of polycythemia have been attributed to myxoma of the atrium,³⁶ hamartoma of the liver,³⁷ and focal hyperplasia of the liver.³⁸ Erythrocytosis and inappropriate secretion of erythropoietin may be found in approximately 15 percent of patients with cerebellar hemangioma.^39,40

Self-Administration of Erythropoietin

Athletes have attempted for decades to manipulate their blood to gain a competitive advantage either by blood transfusions or erythropoietin. This evolving, but continuous, problem has been the subject of a review.⁴¹

Polycythemia of High Altitude

Elevation of red cell mass as a response to high altitude hypoxia represents an appropriate adjustment to reduced blood oxygen content and delivery. The exponentially decreasing atmospheric oxygen pressure with altitude stimulates the body to accommodate by an increase in respiratory rate and volume. Such adaptation is possible only in the short-term, because the body may not always be able to adequately enhance respiration. Polycythemia is considered to be a universal, uniform adaptation response to hypoxia that arises in normal individuals, but when it is exaggerated, in some cases it results in chronic mountain sickness with associated symptoms of fatigue, headache, and pulmonary hypertension.⁴²

Altitude above sea level should be used as an independent variable for the definition of polycythemia⁴³ and Centers for Disease Control data lists the appropriate adjustment values.⁴⁴

Post–Renal Transplant Erythrocytosis

This syndrome, defined as a persistent elevation of hematocrit at greater than 51 percent, is a relatively common condition found in approximately 5 to 10 percent of renal allograft recipients.^45,46 Post–renal transplant erythrocytosis usually develops within 8 to 24 months after transplantation, despite persistently good function of the allograft, and resolves spontaneously within 2 years in approximately 25 percent of patients.⁴⁷ Factors that increase the likelihood of its development are lack of erythropoietin therapy prior to transplantation, a history of smoking, diabetes mellitus, renal artery stenosis, low serum ferritin levels, and normal or higher pretransplant erythropoietin levels. Post–renal transplant erythrocytosis is also more frequent in patients who are not undergoing graft rejection.

Chuvash Polycythemia

A Russian hematologist, Lydia A. Polyakova, described polycythemia in the Chuvash population (an ethnic isolate in the mid-Volga River region of Russia of Turkish descent) in the early 1960s,⁴⁸ and by 1974, 103 cases from 81 families had been described.⁴⁸ Since then, more cases have come to light; hundreds of children and adults suffer from this condition, indicating that Chuvash polycythemia is the only known endemic congenital polycythemia in the world.⁴⁹ Outside of Chuvashia, Chuvash polycythemia has also been found sporadically in diverse ethnic and racial groups,^50,51 and recently a high prevalence of this disorder has also been reported on the Italian island of Ischia.⁵²

PRIMARY POLYCYTHEMIAS

Primary Familial and Congenital Polycythemia

In contrast to polycythemia vera, PFCP is caused by germline rather than acquired somatic mutations. It is congenital and manifests autosomal dominant inheritance³ and, not infrequently, sporadic occurrence from de novo germline mutations. Like polycythemia vera (Chap. 84), it is primary in that the defect changes intrinsic responses of erythroid progenitors, and erythropoietin levels are low.

To date, 12 mutations of the erythropoietin receptor (EPOR) associated with PFCP have been described (Table 57–1). Nine of the 12 result in truncation of the EPOR cytoplasmic carboxyl terminal, and are the only mutations convincingly linked with PFCP. Such truncations lead to a loss in the negative regulatory domain of the EPOR (Chaps. 32 and 34). Three missense EPOR mutations have also been described, but these have not been linked to PFCP or any other disease phenotype (Table 57–1).

Erythropoietin-mediated activation of erythropoiesis involves several steps (also see Chap. 32). First, erythropoietin activates its receptor by inducing conformational changes of its dimers (Chap. 17). These changes lead to initiation of an erythroid-specific cascade of events. The first signal is initiated by conformation change-induced activation of Janus-type tyrosine kinase 2 (JAK2) and its phosphorylation and activation of a transcription factor, signal transducer and activator of transcription 5 (STAT5), which regulates erythroid-specific genes. This “on” signal is negated by dephosphorylation of EPOR by hematopoietic cell phosphatase (HCP), also known as SHP1, that is, the “off” signal. EPOR truncations lead to a loss of the negative regulatory domain of the EPOR, a binding site for HCP, explaining the gain-of-function properties of these EPOR mutations (Fig. 57–1).

SECONDARY POLYCYTHEMIAS

The morbidity of primary polycythemias, such as polycythemia vera (Chap. 84), is largely the result of increased activated neutrophils and, perhaps, attendant pathologic platelet–endothelial interactions, whereas in secondary polycythemias it is presumably related to an increase in blood viscosity and, in part, to the resulting increased cardiac work.⁵³ In most instances, the etiology of morbidity or mortality, such as associated with congenital disorders of hypoxia sensing, is largely unknown.⁵⁴ The effect of blood viscosity on oxygen delivery is often oversimplified and the emphasis on the hematocrit alone may lead to ill-advised therapeutic interventions. In the normovolemic state, viscosity increases in a log-linear fashion as hematocrit increases, and the effect is particularly pronounced when the hematocrit rises above 50 percent. The prediction is that oxygen delivery decreases as hematocrit rises significantly above 50, as the greatly increased viscosity reduces blood flow, overshadowing the increased oxygen-carrying capacity of blood with a higher concentration of hemoglobin. However, polycythemia is not a normovolemic state, but is accompanied by an increase in blood volume, which, in turn, enlarges the vascular bed and decreases peripheral resistance (Chap. 34). Thus, hypervolemia can increase oxygen transport, and the optimum for oxygen transport occurs at higher hematocrit values than in normovolemic states. Consequently, despite the attendant increase in viscosity, an increase in hematocrit may generally be of benefit in appropriate secondary polycythemias. However, at some point, the high viscosity causes an increase in the work of the heart and a reduction in blood flow to most tissues and may be responsible for cerebral and cardiovascular impairment.

APPROPRIATE POLYCYTHEMIAS

High Altitude Polycythemia

Adaptive adjustments of humans living at high altitude involve a series of steps that reduce the steepness of the oxygen gradient between the atmosphere and mitochondria (Fig. 57–2).⁵⁵ The initial oxygen gradient between atmospheric and alveolar air can be reduced by an increase in respiratory rate and volume. Because dead space and water vapor pressure are constant and acclimatized individuals do not ventilate excessively, the normal sea level gradient of about 60 torr is only reduced to approximately 40 torr at 4540 m (14,900 ft) above sea level.⁵⁵ Further reduction can be achieved, and at the top of Mount Everest, extreme hyperventilation reduces the gradient to less than 10 torr. A shift in the oxygen dissociation curve to the right, which represents decreased affinity of hemoglobin for oxygen, may be of benefit for short-term high-altitude acclimatization,⁵⁶ but its usefulness for chronic acclimatization has probably been exaggerated.⁵⁷ In the unacclimatized subject exposed acutely to high altitude, hyperventilation alkalosis leads initially to a shift of the oxygen dissociation curve to the left, representing an increased affinity of hemoglobin for oxygen, further worsening already present tissue hypoxia. The alkalosis and the hypoxia will, in turn, promote red cell synthesis of 2,3-BPG and cause the oxygen dissociation curve to shift back to a normal or even right-shifted position (Chap. 49). In chronic acclimatization, blood pH is slightly increased, and when this is taken into account, the dissociation curve is shifted approximately to normal.⁵⁸ It is unlikely that a shift to the right would be to the advantage of high-altitude dwellers, except as a partial compensation for respiratory alkalosis.⁵⁹ In addition, a right-shifted curve also has a decrease in oxygen loading in the alveolar capillary, minimizing any net gain in off-loading. There is a relationship between higher altitude and hemoglobin concentration response, best studied among Andean highlanders and Europeans in the United States; hemoglobin concentration is almost 10 percent higher in those Andean highlanders living at 5500 m than in those living at 4355 m. Furthermore, native Andean high-altitude dwellers have a gradual increase in their hemoglobin levels with age⁶⁰ and body weight.⁶¹ Although it has been postulated that high hemoglobin–oxygen affinity in the setting of extremely low ambient oxygen may be one such adaptive change,⁶² increased hemoglobin–oxygen affinity or increased fetal hemoglobin are not adaptive phenotypes of Tibetan or Andean highlanders.⁶³

In a subset of Andean high-altitude native dwellers, namely Quechua and Aymara Indians, polycythemia becomes excessive and often results in chronic mountain sickness with its associated constitutional symptoms and pulmonary hypertension.^42,60 This excessive erythrocytosis, called Monge disease or chronic mountain sickness,^42,64 is also described in Han Chinese living in Tibet⁶⁵ and occurs in whites living at high altitudes.⁶⁶

The polycythemia encountered in high-altitude dwellers is often considered to be a universal, uniform adaptation process to hypoxia that would arise in all normal individuals. In reality, there is marked variability in erythropoietin level and subsequent polycythemic response to chronic hypoxia,^60,67 suggesting that some of these factors may be genetically determined; the same degree of hypoxia induces substantial differences in erythropoietin production in response to high altitude.^62,68,69 Three distinct adaptations to high altitude appear to have evolved. Andean highlanders have higher oxygen saturation than Tibetans at the same altitudes.⁶² Tibetan mean resting ventilation and hypoxic ventilatory response are higher than Andean Aymaras, whereas the mean Tibetan hemoglobin concentration is below the Andean mean. High levels of nitric oxide (NO) in the exhaled breath of Tibetans may represent increased physiologic NO. This effect may improve oxygen delivery by inducing vasodilation and increasing blood flow to tissues, thus making the compensatory increase in red cell volume unnecessary.⁷⁰ Another distinct successful pattern of human adaptation in high-altitude dwellers that contrasts with both the Andean “classic” (arterial hypoxemia with polycythemia) and Tibetan (arterial hypoxemia with normal venous hemoglobin concentration) patterns evolved in Ethiopia. While Ethiopian high-altitude dwellers have hemoglobin concentrations that fall in the normal range (15.9 and 15.0 g/dL for males and females, respectively), they have a surprisingly, as-yet-unexplained high (average of 95.3 percent) oxygen saturation of hemoglobin despite their hypoxic environment (reviewed in Ref. 62). Their cerebral circulation is increased but is insensitive to hypoxia, unlike Peruvian high-altitude dwellers.⁷¹ Thus, Ethiopian highlanders maintain venous hemoglobin concentrations and arterial oxygen saturation within the ranges of sea level populations, despite the decrease in ambient oxygen tension at high altitude.⁷² Tibetans and Ethiopians have lived as mountain dwellers much longer than the Quechua or Aymara Indians,⁷³ suggesting that extreme elevation of red cell mass is a maladaptation that Tibetans avoided by evolving a more efficient, or less detrimental, compensatory mechanism than that which causes Monge disease.

With rapid advances in genomics (Chap. 11), progress has been made in identification of the molecular basis of high-altitude adaptation; most of these advances have been in our understanding of Tibetan adaptation. Several studies reported evidence for positive natural selection in genomic regions in Tibetans; not surprisingly, most are haplotypes comprising genes that are components of hypoxia sensing that are mediated by hypoxia-inducible factors (HIFs) (described in Chap. 32). Two of these selected regions include genes that have undergone the strongest genetic selection, and are thus likely the most beneficial for Tibetan adaptation. These two regions encompass the EPAS1 gene, encoding the α subunit of HIF-2, and the EGLN1 gene, encoding proline hydroxylase 2 (PHD2). PHD2 is the principal negative regulator of both HIF-1 and HIF-2. Both of these haplotypes were shown to be associated with differences in hemoglobin concentration at high altitude by several independent studies.^74,75,76 Intriguingly, the EPAS1 haplotype was previously identified as having the strongest Tibetan positive selection and was found to have an unusual haplotype structure that originated by introgression of DNA from Denisovan or Denisovan-related hominins.⁷⁷ Denisovans, a sister group to the Neanderthals, branched off from the human lineage perhaps 600,000 years ago,⁷⁸ and available evidence suggests that Denisovan and Neanderthal hominins contributed to the modern homo sapiens admixture before their extinction, likely by interbreeding,⁷⁹ and that these hominin species provided genetic variations that helped humans adapt to new environments, such as extreme hypoxia associated with high altitude.⁷⁷ The first Tibetan adaptation gene mutation identified, which changes the encoded PHD2 protein within the selected haplotype, is a missense variant of the EGLN1 gene, c.12C>G,⁸⁰ that is in near complete linkage disequilibrium with a previously reported missense variant, EGLN1:c.380G>C.⁸⁰ Both EGLN1:c.[12C>G; 380G>C] (PHD2^D4E,C127S) are in cis; that is, the constituting PHD2^D4E,C127S locus. Analysis of Tibetans and related populations suggests that 12C>G started on the 380G>C variant that is not Tibetan specific,^80,81 that PHD2^D4E,C127S originated from a single individual approximately 8000 years ago,⁸¹ and that now greater than 80 percent of Tibetans carry this PHD2 variant.⁸¹ Functional assessment of homozygous PHD2^D4E,C127S recombinant proteins showed that the variant protein has increased hydroxylase activity under hypoxic conditions. Furthermore, native homozygous PHD2^D4E,C127S erythroid progenitors have blunted erythropoietic responses to hypoxia by both erythropoietin-specific and erythropoietin-independent mechanisms.⁸¹ Although this is the first identified variant that contributes to the molecular and cellular basis of Tibetan adaptation to high altitude, there are other evolutionarily selected genomic regions, and elucidation of their functional impact is, at the time of this writing, unknown.

Understanding the etiology of polycythemia of high altitude is made more complex by a study of inhabitants of the Peruvian mining community of Cerro de Pasco (altitude 4280 m) with excessive erythrocytosis (mean hematocrit: 76 percent; range: 66 to 91 percent). About half of those individuals with a hematocrit greater than 75 percent had toxic serum cobalt levels,⁸² suggesting that other erythropoiesis promoting factors such as cobalt⁸³ can augment hypoxia induction of erythropoietin, causing extreme polycythemias (Chap. 32). Most high-altitude dwellers do not have measurable levels or a history of exposure to cobalt or other heavy metals.⁸⁴

Erythrocytosis of Pulmonary Disease

Degrees of arterial hypoxia comparable to those observed in individuals at high altitudes are observed in patients with right-to-left shunting resulting from cardiac or intrapulmonary shunts or to ventilation defects, as in COPD.

Many patients with COPD with severe cyanosis are not polycythemic. This has been attributed to infections and inflammation often present in the lungs, resulting in anemia of chronic inflammation, and to an increase in plasma volume. Why some patients with lung disease and congenital heart disease develop polycythemia, while others do not, is not entirely clear.

Erythrocytosis of Eisenmenger Syndrome

Patients with right-to-left shunting (Eisenmenger syndrome) develop a degree of erythrocytosis comparable to that observed with similar degrees of desaturation at high altitudes.⁸⁵ The hematologic changes associated with this syndrome include hyperviscosity caused by erythrocytosis. Erythrocytosis is present in most patients, but excessive phlebotomy may cause microcytosis and some have attributed this effect to the exacerbation of the symptoms of hyperviscosity.²⁸ In view of recent understanding of physiology of HIF regulation, it may not be the microcytosis, per se, that is detrimental, but the induced iron deficiency that inhibits PHD2 and increases HIF, which then directly causes pulmonary vasoconstriction and enhanced pulmonary vascular pressure (Chaps. 32 and 34).

Obstructive Sleep Apnea-Induced Syndrome

In the colorfully named pickwickian syndrome,⁸⁶ polycythemia is characterized by its association with extreme obesity and somnolence. Today, the more widely studied OSA may not always be associated with obesity⁸⁷ but can, if severe, cause arterial hypoxemia, hypercapnia, somnolence, and secondary polycythemia.⁸⁸

Smoker’s Polycythemia

Heavy smoking will result in the formation of carboxyhemoglobin, which does not transport oxygen (Chap. 50), and causes an increase in oxygen affinity of the remaining normal hemoglobin. Carboxyhemoglobin increases in relationship to the number of cigarettes or cigars smoked each day (Table 57–2). This leads to tissue hypoxia, erythropoietin production, and stimulation of red cell production.⁸⁹ Smoking may also cause a reduction in plasma volume.⁹⁰ Either augmentation of red cell mass or shrinkage of plasma volume could easily explain the rise in the hematocrit.

Polycythemia Secondary to High-Affinity Hemoglobins

Hemoglobins with certain amino acid substitutions manifest an increased affinity for oxygen, producing tissue hypoxia and compensatory erythrocytosis (Chap. 49). Mutations affecting amino acids of the α₁β₂-globin chain interface affect normal rotation within molecules and impair the rate of deoxygenation. Changes in the carboxyl terminal and penultimate amino acids also impair intramolecular motion and tend to keep molecules in a high-affinity state. Alterations in the amino acids lining the central cavity of hemoglobin destabilize the binding of 2,3-BPG in this cavity and lead to increased oxygen affinity (Chaps. 47 and 49). Finally, some heme pocket mutations interfere with deoxygenation. Most hemoglobins with a mutation involving amino acids in the heme pocket are unstable and are associated with hemolytic anemia and cyanosis. Inheritance of these disorders is autosomal dominant. High-affinity hemoglobins result from mutations in any of three globin genes; those from α-globin gene mutations are congenital and life-long. β-Globin gene mutations are not apparent at birth but manifest after fetal to adult hemoglobin switching at approximately 6 months of life, while γ-globin gene mutations cause transient increase of hemoglobin concentration at birth lasting only about 6 months.

Polycythemia Secondary to Red Cell Enzyme Deficiencies

Deficiencies of red cell enzymes in early steps of glycolysis sometimes cause a marked decrease in the levels of 2,3-BPG (Chap. 47). Occasionally, mild polycythemia occurs in patients with methemoglobinemia as a result of cytochrome b₅ reductase (methemoglobin reductase) deficiency (Chap. 50).

The same pathophysiology as that seen in high-affinity hemoglobins is also exhibited in mutations of the 2,3-BPG mutase gene, resulting in low 2,3-BPG. Because these mutations are very rare, with only a single family comprehensively studied,⁹¹ it is not clear if the mode of inheritance is recessive or dominant.

This condition, as well as other high-affinity hemoglobins, can only be conclusively confirmed by direct measurement of a hemoglobin dissociation curve, conveniently expressed as the partial pressure of oxygen required to saturate 50 percent of hemoglobin (p50O₂); when equipment for this is not available, p50 can be estimated from pH, pO₂ and hemoglobin oxygen saturation of venous blood.^92,93

Chemically Induced Tissue Hypoxia

A number of chemicals have been suspected of causing histotoxic anoxia and secondary polycythemia, but the only chemical with a predictable capacity to cause erythrocytosis is cobalt.⁸³ Cobalt administration increases erythropoietin production by increasing HIFs (see Chap. 32).⁹⁴

INAPPROPRIATE POLYCYTHEMIAS

Congenital Disorders of Hypoxia Sensing

Chuvash Polycythemia

Chuvash polycythemia is the only known endemic congenital polycythemia. The condition is caused by an abnormality in the oxygen-sensing pathway and causes thrombotic and hemorrhagic vascular complications that lead to early mortality; survival beyond age 65 years is uncommon.^48,95 Inheritance is autosomal recessive, and affected patients tend to have normal blood gases, normal calculated p50, normal to increased erythropoietin levels, absence of genetic linkage to erythropoietin and EPOR loci, and no evidence of abnormal hemoglobin.⁹⁵ In a study of five multiplex Chuvash families with Chuvash polycythemia, a homozygous mutation of the von Hippel-Lindau (VHL) gene (598C>T); that is, VHL^R200W, was found in all affected individuals. This mutation impairs the interaction of VHL protein (pVHL) with both HIF-1α and HIF-2α, thus reducing the rate of ubiquitin-mediated destruction of HIF-1α and HIF-2α (Chap. 32). As a result, the level of HIF-1 and HIF-2 heterodimers increases and leads to the increased expression of target genes, including the erythropoietin (EPO), vascular endothelial growth factor (VEGF), and plasminogen activator inhibitor genes (PAI-1), among others.^4,5 Figure 57–3 depicts the effect of this mutation on hypoxic sensing. The role of circulating erythropoietin in the Chuvash polycythemia phenotype is indisputable. However, there must be other factors associated with the Chuvash polycythemia VHL mutation that contribute to the polycythemic phenotype, as the erythroid progenitors of Chuvash polycythemia patients are hypersensitive in vitro to extrinsic erythropoietin; the mechanism of this observation is not fully explained.^4,5 Some, but not all, patients with other VHL mutations have erythropoietin hypersensitive erythroid colonies^96,97,98; in these patients there is also increased expression of the RUNX1 and NFE1 genes, which can stimulate erythropoiesis.⁶

Despite increased expression of HIF-1α, HIF-2α, and VEGF in normoxia, Chuvash polycythemia patients do not display a predisposition to tumor formation. Imaging studies of 33 Chuvash polycythemia patients revealed unsuspected cerebral ischemic lesions in 45 percent, but no tumors characteristic of VHL syndrome.⁹⁹ There also, is a high prevalence of this disorder on the Italian island of Ischia.⁵² The Chuvash VHL^R200W mutation has also been described in whites in the United States and Europe, and in people of Punjabi/Bangladeshi Asian ancestry.¹⁰⁰ Some patients with congenital polycythemia have proved to be compound heterozygotes for the VHL^R200W mutation and other VHL mutations. Additionally, two distantly related Croatians with polycythemia are homozygous for VHL^H191D, the first example of a homozygous VHL germline mutation other than VHL^R200W causing polycythemia.^{51,54,101,102,103,104,105}

A small number of cases of congenital polycythemia that appear to have a mutation of only one VHL allele confound an obvious pathophysiologic explanation. In a Ukrainian family, two children with polycythemia were heterozygotes for VHL 376G>T (D126Y), but the father with the same mutation was not polycythemic.¹⁰⁴ An English polycythemic patient was a heterozygote for VHL 598C>T¹⁰⁶; but the inheritance of the deletion of a VHL allele, or null VHL allele, in a trans position was not excluded. Subsequently, two polycythemic VHL heterozygous patients were described in whom a null VHL allele was more rigorously excluded^101,102; the molecular mechanism of their polycythemic phenotype remains to be elucidated.

To address the question of whether the VHL 598C>T substitution occurred in a single founder or resulted from recurrent mutational events, haplotype analysis of eight highly informative single nucleotide polymorphic markers covering 340 kb spanning the VHL gene was performed on 101 subjects bearing the VHL 598C>T mutation and 447 normal unrelated individuals from Chuvash, Southeast Asian, white, Hispanic, and African American ethnic groups.⁴⁹ Polymorphism of the VHL locus in normal controls (having a wild VHL 598C allele) and subjects bearing Chuvash polycythemia VHL 598T were in strong linkage disequilibrium. These studies indicated that, in most individuals, the VHL 598C>T mutation arose in a single ancestor between 51,000 and 12,000 years ago. However, this is not the case for a Turkish polycythemic family with a VHL 598C>T mutation wherein the VHL 598C>T mutation occurred independently.¹⁰²

Chuvash polycythemia homozygotes have decreased survival because of thrombotic and hemorrhagic complications, mostly in the venous circulation,⁹⁹ and thus are under negative selection pressure. The high frequency of the mutation in some areas may be the result of random factors (“drift”), but it is also possible that propagation of the VHL 598C>T mutation is the result of a survival advantage for heterozygotes. Such an advantage might be related to a subtle improvement of iron metabolism, erythropoiesis, embryonic development, energy metabolism,¹⁰⁶ or some other as yet unknown effect. Indeed, heterozygotes were shown to be less likely to be anemic compared to control subjects.¹⁰⁷ Another potential protective role of a mildly augmented hypoxic response is improved protection against bacterial infections, as the hypoxia-mediated response has been reported to be essential for the bactericidal action of neutrophils.¹⁰⁸

Classic von Hippel-Lindau Syndrome

VHL syndrome is an autosomal dominant genetic abnormality affecting the posttranslational control of HIF-1α.^109,110,111 The syndrome is characterized by a propensity for developing renal cell carcinomas, retinal hemangioblastomas, cerebellar and spinal hemangioblastomas, pancreatic cysts, and pheochromocytomas. The tumors result from a somatic mutation in addition to the germline mutation, that is, loss-of-heterozygosity. Polycythemia is not part of VHL syndrome but hemangioblastomas of the central nervous system, and, less commonly, pheochromocytoma and renal cancer, have been associated with polycythemia mediated by paraneoplastic erythropoietin production.¹¹¹ Other patients with VHL syndrome also develop acquired polycythemia.^99,111 The VHL gene codes for 213 amino acids, and more than 130 germline mutations associated with classic VHL syndrome have been identified, virtually all of them 5′ to the codon 200 position that is mutated in Chuvash polycythemia.¹¹² Figure 57–4 depicts the schematic effect of the Chuvash polycythemia mutation in the context of other previously found VHL mutations.

It is not clear why mutations of a single gene lead to these two diverse phenotypes. It has been suggested that quantitative differences in loss of activity could explain the variable phenotypes among VHL mutations,¹¹³ but the VHL gene may also have other functions, possibly as a result of interactions with other modifying factors, that can contribute to the onset of disease and that await future clarification. Another plausible explanation of polycythemia versus cancer predisposition syndrome is that almost all polycythemic subjects have germline mutations of both VHL alleles, whereas those with VHL cancer predisposition syndrome have only a single germline mutation and then acquire a somatic mutation that is essential for tumor genesis.

EGLN1 Gene Mutations, Proline Hydroxylase Deficiency

Another principal negative regulator of HIFs is PHD2 (encoded by the EGLN1 gene), which targets the α subunit of HIF for degradation. The first loss-of-function mutation of PHD2 (PHD2^P317R) was identified in a family in which heterozygotes had mild or borderline polycythemia.¹¹⁴ Since then, 25 additional patients with unexplained polycythemia who are heterozygote carriers of different PHD2 mutations have been reported.¹¹⁵ Almost all patients with PHD2-associated polycythemia have normal erythropoietin levels. Whether the cause of polycythemia in this case is haploinsufficiency or a dominant negative effect remains to be determined.

EPAS1 (HIF-2α) Gain-of-Function Mutations

Affected patients have heterozygous missense mutations in the coding sequence of the EPAS1 gene that encodes HIF-2α, and typically have elevated erythropoietin levels.^115,116 There is heterogeneity in these gain-of-function HIF-2α mutations, but their existence supports the critical role of HIF-2α in controlling the expression of erythropoietin. Some patients with EPAS1 mutations, similar to Chuvash polycythemia, have erythropoietin hypersensitive colonies, thus sharing features of both primary and secondary polycythemias.⁹⁷

An explanation for the hypersensitivity of erythroid colonies bearing mutations that augment HIF stabilization remains to be discovered. It has been proposed that mutated VHL^R200W protein hinders suppression of cytokine signaling SOCS1-mediated JAK2 degradation, via binding of a negative regulator of erythropoiesis, SOCS1,¹¹⁷ to the extreme 3′ coding region of the VHL gene. Other observations are not consistent with this proposed mechanism: Another closely positioned VHL polycythemia mutation, VHL^H191D, is not associated with erythropoietin hypersensitivity,⁹⁶ while other, more upstream, mutations such as VHL^P138L are.⁸⁰ Furthermore, the hypersensitivity of erythroid colonies is also seen in some HIF-2α mutations.⁸⁶ Interestingly, in some, but not all, of these families, upregulation of NFE2, which enhances erythropoiesis, has been found.^6,118

Unexplained Congenital Polycythemias with Elevated or Inappropriately Normal Levels of Erythropoietin

The majority of patients with congenital polycythemias with inappropriately normal or elevated erythropoietin levels do not have VHL mutations, EGLN1 or EPAS1 mutations, hemoglobinopathies, or 2,3-BPG deficiency, and the molecular basis of polycythemia in these cases remains to be elucidated. Some such families show dominant inheritance,¹¹⁹ while in others inheritance is recessive, and in some it is sporadic. Lesions in genes linked to hypoxia independent regulation of HIF, as well as oxygen-dependent gene regulation pathways, are leading candidates for mutation screening in polycythemic patients with normal or elevated erythropoietin without VHL, EGLN1 (proline hydroxylase), or EPAS1 (HIF-2α) mutations.

Other Inappropriate Secondary Erythrocytoses

Renal Polycythemia and Post–Renal Transplant Erythrocytosis

Absolute erythrocytosis has been observed in a considerable number of patients with solitary renal cysts, polycystic renal disease, or hydronephrosis. In most of these cases, erythropoietin assays on cyst fluid, serum, or urine have disclosed the presence of erythropoietin.¹²⁰ Patients with polycystic disease have a hematocrit value slightly higher than normal and definitely higher than would have been expected of patients with uremia. In some patients on prolonged dialysis treatment, cystic transformation occurs in the native kidneys. This acquired cystic disease is occasionally associated with marked erythrocytosis.¹²¹ In patients with pheochromocytoma or paraganglioma and erythrocytosis, erythropoietin assays of serum and urine have disclosed higher-than-normal levels, and the erythrocytosis is most likely caused by excessive erythropoietin secretion by the tumor. This assumption has been supported by the presence of erythropoietin mRNA in tumor cells.¹²² Wilms tumors¹²³ and paraganglioma¹²⁴ are also occasionally associated with an erythrocytosis. Many of these cases may have a somatic VHL gene mutation that, in combination with a germline mutation of another allele, may constitute an unrecognized VHL syndrome. A patient with congenital erythrocytosis and recurrent paraganglioma with a PHD2 mutation was described. Tumor tissue exhibited a loss of heterozygosity of PHD2 in the tumor, suggesting that PHD2 could be a tumor-suppressor gene.³⁴

Partial obstruction of the renal artery would be expected to cause renal tissue hypoxia and a physiologic stimulation of erythropoietin production. Nevertheless, it has proved quite difficult to induce erythrocytosis in laboratory animals by placing a Goldblatt clamp on the renal arteries.¹²⁵ Only a few of the many patients who have arteriosclerotic narrowing of the renal arteries have been reported to be polycythemic.¹²⁶

Post–Renal Transplantation Erythrocytosis

Although the full molecular basis of post–renal transplant erythrocytosis remains unknown, angiotensin II (Chaps. 32 and 34) plays an important role in its pathogenesis.¹²⁷ Increased activity of the angiotensin II–angiotensin receptor 1 pathway makes the erythroid progenitors hypersensitive to angiotensin II.^128,129 Furthermore, angiotensin II can modulate release of erythropoiesis stimulatory factors (Chap. 32) including erythropoietin and insulin-like growth factor (IGF)-1.^130,131 Studies of venous effluents have determined that the native rather than the transplanted kidneys are the source of the inappropriate production of erythropoietin,¹³² and in some patients, removal of the native kidneys has led to rapid restoration of normal hematocrit values.¹³³ The condition is rarely seen in patients with nonrenal solid-organ allografts. The role of angiotensin II in augmenting erythropoiesis was confirmed by anemia in angiotensin-converting enzyme–knockout mice.¹³⁴ Prior to the late 1990s when the use of angiotensin-converting enzyme inhibitors increased as a means to reduce proteinuria, the incidence of erythrocytosis in renal transplant patients was approximately 8 to 10 percent within the first 2 years after engraftment.

Polycythemia with Connective Tissue Tumors

Occasionally, there is an association of erythrocytosis with large uterine myomas.³⁵ Usually, the tumor has been huge and extirpation has routinely been followed by a hematologic “cure.” The suggestion that the tumor interferes with pulmonary ventilation has not been supported by the normal arterial blood gas findings in the few patients so studied. Another possible mechanism is that the large abdominal mass causes mechanical interference with the blood supply to the kidneys, resulting in renal hypoxia and erythropoietin production. Inappropriate erythropoietin secretion by smooth muscle cells has been demonstrated both in uterine myomas and in one case of cutaneous leiomyoma.^35,135 Rare cases of polycythemia attributed to a myxoma of the atrium,³⁶ hamartoma of the liver,³⁷ and focal hyperplasia of the liver³⁸ have been documented.

Brain Tumors

In adequately studied patients with erythrocytosis and cerebellar hemangiomas, arterial blood gas tensions have been normal. That the tumors are directly responsible for the polycythemia can be surmised from the identification of erythropoietin in cyst fluid and stromal cells and from a case in which erythropoietin mRNA was present in the tumor.¹³⁶ Although in these cases a mutation of the VHL gene was not sought, it is likely that these tumors were a manifestation of an underlying VHL syndrome as cerebellar hemangiomas are an integral feature of VHL syndrome.

Hepatoma

In 1958, McFadzean and coworkers reported that almost 10 percent of patients in Hong Kong with hepatocellular carcinoma developed erythrocytosis.¹³⁷ Since then, this association has been recognized as an important clinical clue in the diagnostic consideration of patients with liver disease.¹³⁸ The cause of erythrocytosis is probably inappropriate production of erythropoietin by the neoplastic cells.¹³⁹ Normal hepatocytes, and to a lesser degree nonparenchymal liver cells, produce small amounts of erythropoietin, both constitutively and in response to hypoxia.

Congenital Polycythemia and Pheochromocytoma

Pheochromocytomas have been described in association with congenital erythrocytosis.¹⁴⁰ In a growing number of reports, several individuals with congenital polycythemia have developed recurrent pheochromocytomas, paragangliomas, and sometimes somatistatinomas.^141,142,143 The tumors in these patients are heterozygous for gain-of-function mutations of the EPAS1 gene (encoding HIF-2α), and an erythropoietin transcript is present in tumor tissues (Chaps. 32 and 57). Even though these tumors may be recurrent, they bear the same heterozygous mutations of the EPAS1 gene. However, these mutations are generally not found in nontumor tissues, so the etiology of the association of these tumors with polycythemia is not certain; it is also possible that they may be associated with postgonadal genetic mosaicism, wherein the EPAS1 mutation predisposes to tumor development.^141,142,143 However, in one family the EPAS1 mutation was inherited and also associated with the development of recurrent pheochromocytomas/paragangliomas.¹¹⁶

Endocrine Disorders

Chapter 38 has additional discussion.

Aldosterone-producing adenomas,¹⁴⁴ Bartter syndrome,¹⁴⁵ and dermoid cyst of the ovary¹⁴⁶ have been described in association with erythrocytosis. Erythropoietin levels were found to be elevated in the serum and returned to normal after extirpation of the tumors. A number of pathogenetic mechanisms have been suggested (Chaps. 32 and 38), including decreased plasma volume; mechanical interference with renal blood supply; hypertensive damage to renal parenchyma; functional interaction between aldosterone, renin, and erythropoietin; and inappropriate secretion of erythropoietin by the tumors. Mild polycythemia may be present in patients with Cushing syndrome, but its pathophysiologic basis is not entirely clear (Chap. 38).

The erythropoietic effect of androgens is of considerable practical importance.¹⁴⁷ For many years, it was assumed that the higher red cell count in males was caused by androgens because the hemoglobin levels of boys and girls were identical up until the time of puberty. It was not until pharmacologic doses of testosterone were administered to women with carcinoma of the breast that the full erythropoietic potency of androgens was appreciated.¹⁴⁸ Since then, various androgen preparations have been used in the treatment of refractory anemia, occasionally causing dramatic erythropoiesis, with hemoglobin values climbing into the polycythemic range (Fig. 57–5).

The mechanism of androgen action on erythropoiesis appears to be complex, related both to their capacity to stimulate erythropoietin production¹⁴⁹ and their capacity to induce differentiation of marrow stem cells directly.¹⁴⁷ These two effects have specific structural requirements. Androgens with the 5α-H configuration stimulate renal and extrarenal erythropoietin production, whereas androgens with the 5β-H configuration enhance the differentiation of stem cells.¹⁴⁹ Testosterone administration is associated with an increase in erythropoietin levels and a decrease in hepcidin levels.¹⁵⁰ Although erythropoietin levels declined with continued testosterone administration, they remained inappropriately high despite improved hemoglobin levels, suggesting a new set point.¹⁵⁰

Neonatal Polycythemia

Polycythemia at birth is a normal physiologic response to intrauterine hypoxia and to the high oxygen affinity of red cells containing very high proportions of hemoglobin F (Chap. 7). It may become excessive and even symptomatic, especially in infants of diabetic mothers, or if the clamping of the cord is delayed, permitting placental blood to boost the blood volume of the infant.¹⁵¹ Because it is difficult to recognize symptoms of hyperviscosity in the neonate, many pediatricians perform a partial plasma exchange transfusion if the venous hematocrit is above 65 percent at birth.¹⁵²

One study of 25,000 neonates in Utah¹⁵³ showed that the average hematocrit at birth would be considered “polycythemic” in adults, while 2 weeks later it has fallen to “anemic” levels. This dramatic decrease of red blood cells in neonates during their first days of life likely contributes to neonatal jaundice (Chap. 33).¹⁵⁴

APPARENT (RELATIVE) POLYCYTHEMIA

(Refer to section “Secondary Polycythemias/Erythrocytoses” above.)

Some believe that apparent polycythemia is merely a mild absolute polycythemia accentuated by a compensatory reduction in plasma volume. Others suggest that it is caused by a primary reduction in plasma volume and have associated it with hypertension, obesity, and stress. When the red cell mass is documented to be normal, spurious polycythemia is also an appropriate term. Its clinical significance has also been disputed. The high hematocrit with its associated high viscosity is believed by some to be a risk factor heralding cerebral and cardiac complications, while others believe it is merely a well-tolerated anomaly. Because the designation apparent polycythemia¹⁵⁵ is noncommittal, it is used here.

The main clinical associations with apparent polycythemia are obesity, hypertension, and smoking. In obese patients, the finding of a normal red cell volume may be spurious because if the volume is expressed in terms of lean body weight, some of these patients would have a significant increase in red cell mass. In hypertensive patients, there is no adequate explanation for the apparent increase in red cell production or decrease in plasma volume. Sleep apnea (common in patients with congestive failure), excessive production of atrial natriuretic factor, increased adrenal activation, decreased aldosterone secretion, and hypoxic vasoconstriction are all factors that have been invoked,^156,157,158 but with uncertainty. Chronic administration of diuretics to treat hypertension may be a more likely cause.¹⁵⁸

PRIMARY POLYCYTHEMIA

Primary Familial and Congenital Polycythemia

Although PFCP is uncommon, it is frequently misdiagnosed.¹⁸ Unlike patients with polycythemia vera, patients with PFCP lack splenomegaly, neutrophilia, basophilia, thrombocytosis, and a JAK2 mutation. Unless exposed to alkylating agents or radioactive phosphorus, as many have been, these patients do not progress to acute leukemia or myelodysplastic syndrome.¹⁵⁹ Generally thought to be benign, this condition predisposes patients to severe cardiovascular problems because of chronic augmented erythropoietin signaling in all tissues bearing EPOR.¹⁶⁰ An increased incidence of cardiovascular disease has been observed in affected members of PFCP families.¹⁶¹ Erythrocytosis may be very severe, with hemoglobin levels that typically exceed 20 g/dL in men and 18 g/dL in women. Headaches are commonly present. Hypertension, coronary artery disease, and strokes have been reported, but do not clearly appear to be related to an elevated hematocrit as they also occur in aggressively phlebotomized patients with normal hematocrits,¹⁶² and are not a constant feature of the disorder.¹⁶³

Chuvash Polycythemia

The recessive polycythemia that is endemic in the Chuvash Autonomous Republic of the Russian Federation is characterized by elevation of the hemoglobin level to a mean of 22.6 with a standard deviation of 1.4 g/L.⁹⁵ Some patients are symptomatic, with headache, fatigue, and signs that include clubbing, hemorrhaging, and peptic ulcer. Chuvash polycythemia is also associated with thrombosis, relatively low blood pressure (also seen in heterozygotes), and varicose veins.^4,95,99 As of yet, no significant association of thrombosis with elevated hematocrit and history of phlebotomies has been found.⁹⁹ A matched cohort study of 96 patients diagnosed in 1977 (65 spouses and 79 unaffected community members of the same age, sex, and village of birth) found that homozygosity for VHL 598C>T was associated with polycythemia, varicose veins, lower blood pressures, elevated serum VEGF and PAI-1 levels, and premature mortality related to cerebral vascular events and both venous and arterial thromboses.⁹⁹

Because Chuvash polycythemia is characterized by a germline mutation in the VHL gene, it was expected that homozygotes for this mutation may develop certain vascular tumors similar to those associated with classic VHL syndrome. However, tumors typical of classic VHL syndrome, such as spinocerebellar hemangioblastomas, renal carcinomas and pheochromocytomas/paragangliomas, were not found, indicating that increased expression of HIF-1α and VEGF is not sufficient for tumorigenesis. Benign vertebral body hemangiomas (a distinct entity from hemangioblastoma) were found in significantly more patients with Chuvash polycythemia compared to controls (55 percent vs. 21 percent). Imaging studies of 33 Chuvash polycythemia patients revealed unsuspected cerebral ischemic lesions in 45 percent of patients.⁹⁹ Affected patients have elevated systolic pulmonary artery pressures as estimated by echocardiography compared to controls, and iron deficiency associated with phlebotomy therapy may exacerbate this finding.^{164,165,166,167}

Other Congenital Disorders of Hypoxia Sensing

Because of their only recent discovery and apparent rarity, reliable clinical information is lacking. However, these disorders, in view of their global deregulation of hypoxia sensing, are expected to also have extraerythroid manifestation(s). This author is aware of a yet unpublished large family with a gain-of-function EPAS1 gene mutation wherein affected members appear to have early onset of strokes and cardiovascular disease, which are not prevented by control of their erythrocytosis by phlebotomies.

SECONDARY ACQUIRED POLYCYTHEMIA

High-Altitude Erythrocytosis

Tolerance to high altitudes varies greatly, but most normal individuals have no discomfort at altitudes of up to 2130 m (7000 ft). Above this level, and especially if the ascent is rapid, some manifestations of cerebral hypoxia are common. Headaches, sleeplessness, and palpitations are frequently encountered, and weakness, nausea, vomiting, and mental dullness may be present. More-severe manifestations include pulmonary and cerebral edema that may lead to death. Cheyne-Stokes respiration commonly occurs, especially during sleep. These symptoms constitute the syndrome of acute mountain sickness.¹⁶⁸

Ruddy cyanosis and physiologic emphysema are the two characteristic features of some humans living at high altitudes. Venous and capillary engorgement can be observed readily in the conjunctiva, mucous membranes, and skin, and may contribute to the remarkable capacity of Tibetan Sherpas to walk barefoot and sleep on ice and snow.¹⁶⁹ Asymptomatic retinal hemorrhages are seen frequently at high altitudes, but rarely at altitudes of 3000 m (9000 ft) or less.¹⁷⁰ Splenomegaly and jaundice are unusual, although the sustained erythrocytosis is associated with an increased fractional rate of red cell destruction and bilirubin generation. It has been stated that Monge disease includes low fertility,^42,66 but this may not be universally so. It has been suggested that high-altitude native-resident Tibetans exhibit two distinct genotypes for increased oxygen affinity of hemoglobin, and that women with genotypes for high oxygen saturation have more surviving children¹⁷¹ suggesting natural selection on the locus for oxygen saturation of hemoglobin.^72,171 However, these conclusions have been based not on measurement of the hemoglobin-oxygen dissociation curve, but on assumptions based on arterial oxygen saturation. In point of fact, when properly measured, Tibetans have normal oxygen affinity of hemoglobin when the hemoglobin-oxygen dissociation curve is rigorously determined.⁶³

Erythrocytosis Associated with Pulmonary Disease

The erythrocytosis associated with smoking is generally asymptomatic. There may be an increase in thrombotic events, but this may be from smoking rather than erythrocytosis.

When erythrocytosis is present in patients with COPD, with or without smoking, elevated hematocrit is associated with higher survival rates than anemic and normocythemic subjects.^19,20,175 Furthermore, moderate erythrocytosis has no adverse effect on vascular function in COPD¹⁷⁶ and is not associated with venous thromboembolism.¹⁷⁷

Large studies of patients with Eisenmenger syndrome²⁸ and other patients with cyanotic heart disease¹⁷⁸ caution against routine phlebotomy for asymptomatic elevation of the hematocrit; in fact, thrombotic complications were not observed in these studies. Transgenic mice with extreme polycythemia (hematocrit 85 percent) from constitutive overexpression of erythropoietin did not develop the expected thrombotic complications.¹⁷⁹ Adults with cyanotic congenital heart disease are at risk of having cerebrovascular events. This risk is increased in the presence of hypertension, atrial fibrillation, history of phlebotomy, and microcytosis, the latter condition having the strongest significance (p <0.005). The authors of these findings endorsed a more conservative approach toward phlebotomy and more aggressive approach toward treating microcytosis with iron preparations in adults with cyanotic congenital heart disease.¹⁸⁰

In a prospective cohort of U.S. Veterans Health Administration outpatients with stable COPD (n = 683), polycythemia prevalence was low and, unlike anemia, had no association with worsened outcomes.¹⁷⁵

Renal Polycythemia and Post–Renal Transplant Erythrocytosis

Although most patients with kidney failure are anemic, a fraction (often with polycystic kidney disease) display erythrocytosis, which, like the post–renal transplant state, can be very severe. Erythrocyte counts may be as high as 8 × 10¹² cells/L and associated with hypertension and congestive failure.¹⁸¹ At higher hematocrit levels (usually >60 percent), thrombotic events may complicate the clinical course.^46,47,182 Comorbidities that are associated with or causative of renal failure are frequently also factors predisposing to thrombosis, and the risk of erythrocytosis-associated thrombosis has not been submitted to rigorous multivariate rigorous statistical analyses. Thus, reports of increased thrombotic risks must be viewed with great caution.

Tumors

The erythrocytosis that occurs with erythropoietin-secreting tumors is generally mild,¹³⁶ and the predominating clinical manifestations are those of the tumor itself. Even moderate elevations to a hematocrit of 64 percent have been encountered without symptoms referable to the polycythemia.³⁸ Resection of the erythropoietin-secreting tumor cures the associated polycythemia.¹⁰⁵

In the syndrome of congenital polycythemia and pheochromocytoma or paraganglioma (see description in “Etiology” earlier) tumor resection does not lead to normalization of elevated hematocrit.

Neonatal Polycythemia

Of 55 infants with neonatal polycythemia, 47 (85 percent) had signs and symptoms attributed to this disorder. These included “feeding problems” (21.8 percent), plethora (20 percent), lethargy (14.5 percent), cyanosis (14.5 percent), respiratory distress (9.1 percent), jitteriness (7.3 percent), and hypotonia (7.3 percent). Other findings included hypoglycemia (40 percent) and hyperbilirubinemia (21.8 percent). In a larger group of nearly 1000 infants, six had intracranial hemorrhage.¹⁵¹

PRIMARY POLYCYTHEMIA

Primary Familial and Congenital Polycythemia

Characteristic laboratory findings of PFCP are: (1) increased red blood cell mass without increased leukocyte or platelet counts; (2) normal hemoglobin–oxygen dissociation curve; (3) invariably low serum erythropoietin levels; and (4) in vitro hypersensitivity of erythroid progenitors to erythropoietin.⁵ PFCP is often misdiagnosed as polycythemia vera, although with the advent of a reliable polymerase chain reaction-based test for the JAK2^V617F mutation, this should no longer ever happen. Whereas leukocytes are typically normal, platelet counts are often mildly decreased, presumably by dilution of the normal platelet mass by an often-dramatic increase of red cell and whole-blood volumes. Some patients come to attention because of concurrent medical problems that may cause leukocytosis and secondary thrombocytosis, falsely suggesting the phenotype of polycythemia vera.

Chuvash Polycythemia

Blood profiling in patients with Chuvash polycythemia indicates increased hemoglobin and hematocrit and lower white blood cell and platelet counts than in controls. Erythropoietin ranges from normal (but never close to the lower limits of normal) to elevated, at times exceeding 10 times the mean normal value. In larger studies, hemoglobin-adjusted serum erythropoietin concentrations were approximately 10-fold higher in VHL 598C>T homozygotes than in controls.^4,5,99

Affected subjects have lower CD4 counts, elevated levels of both proinflammatory and antiinflammatory cytokines, and altered plasma thiol levels, with elevated homocysteine and glutathione and low cysteine concentrations.^164,183,184 Their serum PAI-1 and VEGF levels are also increased.^4,95,99 Circulating transferrin receptor levels are higher in Chuvash polycythemia homozygotes as compared to their unaffected relatives and spouses.^4,5,99 Ferritin-adjusted transferrin receptor concentrations were approximately threefold higher in VHL 598C>T homozygotes than in unaffected participants (p <0.0005), which is consistent with upregulation by HIF-1.

Chuvash polycythemia is a relatively recently described disorder; thus, additional laboratory findings as a result of augmentation of hypoxia sensing are expected to be described.

Other Congenital Polycythemias from Augmented Hypoxia Sensing

At present, a paucity of data precludes any reliable description of other congenital polycythemias from augmented hypoxia sensing. Some affected subjects have unexpectedly low-normal erythropoietin levels.

SECONDARY ERYTHROCYTOSIS

Characteristically, only the number of erythrocytes in the blood is increased in secondary erythrocytosis. An increase in the leukocyte count and/or splenomegaly may be present as features of the underlying disease, for example, pulmonary infection in COPD with cor pulmonale, or as seen in Monge disease among Andean high-altitude dwellers or patients inheriting a high-affinity hemoglobin that is also unstable (Chaps. 34 and 49).

In patients with appropriate erythrocytosis, the underlying defect is usually demonstrable. Arterial hypoxia can be demonstrated in most cases. Some obese patients who, like Mr. Wardle’s proverbial boy, Joe, in The Pickwick Papers by Charles Dickens, are always half asleep, will be very much awake when exposed to arterial puncture and ventilatory testing, and their apprehensive hyperventilation will result in the disappearance of all abnormalities in arterial oxygen tension. As soon as they return to bed, however, they will go to sleep again and display the characteristic somnolent cyanosis.

In inappropriate erythrocytosis, the laboratory findings will be those of the underlying defect.

Also refer to Chaps. 34 and 59, Table 57–3, and Fig. 57–6.

Distinguishing between polycythemia vera and other polycythemic disorders used to be challenging, but discovery of the JAK2^V617F and JAK2 exon 12 mutations has made it, in most instances, straightforward. Some of the clinical and laboratory features that can be helpful for differential diagnosis are summarized in Chap. 34, Table 34–2, and in Fig. 57–6.

RED CELL MASS DETERMINATION

Determination of red cell mass is invaluable for differentiation of apparent (spurious) polycythemia from true polycythemic states. Unfortunately, determination of the red cell mass is expensive and, when performed by the inexperienced, often inaccurate.¹⁸⁵ It is not useful in distinguishing polycythemia vera from secondary erythrocytoses, the differentiation that is usually needed, because it is increased in both disorders. Ideally, the red cell mass and plasma volume should be measured separately. Unfortunately, the ¹³¹I-labeled albumin necessary to measure the plasma volume is often unavailable. Fortunately, in most cases, the diagnosis of polycythemia vera and other true polycythemic states can be established with confidence without measuring the red cell mass.

ERYTHROID COLONY CULTURES

In vitro assays of erythroid progenitor cells permit the study of their responsiveness to erythropoietin. This can be applied to polycythemia vera and erythroid progenitor burst-forming unit–erythroid (BFU-E) growth without added erythropoietin,¹⁸⁶ referred to as “endogenous erythroid colonies.” Detection of endogenous erythroid colonies in cultures of marrow or blood used to be the most specific test for polycythemia vera.^50,187,188 In one study, all patients with polycythemia vera, but none with secondary or other causes of polycythemia, had endogenous erythroid colonies.¹⁸⁹ Rare endogenous erythroid colonies may at times be observed in PFCP, Chuvash polycythemia, and in a single studied subject with the HIF-2α mutation,¹⁹⁰ but unlike the endogenous erythroid colonies of polycythemia vera, these are abrogated by pretreatment with erythropoietin and EPOR-blocking antibodies.^191,192

In experienced hands, endogenous erythroid colonies is a specific and sensitive means for detecting polycythemia vera and may be useful in diagnosing patients with unusual presentations of polycythemia vera, such as Budd-Chiari syndrome^{193,194,195,196} or isolated thrombocytosis.¹⁹⁷ However, this test has not been standardized, is expensive and laborious, and technical variations make interlaboratory results difficult to compare.

ERYTHROPOIETIN LEVELS

All patients with PFCP encountered⁵⁴ have erythropoietin below normal levels or below levels of detection. Thus, a low erythropoietin level is not pathognomonic of polycythemia vera, as patients with PFCP have as low or even lower erythropoietin levels.¹⁸

Because polycythemia vera is distinguished by the fact that erythroid cells proliferate even in the absence of substantial levels of erythropoietin, one would expect that at high hematocrit levels the production of erythropoietin would be inhibited and serum levels consequently reduced. Older erythropoietin assays were too insensitive to detect subnormal levels of erythropoietin, but using improved technology, several studies have documented serum erythropoietin levels below the normal reference range in patients with polycythemia vera.^198,199,200 Erythropoietin levels remain low even after phlebotomy,¹⁹⁸ which increases erythropoietin levels in normal individuals. Erythropoietin levels in Budd-Chiari syndrome may be normal or even increased.²⁰¹

Patients with secondary erythrocytosis usually have normal to elevated erythropoietin levels, although considerable overlap exists in the range of erythropoietin levels between some patients with polycythemia vera and those with secondary erythrocytosis.^199,202

CLONALITY IN FEMALE SUBJECTS USING ASSAYS EMPLOYING X-CHROMOSOME–BASED POLYMORPHISM ASSAYS

The principal role of the clonality assay is to differentiate polycythemia vera with an incomplete phenotype or atypical presentation from idiopathic, or as-yet undiagnosed erythrocytosis. Polycythemia vera results from an acquired mutation in a pluripotential hematopoietic stem/progenitor cell. Clonality studies based on the phenomenon of X-chromosome inactivation²⁰³ show that red cells, granulocytes, platelets, monocytes, and B lymphocytes are all part of the clone.^204,205 The majority of T lymphocytes and natural killer (NK) cells are polyclonal, but a small proportion of these cells are also derived from the polycythemia vera clone²⁰⁶; this is presumed to be a result of the presence of long-lived normal T cells that preceded the development of the clone. Unfortunately, interpretation of publications on the applicability of X-chromosome inactivation for differential diagnosis of polycythemia vera is hampered by the many methodologic and conceptual differences that have drawn conflicting conclusions.²⁰⁷ Some discrepancies are caused by the fact that two different approaches, which are not comparable, are used to distinguish the active from the inactive X-chromosome; one approach uses X-chromosome differential methylation,²⁰⁸ typically using the polymorphic CAG repeat in the human androgen-receptor gene (HUMARA),²⁰⁹ while the other approach uses more biologically sound but more technically demanding transcriptional analysis of the active X-chromosome.^208,210 Furthermore, the wide range of skewing of the X-chromosome allelic usage that is normally present²¹¹ is often misinterpreted as clonality, and the potentially clonal myeloid cells are not compared to the polyclonal control cells of the same origin.⁵⁰ In our study of 56 PV female polycythemia vera patients, reticulocytes, platelets, and granulocytes were always clonal, with the exception of a few patients who converted to polyclonal hematopoiesis after therapy with interferon-α.⁵⁰ While it was previously reported that clonality assays based on X-chromosome inactivation are not suitable for studies of older women using the HUMARA assay,^212,213,214 that was not confirmed by studies using quantitative transcriptional analysis of active X-chromosomes.^215,216

OTHER POLYCYTHEMIAS

Clinical history is of the utmost importance for the differential diagnosis of polycythemic states. Differentiation of an acquired from congenital disorder, and distinction between sporadic versus familial occurrence of polycythemia, when possible, will streamline the diagnosis. Thus, an autosomal dominant disorder is likely a result of polycythemia from gain-of-function EPOR, EPAS1 (HIF-2α) or EGLN1 (PHD2) mutations, or a high-affinity hemoglobin. Recessively inherited conditions may be from VHL gene mutations. Although rare patients with polycythemia vera may have a history of other affected family members (Chap. 84), polycythemia vera is always an acquired condition. Many familial polycythemias are the result of yet-to-be-discovered genetic events.

Patients with a low level of erythropoietin and autosomal dominant inheritance should have sequence analysis of the EPOR gene. This will define the defect in some patients with PFCP; if the polycythemia is acquired and present in multiple relatives, a diagnosis of familial clustering of polycythemia vera should be pursued.²¹⁷ Patients with secondary erythrocytosis have a genuine increase in the number of circulating erythrocytes and the red cell mass. Such patients do not typically have increased platelet or leukocyte counts or splenomegaly, which are often seen in polycythemia vera. The lack of involvement of other cell lineages in hematopoietic proliferation should arouse suspicion that the patient may have erythrocytosis other than polycythemia vera.

However, reactive thrombocytosis, leukocytosis, and splenomegaly may occasionally also be present in secondary erythrocytosis, which then renders the distinction from polycythemia vera more difficult. In patients in whom secondary erythrocytosis is caused by lung or cardiac disease, clubbing is often present. In some cases, determining the arterial oxygen saturation will clarify the diagnosis, but modest arterial oxygen desaturation may also be present in polycythemia vera.^85,218 Imaging of the kidneys may reveal a neoplasm or cyst in some patients. Determining the hemoglobin oxygen dissociation curve (Chaps. 32, 34, 50), or estimation of p50 from venous blood⁹² will detect abnormalities related to increased oxygen affinity either because of inheritance of a high-affinity hemoglobin (Chap. 49), or because of very rare 2,3-BPG depletion, as in phosphoglyceromutase deficiency (Chap. 47). The mild erythrocytosis associated with hereditary methemoglobinemia (Chap. 50) is readily diagnosed because of coexistent cyanosis.

In patients with elevated erythropoietin, or with erythropoietin levels inappropriately normal for the degree of hemoglobin elevation, analysis of VHL, EGLN1, and EPAS1 genes may be in order; some of these patients may have a history of autosomal recessive inheritance and have a typical history of congenital polycythemia. It may also be useful to determine the carboxyhemoglobin level of the blood if smoker’s polycythemia is suspected.

SPURIOUS POLYCYTHEMIA

The erythrocytosis observed in patients with spurious polycythemia (apparent polycythemia, stress polycythemia) is the consequence of a decrease in plasma volume.¹⁵⁵ The observed erythrocytosis does not represent a true increase in the red cell mass. Usually, the increase in hematocrit is very modest. Such patients do not have an increased white blood count, thrombocytosis, splenomegaly, or JAK2 mutation. The arterial oxygen saturation is normal. Estimation of the red cell mass and plasma volume is required to establishing a diagnosis of spurious polycythemia, but it should be recognized that during the natural history of patients who develop primary or secondary polycythemia, their red cell mass is, at some point, within the normal range while it is rising to abnormal values. Because of the significant error rate of red cell mass measurement, it is recommended that both red cell mass and plasma volume be measured simultaneously.

POLYCYTHEMIAS OTHER THAN POLYCYTHEMIA VERA

Treatment of patients with post–renal transplant erythrocytosis with drugs that suppress the renal–angiotensin system has virtually eliminated the need for therapeutic phlebotomy. The maximal reduction of hemoglobin and hematocrit levels usually manifests by 6 months after starting therapy with either the angiotensin-converting enzyme inhibitor, enalapril, or the angiotensin II receptor type 1 blocker, losartan.¹²⁷ Some patients are exquisitely sensitive and may become severely anemic.

High-altitude erythrocytosis is also associated with pulmonary hypertension, proteinuria, and elevated blood pressure. A prospective randomized trial of enalapril reported decreased hemoglobin concentration, proteinuria and beneficial effects on elevated blood pressure.²¹⁹

When erythrocytosis is secondary to a renal tumor or cyst, pheochromocytoma, myoma, or brain tumor, removal of the neoplasm usually results in disappearance of the erythrocytosis, but in the syndrome of congenital polycythemia with pheochromocytoma caused by EPAS1 mutations (see above paragraphs) erythrocytosis persist after tumor resection.

No specific therapy is currently available for polycythemic subjects with VHL, EGLN1, or EPAS1 mutations.

Lowering the hematocrit to a normal or near-normal level by phlebotomy is the usual, but empiric, treatment of secondary erythrocytosis,^220,221 but should always be viewed in the context of a particular subject.^28,178 The appropriate level is that at which the patient becomes asymptomatic. Although cytotoxic agents are sometimes used for this purpose, phlebotomy is preferred, if indeed needed, because of the leukemogenic risk of the agents that are used in polycythemia vera. This author favors, in most instances, benign neglect, unless specific therapy, such as that seen in erythropoietin-secreting tumors or post–renal transplant erythrocytosis, is available. One should phlebotomize only those patients who are symptomatic from the elevated red cell mass and continue to do so cautiously only if symptoms respond promptly to phlebotomy.

CHUVASH POLYCYTHEMIA

A study compared 96 patients with Chuvash polycythemia diagnosed before 1977 to 65 spouses, and 79 unrelated community members with normal hemoglobin concentration, same age, sex, and village of birth; the estimated survival to 65 years was 31 percent or less for Chuvash polycythemia patients versus 67 percent or greater for spouses and community members (p ≤0.002).⁹⁹

OTHER POLYCYTHEMIAS

The clinical course of secondary erythrocytosis is largely a function of the underlying disorder. In patients with PFCP secondary to mutations of the EPOR gene, coronary artery disease and strokes have been reported,¹⁶² although not in all series.¹⁶³ The rarity of patients having mutations of the EPOR, VHL, EGLN1, and EPAS1 genes and polycythemias from globin mutations and or red cell enzyme deficiencies precludes any meaningful prognostic evaluation.