Increased blood red cell mass can be termed either polycythemia or erythrocytosis; no clear consensus for either term has been achieved. Primary polycythemias are caused by acquired or inherited mutations causing functional changes within hematopoietic stem cells or erythroid progenitors leading to an accumulation of red cells. The most common primary polycythemia, polycythemia rubra vera, which is a clonal disorder, is discussed in Chap. 84; other primary polycythemias, such as those inherited from mutations in the erythropoietin receptor or congenital disorders of hypoxia sensing, are discussed herein. In contrast, secondary polycythemias are a result of either an appropriate or inappropriate increase in the red cell mass, most often as a result of augmented levels of erythropoietin; these polycythemias can also be either acquired or hereditary. Although the clinical presentations of primary and secondary polycythemias may be similar, distinguishing amongst them is important for accurate diagnoses and proper management.
For example, those secondary polycythemic states that represent an appropriate physiologic compensation to tissue hypoxia, should not be treated by phlebotomies. An occasional patient may experience hyperviscosity symptoms and may benefit from isovolemic reduction of hematocrit. Inappropriate polycythemias are caused by erythropoietin-secreting tumors, self-administration of erythropoiesis-stimulating agents, including androgens, inherited disorders of hypoxia sensing, or, rarely, some endocrine disorders (described in Chap. 38). Correction of hypoxia, discontinuation of erythropoiesis-stimulating agents or resection of erythropoietin-secreting tumors will typically correct the associated polycythemia.
Acronyms and Abbreviations:
BFU-E, burst-forming unit–erythroid; 2,3-BPG, 2,3-bisphosphoglycerate; COPD, chronic obstructive pulmonary disease; EGLN1, a gene encoding for PHD2; EPAS1, a gene encoding for HIF-2α; EPOR, erythropoietin receptor (protein); HCP, hematopoietic cell phosphatase; HIF, hypoxia-inducible factor; HUMARA, human androgen-receptor gene; JAK, Janus-type tyrosine kinase; OSA, obstructive sleep apnea; PAI-1, plasminogen activator inhibitor; PFCP, primary familial and congenital polycythemia; PHD2, proline hydroxylase 2; STAT, signal transducer and activator of transcription; VEGF, vascular endothelial growth factor; VHL, von Hippel-Lindau syndrome.
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.
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...