Polycythemie

Polycythemia, or polycythemia vera in its most common form, refers to an increase in red blood cell mass, which raises hemoglobin and hematocrit levels. It is important to distinguish between absolute polycythemias, where there are more red cells, and relative polycythemia, where plasma volume is reduced. Absolute polycythemias are further classified as primary (a clonal bone marrow disorder) and secondary (a response to other conditions that raise erythropoietin).

Primary polycythemia, most notably polycythemia vera (PV), is a myeloproliferative neoplasm characterized by uncontrolled red cell production. Most patients harbor a JAK2 mutation (JAK2 V617F or, less commonly, JAK2 exon 12). PV often presents with elevated hematocrit, enlarged spleen, headaches, ruddy complexion, and an increased risk of blood clots. Management centers on reducing thrombotic risk and controlling red cell mass: regular therapeutic phlebotomy to maintain hematocrit below about 45%, low-dose aspirin in many patients, and, for higher-risk individuals, cytoreductive therapy such as hydroxyurea or interferon. Leukemic transformation is a rare but recognized complication.

Secondary polycythemia arises from increased erythropoietin production due to chronic hypoxia or ectopic EPO production (for example, from kidney tumors). Common causes include chronic lung disease, living at high altitude, sleep apnea, and tobacco use. Treatment targets the underlying cause; supplemental oxygen may be used for hypoxic patients, and phlebotomy is considered cautiously.

Relative polycythemia results from plasma volume loss (dehydration, diuretics, or stress) with a normal or high red cell mass; management focuses on fluid resuscitation and addressing contributing factors.

Diagnosis combines elevated hemoglobin/hematocrit, erythrocyte mass assessment, erythropoietin level, JAK2 testing, and sometimes bone marrow examination.