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Prevalence is unknown.

The hematological disorder is present at birth but the clinical symptoms, if they develop, can be discovered at any time during childhood or adulthood. Clinical features can include headache, dizziness, epistaxis and exertional dyspnea. Thrombotic events have been observed. The hematological features include the presence of isolated erythrocytosis without evolution into leukemia or other myeloproliferative disorders, absence of splenomegaly, normal white blood cell and platelet counts, and low plasma EPO levels.

Primary familial polycythemia is caused by mutations in the EPO receptor (EPOR) gene (19p13.3-p13.2) resulting in hypersensitivity to EPO. The mutations result in a receptor that is ''switched on'' to stimulate red blood cell production by erythroid progenitor cells but that has no ''switch off'' mechanism.

Diagnosis is based on evidence in kindreds of isolated erythrocytosis without splenomegaly, low serum EPO levels, normal hemoglobin oxygen affinity and bone marrow erythroid progenitors that exhibit EPO hypersensitivity.

Differential diagnoses include polycythemia vera (see this term), although there is no propensity to leukemic transformation or development of other myeloproliferative neoplasms in primary familial polycythemia, and secondary polycythemia (see these term). Polycythemia vera can be excluded on the basis of the absence of mutations in the JAK2 gene (9p24), and secondary polycythemia should be suspected if EPO levels are normal to high.

Transmission is usually autosomal dominant although sporadic cases have been reported. At least 14 different mutations have been described in single families.

Patients with polycythemia should be individually assessed. Reducing the hematocrit (Hct) by phlebotomy reduces the blood viscosity and may be of benefit. Clinical symptoms are effectively relieved by phlebotomy. However, the increased risk of cardiovascular morbidity is not ameliorated by maintaining a normal Hct. If venesection is judged appropriate, the sparse evidence suggests it can reduce the Hct when it is over 54%. In patients with an increased risk of thrombosis, previous thrombosis, peripheral vascular disease, diabetes or hypertension, venesection should be considered at Hct less than 54%. In those with no specific contraindication, low-dose aspirin is relatively safe and may be of benefit to patients with erythrocytosis.

Primary familial polycythemia does not necessarily carry an adverse prognosis in early life and most patients have a benign clinical course, but it is associated with an increased risk of thrombosis and vascular mortality in later life.