polycythemia

polycythemia. Polycythemia is a condition characterized by an abnormal increase in the number of red blood cells circulating in the blood. This elevated hematocrit or hemoglobin concentration increases blood viscosity, which can impair circulation and lead to serious complications. The disorder is broadly classified into primary forms, which are myeloproliferative neoplasms, and secondary forms, driven by external factors such as chronic hypoxia.

Definition and classification

Polycythemia is defined by laboratory parameters, specifically a hematocrit level above 52% in men and 48% in women, or a hemoglobin level exceeding 18.5 g/dL in men and 16.5 g/dL in women. It is classified into two principal categories: primary and secondary. Primary polycythemia, known as polycythemia vera, is a clonal disorder of the hematopoietic stem cell and is one of the classic Philadelphia chromosome-negative myeloproliferative neoplasms. Secondary polycythemia arises from an appropriate or inappropriate increase in the hormone erythropoietin, often in response to chronic hypoxia from conditions like chronic obstructive pulmonary disease or cyanotic heart disease, or from erythropoietin-secreting tumors such as renal cell carcinoma. A rare, inherited form known as familial erythrocytosis is caused by mutations in genes like the EPO receptor or VHL gene.

Causes and pathophysiology

The fundamental pathophysiology involves the excessive production of red blood cells. In polycythemia vera, over 95% of cases are driven by an acquired mutation in the JAK2 gene, specifically the JAK2 V617F mutation, which leads to constitutive activation of the JAK-STAT signaling pathway and cytokine-independent proliferation of erythroid progenitors. Secondary polycythemia is physiologically appropriate when it results from tissue hypoxia, as seen in individuals living at high altitude, patients with sleep apnea, or those with chronic carbon monoxide poisoning. Inappropriate secondary polycythemia can occur due to erythropoietin-producing tumors, such as hepatocellular carcinoma or cerebellar hemangioblastoma, or from the abuse of erythropoiesis-stimulating agents like in blood doping. The increased red blood cell mass directly increases blood viscosity and contributes to a hypercoagulable state.

Signs and symptoms

Clinical manifestations are often related to increased blood viscosity and hypervolemia. Common symptoms include headache, dizziness, visual disturbance, and tinnitus. Patients frequently experience a characteristic burning pain in the hands or feet, known as erythromelalgia, and severe pruritus, especially after a warm shower, believed to be related to mast cell and basophil activation. Physical examination may reveal plethora, conjunctival injection, and splenomegaly due to extramedullary hematopoiesis. Hypertension and gout are also commonly associated findings. Severe complications often present as thrombotic events, including deep vein thrombosis, Budd-Chiari syndrome, or transient ischemic attack.

Diagnosis

Diagnosis begins with a complete blood count showing elevated hematocrit, hemoglobin, and often increased red blood cell count. For suspected polycythemia vera, testing for the JAK2 V617F mutation is essential and is included in the diagnostic criteria established by the World Health Organization. Additional tests may include serum erythropoietin level, which is typically low in polycythemia vera but normal or high in secondary forms. A bone marrow biopsy often shows panmyelosis with prominent erythroid hyperplasia. To distinguish absolute polycythemia from relative polycythemia (Gaisböck syndrome), measurement of red cell mass using chromium-51 labeling may be performed. Investigations for secondary causes include arterial blood gas analysis, pulse oximetry, renal ultrasound, and imaging studies to rule out renal cell carcinoma.

Treatment and management

The primary goal of treatment is to reduce the risk of thrombosis and manage symptoms. For polycythemia vera, the cornerstone of therapy is phlebotomy to maintain a hematocrit below 45%, as demonstrated by the CYTO-PV clinical trial. Cytoreductive therapy is indicated for high-risk patients, typically those over 60 or with a history of thrombosis; first-line agents include hydroxyurea or interferon alfa. The JAK inhibitor ruxolitinib is approved for patients resistant or intolerant to hydroxyurea. Low-dose aspirin is routinely used to mitigate thrombotic risk. Management of secondary polycythemia focuses on correcting the underlying cause, such as supplemental oxygen for hypoxia or surgical resection of an erythropoietin-secreting tumor. All patients are advised to avoid dehydration and smoking.

Prognosis and complications

The prognosis varies significantly between primary and secondary forms. Untreated polycythemia vera carries a high risk of life-threatening complications, primarily arterial and venous thrombosis, including myocardial infarction and stroke. Long-term sequelae include progression to myelofibrosis or transformation to acute myeloid leukemia. With modern management, median survival can exceed 10-15 years. The major cause of mortality remains thrombosis, followed by hematological transformation. Other complications include peptic ulcer disease, due to increased histamine release, and iron deficiency from chronic phlebotomy. The prognosis for secondary polycythemia is largely dependent on the control of the underlying disorder, such as successful management of chronic obstructive pulmonary disease or removal of a renal cell carcinoma. Category:Hematology Category:Myeloproliferative neoplasms