What is polycythemia, including its classification, clinical features, diagnostic criteria, and management?

Polycythemia: Definition and Overview

Polycythemia refers to an increase in red blood cell mass, which can be either true (actual increase in red blood cells) or apparent (spurious elevation due to decreased plasma volume). 1

Classification

Polycythemia is fundamentally divided into three categories:

1. Primary Polycythemia (Polycythemia Vera)

• A clonal myeloproliferative neoplasm caused by acquired mutations
• More than 95% of patients harbor JAK2 V617F mutation 2, with JAK2 exon 12 mutations accounting for most remaining cases 3
• Represents autonomous, clonal proliferation of red blood cells independent of erythropoietin (EPO) control 1

2. Secondary Polycythemia

• Non-clonal increase in red cell mass, often EPO-mediated
• Hypoxia-driven causes: Chronic lung disease, sleep apnea, high altitude, carbon monoxide exposure (smoker's polycythemia), cyanotic heart disease 1
• Hypoxia-independent causes: EPO-secreting tumors (renal cancer, uterine leiomyoma, pheochromocytoma, meningioma), congenital disorders (Chuvash polycythemia), exogenous EPO or androgen administration 1

3. Apparent (Relative) Polycythemia

• Elevated hemoglobin/hematocrit due to contracted plasma volume rather than true increase in red cell mass
• Causes include dehydration, diuretic use, severe burns, capillary leak syndrome 1
• Gaisböck syndrome and stress polycythemia are poorly understood entities with little foundation 1

Diagnostic Criteria for Polycythemia Vera

The 2016 WHO criteria require either all 3 major criteria OR the first 2 major criteria plus the minor criterion 4:

Major Criteria:

1. Hemoglobin >16.5 g/dL in men or >16.0 g/dL in women (or hematocrit >49% in men, >48% in women, or increased red cell mass) 4
2. Bone marrow biopsy showing hypercellularity with trilineage growth (panmyelosis) with pleomorphic, mature megakaryocytes 3
3. Presence of JAK2 V617F or JAK2 exon 12 mutation 3

Minor Criterion:

• Subnormal serum erythropoietin level 4

Note: Bone marrow biopsy may not be required in cases with sustained absolute erythrocytosis (hemoglobin >18.5 g/dL in men or >16.5 g/dL in women) if major criterion 3 and the minor criterion are present, though initial myelofibrosis can only be detected by bone marrow examination 4.

Clinical Features

Symptoms and Signs:

• Microvascular disturbances: Erythromelalgia (5.3%), headache, visual disturbances (14%), dizziness 2
• Pruritus (33% of patients), often aquagenic (triggered by water exposure) 2
• Splenomegaly (36%) with abdominal discomfort 2
• Thrombocytosis (53%) and leukocytosis (49%) commonly accompany erythrocytosis 2

Complications:

• Thrombosis: Most important complication
  • Arterial thrombosis occurs in 16% of patients prior to or at diagnosis 2
  • Venous thrombosis in 7%, including unusual sites like splanchnic veins 2
  • 20-year thrombosis risk approximately 26% 5
• Bleeding risk: Especially with extreme thrombocytosis (platelet count ≥1000 × 10⁹/L) due to acquired von Willebrand disease, which occurs in more than one-third of PV patients 1
• Transformation: 20-year risk of post-PV myelofibrosis is 16%, acute myeloid leukemia is 4% 5

Risk Stratification

For Polycythemia Vera, patients are classified as high-risk if age >60 years OR history of previous thrombosis 6. All others are low-risk.

Additional thrombosis predictors include:

• Arterial thrombosis: Cardiovascular risk factors 5
• Venous thrombosis: Higher absolute neutrophil count and JAK2V617F allele burden 5

Management

All Patients (Regardless of Risk):

1. Phlebotomy to maintain hematocrit <45% 6, 1, 2
2. Low-dose aspirin (81 mg) once or twice daily, unless contraindicated 6, 1, 2

High-Risk Patients:

Add cytoreductive therapy 6:

• First-line: Hydroxyurea (starting dose 500 mg twice daily) or interferon-α (3 million units subcutaneously 3 times weekly) at any age 6, 1
• Second-line: Busulfan (initial dose 4 mg/day) or ruxolitinib 7, 5

Ruxolitinib is specifically recommended for patients intolerant or resistant to hydroxyurea, particularly those with severe pruritus or marked splenomegaly unresponsive to other agents 7, 5

Special Considerations:

• Systemic anticoagulation is advised for patients with venous thrombosis history 5
• Interferon-α is preferred in younger patients and women of childbearing age due to theoretical teratogenicity concerns with other cytoreductive agents 1

Prognosis

• Median survival approximately 15 years overall, but exceeds 35 years for patients aged ≤40 years 5
• Prognostically adverse factors include older age, leukocytosis, abnormal karyotype, and presence of SRSF2, IDH2, RUNX1, or U2AF1 mutations 5

Diagnostic Pitfalls

When evaluating polycythemia, if erythrocytosis is isolated, consider secondary and hereditary causes first 8. If accompanied by leukocytosis or thrombocytosis, initial investigations should include JAK2 mutation testing and serum erythropoietin levels 8.

Avoid unnecessary red cell mass measurements in clinically obvious cases of relative polycythemia (dehydration, diuretic use) or when modern diagnostic tests (JAK2 mutation, EPO levels) can distinguish PV from secondary causes 1.