Polycythaemia

Overview

Polycythaemia refers to an increased concentration of red blood cells (RBCs) in the bloodstream, which may be true (due to overproduction of RBCs) or relative (due to plasma volume loss). It is an important cause of hyperviscosity-related symptoms and thrombotic events. The most notable cause is Polycythaemia Vera, a clonal myeloproliferative neoplasm. Other causes include secondary hypoxic states or renal pathologies. Diagnosis depends on careful differentiation between primary, secondary, and spurious causes, and management is tailored to the underlying mechanism.

Definition

• Increased RBC concentration in peripheral blood
• Often measured as ↑ haemoglobin, ↑ haematocrit, or ↑ RBC count
• Can be:
  • True polycythaemia (absolute ↑ in RBC mass)
  • Relative (spurious) polycythaemia due to ↓ plasma volume

Aetiology

True Polycythaemia

Primary:

• Polycythaemia Vera
  • Myeloproliferative disorder with autonomous RBC production
  • Often associated with JAK2 mutation
  • May also affect platelets and WBCs

Secondary:

• Tissue hypoxia → ↑ Erythropoietin (EPO)
  • Smoking (carbon monoxide exposure)
  • High altitude
  • Chronic lung disease
  • Cyanotic congenital heart disease
• Renal hypoxia or tumours → ↑ EPO
  • Renal cell carcinoma
  • Hydronephrosis
  • Polycystic kidney disease

Relative (Spurious) Polycythaemia

• Dehydration (e.g. vomiting, diuretics, burns)
• Results in apparent rise in haematocrit due to ↓ plasma volume

Pathophysiology

Polycythaemia Vera

• Clonal stem cell disorder (myeloproliferative neoplasm)
• Malignant proliferation of erythroid progenitor cells independent of EPO
• Also involves increased platelet and WBC production
• Risk of thrombosis, bleeding, and progression to myelofibrosis or AML

Secondary Polycythaemia

• Triggered by chronic hypoxia → ↑ EPO
• EPO drives bone marrow to increase RBC mass

Relative Polycythaemia

• RBC mass is normal
• Plasma volume is reduced

Morphology

• Hypercellular marrow with erythroid hyperplasia in true polycythaemia
• Bone marrow biopsy may be used to confirm diagnosis in PV

Clinical Features

General Symptoms

• Often asymptomatic early
• Hyperviscosity symptoms:
  • Headaches
  • Dizziness
  • Tinnitus
  • Visual disturbances

Pathognomonic Features (esp. Polycythaemia Vera)

• Itching after hot showers (aquagenic pruritus)
• Erythromelalgia (burning pain in hands/feet, relieved by cold)

Signs

• Facial plethora
• Splenomegaly
• Thrombotic events (DVT, PE, stroke, MI)
• Hypertension

Investigations

• FBC: ↑ Hb, ↑ Hct, possibly ↑ WBC and platelets
• EPO level: ↓ in PV, ↑ in secondary causes
• JAK2 mutation: present in >95% of PV cases
• Oxygen saturation, carboxyhaemoglobin: if hypoxia suspected
• Abdominal imaging: for renal lesions
• Bone marrow biopsy: confirms PV or excludes malignancy

Management

• Treat underlying cause (e.g. oxygen for hypoxia, nephrectomy if renal carcinoma)
• Venesection: first-line for PV to reduce haematocrit
• Antiplatelet therapy: e.g. low-dose aspirin to reduce thrombosis risk
• Cytoreductive therapy (hydroxyurea): if high-risk PV
• Manage cardiovascular risk factors (BP, smoking, cholesterol)

Summary

Polycythaemia refers to increased red cell mass and may be primary (e.g. PV), secondary to hypoxia or EPO excess, or relative from dehydration. It often presents with hyperviscosity symptoms and may lead to thrombotic complications. Diagnosis is based on clinical findings, blood counts, and EPO levels, with JAK2 mutation supporting PV. For broader context, see our Blood & Haematology Overview page.