🌡️ Polycythaemia = an abnormally raised red blood cell (RBC) mass, leading to increased blood viscosity. It may be true (increased RBC production) or apparent (reduced plasma volume). ⚠️ Important because it predisposes to thrombosis, haemorrhage, and progression to myelofibrosis/leukaemia.

🧬 Pathophysiology

Primary (Polycythaemia Vera, PV): Myeloproliferative neoplasm, usually due to JAK2 mutation → uncontrolled RBC production, often with ↑ WBC and platelets.
Secondary: Driven by ↑ erythropoietin (EPO) due to hypoxia (lung disease, high altitude, cyanotic heart disease) or EPO-secreting tumours (renal/hepatic carcinoma).
Relative (Apparent): Normal RBC mass but ↓ plasma volume → dehydration or stress-related polycythaemia (Gaisböck syndrome).

📋 Causes

Cause	Clinical Features	Key Tests	Management
Primary Polycythaemia (PV) 🎗️	- Hyperviscosity: headache, dizziness, blurred vision - Aquagenic pruritus 🛁 (after hot shower) - Splenomegaly, thrombosis, erythromelalgia (burning red hands/feet)	- FBC: ↑ Hb/Hct, often ↑ WCC & platelets - JAK2 mutation (>95%) - Low serum EPO - Bone marrow biopsy: hypercellularity	- Venesection to keep Hct <0.45 (♀) / <0.50 (♂) - Low-dose aspirin - Hydroxyurea in high-risk patients - Ruxolitinib (JAK inhibitor) if resistant
Secondary (Hypoxia-driven) 🌬️	- Features of underlying disease (COPD, OSA, cyanotic heart disease, high altitude) - Dyspnoea, cyanosis, clubbing possible	- FBC: ↑ Hct - ABG: hypoxaemia, hypercapnia - Serum EPO: high	- Treat cause (O₂ for COPD, CPAP for OSA) - Venesection if symptomatic or very high Hct
Secondary (EPO-producing tumour) 🦠	- Systemic symptoms: fatigue, weight loss, night sweats - Possible abdominal mass (renal tumour)	- FBC: ↑ Hct - Serum EPO: high - CT/MRI: renal or hepatic tumour	- Treat underlying malignancy (surgery, chemo, radiotherapy) - Venesection for symptom relief
Relative (Dehydration) 💧	- Dry mucous membranes, hypotension - Dizziness, fatigue	- FBC: ↑ Hct, normal RBC mass - Electrolytes: hypernatraemia - Urinalysis: concentrated urine	- IV/oral fluids - Treat cause (vomiting, diarrhoea, diuretics)
Relative (Gaisböck Syndrome) ⚡	- Middle-aged, obese, hypertensive, stressed males - No true ↑ RBC mass	- FBC: high Hct, normal RBC mass - EPO: normal	- Lifestyle modification (weight loss, BP control) - Venesection in select cases - Aspirin if cardiovascular risk
Drug-Induced (Testosterone, EPO use) 💊	- Athletes, anabolic steroid users, testosterone therapy - Headache, visual disturbance	- FBC: high Hct - Drug history - EPO: normal or slightly raised	- Stop/reduce offending drug - Monitor Hct - Venesection if persistent

🩺 Clinical Features

Hyperviscosity ➝ headache, confusion, blurred vision, TIAs
Thrombosis ➝ DVT, PE, stroke, MI
Haemorrhage (paradoxical, due to abnormal platelets)
Aquagenic pruritus (PV hallmark)
Plethoric, ruddy complexion & splenomegaly

🔎 Investigations

Repeat FBC – confirm ↑ Hb/Hct
Blood film – exclude leukaemia (look for ↑ WCC, platelets)
JAK2 mutation – confirms PV
EPO levels – low in PV, high in secondary causes
Bone marrow biopsy – hypercellular in PV
ABG/oximetry – hypoxia in lung/cardiac disease
CXR – emphysema, lung pathology
USS abdo/pelvis – exclude renal/hepatic tumours
U&E, uric acid – renal function and gout risk

💊 Management Overview

PV: Regular venesection, aspirin, hydroxyurea/ruxolitinib for high-risk
Secondary (lung disease): Oxygen, treat underlying cause, venesection if needed
Tumour-related: Treat malignancy, venesection if symptomatic
Relative: Rehydration or lifestyle modification
⚠️ Always address cardiovascular risk (BP, smoking, diabetes, statins if needed)

💡 Teaching Pearl: Polycythaemia vera = JAK2+, low EPO, splenomegaly, aquagenic pruritus. Secondary = high EPO, often hypoxia or tumour. Relative = normal RBC mass, ↓ plasma volume.

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Polycythaemia

🧬 Pathophysiology

📋 Causes

🩺 Clinical Features

🔎 Investigations

💊 Management Overview

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