Polycythemia

Classification & Pathophysiology

• Relative Polycythemia: ↓ Plasma volume (hemoconcentration) with normal RBC mass.
  • Etiology: Dehydration, excessive diuresis, “Stress Polycythemia” (Gaisböck syndrome - obese, hypertensive males).
• Absolute Polycythemia: ↑ RBC mass. Subdivided into Primary (Bone marrow disorder) and Secondary (EPO-driven).

Secondary Polycythemia

Caused by increased Erythropoietin (EPO).

• Etiology
  • Hypoxic (Appropriate ↑ EPO):
    • Chronic hypoxia triggers renal EPO release.
    • Causes: High altitude, COPD, Obstructive Sleep Apnea (OSA), Cyanotic heart disease (R→L shunts).
  • Ectopic Production (Inappropriate ↑ EPO):
    • Tumors secreting EPO.
    • “Potentially Really High Hematocrit” Mnemonic:
      • Pheochromocytoma
      • Renal Cell Carcinoma (RCC) t
      • Hepatocellular Carcinoma (HCC)
      • Hemangioblastoma (Cerebellar)
      • Also: Uterine Leiomyoma (Fibroids).
• Diagnostics
  • EPO Level: High.
  • SaO2:
    • Low in Hypoxic causes (COPD, OSA).
    • Normal in Ectopic causes (RCC, HCC).
• Treatment
  • Treat the underlying cause (e.g., O2 for COPD, CPAP for OSA, tumor resection).

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Diagnostic Algorithm (High-Yield)

1. Check Hematocrit (Hct) & RBC Mass:
   • Normal RBC mass + Low Plasma volume = Relative Polycythemia.
   • High RBC mass = Absolute Polycythemia → Check SaO2.
2. Check SaO2:
   • Low (<92%) = Secondary Polycythemia (Hypoxic).
   • Normal = Primary or Secondary (Ectopic) → Check EPO.
3. Check EPO:
   • Low EPO = Polycythemia Vera (Confirm with JAK2).
   • High EPO = Secondary Polycythemia (Ectopic source).