Etiology

• Primary ITP: idiopathic (most common)
• Secondary ITP associated with:
  • Autoimmune disorders: SLE, Antiphospholipid syndrome
  • Malignancy: lymphoma, leukemia (particularly CLL)
  • Infection: HIV, HCV
  • Drugs: e.g., quinine, beta-lactam antibiotics, carbamazepine, heparin, vaccines, linezolid, sulfonamides, vancomycin, TMP-SMX, antiepileptics

The acute form is typically seen in children after a viral disease.

Pathophysiology

Antiplatelet antibodies (mostly IgG directed against, e.g., GpIIb/IIIa, GpIb/IX) bind to surface proteins on platelets → sequestration by spleen and liver → ↓ platelet count → bone marrow megakaryocytes and platelet production increase in response (in most cases).

Clinical features

• Mostly asymptomatic. Splenomegaly is very unusual in ITP and makes other diagnoses more likely!
  • Although platelets can undergo splenic sequestration in ITP, the primary mechanism responsible for the thrombocytopenia is immune destruction.