immune thrombocytopenic purpura

ITP is a diagnosis of exclusion characterized by isolated thrombocytopenia (platelet count <100,000/µL) without other causes of low platelets. Pathophysiology involves IgG autoantibodies (often against platelet glycoprotein IIb/IIIa or Ib/IX complexes) that bind platelets, which are then opsonized and destroyed by splenic macrophages via Fc receptor-mediated phagocytosis. There are two clinical patterns: acute ITP, common in children, often following a viral infection, typically self-limited and resolving within weeks to months; and chronic ITP, more common in adults (especially women 20-40 years old), often associated with autoimmune conditions (SLE), lymphoproliferative disorders, or HIV/HCV infection.

Clinical presentation includes mucocutaneous bleeding manifestations: petechiae, purpura, easy bruising, epistaxis, gingival bleeding, and menorrhagia. Unlike coagulation factor deficiencies, ITP does not typically cause deep tissue hematomas or hemarthroses. Physical exam may reveal petechiae and purpura but typically no splenomegaly (splenomegaly should prompt consideration of other diagnoses).

Laboratory findings show isolated thrombocytopenia with normal PT/PTT and normal or increased megakaryocytes on bone marrow biopsy (reflecting compensatory increased platelet production). Peripheral blood smear shows large platelets (reflecting younger, more reactive platelets) but no schistocytes (which would suggest microangiopathic processes like TTP/HUS or DIC).

Treatment approach depends on severity and bleeding risk. Asymptomatic patients with platelet counts >30,000/µL often require only observation. First-line treatment for symptomatic or severe thrombocytopenia includes corticosteroids (prednisone) and/or IVIG (which saturates Fc receptors, reducing platelet destruction). Second-line options include rituximab (anti-CD20, targeting antibody-producing B cells), thrombopoietin receptor agonists (romiplostim, eltrombopag), and splenectomy for refractory cases (removes primary site of antibody production and platelet destruction). Platelet transfusions are generally avoided except in life-threatening bleeding since transfused platelets are also rapidly destroyed.

Key distinguishing feature from thrombotic thrombocytopenic purpura (TTP): ITP lacks the microangiopathic hemolytic anemia, neurological symptoms, renal dysfunction, and fever seen in TTP. ITP is also associated with pregnancy and can cause neonatal thrombocytopenia due to transplacental passage of maternal antiplatelet antibodies.