Henoch-Schönlein purpura

HSP is a small-vessel leukocytoclastic vasculitis mediated by IgA immune complex deposition in vessel walls, most commonly affecting children ages 3-10, often 1-3 weeks after a URI (frequently group A Streptococcus). Pathophysiology involves abnormal glycosylation of IgA1, leading to immune complex formation and deposition in skin, joints, GI tract, and kidneys, activating complement and causing vascular inflammation.

Clinical tetrad: (1) Palpable purpura - symmetric, typically on buttocks and lower extremities, due to vasculitis without thrombocytopenia (platelet count normal); (2) Arthralgia/arthritis - typically knees and ankles, non-deforming; (3) Abdominal pain - due to bowel wall edema/hemorrhage, can cause intussusception (a key complication, typically ileo-ileal rather than the more common ileocolic type); (4) Renal disease - IgA nephropathy identical histologically to Berger disease, presenting with hematuria +/- proteinuria; can progress to nephritic or nephrotic syndrome, and rarely to renal failure.

Diagnosis is primarily clinical; skin biopsy shows leukocytoclastic vasculitis with IgA deposition on immunofluorescence. Labs show normal platelet count and coagulation studies (helps distinguish from other purpura causes like ITP, meningococcemia, or coagulopathy). Serum IgA may be elevated.

Management is largely supportive (NSAIDs for arthralgia, hydration) as the disease is typically self-limited, resolving within 4 weeks. Corticosteroids may be used for severe GI or renal involvement. Most children recover fully, but renal involvement requires monitoring as a minority develop chronic kidney disease. Recurrence occurs in about 1/3 of patients.