HSP

**Note on ambiguity:** "HSP" can also mean heat shock protein (chaperones such as Hsp60/Hsp70/Hsp90 that refold denatured proteins and are upregulated by stress), hereditary spastic paraplegia (inherited, progressive lower-limb spasticity), or hereditary spherocytosis (abbreviated HS). On the USMLE, HSP almost always means Henoch-Schönlein purpura, which is covered below.

**Henoch-Schönlein purpura (HSP) = IgA vasculitis**

**Epidemiology** - Most common systemic vasculitis in children, typically ages 3-15. - More common in boys. - Peaks in fall/winter/spring. - Often follows an upper respiratory infection (e.g., group A strep). Other triggers include drugs and vaccines.

**Pathophysiology** - A type III hypersensitivity reaction. - Abnormal (galactose-deficient) IgA1 forms IgA-containing immune complexes. They deposit in small vessels (capillaries, venules, arterioles) of the skin, GI tract, joints, and kidney. - Complement activation (alternative pathway) recruits neutrophils, causing leukocytoclastic vasculitis. - Biopsy shows neutrophil infiltration with nuclear debris and fibrinoid necrosis. Immunofluorescence shows IgA and C3 deposition. - Renal involvement is histologically the same as IgA nephropathy (Berger disease), with mesangial IgA deposits.

**Clinical tetrad** 1. **Palpable purpura** on the buttocks and extensor surfaces of the legs. This is non-thrombocytopenic, so the platelet count and coagulation studies are normal. 2. **Arthralgia/arthritis**, typically of the knees and ankles. It is transient and non-deforming. 3. **Abdominal pain**, which is colicky, with possible GI bleeding. Intussusception is a classic complication (usually ileo-ileal). 4. **Renal disease**, with hematuria and mild proteinuria. It can progress to nephritic or nephrotic syndrome.

Scrotal pain or swelling (orchitis) can also occur.

**Diagnosis** - Mainly clinical. - Skin or renal biopsy (IgA deposition) is used if the presentation is atypical. - Urinalysis (hematuria, proteinuria) and blood pressure monitoring are needed to assess renal involvement. - Normal platelets and coagulation studies help exclude ITP and DIC. - Serum IgA may be elevated but is not required for diagnosis.

**Management** - Supportive care (hydration, analgesia) is the mainstay. It is usually self-limited over about 4 weeks. - NSAIDs can help joint pain, but use caution with renal involvement. - Corticosteroids are used for severe abdominal pain, GI bleeding, or severe renal disease. They do not prevent nephritis. - Severe or crescentic nephritis may need immunosuppression.

**Prognosis** - Excellent in children. Recurrences are possible. - Adults tend to have a worse renal prognosis. - Long-term risk is mainly from renal disease, so follow urinalysis and blood pressure for several months.

**High-yield associations and differentials** - Small-vessel vasculitis with IgA = HSP. Compare with other vasculitides: - Kawasaki disease: medium vessel, with fever for 5 or more days, conjunctivitis, and coronary aneurysms. - GPA: c-ANCA, upper respiratory, lung, and kidney involvement. - Cryoglobulinemic vasculitis: associated with hepatitis C. - Think HSP when a child has purpura plus abdominal pain, arthralgia, and hematuria with a normal platelet count. - A child with HSP and severe colicky pain, vomiting, or currant-jelly stools should be evaluated for intussusception.