15 years with a peak incidence at to years of age In pediatric cases, a male predominance has been noted with male-to-female ratio as high as 1.8:1 Presentation during adolescence or adulthood portends a worse prognosis The disease is more prevalent during fall, winter, and early spring The onset is usually sudden and frequently preceded by an acute illness, often of the upper respiratory tract Though the acute illness is often predominated by rash, joint pain, and gastrointestinal symptoms, the long-term prognosis for children with HSP depends upon the extent of the renal involvement Nephritis occurs in 30% to 50% of affected patients, and the severity of renal involvement is not correlated with the severity of the other symptoms Most children who develop nephritis will have relatively mild renal involvement such as asymptomatic hematuria with mild or no proteinuria and preserved or mildly and transiently impaired renal function These patients are expected to have a favorable long-term prognosis A subset of children will develop more severe renal involvement and develop nephritic syndrome or combined nephritic–nephrotic syndrome Severe disease can be associated with decreased renal function, hypertension, hypoalbuminemia, and edema, and severe acute involvement would increase the risk for long-term renal sequelae Approximately 1% develop end-stage renal disease (ESRD) Clinical assessment The diagnosis of HSP is clinical, and the history and physical examination should focus on evaluation for the clinical manifestations of the small-vessel vasculitis associated with HSP The hallmark signs and symptoms include nonthrombocytopenic purpuric rash, arthralgias, nonerosive arthritis, abdominal pain, and nephritis in individuals ≤20 years of age The rash, often the most distinctive feature of the disease, characteristically involves the buttocks and extensor surfaces of the lower extremities Purpura and joint pain are the most common presenting symptoms, though studies have revealed that abdominal symptoms may precede the rash in as many as 15% to 35% of cases Overall, gastrointestinal symptoms occur in approximately half of children with HSP, and the most common abdominal symptoms are periumbilical pain, vomiting, diarrhea, and hematochezia Surgical emergencies such as intussusception, most often isolated to the small bowel, and bowel perforation develop in 1% to 5% of patients