It usually affects people in their 20s to 50s and occurs in both men and women. Typically, PG affects the legs in adults. In children it may affect the legs, buttocks, head, and neck. Another common area of involvement is around surgical sites, especially ostomy sites after surgery for Crohn disease. Rarely, PG affects the eyes, lungs, spleen, and musculoskeletal system. It sometimes occurs with other medical problems or is caused by a medication.