Allergic Reactions: Urticaria Pigmentosa

An 8-year-old boy was evaluated because of widespread lesions on his face, arms, and legs. The lesions had been present for 3 years and were occasionally pruritic, especially when scratched. The child was otherwise healthy. Hundreds of faintly brown, uniformly distributed macules, patches, and nodules were noted on most of the patient’s body and parts of his face. The lesions readily urticated on stroking. This patient has urticaria pigmentosa—the most common form of cutaneous mastocytosis. The condition is caused by lesional collections of increased numbers of mast cells. When the skin is stroked, the mast cells degranulate and release mediators that increase vascular permeability and promote localized edema, wheal formation, and mild to intense stinging pain and pruritus. Although treatment is often not required, symptomatic patients may benefit from an oral antihistamine, a mast cell stabilizer (such as disodium cromoglycate), or photochemotherapy. Avoidance of known mast cell degranulators (eg, allergens and certain medications) is recommended when there is extensive urticaria pigmentosa. In severe cases, corticosteroids, or even epinephrine, are occasionally used. For children with urticaria pigmentosa, the prognosis is excellent—more than half of those affected have spontaneous remission by adolescence. The remaining patients have minor residual, isolated lesions.