A histological study of the evolution of solar urticaria.

The histological evolution of solar simulator-induced lesions of solar urticaria was investigated in four severely affected white patients. A series of two to 32 minimal whealing doses of radiation, each much lower than the 24-hour minimal erythema dose, was administered to separate buttock sites. Biopsy specimens were obtained from the exposed areas at five minutes and two and 24 hours later, as well as from adjacent nonexposed skin. Lesions showed a statistically significant dose-dependent increase, predominantly perivascular, in upper dermal neutrophil and eosinophil numbers at five minutes and two hours, but not at 24 hours, and at higher radiation doses in mononuclear cell numbers by 24 hours. Nonirradiated patient skin and irradiated control subject skin was not similarly affected. These changes may be associated with the pathogenesis of solar urticaria.