Histopathology of actinic prurigo.

Actinic prurigo (AP) is an idiopathic familial photodermatosis seen in American Indians. We report on 17 patients; 16 had dermatitis and one had actinic cheilitis. Ten patients had acute dermatitis and six had chronic dermatitis. The histologies of acute AP and polymorphous light eruption (PLE; limited concept) are eczematous and indistinguishable. Both show spongiosis, superficial (and sometimes deep) perivascular lymphocytic infiltrates, and papillary dermal edema. Both also show vacuolar degeneration of the basal layer. In contrast, the chronic lichenified AP lesions are associated with marked hyperkeratosis, acanthosis, elongation of the rete ridges, and tissue repair. The large lymphoid germinal centers in the lamina propria are the main features of the lip histology. Seven biopsies were positive in the basal membrane zone on direct immunofluorescent testing, four were negative, and one was inconclusive. IgM was present in six and C3 in two. These immunofluorescent results are probably not significant. Immunofluorescent testing of the lip was negative. Although it is not possible to distinguish acute AP from PLE histologically, it is possible to differentiate the two conditions when chronic AP changes are present.