Histologic alterations of psoralen- and longwave ultraviolet radiation-exposed skin.

Histologic changes in skin exposed to psoralen photochemotherapy (PUVA) may be classified as acute or chronic. The acute set of histologic alterations includes changes associated with lesion regression and PUVA toxicity. The chronic set of changes related primarily to the side effects of PUVA therapy. In psoriatic epidermis, morphologic evidence of cellular hyperactivity ceases with accompanying loss of psoriasiform hyperplasia, whereas in other inflammatory disorders the lymphocytic infiltrate is cleared in the epidermis and papillary dermis. In short-term therapy, changes include melanocytic hyperplasia and increased activity with marked keratinocyte hypermelaninosis. In skin chronically exposed to PUVA, all cutaneous compartments are affected with an epidermal maturation disorder, melanocyte activation, as well as dermal collagen and elastic tissue degeneration, some of which persist after cessation of therapy.