Role of ultraviolet A in phototherapy for psoriasis

Multiple studies have demonstrated that the doses of ultraviolet A (UVA) (320-400 nm) achieved with ultraviolet sources presently used for phototherapy for psoriasis are inadequate to induce coal tar phototoxicity (as manifested by delayed erythema). Some centers still use a phototherapy protocol that combines UVA, ultraviolet B (UVB), and tar for the treatment of generalized psoriasis. We designed a bilateral comparison study to determine whether the addition of UVA to one side, in doses sufficient to induce an immediate burning or smarting sensation in tar-treated skin, would add to the beneficial effects of UVB. The psoriasis of ten of thirteen ambulatory patients cleared in a mean of 26.1 treatments. Despite a mean cumulative UVA dose of 130.8 joules/cm2, none of the thirteen patients showed a better response on the side that received additional UVA. A "nonaggressive" inpatient protocol was designed to maximize the chances of demonstrating a beneficial effect of UVA. The psoriasis of eight of twelve patients cleared in a mean of 21.0 treatments. Despite a mean cumulative UVA dose of 40.3 joules/cm2, the twelve patients showed no difference in clearing between sides. The threshold for smarting increased throughout the treatment and provided a convenient guide to the delivery of increasing doses of UVA. In doses sufficient to induce coal tar phototoxicity manifested by the smarting reaction, UVA does not add to the known benefits of UVB phototherapy.