Successful and long-lasting treatment of solar urticaria with ultraviolet A rush hardening therapy

Summary Background Solar urticaria (SU) is a photodermatosis that is thought to be caused through the effects of mast cell mediators released because of an altered chromophore, possibly a photoallergen recognized by IgE. Phototherapy for SU to induce a tolerant state appears to be most effective, but is often time consuming and provides only short‐lived remission. Ultraviolet (UV) A rush hardening has been successful and less time consuming in serum factor‐negative patients with SU. However, the mechanism of action and long‐lasting effects of UVA rush hardening therapy remain unclear. Objectives We aimed to evaluate whether UVA rush hardening exhibits long‐lasting therapeutic effects in serum factor‐positive patients with SU and to examine the action mechanism of tolerance. Methods Two serum factor‐positive patients with SU were exposed to multiple UVA irradiations at 1‐h intervals per day for 2 or 3 days. Intradermal injection of their in vitro‐irradiated autologous serum or compound 48/80 and a prick test for histamine were performed before and after UVA rush hardening. Results The two serum factor‐positive patients with SU benefited greatly from UVA rush hardening, as documented by a marked increase in minimal wealing dose, and remained symptom free without using sunscreen in their daily life. Intradermal injection of in vitro‐irradiated autologous serum induced wealing before hardening, but not in tolerized skin after hardening. The responses to compound 48/80 and histamine were unaltered. Conclusions UVA rush hardening is an effective and long‐lasting treatment even in serum factor‐positive patients with SU. The mechanism of tolerance may involve continued blockade of photoallergen binding to IgE on mast cells, rather than depletion of mast cell mediators or histamine tachyphylaxis. See also the Commentary by Dawe