Localized bullous pemphigoid induced by photodynamic therapy

Topical photodynamic therapy (PDT) causes localized phototoxicity and has been shown both in vitro and in humans to have immunomodulatory and immunosuppressive effects. We report a case of localized bullous pemphigoid (BP) developing after PDT. Although BP has been reported to develop following cutaneous insults such as surgery, radiotherapy, psoralen ultraviolet A (PUVA) and ultraviolet B phototherapy, PDT has not previously been reported as a trigger. Possible mechanisms include direct mechanical injury to the basement membrane and subsequent autoantibody formation, an indirect immunomodulatory effect of PDT, or most likely, precipitation of BP in individuals with pre-existing low titres of epidermal autoantibodies (so-called subclinical BP). PDT should be added to the list of possible exogenous triggers for BP and this condition should be considered if blistering develops following PDT.