Ultraviolet light hardening in polymorphous light eruption--a controlled study comparing different emission spectra

Polymorphous light eruption (PLE) is a common disorder characterized by a delayed, abnormal response to ultraviolet (UV) radiation, with a varied morphology of itching efflorescences on sun-exposed areas of the skin. Thirty-one PLE subjects were treated with either UVA (340-400 nm) or UVA and UVB (300-400 nm) phototherapy during spring 1987 (10 exposures to UV light). They were randomly allocated to these 2 groups. For subjects of the UVA group, the applied dose corresponded to their individual minimal tanning dose; for subjects of the UVA and UVB group it corresponded to approximately 3/4 of their individual minimal erythema dose. The sun protection effect was studied by a high dose of UVA (80-160 J/cm2; 340-440 nm) after the treatment period, by analysing the histidine content of the stratum corneum and the urocanic acid photoisomerization, and by evaluating the subjects' diaries. The patients were asked to expose their skin to sunlight at least 3 times after UV hardening in the following 2-10 weeks. The results of both the UVA provocation and of the natural sun exposure confirmed the success of UV hardening without the occurrence of severe side effects. The content of histidine and of its metabolite urocanic acid in stratum corneum was significantly increased during the treatment. These data are interpreted to be biochemical markers for improved sun protection.