Pathogenesis of Dermatitis
Although the pathogenesis of photosensitivity in pellagra is reasonably complex, Karthikeyan et al considered excess production of a photosensitiser, kynurenic acid and lack of urocanic acid. Production of kynurenic acid via the tryptophan-kynurenine-nicotinic acid pathway results from niacin deficiency. In its excess, the photo-exposed skin becomes more photosensitive.9 Urocanic acid is produced by the enzyme histidase in the stratum corneum and crucially protects the skin against harmful effects of UVB radiation. Niacin deficiency lowers the concentration of urocanic acid that leads to UVB-induced photosensitivity.9, 10