Porphyrins excreted in the urine are responsible for the unusual hues and have led to many misguided workups for hematuria. Porphyrins also account for the fluorescence of urine viewed with a Wood's lamp. Porphyria cutanea tarda is the most prevalent of the porphyrias and can be acquired sporadically or inherited through autosomal dominant transmission with variable, usually low, penetrance. Clinical onset is typically during the fourth or fifth decade of life. The disease tends to develop, recur, or worsen during the spring and summer, when exposure to sunlight is greatest.
Photosensitivity is the hallmark of porphyria cutanea tarda.
Ultraviolet light transforms accumulated porphyrins in the skin into toxins that cause skin fragility. The dorsal surfaces of the hands are the principal sites of bullae formation because of exposure to sun and trauma. Shortly after bullae form, they rupture and become painful erosions that heal as atrophic scars and milia. Laboratory abnormalities are common in porphyria cutanea tarda. Liver transaminase levels often are mildly elevated.