Abstract
Prurigo pigmentosa is an uncommon dermatosis presenting with pruritic papulovesicles on the trunk. It most commonly presents in young adult females and has been increasingly associated with ketosis; it has been proposed that ketone bodies deposit in the skin, resulting in a perivascular neutrophilic infiltrate. Histopathology varies according to the stage of the dermatitis, with early, intermediate, and late stages recognized. Cases not associated with ketosis have been reported in Japan, where most cases have arisen, and have been speculated to result from genetic or environmental triggers. Here we present a 14-year-old Caucasian female with a one-year history of pruritic papulovesicles coalescing into plaques on the back and chest. Her rash progressed despite topical steroid treatment, and punch biopsy showed a spongiotic dermatitis with numerous dyskeratotic keratinocytes, microvesicle formation, and mixed inflammatory infiltrate with neutrophils and eosinophils. Clinicopathologic correlation allowed for a diagnosis of intermediate-phase prurigo pigmentosa. The patient was started on doxycycline with rapid improvement. This case is significant as our patient has no underlying risk factors for developing prurigo pigmentosa. She is not following a ketogenic diet or intermittent fasting, and she has no underlying diseases that would result in ketosis. Given the history of rash ongoing for one year and the biopsy suggesting an intermediate stage of disease, the patient is being referred for patch testing to rule out an ongoing allergen. This case illustrates that prurigo pigmentosa should be considered even in patients who do not have known underlying risk factors.