Prurigo pigmentosa , also known as Nagashima’s disease and “keto rash,” is an uncommon inflammatory skin condition of unknown etiology.
Although PP has been described in people of all races, ages, and sexes, it is predominantly observed in Japan, often in female young adults. Triggers may include a ketogenic diet, diabetes mellitus, and pregnancy. Friction and contact allergic reactions to chrome or nickel have been proposed as exogenous trigger factors. Individual cases of Sjögren’s syndrome,infections, and adult Still syndrome have also been associated with recurrent eruptions.
The first-line therapy for prurigo pigmentosa is oral minocycline. However, for some patients, doxycycline, macrolide antibiotics, or dapsone may be indicated. Adding carbohydrates to a keto diet may be helpful. In this patient, a punch biopsy was performed, which revealed an interface dermatitis with eosinophils and neutrophils, consistent with prurigo pigmentosa. The cause of her PP remains idiopathic.