Prurigo Pigmentosa (Keto Rash) Secondary to an Eating Disorder: A Case Report and Proposed Diagnostic Criteria

Prurigo pigmentosa is a rare inflammatory dermatosis increasingly reported in association with ketogenic diets, fasting, diabetes, and eating disorders. We describe a 21-year-old female admitted after an intentional overdose of nonsteroidal anti-inflammatory drugs (NSAIDs) and aspirin, who presented with a pruritic, symmetric erythematous papular eruption involving the trunk, neck, upper arms, and back, which evolved into a reticulated hyperpigmented pattern. Urinalysis confirmed ketonuria. Histology demonstrated chronic lymphohistiocytic perivascular inflammation, melanophages, and pigment incontinence, consistent with the evolving subacute prurigo pigmentosa. This case highlights a frequently misdiagnosed eruption that can mimic common dermatoses. To address this diagnostic challenge, we propose a set of practical diagnostic criteria based on clinical features, disease course, and the context of ketosis. Early recognition, aided by these criteria, is essential for effective management through dietary modifications, thereby reducing unnecessary investigations and facilitating prompt resolution.