PA43 An infrequently diagnosed cause of pruritus

Abstract A 19-year-old woman presented with three episodes of intensely pruritic rash on her back, and weight loss of > 40 kg in 5 months. There was no personal or family history of similar skin conditions or atopy, and she was not on any medication. She had a background history of anorexia and bulimia nervosa necessitating admissions for starvation ketosis. The rash flared up during periods of extreme starvation and improved spontaneously with food intake. Skin examination showed erythematous papules and brown macules on her back in a reticular pattern. Blood tests were normal except for elevated serum ketone levels (6.2 mmol L−1, reference 0.6–1.6). Skin biopsy showed mild hyperkeratosis and acanthosis with marked infiltrate of neutrophils and eosinophils with dermal necrosis. A diagnosis of prurigo pigmentosa secondary to starvation ketosis was made based on the clinical and pathological findings. The rash resolved with hyperpigmentation following improvement in food intake and follow-up with the mental health team. Prurigo pigmentosa is a rare inflammatory dermatosis associated with ketosis, first described by Nagashima et al. in 1971 (Nagashima M, Ohshiro A, Simizu N. A peculiar pruriginous dermatosis with gross reticular pigmentation. Jpn J Dermatol 1971; 81: 38–9). The majority of the reports are from East Asia, and described in young female patients in late adolesce or early adulthood. However, it has been described in people of all ages, sex and ethnicities. Over 300 cases of prurigo pigmentosa have been reported in the literature. The exact pathogenesis remains unclear. Metabolic disorders such as poorly controlled diabetes, intake of low-carbohydrate diets, anorexia nervosa and fasting have been suggested as possible triggers, as they increase serum ketones, which in turn induce neutrophilic inflammation. Between 2015 and 2021, 19 cases of pruritus pigmentosa induced by ketogenic diet were reported, with a significant female preponderance and an age range of 16–43 years [Xiao A, Kopelman H, Shitabata P et al. Ketogenic diet-induced prurigo pigmentosa (the keto rash): a case report and literature review. J Clin Aesthet Dermatol 2021; 14: S29–32]. The rash usually appears within weeks of initiating a ketogenic or restrictive diet and resolves upon discontinuing the ketogenic diet or consumption of a high-calorie diet. Prurigo pigmentosa presents with erythematous, coalescing papules typically on the upper back, rarely affecting the face and limbs. The inflammatory phase resolves with postinflammatory reticulated hyperpigmentation. Most cases of prurigo pigmentosa resolve spontaneously. Treatment with tetracycline antibiotics or dapsone has been shown to reduce the duration of the inflammatory phase. The residual hyperpigmentation usually persists for many months. This case is presented to enhance awareness, and facilitate early identification and appropriate treatment of this characteristic inflammatory dermatosis.