Make the Diagnosis

A 31-year-old with a 3-week history of a waxing and waning, mildly pruritic eruption on his neck, chest, and back

A 31-year-old male with no significant past medical history presented with a 3-week history of a waxing and waning, mildly pruritic eruption on the posterior neck, upper chest, and mid- to lower back. He denied any recent changes in his day-to-day routine.

What's the diagnosis?

Dermatitis herpetiformis

Erythema ab igne

Prurigo pigmentosa

Confluent and reticulated papillomatosis

Prurigo pigmentosa is an inflammatory disorder of uncertain etiology characterized by the eruption of erythematous, markedly pruritic, urticaria-like papules and vesicles on the posterior neck, mid- to upper back, and chest. Crops of papules appear rapidly and then involute within days, leaving behind postinflammatory hyperpigmentation in a netlike configuration. New papules may appear prior to resolution of hyperpigmented macules, resulting in a mixed presentation of erythematous papules overlying reticulated hyperpigmentation.1

Dr. Nathan Johnson

The condition was initially described in Japanese individuals, and to date, most cases have occurred in this population.2 However, the incidence of prurigo pigmentosa is increasing worldwide, including in the United States, which has led to the identification of several metabolic risk factors including diabetes mellitus, fasting, and dieting, with the common etiologic endpoint of ketosis.3With the increasing popularity of diets with strict carbohydrate limits, often with the goal of ketosis, dermatologists should be aware of the clinical appearance and common history of this rash to facilitate prompt diagnosis and treatment.

Clinical exam with appropriate history is usually sufficient for diagnosis. However, biopsy with histopathologic analysis can be utilized to confirm atypical cases. Histopathologic findings depend on the stage of the lesion biopsied. The earliest finding is a shallow perivascular neutrophilic infiltrate, neutrophil exocytosis, and epidermal and superficial dermal edema. As lesions progress, the prominent findings include epidermal vesiculation with necrotic keratinocytes and a lichenoid infiltrate dominated by lymphocytes and eosinophils. In the final stages, lesions demonstrate variable parakeratosis and acanthosis, as well as prominent dermal melanophagia.1

Treatment of prurigo pigmentosa includes modification of the patient’s underlying health issues to avoid ketosis, and in the case of diet-induced ketosis, reinstitution of a more balanced diet with sufficient carbohydrates. In the case of the patient presented here, rash resolved 1 week following instruction to include more carbohydrates in his diet. For recalcitrant cases or those without a clear precipitating factor, the addition of oral antibiotics is often helpful. Tetracyclines or dapsone are typically employed, usually in courses of 1-2 months.3,4

Dr. Johnson is a PGY-4 dermatology resident at Carilion Clinic in Roanoke, Va. He provided the case and photos. Donna Bilu Martin, MD, is the editor of the column.


1. Boer A et al. Am J Dermatopathol. 2003 Apr;25(2):117-292.

2. Satter E et al. J Cutan Pathol. 2016 Oct;43(10):809-14.

3. Alshaya M et al. JAAD Case Rep. 2019 Jun 8;5(6):504-7.

4. Hartman M et al. Cutis. 2019 Mar;103(3):E10-3.

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