The patient was given a diagnosis of prurigo pigmentosa based on the characteristic pruritic rash that had developed after the patient started a strict ketogenic diet.
Prurigo pigmentosa is a benign, pruritic rash that most commonly presents with erythematous or hyperpigmented, symmetrically distributed urticarial papules and plaques on the chest and back. Females represent approximately 70% of cases with a predominant age range of 11 to 30. It more commonly is seen in people of Asian descent.
While the pathophysiology remains unknown, the rash most commonly occurs in association with diabetes, ketosis, and more recently with ketogenic diets. Despite occurring in only a fraction of patients on the ketogenic diet, the characteristic presentation has led to the alternative name of the “keto rash” in online nutritional forums and blogs.
The diagnosis is made clinically, so the appearance of a symmetric pruritic, hyperpigmented rash on the chest and back should prompt the physician to ask about any recent changes in diet. Laboratory analysis is unnecessary, as a complete blood count, basic metabolic panel, and liver function panel are almost always normal.
Other conditions can mimic prurigo pigmentosa such as urticaria, irritant contact dermatitis, confluent and reticulated papillomatosis, and pityriasis rosea.
Primary treatment includes resumption of a normal diet. This often leads to rapid resolution of pruritis. Residual hyperpigmentation may take months to fade. If additional treatment is required, minocycline 100 to 200 mg/d has been reported most effective, likely due to its anti-inflammatory properties. Topical corticosteroids and oral antihistamines provide symptomatic relief in some patients.
This patient had resolution of the pruritis and urticarial lesions within 2 days of resuming a normal diet; however, residual asymptomatic hyperpigmentation persisted. A retrial of the ketogenic diet initiated a flare of the rash in the same distribution. It rapidly resolved with carbohydrate intake.
This case was adapted from: Croom D, Barlow T, Landers JT. Pruritic rash on chest and back. J Fam Pract. 2019;68:113-114,116