Diagnosis: Prurigo pigmentosa
We recognized that this was a case 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 30.1
While the pathophysiology remains unknown, the rash most often is reported in association with ketogenic diets.1 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.
Although prurigo pigmentosa is relatively uncommon (with an unknown incidence), primary care physicians may begin to encounter the characteristic rash more frequently, given the number of articles over the past 5 years in the primary care and nutritional literature highlighting the diet’s health benefits.2 The ketogenic diet is a high-fat, low-carbohydrate diet with preliminary evidence of improved weight loss, cardiovascular health, and glycemic control suggested by a meta-analysis of 13 randomized controlled trials.3 Additionally, the popular press and general public’s rising interest are likely to increase the number of patients on this diet.
A clinical diagnosis
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. Urinary ketones are present in only 30% to 50% of patients; there is, however, an absence of blood ketones.1,4
Continue to: Other conditions can mimic prurigo pigmentosa