Prurigo pigmentosa

Prurigo pigmentosa (PP) is an uncommon condition consisting of a network of erythematous, pruritic papules evolving into reticulated hyperpigmentation with a specific predilection for the trunk. It is seen mostly in young adults, more often in women.

• While the exact pathogenesis is unknown, in many cases rapid weight loss appears to play an important role
• While PP has historically been reported primarily in Asia, with the rise in popularity of the ketogenic diet in recent years, there has been an emergence of PP cases worldwide. The ketogenic diet is a very low-carbohydrate, moderate-protein, high-fat diet that restricts carbohydrate intake to under 50 grams daily
• Anorexia nervosa, insulin-dependent diabetes mellitus, bariatric surgery, and other causes of rapid weight loss can also cause PP
• Possible triggers include sweating, friction from clothing, acupuncture, atopic dermatitis, and pregnancy

• In terms PP induced by a ketogenic diet a review of the literature from 2015-2021 identified 19 cases. Patient's age ranged from 16 to 43 years, with a mean age at diagnosis of 24 years. There was a strong female predominance (79%), which could be due to higher prevalence of dieting behaviours among young women compared to young men. On average, the rash presented 31 days after initiating the diet and subsided 18 days after diet cessation

• PP progresses through several stages, commencing as erythematous macules which evolve to urticarial papules and papulovesicles. Subsequently, the lesions become crusted or scaly. A few weeks later, they spontaneously resolve, leaving behind reticulated pigmentation

• In many cases the skin changes resolve as the ketosis diminishes
• Topical steroids may help pruritus but have little effect on the rash itself
• First-line therapy is with oral tetracycline antibiotics (lymecycline, doxycycline) or macrolides - a one-week course can be effective; some patients require a longer duration
• In recalcitrant cases dapsone has been used 
• PP can be recurrent