A 30+ year old man was seen for a chief complaint of itchy, raised, red rash, located on the trunk. The rash had been present for 1 week. Over the course of a week, he tried topical steroids: hydrocortisone 2.5% cream and betamethasone dipropionate 0.05% cream without improvement. About 3 weeks prior to this presentation, he had had a faint eruption on the trunk which cleared with hydrocortisone 2.5%. Rash recurred and worsened after working outside on a hot day one week prior to this presentation.

Patient is otherwise healthy and is not on any medications other than the prescribed topical steroids. Patient has been following a strict low carb paleo diet for one month. Patient also has history of scalp psoriasis which is treated with topical clobetasol solution when flared.

Biopsy was taken from the mid chest. 

What is the cause of this eruption?

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The patient is following a strict ketogenic diet.


The stratum corneum is compacted with collections of neutrophils on the surface of the epithelium. The epidermis is mildly acanthotic and mildly spongiotic. A superficial perivascular and interstitial infiltrate of lymphocytes and many eosinophils is present. There are mild interface changes with vacuolar change and apopotic keratinocytes.


The differential diagnosis includes allergic contact dermatitis and dermal hypersensitivity reaction, given the prominence of eosinophils. Tying in the clinical presentation, however, with the histology, the findings are most consistent with prurigo pigmentosa.

Prurigo pigmentosa aka the “keto rash” is an uncommon dermatosis 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. It is also known as Nagashima disease and ‘keto rash’. Prurigo pigmentosa has been described in people of all ages, sex and ethnicities. It has increasingly been associated with ketotic states associated with diabetes, fasting and post-bariatric surgery. The exact role of ketosis and the exclusion of carbohydrates in the development of prurigo pigmentosa has not yet been elucidated.

Main clinical features are:

  • A pruritic (itchy) rash, which may recur
  • Inflamed, red raised spots (papules) that merge to form a reticulate (network-like) pattern
  • Symmetrical distribution on the trunk most often affecting the upper back, sacrum (natal cleft), abdomen and chest
  • Rare involvement of the face or limbs
  • Sparing of mucous membranes, hair and nails
  • Reticulated hyperpigmented patches following resolution of the inflammatory phase of the rash.


The patient is resistant to stopping his low carb diet.  He is lost to follow up at this time due to his travels.  Typically, after the acute eruption, reticulated hyperpigmentation develops in the places where the eruption occurred.

By: Ms. Beth Schroth-Seiler, PA-C and Dr. K. Mirielle Chae, MD