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Prurigo Pigmentosa

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Also known as: Nagashima disease or keto rash

What is prurigo pigmentosa?
Prurigo pigmentosa is a rare skin condition characterised by a recurrent itchy, red rash that can leave behind a dark net like mark.

Who develops prurigo pigmentosa?
The exact cause of prurigo pigmentosa is not known. The skin condition is more common in females, and young adults. The average age of onset has been between 23 and 27 years, however it can occur at any age. Most reports of prurigo pigmentosa have come from Japan.

What causes prurigo pigmentosa?
The cause of prurigo pigmentosa is not known. There have been proposed theories suggesting associations with allergies and irritation from contact with clothing material. Prurigo pigmentosa has been associated with fasting or very low carbohydrate diets (ketogenic diets).

What does prurigo pigmentosa look like?
The rash usually develops rapidly on the back, neck and chest in a symmetric fashion. It typically presents as red, inflamed, raised spots that appear quickly and as they heal they leave a flat brown net like pattern behind. Face, arms, legs, hair, nails and mucous membranes are rarely affected. There have been reports of blistering forms of prurigo pigmentosa.

How is prurigo pigmentosa diagnosed?
A dermatologist usually makes the diagnosis through history and examination of the skin. It can sometimes require a skin biopsy if the diagnosis is unclear.

How is prurigo pigmentosa treated?
Treatment options include:
• Tetracycline antibiotics (Minocycline or Doxycycline)
• Macrolide antibiotics
• Dapsone
• Isotretinoin
• Narrowband UV-B therapy

Prurigo pigmentosa associated with ketosis can be treated with correction of ketosis through increasing carbohydrate intake or administering insulin in particular cases.

What is the likely outcome of prurigo pigmentosa?

The skin changes of prurigo pigmentosa usually heal within weeks. It can leave a net like appearance of darker mottled skin. This eventually fades with time.

This information has been written by Dr Subashini Gnanendran and Rachael Anforth

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