What is prurigo pigmentosa?
Prurigo pigmentosa is a rare inflammatory skin condition associated with ketosis. It is characterised by a recurrent itchy rash with netlike hyperpigmentation. Prurigo pigmentosa responds well to tetracycline and has an excellent prognosis.
Prurigo pigmentosa is also known as Nagashima disease and 'keto rash'.
Who gets prurigo pigmentosa?
Prurigo pigmentosa has been described in people of all ages, sex and ethnicities. However, it is more common amongst Asians, particularly young women. Women are affected twice as commonly as men.
It has increasingly been associated with ketotic states associated with diabetes, fasting and post-bariatric surgery.
In some patients, prurigo pigmentosa has been associated with systemic diseases such as Sjogren syndrome and eating disorders such as anorexia nervosa. It has also been described in people with atopy, as well as in pregnancy.
What causes prurigo pigmentosa?
The exact role of the exclusion of carbohydrates and ketosis in the development of prurigo pigmentosa has not yet been elucidated.
Several other mechanisms for prurigo pigmentosa have been proposed, including friction with clothes or a contact allergy to trichlorphenol, chromium in acupuncture needles, chrome in detergent, and nickel.
How does prurigo pigmentosa present?
The clinical features of prurigo pigmentosa 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.
Histopathology of prurigo pigmentosa
There is distinct histopathology in prurigo pigmentosa.
- Early lesion: superficial perivascular neutrophilic infiltrate, and a few necrotic keratinocytes
- Developed lesion: patchy lymphocytic infiltrate, and necrosis of numerous keratinocytes
- Late lesion: lymphocytic infiltrate, scale-crust, and melanophages
What is the treatment for prurigo pigmentosa?
The addition of carbohydrates to the diet may be beneficial if the patient is following a ketogenic diet or has undergone prolonged fasting or stomach surgery.
Dapsone and tetracycline antibiotics are effective in treating prurigo pigmentosa during the inflammatory phase of the disease. These treatments are thought to work by interfering with the movement and function of neutrophils. Recently macrolide antibiotics such as erythromycin have also been demonstrated to be helpful. It is unclear if the antibacterial action of the antibiotics is relevant.
To date, there are no effective treatments for the hyperpigmentation that develops in the later stages of the disease. It eventually fades.