Prurigo pigmentosa, a rare inflammatory skin condition also known as Nagashima disease, is taking skin conditions common in patients of Asian descent in the Skin of Color Update 2021, according to a dermatologist.
“Ketogenic diets are gaining popularity around the world for weight loss. After 2-4 weeks [on a strict ketogenic diet], some patients notice very itchy papules on their trunk called a keto rash, “reported Hye Jin Chung, MD, director of the Asian Skin Clinic, Beth Israel Deaconess Medical Center, Boston.” Keto rash is actually prurigo pigmentosa. “
The exact pathogenesis of prurigo pigmentosa, a highly itchy macular and papular rash with severe reticular pigmentation, is unclear, but Chung reported that the strong association with ketosis could explain why more cases are now occurring outside of East Asia. Ketosis, or conditions associated with a high risk of ketosis, such as anorexia nervosa, diabetes mellitus, or recent bariatric surgery, have been linked to prurigo pigmentosa in all skin types and races.
“I tell my residents that this is a disease they will never forget after your first fall,” she said.
The differential diagnosis includes contact dermatitis and other inflammatory diseases, but Chung said the reticular pattern of the lesions is a relatively unique feature. Confluent and reticulated papillomatosis (CARP) shares a pattern of reticulated lesions, but Chung said it lacks the inflammatory erythematous papules and the severe pruritus that is common with prurigo pigmentosa.
The pattern develops histologically. It begins as a perivascular infiltration, which is dominated by neutrophils and eosinophils, with hyperkeratosis, acanthosis and spongiosis. Over time, Chung shows that as the keratinocytes die, the histological picture shows an increasing degree of dyskeratosis.
Prurigo pigmentosa was first described 50 years ago by Masaji Nagashima, MD, who published a report of eight patients in Japan with pruriginous stem dermatosis with symmetrical pigmentation. Most of the subsequent reports also came from Japan or other East Asian countries, but have now spread.
This global diffusion was captured in a recently published review of 115 published studies and case reports from 24 countries. In this review, the proportion of studies from Europe (36.5%) approached that of those from East Asia (38.2%), even though 76% of the patients for whom race was reported were of Asian descent.
Of the 369 patients evaluated in these studies and case reports, 72.1% were female. The median age was 25.6 years. In the studies originating outside of Asia, prurigo pigmentosa has been reported in a range of skin types and races, including whites, blacks, and Hispanic Americans. The lowest reported incidence was in the latter two groups, but the review authors speculated that this condition is likely to be underdiagnosed in non-Asian individuals.
Chung agreed and warned that the consequences usually lead to significant delays in disease control. When she shared a recent case of prurigo pigmentosa at her center, she said the 59-year-old Asian patient had started topical steroids and oral antihistamines from her GP prior to her referral. This is a common and sensible strategy for a severe itchy rash that may be caused by contact dermatitis, but it is ineffective for the condition.
“Prurigo pigmentosa requires anti-inflammatory drugs,” she explained. She said doxycycline and minocycline were the treatments of choice, but noted that there have also been reports of the effectiveness of dapsone, macrolide antibiotics, and isotretinoin.
In her most recent case, she gave the patient 100 mg of doxycycline twice a day. There was a significant improvement within 2 weeks, and the rash went away within a month with no relapse at the follow-up visit, which is now past 12 months, Chung said.
Asian Americans are the fastest growing ethnic group in the United States, according to Chung, so it is becoming increasingly important to be familiar with conditions common or unique to Asian skin, but prurigo pigmentosa is no longer limited to those of Asian ancestry. She encouraged clinicians to recognize this disorder in order to reduce the usual delays in effective treatment.
The lead author of the recently published study review, Jensen Yeung, MD, of the Department of Dermatology at the University of Toronto, agreed. He too is of the opinion that dermatologists need to sharpen their awareness of the signs and symptoms of prurigo pigmentosa – and not just in Asian patients or patients of Asian descent.
“This diagnosis is often overlooked,” he claimed in an interview. “In my clinical experience, this condition has become more common over the past 5 years.” He added that the increasing incidence may not be related solely to better diagnostic accuracy, although the most important of the other possible explanations is “not yet well understood”.
Chung reports that she has no relevant financial relationships to disclose. Yeung reports financial relationships with more than 25 pharmaceutical companies, some of which manufacture treatments to control prurigo pigmentosa.
This article originally appeared on MDedge.com, part of the Medscape Professional Network.