, according to a dermatologist, who reviewed skin conditions common to patients of Asian descent at the Skin of Color Update 2021.
Dr. Hye Jin Chung
“Ketogenic diets are gaining popularity globally for weight loss. After 2-4 weeks [on a strict ketogenic diet], some patients start to notice very pruritic papules on their trunk, the so-called 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 pruritic macular and papular rash with gross reticular pigmentation, is unclear, but Dr. Chung reported that the strong link with ketosis might explain why more cases are now being encountered outside of east Asia. Ketosis or conditions associated with a high risk for ketosis, such as anorexia nervosa, diabetes mellitus, or recent bariatric surgery, have been linked to prurigo pigmentosa in all skin types and ethnicities.
“I tell my residents that this is a disease you will never forget after your first case,” she said.
The differential diagnosis includes contact dermatitis and other inflammatory disorders, but Dr. Chung said that the reticular pattern of the lesions is a relatively unique feature. Confluent and reticulated papillomatosis (CARP) shares a pattern of reticulated lesions, but Dr. Chung said it lacks the inflammatory erythematous papules and the severe pruritus common to prurigo pigmentosa.
Histologically, the pattern evolves. It begins as a perivascular infiltration dominated by neutrophils and eosinophils with hyperkeratosis, acanthosis, and spongiosis. Over time, Dr. Chung said that the histologic picture shows an increasing degree of dyskeratosis as keratinocytes die.
Prurigo pigmentosa was first described 50 years ago by Masaji Nagashima, MD, who published a report on eight patients in Japan with a pruriginous truncal dermatosis featuring symmetrical pigmentation. Most subsequent reports were also from Japan or other east Asian countries, but it has since spread.
This global spread 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 if 76% of the patients for whom race was reported were of Asian ethnicity.
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Of the 369 patients evaluated in these studies and case reports, 72.1% were female. The mean age was 25.6 years. In the studies originating outside of Asia, prurigo pigmentosa was reported in a spectrum of skin types and ethnicities, including Whites, Blacks, and Hispanics. The lowest reported incidence has been in the latter two groups, but the authors of the review speculated that this condition is likely being underdiagnosed in non-Asian individuals.
Dr. Chung agreed, and she cautioned that the consequences typically result in a significant delay for achieving disease control. In recounting a recent case of prurigo pigmentosa at her center, she said that the 59-year-old Asian patient had been initiated on topical steroids and oral antihistamines by her primary care physician before she was referred. This is a common and reasonable strategy for a highly pruritic rash potentially caused by contact dermatitis, but it is ineffective for this disorder.
“Prurigo pigmentosa requires anti-inflammatory agents,” she explained. She said that doxycycline and minocycline are the treatments of choice, but noted that there are also reports of efficacy with dapsone, macrolide antibiotics, and isotretinoin.
In her most recent case, she initiated the patient on 100 mg of doxycycline twice daily. There was significant improvement within 2 weeks, and the rash resolved within a month with no relapse in follow-up that now exceeds 12 months, Dr. Chung said.
According to Dr. Chung, Asian-Americans are the most rapidly growing ethnic group in the United States, making it increasingly important to be familiar with conditions common or unique to Asian skin, but prurigo pigmentosa is no longer confined to those of Asian descent. She encouraged clinicians to recognize this disorder to reduce the common delays to effective treatment.
The senior author of the recently published review of studies, Jensen Yeung, MD, of the department of dermatology, University of Toronto, agreed. He, too, believes that dermatologists need to increase 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 missed,” he contended in an interview. “This condition has become more common in the past 5 years in my clinical experience.” He added that the increasing incidence might not just be related to better diagnostic accuracy, although the most significant of other possible explanations “is not yet well understood.”
Dr. Chung reports that she has no relevant financial relationships to disclose. Dr. Yeung reports financial relationships with more than 25 pharmaceutical companies, some of which produce treatments employed in the control of prurigo pigmentosa.