The main feature of retronychia, a nail disorder of the toes, is the growth of new nail under prior nail growth. But this layering of nails is not always readily apparent on clinical examination, commonly leading to the wrong therapy and no resolution to the problem, according to an expert update at the 2022 American Academy of Dermatology 2022 annual meeting in Boston, Massachusetts.

Misinterpretation of the yellow discoloration, a common feature of retronychia, means “many patients are maintained on antifungal therapy for years and years with no change in their condition,” reported Phoebe Rich, MD, director of the Nail Disorders Clinic, Oregon Health & Science University, Portland.

Infection is not commonly involved in retronychia, but importantly, antifungals and antibiotics “have no role in treating the underlying disorder,” Rich said.

The term retronychia and its description is only about 20 years old, according to Rich, who cited work by David A. de Berker, MBBS, PhD, a consultant dermatologist at University Hospitals in Bristol, UK. His publication on this disorder appeared in 1999, with a more detailed description published about 10 years later.

Recently, the body of literature on this disorder has been growing, contributing to an increasing consensus about etiology, diagnosis, and treatments to consider in the context of causes and severity, Rich said.

Some but not all patients have abnormal formation of the nail bed, increasing susceptibility to retronychia, but trauma or microtrauma typically serve as a trigger in most cases. Dancing, high heels, steel-toed shoes, and other sources of trauma to the toes are implicated.

Whether or not patients have an inherent susceptibility, injury separates the existing nail from the matrix and nail bed so that newly forming nail begins to grow under the nail rather continuing to push out the old nail.

Susceptibility is increased substantially in individuals with a shortened nail bed, according to Rich. In severe cases, when there is simply inadequate nail bed for the nail growth to attach, recurrence is common or even inevitable. Even when the nail is removed and regrowth appears normal at the end of a year, those patients with very short nail beds cannot count on a cure.https://www.medscape.com/index/list_13472_0

“Due to the slow growth of nails, it might take 2 or 3 years for the problem to recur,” Rich cautioned. For this reason, cure rates reported for the various interventions at 1 year might not predict longer-term benefit.

Retronychia is usually a clinical diagnosis based on the presence of the increased bulk of the toenail when overlapping nails cannot be seen. This is not necessarily a single overgrowth. In some cases, multiple layers of nails are stacked one on top of the other. Xanthonychia (yellow nail) is usually present.

“The layering might not be visible without removing the nail,” said Rich, explaining one reason that the diagnosis is sometimes missed. Ultrasound is a noninvasive means to confirm the problem, although Rich warned that imaging is not necessarily reimbursed.

“There is no diagnosis by histopathology, so it cannot be confirmed with biopsy,” Rich said.

Treatments range from conservative strategies, particularly topical or intralesional steroids in mild cases, to more invasive procedures such as clipping of the nail plate or surgical avulsion. All can be effective when used appropriately, according to Rich.

“The more invasive procedures are the more effective, but the caveat is they are also associated with more complications,” said Rich, citing, for example, the risk of nail dystrophies. Due to the increasing number of studies, the relative benefits and risks of retronychia treatment have now been summarized in a recent review. Rich suggested the review is one of the most recent and detailed evaluations of the topic that “I encourage everyone to read.”

Despite progress in describing retronychia, Rich said that there might be more to learn about risk. In particular, she cited the work of Dana W. Stern, MD, a specialist in nail disorders who is in private practice in New York City. Stern is pursuing a hypothesis that at least some cases are due to potentially targetable biomechanical issues.

“I have observed that many of the younger patients in my practice with retronychia seem to have atypical foot anatomy,” said Stern, who was contacted by Medscape Medical News to explain this area of research. “I am collecting cases and hoping to explore this issue in more depth.”

She said that foot anatomy in relationship to retronychia has not been adequately evaluated.

“In my review of the literature, I could not find a single study that showed imagery of the feet,” she said. She is considering a collaboration with others, including Rich, to explore this as a factor in retronychia.

Asked about risk of misdiagnosis, Stern reiterated some of the points made by Rich. In particular, she agreed that discolored nails alone should not be a reason to initiate antimycotic therapy without considering the possibility of retronychia.

“So many providers are not familiar with the diagnosis, and only 50% of yellow thickened nails are in fact onychomycosis,” she said. “We end up seeing a plethora of patients [with retronychia] who are unfortunately misdiagnosed for years.”

Rich has reported financial relationships with AbbVie, Boehringer Ingelheim, Bristol-Myers Squibb, Cassiopea SpA, Dermavant Sciences, Eli Lilly and Company, Galderma Laboratories, Janssen-Ortho, Kadmon Corporation, LEO Laboratories, Moberg Derma, Novartis Pharmaceuticals, Sun Pharmaceutical Industries, and UCB. Stern has reported a financial relationship with Rare Beauty Brands. Both Rich and Stern said they have no disclosures related to this topic.

American Academy of Dermatology 2022. Presented March 27, 2022.

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