The prevalence of dermatomyositis without dermatitis among patients with biopsy-confirmed dermatomyositis was approximately 8% in a Japanese cohort study. “Dermatomyositis sine dermatitis does exist and is significantly associated with anti–nuclear matrix protein 2 [anti-NXP-2] autoantibodies,” the researchers reported in JAMA Neurology.
Few case reports of dermatomyositis sine dermatitis have been documented. To confirm the existence of the condition, study its prevalence, and characterize its serologic features, Michio Inoue, MD, PhD, of the National Center of Neurology and Psychiatry in Tokyo, and colleagues conducted a cohort study of patients seen at the center between January 2009 and August 2019.
Of more than 8,800 patients whose muscle biopsies were examined for diagnostic purposes, 199 were tested for dermatomyositis-specific autoantibodies. The investigators excluded patients who did not have myxovirus resistance protein A expression in myofibers on muscle biopsy. In all, 182 patients with dermatomyositis were enrolled in the study (51% women; median age at biopsy, 56 years). Fourteen patients without a skin rash at the time of muscle biopsy received a diagnosis of dermatomyositis sine dermatitis. Before the muscle biopsy, most patients without a rash had a diagnosis of polymyositis.
Association with anti-NXP-2 autoantibodies
Anti-NXP-2 autoantibodies were detected in 86% of the patients without a rash at the time of biopsy, compared with 28% of the patients with rashes. “No other clinical or pathological characteristics were associated with [dermatomyositis sine dermatitis] except increased probability of developing perifascicular atrophy (71% vs. 43%),” Dr. Inoue and colleagues said.
During a median follow-up of 34 months, patients with dermatomyositis sine dermatitis received oral prednisolone with or without additional immunotherapy, and two patients had subcutaneous edema. Calcification was not seen during follow-up. “One patient with ... anti-NXP-2 autoantibodies had severe interstitial lung disease and needed noninvasive positive-pressure ventilation support,” the researchers said.
Four of the 14 patients with dermatomyositis sine dermatitis “developed skin rashes after muscle biopsy,” the researchers noted. “Similarly, a patient with [dermatomyositis sine dermatitis] was reported to have developed a skin rash 2 years after muscle biopsy.”
Potential therapies for refractory dermatomyositis, such as Janus kinase inhibitors, may not be effective for other types of myositis, so identifying patients with dermatomyositis may be “more essential than ever,” the authors said.
Effects on organ systems vary
The study is the first to systematically examine dermatomyositis sine dermatitis, said David Fiorentino, MD, PhD, professor of dermatology and director of the multidisciplinary rheumatic skin disease clinic at the Stanford (Calif.) University.
Dr. David Fiorentino
On the one hand, the results are not surprising because dermatomyositis is a systemic autoimmune disease. “There are no rules about which organs it will or won’t affect in a given individual,” Dr. Fiorentino said in an interview.
At the same time, dermatomyositis’s historical association with rash persists even though there is “no biological reason why that would have to be the case.”
Some patients with dermatomyositis have skin-predominant disease without clinically significant muscle involvement. Lung-predominant disease also may exist, although it has not been carefully studied, he said.
The findings remind clinicians that they need to consider the diagnosis of dermatomyositis “even if they do not have the skin findings,” he said. Dr. Fiorentino cautioned against interpreting the results to mean that certain patients never have signs of cutaneous inflammation. In the study, about a one-third of patients without dermatitis at the time of biopsy developed a rash. In addition, clinicians often miss subtle disease under the fingernails or on the scalp, or mild rash on the elbows.
The cohort of patients who underwent muscle biopsy may not be representative of the spectrum of patients with dermatomyositis, and the findings need to be verified in other populations, Dr. Fiorentino said.
The study was supported by an intramural research grant of the National Center of Neurology and Psychiatry and a grant from the Japan Society for the Promotion of Science. Authors disclosed personal fees from pharmaceutical companies and government and corporate grants outside the submitted work. Dr. Fiorentino had no relevant disclosures.
SOURCE: Inoue M et al. JAMA Neurol. 2020 Apr 20. doi: 10.1001/jamaneurol.2020.0673.