Association of Dermatomyositis Sine Dermatitis With Anti–Nuclear Matrix Protein 2 Autoantibodies

IMPORTANCE: Reports on dermatomyositis (DM) sine dermatitis (DMSD) are scarce, and the concept of the disease has not been widely accepted. OBJECTIVE: To confirm the existence of DMSD, determine its prevalence, and characterize its serologic features. DESIGN, SETTING, AND PARTICIPANTS: This is a cohort study that reviewed clinical information, laboratory data, and muscle pathology slides from January 2009 to August 2019. We further assessed the follow-up data of 14 patients with DMSD. The median (interquartile range) follow-up period was 34 (16-64) months.Muscle biopsy samples, along with clinical information and laboratory data, were sent to a referral center for muscle diseases in Japan for diagnosis.Of patients whose myopathologic diagnosis was made at the National Center of Neurology and Psychiatry between January 2009 and August 2019, 199 patients were eligible for inclusion. These patients underwent full investigation for DM-specific autoantibodies (against transcriptional intermediary factor γ, Mi-2, melanoma differentiation–associated gene 5, nuclear matrix protein 2 [NXP-2], and small ubiquitin-like modifier activating enzyme ); however, 17 patients were excluded because their muscle fibers did not express myxovirus resistance protein A, a sensitive and specific marker of DM muscle pathology. MAIN OUTCOMES AND MEASURES: Diagnosis of DMSD was based on the absence of a skin rash at the time of muscle biopsy. RESULTS: Of the 182 patients, 93 were women (51%) and 46 were children (25%) (