Inflammatory myositis associated with anti‐U1‐small nuclear ribonucleoprotein antibodies: a subset of myositis associated with a favourable outcome

Objectives. Inflammatory myositides are rare chronic disorders which may be either isolated or associated with other conditions such as connective tissue diseases or neoplasia. A large variety of autoantibodies can be detected in patients with myositis, some of which have a diagnostic and/or a prognostic value. Myositis associated with anti‐U1‐small nuclear ribonucleoprotein antibodies (anti‐U1‐snRNP Abs) are usually considered as overlapping syndromes, mainly mixed connective tissue diseases (MCTD) in which muscle symptoms occur insidiously during the disease course and are characterized by a favourable outcome. Methods. The clinical, biological, immunological and pathological findings as well as the outcome of five patients with anti‐U1‐snRNP‐associated myositis were retrospectively analysed. Results. Patients were mainly black females. In all five patients, myositis was the predominant manifestation at presentation. Associated conditions consisted of interstitial lung disease (ILD) (three), arthritis (three) and neurological symptoms (two). No patient presented Raynaud's phenomenon nor met criteria for MCTD. Biological inflammatory features, rheumatoid factor and polyclonal hypergammaglobulinaemia were present in all cases. Besides anti‐U1‐snRNP Abs, one patient had anti‐Ro/SSA and anti‐La/SSB Abs at presentation and one additional patient developed anti‐double‐stranded‐DNA and anti‐Sm Abs after a follow‐up of more than 4 yr. No patient had anti‐PM/sclerosis (Scl) nor anti‐aminoacyl‐tRNA synthetase Abs. All patients dramatically improved with steroids, and reached complete remission (CR) within 3 weeks. Two patients relapsed 18 months after CR. They both reached rapidly second CR using steroids associated or not with oral methotrexate. Conclusion. Our data suggest that anti‐U1‐snRNP Abs may define a subset of myositis characterized by a favourable outcome, though often associated with ILD and/or neurological manifestations.