Muscular and extramuscular features of myositis patients with anti-U1-RNP autoantibodies.

OBJECTIVE: To define the clinical phenotype of patients with myositis with anti-U1-ribonucleoprotein (RNP) autoantibodies. METHODS: In this longitudinal cohort study, the prevalence and severity of clinical features at disease onset and during follow-up in patients with anti-U1-RNP-positive myositis were compared to those with dermatomyositis (DM), immune-mediated necrotizing myopathy (IMNM), and the antisynthetase syndrome (AS). RESULTS: Twenty anti-U1-RNP-positive patients, 178 patients with DM, 135 patients with IMNM, and 132 patients with AS were included. Anti-U1-RNP-positive patients were younger (∼37 years) and more likely to be black (60%) than patients with AS, DM, or IMNM. Muscle weakness was a presenting feature in 15% of anti-U1-RNP-positive patients; 80% eventually developed weakness. Four of 7 anti-U1-RNP-positive patients had necrotizing muscle biopsies. Arthritis occurred in 60% of anti-U1-RNP-positive patients; this was increased compared to DM (18%) or IMNM (6%) (all p < 0.01). DM-specific skin features developed in 60% of anti-U1-RNP-positive patients. Interstitial lung disease (ILD) occurred in 45% of anti-U1-RNP-positive patients; fewer patients with DM (13%) and IMNM (6%) and more patients with AS (80%) developed ILD (all p < 0.01). Glomerulonephritis and pericarditis occurred in 25% and 40% of anti-U1-RNP-positive patients, respectively, but rarely in the other groups; these features occurred only in those with coexisting anti-Ro52 autoantibodies. No anti-U1-RNP patient had cancer-associated myositis or died during the study period. CONCLUSIONS: Patients with anti-U1-RNP myositis typically present with proximal weakness and necrotizing muscle biopsies. Arthritis, dermatitis, and ILD are the most common extramuscular clinical features. Pericarditis and glomerulonephritis are uniquely found in patients with anti-U1-RNP-positive myositis.