Longterm Survival and Associated Risk Factors in Patients with Adult-onset Idiopathic Inflammatory Myopathies and Amyopathic Dermatomyositis: Experience in a Single Institute in Japan

OBJECTIVE: To analyze clinical characteristics, survival, causes of death, and risk factors associated with mortality in patients with adult-onset idiopathic inflammatory myopathies (IIM) in Japan. METHODS: We retrospectively investigated 197 patients diagnosed with adult-onset IIM at our hospital from 1984 to 2009 according to Bohan and Peter criteria for polymyositis (PM)/dermatomyositis (DM) and modified Sontheimer's criteria for clinically amyopathic DM (ADM). RESULTS: Survival in the whole group at 1, 5, and 10 years was 85%, 75%, and 67%, respectively. Mortality in cancer-associated myositis was the worst (25% at 5 yrs), followed by clinically ADM (61% at 5 yrs) and primary DM (77% at 5 yrs). Primary DM had significantly low survival compared to primary PM (91% at 5 yrs; p = 0.0427). Among the 53 patients who died were 6 patients with ADM (11%) and 20 patients with primary DM (38%). Interstitial lung disease (ILD) was the main cause of death in clinically ADM (71%) and primary DM (60%), most of which occurred within the first few months. Fewer patients died in primary PM (9%) and overlap myositis (13%). Independent risk factors for death were older age (HR 1.031; 95% CI 1.009-1.053) and skin ulcers (HR 3.018; 95% CI 1.340-6.796) in the whole group and ILD with mild serum creatine kinase levels (< 500 IU/l; HR 3.537; 95% CI 1.260-9.928) in primary DM. CONCLUSION: Survival of clinically ADM and primary DM was low, mainly due to fatal ILD, compared to primary PM. Establishing therapeutic strategy for ILD may improve the survival in our patient population.