Epidemiology and Natural History of Inclusion Body Myositis: A 40-Year Population-Based Study

OBJECTIVES: To determine the prevalence and natural history of sporadic inclusion body myositis (sIBM) and to test the hypothesis that patients with sIBM have higher cancer or mortality rates than the general population. METHODS: We sought patients with sIBM defined by the 2011 European Neuromuscular Centre (ENMC) diagnostic criteria among Olmsted County, Minnesota, residents in 40-year time period. RESULTS: We identified 20 patients (10 clinicopathologically defined, 9 clinically defined, and 1 probable) according to the ENMC criteria and 1 patient with all features of clinicopathologically defined sIBM except for symptom onset at <45 years of age. The prevalence of sIBM in 2010 was 18.20 per 100,000 people ≥50 years old. Ten patients developed cancers. The incidence of cancers in sIBM did not differ from that observed in the general population (odds ratio 1.89, 95% confidence interval [CI] 0.639-5.613, p = 0.24). Two-thirds of patients developed dysphagia, and half required a feeding tube. Nine patients required a wheelchair. The median time from symptom onset to wheelchair dependence was 10.5 (range 1-29) years. Overall life expectancy was shorter in the sIBM group compared to the general population (84.1 [95% CI 78-88.4] vs 87.5 [95% CI 85.2-89.5] years, p = 0.03). Thirteen patients died; 9 deaths were sIBM related (7 respiratory and 2 unspecified sIBM complications). Female sex (p = 0.03) and dysphagia (p = 0.05) were independent predictors of death. CONCLUSION: Olmsted County has the highest prevalence of sIBM reported to date. Patients with sIBM have similar risk of cancer, but slightly shorter life expectancy compared to matched patients without sIBM. CLASSIFICATION OF EVIDENCE: This study provides Class II evidence that patients with sIBM have similar risks of cancers and slightly shorter life expectancy compared to controls.