Epidemiology, Survival, and Clinical Characteristics of Inclusion Body Myositis

OBJECTIVE: We performed a population‐based study on inclusion body myositis with the primary aims to define the prevalence, survival rate, and incidence, and to investigate the symptom profiles associated with disease duration and sex over a 33‐year period. METHODS: Patients diagnosed between 1985 and 2017 in Region Västra Götaland, Sweden, were identified according to the European Neuromuscular Centre diagnostic criteria from 2011. RESULTS: We identified 128 patients, 89 men and 39 women, with the strict clinicopathological definition of inclusion body myositis. The prevalence was 32 per million inhabitants, 19 per million women and 45 per million men, by December 31, 2017. Mean incidence was 2.5 per million inhabitants and year. Mean age at symptom onset was 64.4 years with quadriceps weakness being the most common presenting symptom followed by finger flexor weakness. Dysphagia was a common presenting symptom being more frequent in women (23%) than men (10%) and was during the disease course reported in 74% of men and 84% of women. Seventy‐three patients were deceased, with a mean survival of 14 years from symptom onset. Survival rates from both diagnosis date and symptom onset were decreased compared to the matched population. Twenty‐one percent of the patients had an additional autoimmune disease. A cross‐sectional analysis of autoantibodies in 50 patients and 28 matched controls showed autoantibodies to cytosolic 5′‐nucleotidase 1A in 40% of the patients and 3.6% of controls. INTERPRETATION: Inclusion body myositis is an autoimmune disease with decreased survival rate and with marked sex differences in both prevalence and clinical manifestations. ANN NEUROL 2022;92:201–212