Objective: The incidence of sporadic inclusion body myositis (sIBM) in the Japanese population has increased, and some researchers have suggested that race and genetic background may influence the clinical features of the disease. The aim of this study was to clarify the demographic features of Japanese patients with sIBM.
Methods: We retrospectively evaluated the demographic features of consecutive patients who were referred to our institution between 1995 and 2011 for diagnostic muscle biopsies and who subsequently were diagnosed to have sIBM.
Results: Seventy-three patients comprising 54 men and 19 women received a diagnosis of sIBM during the study period. The patients were divided into two groups based on the date of diagnosis (before and including 2002, and after 2002). The annual number of patients who received a diagnosis of sIBM increased significantly from 3.6±1.6 (mean ± SD) before and including 2002 to 4.9±3.1 (mean ± SD) after 2002 (p<0.05), whereas the annual number of patients who received a diagnosis of polymyositis (PM) or dermatomyositis (DM) remained consistent from 1995 to 2011. The ratio of PM and DM to sIBM was 7.6 during the period from 1995 to 2002 and 5.5 during the period from 2003 to 2011. However, the age-adjusted annual number of patients newly diagnosed with sIBM did not increase significantly after 2002.
Conclusion: The number of Japanese patients with sIBM appears to have increased in recent years; however, the characteristics of the patients have not changed. Considering the increased size of the elderly population, prolonged lifespans could explain the demographic movement of sIBM in Japan.