Clinical features of Japanese patients with inclusion body myositis.

Background: The incidence of sporadic inclusion body myositis (sIBM) has been much lower in Japanese than in Western populations. Because of a few reports on Asian populations, it is unclear whether the clinical characteristics of sIBM are identical in Caucasian and Japanese patients.

Methods: We compared 18 patients with sIBM, divided into 3 groups by age-of-onset, with previous cohort studies. We calculated the ΔIBM functional rating scale/time duration (ΔIBMFRS/Δtime) as an index of functional disability progression. Patients' electrophysiology was analyzed in relation to their clinical characteristics.

Results: The cohort was 83.3% male and showed uniform initial muscle weakness in the lower and/or upper limbs. An older age-at-onset was associated with a more rapid progression, and patients with a longer duration frequently showed F-wave abnormalities and findings of chronic denervation.

Conclusions: The clinical characteristics of sIBM were relatively homogeneous beyond the ethnic differences. Aging might be a synergistic factor for the progression of sIBM pathology.