Sporadic Inclusion Body Myositis (sIBM) is a refractory myositis developing in the elderly. Recently, anti-cytosolic 5'-nucleotidase 1A (cN1A) antibodies were reported to be present specifically in sIBM. To assess the clinicopathological features and usefulness of anti-cN1A positivity in muscle specimens, we evaluated a group of Japanese patients with sIBM. Among Japanese patients with myositis who underwent a muscle biopsy from 1991 through 2014 in our hospital, we identified cases of sIBM that met the European Neuromuscular Center (ENMC) criteria. We then analyzed clinical course, pathological findings, and treatment response. Of 282 patients with myositis, 35 (12%) were given a diagnosis of sIBM (24 men and 11 women). The median age at onset was 67 ± 7 (55-81) years, and that at diagnosis was 70 ± 7 years. Remarkably, 5 (14%) patients concurrently had hepatitis C virus (HCV) infection and 4 (11%) had heart disease. Pathologically, we found rimmed vacuoles in 32 (91%) patients and anti-cN1A positivity in perinuclear regions and rimmed vacuoles in 31 (89%) patients. Remarkably, anti-cN1A antibody positivity was significantly higher in sIBM than in polymyositis (30%), dermatomyositis (10%), and morphologically normal cases (0%), suggesting its high specificity for sIBM. Twelve (34%) patients received immunosuppressants, 8 (23%) received prednisone, and 5 (14%) received intravenous immunoglobulins (IVIg). Clinical symptoms transiently improved in 5 patients given prednisone and in 4 patients given IVIg, but these treatments failed to be radical therapies. In our study of Japanese patients with sIBM, anti-cN1A antibody positivity in muscle specimens was highly sensitive and specific pathologically. Therefore, the expression of anticN1A antibody may be greatly useful for the diagnosis of sIBM. Although the concurrent presence of HCV infection and heart disease was notable, other epidemiological characteristics were consistent with the results of previous studies.
Nobuyuki Eura, Kazuma Sugie, Kaoru Kinugawa, Hitoki Nanaura, Hiroya Ohara, Naoki Iwasa, Ryogo Shobatake, Takao Kiriyama, Tesseki Izumi, Hiroshi Kataoka and Satoshi Ueno