Quantitative muscle MRI study of patients with sporadic inclusion body myositis

Background Fat infiltration in individual muscles of sporadic inclusion body myositis (sIBM) patients has rarely been assessed. Methods Sixteen sIBM patients were assessed using MRI of the thighs and lower legs (LL). The severity of fat infiltration, proximal‐to‐distal and side asymmetries, and the correlations with clinical and functional parameters were investigated. Results All the patients had fat‐infiltrated muscles, and thighs were more severely affected than LL. A proximal‐to‐distal gradient of fat infiltration was mainly observed for adductors, quadriceps, sartorius, and medial gastrocnemius muscles. A strong negative correlation was observed between the whole muscle fat fraction in the thighs and LL and the Inclusion Body Myositis Functional Rating Scale and Medical Research Council scores for the lower limbs. Conclusions Fat infiltration in individual muscles of sIBM patients is heterogeneous in terms of proximal‐to‐distal gradient and severity was correlated with clinical scores. These results should be considered for both natural history investigation and clinical trials.