Sporadic inclusion body myositis: overreliance on histology may lead to misdiagnosis.

We describe a 57-year-old man who had initial isolated weakness in the deep finger flexors of his non-dominant hand, followed by quadriceps weakness and wasting a year later. Despite the distribution specific for sporadic inclusion body myositis (s-IBM) and early myogenic abnormalities found in deep finger flexors, the case was initially misdiagnosed as polymyositis based on the initial biopsy results, which revealed endomysial inflammation with partial invasion/compression of non-necrotic fiber, without rimmed vacuoles. As a result, the patient received unnecessary treatment, including immunosuppressants, which caused serious side effects. The second biopsy revealed rimmed vacuoles, and the diagnosis of s-IBM was confirmed. The role of clinical, pathological, and electrophysiological features for early diagnosis of s-IBM and differentiation from PM is discussed.