Inclusion body myositis mimicking motor neuron disease

Objective: To describe the clinical and electrophysiologic features of pati ents with inclusion body myositis that was misinterpreted as motor neuron d isease. Patients and Methods: We retrospectively retrieved the medical records of 7 0 patients with a pathologic diagnosis of inclusion body myositis. From thi s group, we selected those who had been first diagnosed as having motor neu ron disease or amyotrophic lateral sclerosis. We reviewed the clinical, ele ctrophysiologic, laboratory, and morphologic studies. Results: Nine (13%) of 70 patients with inclusion body myositis had been di agnosed as having motor neuron disease. Six of the 9 patients had asymmetri c weakness; in 4 the distal arrn muscles were affected. Eight patients had finger flexor weakness. Tendon reflexes were preserved in weak limbs in 6, hyperactive in 2, and absent in 1. Four patients had dysphagia. Fasciculati on was seen in 2 patients. None had definite upper motor neuron signs or mu scle cramps. Routine electromyographic studies showed fibrillation potentia ls and positive sharp waves in all 9. Fasciculation potentials were seen in 7 and long-duration polyphasic motor unit potentials were seen in 8. There was no evidence of a myogenic disorder in these 9 patients. Muscle biopsy was done because of slow progression or prominent weakness of the finger fl exors and was diagnostic of inclusion body myositis. A quantitative electro myogram was myopathic in 4 of the 5 patients studied. Conclusions: inclusion body myositis may mimic motor neuron disease. Muscle biopsy and quantitative electromyographic analysis are indicated in patien ts with atypical motor neuron disease, especially those with slow progressi on or early and disproportionate weakness of the finger flexors.