Serial quantitative electrophysiologic studies in sporadic inclusion body myositis

Sporadic inclusion body myositis (S-IBM) is a progressive, acquired myopathic process of unknown etiology. No known, successful or proven treatment exists. Quantitative EMG studies including concentric needle motor unit action potentials, interference pattern, macro-EMG and fiber density have allowed different measures to be made of the motor unit. These different measures allow inferences to be made in how the muscle fibers are distributed within both the normal and diseased motor unit. The present study is an effort to use multiple quantitative EMG measurements from the biceps brachii on a serial basis in order to study chronic changes in the motor unit with disease progression. Twenty-eight studies from 9 patients over a four-year period are shown. We conclude that while the concentric needle electrode is most helpful for diagnosing abnormality, the less selective macro-EMG and surface electrodes are better suited to monitor disease progression, especially in very weak muscles. These observations have practical applications for monitoring disease progression, or conversely, response to treatment.