The diagnostic significance of large action potentials in myopathy

Electromyographic and histopathological studies were performed on 112 skeletal muscles in 101 subjects with myopathy. The diagnostic significance of large action potentials (LAPs) in myopathy was studied. LAPs were defined as those action potentials with a duration of over 13 ms and an amplitude of over 3 mV (peak to peak). The following results were obtained: Most muscles with LAPs showed the grouped atrophy of small fibers of neuropathic change in addition to myopathic findings. Even in myopathy most LAPs reflected neuropathic change, except in thyrotoxic myopathy. LAPs were not related to an increase of connective tissue increasing the impedance in volume conduction of the action potentials. LAPs were frequently seen in: progressive muscular dystrophy of limb-girdle type; scapuloperoneal dystrophy; distal myopathy; oculopharyngeal dystrophy; myotonic dystrophy; polymyositis; and thyrotoxic myopathy. Other types of myopathy had few LAPs. There were two types of progressive muscular dystrophy. One had LAPs frequently and the other, rarely. In myotonic dystrophy the muscles with LAPs showed scattered small angular fibers, possibly indicating neurogenic changes. Interstitial myositis had LAPs more frequently than parenchymatous polymyositis. The chronic phase of polymyositis had LAPs more frequently than the acute or subacute phases. In thyrotoxic myopathy the muscles with LAPs rarely showed definite neuropathic change histopathologically. Therefore, LAPs in thyrotoxic myopathy may not indicate denervation.