myotonic muscular dystrophies

The concept of the muscular dystrophies as primary degenerative diseases of muscle has been increasingly challenged in recent years (Fenichel et al., 1967; Gardner-Medwin et al., 1967; Munsat et al., 1972; Gallup and Dubowitz, 1973). Considerable support for the neurogenic hypothesis in muscular dystrophy has come from the results obtained using an electrophysiological technique for the estimation of the number of motor units in the human extensor digitorum brevis (EDB) muscle (McComas et al., 1971b). Using this method a significant reduction in motor unit numbers has been claimed in Duchenne type (McComas et al., 1971c), limbgirdle and facioscapulohumeral (Sica and McComas, 1971), and myotonic muscular dystrophy (McComas et al., 1971a). We have recently presented a critique of the technique used by these authors and have proposed that the results so derived may be erroneously low in diseases where qualitative alteration in the configuration of motor unit potentials occurs, as, for example, in the myopathies (Ballantyne and Hansen, 1974). We have also described a computerized method for this estimation which has theoretical advantages over the earlier technique of McComas et al. (1971b). The usefulness of this method has been demonstrated in practice in a group of patients with myasthenia gravis (Ballantyne and Hansen, 1974). The purpose of this paper is to present the results obtained by the application of this new method to patients with Duchenne type, limb-girdle and facioscapulohumeral, and myotonic muscular dystrophies.