Conventional electromyography in myotonic dystrophy

In 31 subjects of 23 different families with unequivocal signs and symptoms of myotonic dystrophy EMG was performed with conventional technique on brachial biceps, extensor digitorum communis and tibialis anterior muscles. In 14 of these patients motor and orthodromic sensory conduction velocities were measured in median nerve as well. During full muscular effort, an interference pattern of low amplitude was seen in most of the explored muscles, while "after discharge" of different duration was present in 57-83% of the patients. At weak effort, mean potential duration was nearly always significantly reduced if polyphasic potentials were not included in the calculation; polyphasic potentials, however, exceeded the limits of normal incidence and their duration was longer than that of motor unit potentials of simple shape. At rest, spontaneous activity of short duration and positive sharp waves occurred in about 50% of the patients, but rarely in more than two places. Bursts of myotonic or pseudomyotonic discharges occurred either spontaneously or mechanically in all subjects. Conduction velocity along motor, or sensory, peripheral nerve fibres was constantly within normal values. Results were discussed in relation to the hypothesis of a primary neuronal pathogenesis of muscle wasting, and the conclusion was that nerve changes are to be considered as a collateral disorder dependent on the pleiotropic action of the gene.