THE ELECTROMYOGRAM IN MYOPATHY: ANALYSIS WITH THE AUDIO-FREQUENCY SPECTROMETER

Despite the fact that in most established cases of muscular dystrophy the clinical diagnosis is selfevident, occasional cases arise in which it is difficult to decide whether muscular wasting is of neuropathic or myopathic origin. Careful work by Kugelberg (1949) and others has shown that electromyography may be of considerable value in making this fundamental distinction, but in many centres the techniques for diagnosis are not yet firmly established. In this paper the electromyographic findings in 100 cases of myopathy are reviewed, and a method of frequency analysis is described which appears to enhance the value of the electromyogram in diagnosis. The results obtained by applying this analytical technique in the examination of 26 normal subjects and 48 patients with other neurological disorders are compared with those noted in the cases of myopathy. Before discussing the methods and results of the investigation, however, it is essential to discuss briefly the physiological principles involved and the findings of other authors. The unit of muscle structure is the muscle fibre; under normal conditions muscle fibres never contract singly, but single fibre or fibrillation potentials can be recorded from resting muscle after denervation in the form of monophasic or diphasic spikes (Fig. 1) of about 1 millisecond duration (Weddell, Feinstein, and Pattle, 1944). Normal volitional activity of voluntary muscle is made up by the asynchronous contraction of individual motor units; each unit consists of the bundles of muscle fibres supplied by one anterior horn cell and its motor neuron. Electrically, the activity of the fibres of the unit summates smoothly to give a monophasic, diphasic, or triphasic wave, the motor unit action potential (Fig. 2), which varies between 4 and 10 milliseconds in duration (Petersen and Kugelberg, 1949). Hence it will be seen that in sustained contraction of voluntary muscle the dominant frequency of the electrical discharge, depending on the duration of the individual waves from which the complex wave form of the interference pattern is synthesized, will be of the order of 100 to 250 cycles per second. In disease of the anterior horn cell and the proximal part of the lower motor neuron it is clear that whole motor units, supplied by the diseased.neuron, will perish, and for this reason the interference pattern will be reduced; eventually after the death of many neurons a needle electrode may record the activity from isolated surviving units, the " single discharge" of Buchthal and Clemmesen (1941). Under these circumstances, since the remaining motor units are of normal duration, the dominant frequency of the wave form will be unchanged. In primary disease of the tnuscle, on the other hand (e.g. muscular dystrophy), no such anatomical pattern of degeneration obtains and individual muscle fibres die sporadically. No longer, therefore, is it possible for the activity of the fibres of a motor unit to summate smoothly, as some components of the unit will be missing, and for this reason many polyphasic potentials are seen (Fig. 3). If degeneration has proceeded further it may be that one or two fibres are all that remain of a motor unit and fibrillary potentials may be obtained on volition. Hence it will be expected that the dominant frequency of the electrical discharge from contracting dystrophic muscle will approach more nearly to 1,000 cycles per second. Kugelberg (1949) examined 16 cases of muscular dystrophy and carried out careful measurements of the motor unit potentials recorded. These findings were compared with those in 142 cases of neuropathy and it was confirmed that in the cases of myopathy a greatly increased proportion of polyphasic potentials and attenuated or short duration potentials was produced on contraction of the biceps brachii. It should be remarked at this juncture that a number of polyphasic potentials are to be found on recording from normal muscle (Denslow and Hassett, 1943; Weddell and others, 1944); the proportion is about 2 to 4% in the biceps 219 gest. P rocted by coright.