Refractory and irresponsive periods of muscle in progressive muscular dystrophy and paresis due to lower motor neuron involvement

Abstract

The course of recovery was determined after the passage of a propagated action potential in directly stimulated small fiber bundles in muscles of patients with progressive muscular dystrophy and with paresis due to lower motor neuron involvement. In dystrophic muscle, the absolute refractory period averaged 2.5 [plus or minus] 0.1 msec. as compared with 3.6 [plus or minus] 0.1 msec. in normal muscle. This decrease was due to an over-all reduction in refractory period rather than to a selective loss of the fibers with long refractory periods found in normal muscle. The reduction in refractory period was found even in muscles with no or only a minimal force reduction and had the same degree in the pseudohypertrophic and facioscapulohumeral or limb-girdle types of the disease. It can be explained by the decrease in intracellular potassium and the resulting increase in excitability. The absolute irresponsive periods of dystrophic muscle did not deviate from those of normal muscle and exceeded the absolute refractory period by, on the average, 1 msec. as compared with 0.1 msec. in normal muscle. In paresis due to peripheral nerve injury, the absolute refractory period was 50% longer than in normal muscle. This can be explained by the changes in membrane properties caused by denervation and evidenced by the slower rate of repolarization and the reduced conduction velocity. Difference between refractory and irresponsive periods was slight. In paresis secondary to anterior horn cell involvement, the absolute refractory and irresponsive periods were within the limits of normal muscle. This was probably due to the relatively larger number of intact muscle fibers in these muscles (average force graded as 2+) as compared with paresis in peripheral nerve injury (average force graded as 1).