Myoclonus in a patient with spinal cord transection. Possible involvement of the spinal stepping generator.

A patient with clinically complete cervical spinal cord transection developed rhythmic myoclonic movements of the trunk and lower limbs, demonstrating that, in man, such movements can be generated within the spinal cord itself when deprived of supraspinal control. Electromyographic (EMG) recordings used to define the features of the myoclonus, which had a frequency of 0.3-0.6 Hz, was bilaterally symmetric, and involved extensor muscles. The EMG bursts always appeared in phase in all muscles involved. Peripheral stimulation of flexor reflex afferents (FRA) could induce, slow or interrupt the rhythmic activity. When FRA stimulation induced a flexion reflex, it occurred between extensor EMG bursts and induced alternating flexion-extension activity which could be sustained for several cycles. Soleus and quadriceps monosynaptic reflexes were depressed during the silent period of the rhythmic activity. Several arguments, mainly the great sensitivity of the myoclonus to flexor reflex afferent stimulation, suggest that the myoclonus observed in this patient was due to partial release of a spinal stepping generator.