Several studies are being conducted using magnetic stimulation to examine the conduction time of the superior and recurrent laryngeal nerves from where they emit from the skull to the laryngeal muscles on the right and left sides. Our purpose is to determine the normal variation of conduction times to be used for comparisons with patients with suspected nerve injury. We compared the latencies of responses in the right and left thyroarytenoid muscles and the right and left cricothyroid muscles to magnetic stimulation on the right and left sides at the angle of the jaw in normal controls and patients with known injury to the recurrent laryngeal nerve with subsequent reinnervation but persistent laryngeal paralysis. The conduction times were significantly slower on the side of injury in patients. A somewhat surprising finding was that the long latency responses were more pronounced in the patients in comparison with the controls on the side of injury. This finding is being explored further in continuing study as it suggests that some brainstem changes in the control of afferent reflex responses may take place as a result of peripheral nerve injury. A study of the long latency responses to peripheral nerve stimulation at different levels between the angle of the jaw and up to the cortex demonstrated that most of the responses seen during cortical stimulation could be explained by peripheral nerve responses and long latency responses to stimulation of the laryngeal afferents in the superior laryngeal nerve. This suggests that only contralateral muscle responses to cortical stimulation can be assumed to be responses to stimulation of corticobulbar pathways.
