Electrophysiological study of vocal-fold mobility disorders using a magnetic stimulator

In the field of neurolaryngology, there has been a great interest in neurophysiological studies, such as neurography, for the assessment of the integrity of the laryngeal neural pathway. Such tools provide an indication about the site and the nature of the nerve lesion. We have tried to use a magnetically evoked potential to assess the corticolaryngeal pathway in order to provide normative data on laryngeal nerve conductivity and to evaluate the integrity of the laryngeal neural system in patients with vocal-fold mobility disorders. This study was conducted on 26 subjects (10 normal volunteers and 16 patients with vocal-fold immobility) who were primarily selected on the basis of a comprehensive laryngeal evaluation including laryngo-videostroboscopy assessment. Transcranial (cortical) and mastoid (peripheral) magnetic stimulations were performed to evoke muscle action potentials of the thyro-arytenoid (TA) and cricothyroid muscles (CT). In normal volunteers, cortical stimulation leads to contralateral responses (cortical latency) after 10.9 and 11.3 ms and ipsilateral responses after 8.3 and 9.4 ms for right CT and TA muscles, respectively. There was a significant prolongation of cortical latency of the left TA compared with the right TA muscle, whilst no such significant difference was observed in the CT muscles. Peripheral stimulation evoked response (peripheral latency) after 2.8 and 2.7 ms in the right CT and TA, respectively, with the same significant prolongation of the left TA response compared with the right side. Amongst the patient groups, variable patterns of laryngeal muscle response latencies occurred, including normal response latency, lack of response of CT and TA muscles, prolonged peripheral latency with secondary prolonged cortical latency and prolonged cortical latency with normal peripheral latency. The results indicate that the magnetically evoked potential of laryngeal muscles offers an easy, non-invasive technique and could have a role in the assessment of the integrity of corticolaryngeal pathways.