Electrophysiological study of vocal-fold mobility disorders using a magnetic stimulator E. M. Khedr a and E-E. M. Aref b a Department of Neurology; and b Unit of Phoniatrics, ENT Department, Assiut University Hospital, Assiut, Egypt Keywords: magnetic stimulation, neurolaryngology, neuro- physiology, vocal-fold paralysis Received 2 July 2001 Accepted 6 December 2001 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) mag- netic 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. Introduction The neuromuscular system of the larynx may be injured at the motor (efferent) or the sensory (afferent) level. A variety of neurological diseases appear with central or peripheral impairment of laryngeal muscles, or complete loss of function causing alterations in vocal function. In clinical practice, the motor injury is more readily exhibited as a complete loss or restriction of movement of one or both vocal folds, leading to clear asymmetry of the glottic configuration. Despite the help the clinician may get from pertinent evaluation of laryngeal movement, a vocal-mobility disorder case cannot be diagnosed with precision (cause, degree and distribution of the lesion) using videostroboscopy alone (Kotby et al., 1992). Insight into the neuromuscular status by various neurophysiological tools such as EMG and evoked potentials may contribute to the understanding of the pathological condition underlying vocal-fold immobility. Magnetic stimulation has recently been proposed as a non-invasive, painless technique for the stimulation of peripheral nerves, including the laryngeal nerves (Thumfart et al., 1992). It has advantage over electrical stimulation in that the nerve response can be obtained even when the exact nerve location is not known (Barker et al., 1987). However, for such a technique to have clinical utility, consistent responses to magnetic stimulation must be obtained in normal subjects so that delays in nerve conduction time can be easily identified. Thumfart et al. (1992) placed a large round coil 2 cm posterior and 6 cm superior to the external acoustic meatus to stimulate the intracisternal region of the nerve in more than 50 normal subjects whilst recording from the laryngeal muscles. Responses in the thyro-arytenoid (TA) were at 5 ms (range 4–6.6 ms, SD 0.7 ms). In contrast, cortical stimulation at the vertex produced responses between 9.5 and 12 ms. Also, Ludlow et al. Correspondence: Eman M. Khedr, Department of Neurology, Assiut University Hospital, Assuit, Egypt (fax: +88 333327; e-mail: emankhedr99@yahoo.com). ª 2002 EFNS 259 European Journal of Neurology 2002, 9: 259–267