[Electrophysiologic evaluation of the facial nerve in patients with acoustic neuromas. Preliminary results of a comparative study between conventional electroneurography and transcranial magnetic pulse stimulation]

Abstract

Subclinical invasion of the facial nerve by acoustic neuromas may be identified preoperatively using conventional electroneurography (ENOG). The clinical application of extratemporal stimulation distal to the stylomastoid foramen is limited in these cases because of the more proximal site of the lesion. Transcranial magnetic coil stimulation (MCS) is a non-invasive means by which the facial nerve is stimulated at the level of the motor cortex or the brain stem before it enters the internal auditory canal. Topographically such an assessment may have more diagnostic relevance than other forms of electrical stimulation in acoustic neuroma patients. To test this theory the facial nerves of 20 patients with an acoustic neuroma were stimulated using ENOG and MCS before and 1 week after operation. Stimulation parameters were comparable and included threshold, double-threshold and supra-threshold levels of stimulation while compound action potential amplitudes and early and late response latencies were monitored. All the patients had clinically normal facial nerve function preoperatively. Using ENOG 65% of the patients showed amplitude reduction on the tumour side, whereas 70% of these patients had a reduction of amplitude to MCS. Combining both techniques, 88% of the patients had a significant amplitude reduction on the neuroma side. Comparison of the preoperative threshold measurements of both techniques showed that there was a significantly higher incidence of detection of lesions on the diseased side in neuromas larger than 2 cm by the use of MCS.