Direct cochlear nerve stimulation monitoring through evoked muscle responses during retrosigmoid vestibular schwannoma resection surgery: technical note

OBJECTIVE Cochlear nerve preservation during surgery for vestibular schwannoma (VS) may be challenging. Brainstem auditory evoked potentials and cochlear compound nerve action potentials have clearly shown their limitations in surgeries for large VSs. In this paper, the authors report their preliminary results after direct electrical intraoperative cochlear nerve stimulation and recording of the postauricular muscle response (PAMR) during resection of large VSs. METHODS The details for the electrode setup, stimulation, and recording parameters are provided. Data of patients for whom PAMR was recorded during surgery were prospectively collected and analyzed. RESULTS PAMRs were recorded in all patients at the ipsilateral vertex-earlobe scalp electrode, and in 90% of the patients they were also observed in the contralateral electrode. The optimal stimulation intensity was found to be 1 mA at 1 Hz, with a good cochlear response and an absent response from other nerves. At that intensity, the ipsilateral cochlear response had an initial peak at a mean (+/- SEM) latency of 11.6 +/- 1.5 msec with an average amplitude of 14.4 +/- 5.4 mu V. One patient experienced a significant improvement in his audition, while that of the other patients remained stable. CONCLUSIONS PAMR monitoring may be useful in mapping the position and trajectory of the cochlear nerve to enable hearing preservation during surgery.

Automated summary

Direct electrical stimulation and recording of postauricular muscle response (PAMR) can be used to preserve cochlear nerve and hearing during surgery. Stimulation parameters of 1mA at 1Hz were found to induce cochlear response without affecting other nerves. Most patients maintained stable hearing during the surgery, with some even experiencing significant improvement.