Hypothesis: Intraoperative monitoring by distortion-product otoacoustic emissions reflects the cochlear function changes in the real-time domain during removal of cerebellopontine angle tumors.
Background: Cerebellopontine angle tumor surgery is associated with a significant risk of damaging internal auditory canal contents. Although monitoring facial nerve function intraoperatively has already been effectively developed, such efficacious monitoring of auditory function remains to be established. The aim of this study was to investigate the utility of distortion-product otoacoustic emissions for intraoperative monitoring of the cochlear function in humans during removal of cerebellopontine angle tumors.
Methods: Continuous intraoperative monitoring of distortion-product otoacoustic emissions was performed in 20 of 62 patients undergoing surgical removal of cerebellopontine angle tumors. All of these 20 patients, who underwent the retrosigmoid approach, had distortion-product otoacoustic emissions present preoperatively. Depending on the amplitude and frequency band at which distortion-product otoacoustic emissions were present, monitoring was carried out at 2.0 to 6.0 kHz with primary stimulus tone amplitudes of 60 to 70 dB sound pressure level.
Results: In patients operated on for cerebellopontine angle tumors, various patterns of distortion-product otoacoustic emission amplitude reductions and recoveries were observed. Distortion-product otoacoustic emissions recorded from the basal part of the cochlea (i.e., high frequencies) changed earlier and more profoundly than those from the middle and apical sections (i.e., lower frequencies). In some cases, cochlear function was affected irreversibly as reflected by loss of distortion-product otoacoustic emissions. Microcoagulation of small vessels, tumor debulking, and compression or stretch of the internal auditory canal contents were found to be procedures affecting distortion-product otoacoustic emissions. The status of distortion-product otoacoustic emissions at the conclusion of tumor dissection correlated with postoperative hearing levels.
Conclusion: Distortion-product otoacoustic emissions were used to monitor in the real-time domain auditory function during cerebellopontine angle tumor removal operations. The status of distortion-product otoacoustic emissions at the conclusion of the operations was related to postoperative hearing.