Neurophysiology

Fig. 1.5. Normal brainstem auditory evoked potentials with labeling of peaks I-V. During CPA surgery, peak V commonly exhibits a gradual prolongation of latency and reduction of amplitude that is not predictive of hearing loss (AS = left ear, AD = right ear).

potentials detected in this fashion are difficult to record and fail to provide insight into the functional status of ascending pathways in the brainstem.

The most frequent indication for the use of BAEP monitoring has been in the resection of vestibular schwannomas. There are a number of steps in the surgical procedure that are known to place auditory function at risk, including opening of the dura, cerebellar retraction, coagulation of tumor vessels, and removal of tumor present in the auditory canal, particularly from the most lateral portion. A series presented by Fischer et al. demonstrated that hearing was preserved in an average of 45% of patients who underwent resection of a vestibular schwannoma with concomitant recording of BAEP; this number was variable depending on the particular grade of the tumor [20]. Similar findings were described by Fahlbusch et al. in a series of 61 patients who underwent resection of large vestibular schwannomas via a lateral suboccipital approach with pre-operative and intraoperative BAEP monitoring. In this cohort, hearing was preserved in approximately 43% of patients in the early post-operative phase; however, a number of patients had subsequent decreases in hearing, resulting in a decrease in the final number of hearing-intact individuals to 27% [21]. BAEP monitoring has also been used in cases focusing on microvascular decompression of the facial or trigeminal nerves. Both retrospective and prospective studies have suggested that post-operative hearing loss can be reduced in these cases with the use of BAEP monitoring before and during the procedure.

Changes in BAEP that have been referred to as significant indicators of post-operative hearing loss are: decreases in waveform amplitude of 50%, prolongation of waveform latency of 10% or greater, and dramatic alterations in waveform morphology. Obviously, disappearance of the waveform is most concerning and is most likely to correlate with subsequent loss of hearing. Upon exposure of the CPA, wave V may exhibit prolonged latency and amplitude reduction that gradually continues until the potential is lost. When wave V is lost, no prediction of post-operative hearing can be made. If the potential is unchanged throughout surgery, the patient's post-operative auditory function will be stable. Loss of wave I occurs more acutely over minutes and is always associated with loss of wave V; return of this potential will occur within 15 minutes or will not return at all. If it fails to reappear within this time, hearing will

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