Intraoperative Auditory Monitoring in Acoustic Neuroma Surgery

Abstract

Intraoperative auditory monitoring is a useful adjunct that is currently evolving. Near-field monitoring techniques such as electrocochleography and direct eighth nerve compound action potential are being used more frequently. The use of these two techniques is compared in 26 patients undergoing hearing preservation acoustic neuroma resection. Overall, 9 (35%) of the 26 patients had their hearing preserved. Three (23%) of the 13 with electrocochleography monitoring and 6 (46%) of the 13 with direct compound action potential monitoring had hearing preserved after surgery. Although there was a suggestion of improved results with direct compound action potential monitoring, the results were not statistically different. It was noted that lack of electrical response at the completion of the procedure (regardless of monitoring technique) was correlated with poor postoperative hearing, whereas the presence of a waveform at termination in no way predicted satisfactory postoperative hearing. The two techniques and their advantages and disadvantages are discussed.

Intraoperative neuromonitoring techniques in the surgical management of acoustic neuromas

“…23,89,147 Operative damage to the vestibulocochlear nerve can be induced in various ways. 63,64,149 Direct operative trauma is a potential avenue, with the nerve most prone to exposure during maneuvers, such as drilling into the internal auditory canal, operative traction, or subsequent tumor resection. 1,17,19,51,79,96,129 Cranial nerves are inherently more susceptible to trauma because they are ensheathed in central myelin, thus lacking the extra protective layers, such as the perineurium, that are more prevalent in peripheral myelin.…”

“…63,97,129 In comparison with the background electrical brain activity, 54,79,142 these BAEP waves are diminutive and difficult to detect. 18,149 To facilitate distinction between BAEPs and background "noise," several thousand samples of the electrical stimulus must be acquired and subsequently averaged to create a distinct auditory evoked potential. 1,50,64,79,97,129,149 On BAEP recordings, the auditory response is extracted from several locations in the entire vestibular nerve pathway, as it travels peripherally to centrally.…”

“…18,149 To facilitate distinction between BAEPs and background "noise," several thousand samples of the electrical stimulus must be acquired and subsequently averaged to create a distinct auditory evoked potential. 1,50,64,79,97,129,149 On BAEP recordings, the auditory response is extracted from several locations in the entire vestibular nerve pathway, as it travels peripherally to centrally. 97 The peaks of the evoked electrical potentials are classified as Wave I through Wave V, which correspond to the peripheral cochlear nerve and the inferior colliculus, respectively.…”

“…80 In BAEP monitoring, scalp and earlobe electrodes are placed, and an auditory stimulator discharges acoustic clicks to the operated ear through an earphone-transducer apparatus. 59,64,79,97,129,149 The electrical pulse rate is set at a range of 20-50 clicks per second. 16,54,79,97,99,148 Before commencing with the operation, the stimulus intensity, as measured in decibels, is adjusted until the patient can hear the click; the stimulus is eventually delivered at several decibels higher than the measured threshold.…”

“…54,98,129,149 In contrast, ECOG and direct CNAPs are "near-field" techniques because the stimulation evokes and records an electrical response close to its origin on the auditory nerve.…”