Hearing Preservation in Acoustic Neuroma Surgery

Objective To report the authors' experiences in hearing preservation during acoustic neuroma (AN) resection procedures. Methods Two cases of AN removal via retrosigmoid approach were reviewed. Hearing preservation was attempted in the aid of endoscopic technique along with continuous monitoring of the compound action potential (CAP) and auditory brainstem response(ABR) during the surgery. Results The tumor in Case 1 was 1.5 cm in diameter. The average pure-tone hearing threshold was 30 dB HL and ABR was normal. Waves I, III and V of ABR were present following tumor removal. At 7th month follow-up, audiometric thresholds and ABR inter-peak intervals had recovered to pre-operative levels, with normal facial nerve function. The patient in Case 2 had bilateral AN. The tumors measured 4.0 cm(left) and 5.0 cm (right) on MRI scans. The AN on the right side was removed first, followed by removal of the left AN four months later. Intraoperative CAP monitoring was employed during removal of the left AN. While efforts to preserve the cochlear nerve were not successful, CAPs were still present after tumor removal. Conclusions Intraoperatively recorded CAPs are not reliable in predicting postoperative hearing outcomes. In contrast, ABRs are an indicator of function of the peripheral auditory pathway. Presence of waves I, III and V following tumor removal may represent preservation of useful hearing.

Electrode selection for hearing preservation in cochlear implantation:A review of the evidence

Objective:To review and assess the ideal length of electrode in cochlear implant patients for hearing preservation. Methods:The English language literature was reviewed for studies including hearing preserva-tion and speech understanding for electrodes of different lengths. Results:One prospective trial was found, and there were no studies that randomized patients into different length electrodes with an intent to preserve hearing. Eight studies total included multiple length electrodes and contained data regarding hearing preservation. Conclusions: Although there is some evidence that indicates that shorter electrodes may improve both short and long-term hearing preservation rates in cochlear implant patients, no study has directly compared implant length on hearing preservation in a similar patient pop-ulation. A randomized trial of short and standard length electrodes for hearing preservation is warranted. In the interim, utilization of current electrodes measuring 20e25 mm could seem to be a prudent approach when seeking to preserve residual hearing without unduly compro-mising cochlear coverage.

Significance of Vestibular Testing on Distinguishing the Nerve of Origin for Vestibular Schwannoma and Predicting the Preservation of Hearing

Background： Determining the nerve of origin for vestibular schwannoma （VS）, as a method for predicting hearing prognosis, has not been systematically considered. The vestibular test can be used to investigate the function of the superior vestibular nerve （SVN） and the inferior vestibular nerve （IVN）. This study aimed to preoperatively distinguish the nerve of origin for VS patients using the vestibular test, and determine if this correlated with hearing preservation. Methods： A total of 106 patients with unilateral VS were enrolled in this study prospectively. Each patient received a caloric test, vestibular-evoked myogenic potential （VEMP） test, and cochlear nerve function test （hearing） before the operation and 1 week, 3, and 6 months, postoperatively. All patients underwent surgical removal of the VS using the suboccipital approach. During the operation, the nerve of tumor origin （SVN or IVN） was identified by the surgeon. Tumor size was measured by preoperative magnetic resonance imaging. Results： The nerve of tumor origin could not be unequivocally identified in 38 patients （38/106, 35.80%）. These patients were not subsequently evaluated. In 26 patients （nine females, seventeen males）, tumors arose from the SVN and in 42 patients （18 females, 24 males）, tumors arose from the IVN. Comparing with tile nerve of origins （SVN and IVN） of tumors, the results of the caloric tests and VEMP tests were significantly different in tumors originating from the SVN and the IVN in our study. Hearing was preserved in 16 of 26 patients （61.54%） with SVN-originating tumors, whereas hearing was preserved in only seven of 42 patients （16.67%） with IVN-originating tumors. Conclusions： Our data suggest that caloric and VEMP tests might help to identify whether VS tumors originate from the SVN or IVN. These tests could also be used to evaluate the residual function of the nerves after surgery. Using this information, we might better predict the preservation of hearing for patients.

Electrocochleoeraphy in cochlear implantation:Development,applications,and future directions

Electrocochleography(ECochG)is an electrophysiological technique that records electrical potentials generated by different components of the inner ear and peripheral cochlear nerve in response to acoustic stimulation.ECochG responses can be analyzed into(1)cochlear microphonics(CM),(2)auditory nerve neurophonics,(3)summating potential,and(4)compound action potential.Over the past few decades,there have been ongoing refinements in technique and updates in the understanding of recorded potentials.Historically,ECochG found its main application in the diagnostic evaluation of Meniere^disease(MD).However,in the last decade,the focus has shifted towards cochlear implantation(Cl).In patients with residual hearing after Cl,combined electric and acoustic stimulation has resulted in improved hearing and speech outcomes.Despite efforts to mitigate trauma during electrode insertion,hearing preservation rates vary after surgery.During implantation,real-time ECochG offers an opportunity to measure frequency specific CMs elicited from a localized region in the cochlea as the surgeon inserts the electrode array.In extracochlear ECochG recordings,the recording electrode can be placed on the promontory,the stapes,or the tympanic membrane.Intracochlear ECochG can be performed by inserting a recording electrode into the cochlea or by using one of the Cl electrodes as the recording electrode.The loss of intraoperative ECochG signal may indicate cochlear trauma from electrode insertion,but the association between intraoperative ECochG changes and cochlear trauma remains controversial.The ability to monitor cochlear trauma during Cl electrode placement holds promise to improve hearing preservation outcomes,modify surgical techniques,and change electrode design.The goal of this review is to provide a comprehensive overview of the electrophysiology and history of ECochG,discuss its recent applications in Cl,and explore the ongoing research in this expanding field.