ABSTRACT
The use of Intraoperative brainstem auditory evoked potential monitoring [auditory brainstem response, ABR] has been shown to reduce the risk of hearing impairment during cerebello-pontine angle [CPA] surgeries that place the cochlear nerve at risk. Despite its wide use however, studies defining the changes in intraoperative ABR that correlates to hearing impairment are inconsistent. Loss of ABR wave V although a specific sign of postoeprative hearing impairment is not helpful for hearing preservation because its occurrence is a late indication of compromise of hearing. For increasing the efficacy of hearing preservation its would be more practical to rely on earlier warning criteria. To evaluate the value of wave III as an early warning sign for ABR changes and in predicting postoperative hearing outcome during surgery for microvascular decompression [MVD] procedures and vestibular neurectomies. The study was conducted on a total of 30 patients who underwent surgery for MVD procedures and vestibular neurectomies. All patients underwent pure-tone audiometery and speech discrimination immediately before and 10-12 days following surgery. Hearing was classified into four classes based on pure tone average [PTA] and speech discrimination scores. Hearing preservation was attempted in all surgeries using ABR. During the operation ABR was recorded continuously and the latencies and amplitudes of waves I, III and V were compared with those of the baseline recordings. The amplitude of the wave was measured between its peak and the following trough, the ratio of that amplitude value to that of the baseline recording was expressed as a percentage. Out of 30 patients, 21 [70%] had preserved postoperative hearing and 9 [30%] had reduced hearing postoperatively. ABR changes [latency and/or amplitude] in wave III and V occurred in 25 [83.3%] of cases. Of those 25 cases, wave III changes preceded wave V changes in 14 cases [56%]. Delay of latencies of both waves III and V could significantly predict postoperative outcome [cutoff values: 0.6 and 0.62msec respectively]. Comparison between areas under the ROC curve [receiver operated characteristic curve] of latency delay of wave III and wave V in relation to postoperative hearing outcome revealed that although delay of latency of wave V had higher value of area under the curve [0.852] than delay of wave III latency [0.733], there was no statistical significant difference between both variables [p=0.240]. In addition, percent decrease of amplitude of both waves III and V could significantly predict hearing outcome [cutoff values 40% and 42.8% respectively]. Comparison between areas under the ROC curve of percent decrease of amplitude of waves III and wave V in relation to postoperative hearing outcome revealed that the area under the ROC curve of percent decrease of amplitude of wave III [0.929] was significantly higher than that of wave V [0.725] [p=0.028]. Further more, the study revealed that 0.4 msec delay of wave III amplitude was an early alert to more significant changes. Changes in wave III latency and amplitude can predict postoperative hearing outcome as efficient as wave V changes. Furthermore, a more gradual line of ABR changes should be adopted as warning criteria starting with 0.4 msec delay of wave III. A second level of warning at which the surgeon must be alerted includes any of the following single or in combination: a 0.6 msec delay or more of latency of either waves III or V, and more than 40% decrease amplitude of waves III and or V