gms | German Medical Science

Objective: In surgical resection of vestibular schwannomas, preservation of hearing is far less often achieved than facial nerve function. Even in preserved auditory brainstem responses (ABR) at the end of surgery, subsequent auditory function is not guaranteed. Goal of the present study was to find implications for the surgical strategy by analysis of intra- and early post-operative ABR.

Method: In a prospective study, 46 patients (median age 47, 27 female, 22 male) with some pre-operative hearing (up to 80 dB hearing loos, >10% speech discrimination) and all tumor sizes (intra-extrameatal to brainstem compressive) were investigated by continuous intraoperative and intermittent postoperative ABR monitoring at defined intervals up to the fifth day after surgery. ABR quality was graduated according the Hannover classification (Class 1: normal ABR, Class 2: latency delay, Class 3: wave III is lost, Class 4: only wave I or V present, Class 5: loss of all waves) and analyzed for changes due to specific surgical phases and for their stability or variability.

Results: At the end of surgery, there was a complete ABR loss, Class 5, in 16 patients while 30 had some ABR preserved, either stable (17 cases) or unstable (13 cases). In 30 patients investigated by postoperative ABR, there were 22 with further temporary ABR quality changes during the first five days. Those patients with post-operative unstable ABR had shown intra-operative ABR instability as well. Most critical phases for ABR instability and/or loss were the opening of the internal auditory canal, the intrameatal tumor dissection and the final tumor dissection at the extrameatal region. The chance for ABR recovery and permanent preservation after a phase of instability was significantly higher in tumors without brainstem contact and/ or compression.

Conclusions: While it is well known that intra-operative ABR loss usually leads to deafness, this study shows that intraoperative critical ABR developments often continue in the early postoperative phase. Severe intra-operative fluctuations have to be avoided, as thereafter ABR preservation may be only temporary with permanent loss later on. As a consequence, microsurgical resection during the last phase at the intra- and extra-meatal nerve segments must be performed with utmost care, fast and reliable ABR monitoring and with variation of the surgical active sites depending on the monitoring findings.