gms | German Medical Science

Objective: Monitoring of auditory brainstem responses (ABR) is a standard method during vestibular schwannoma surgery. As a farfield technique it has some limitations. The goal of this study is to evaluate, if additional extra-tympanic electrocochleography (ECochG) as a nearfield technique can ensure a continuous monitoring of the auditory function during surgery.

Method: Combined monitoring was performed in 58 patients (32 female, 26 male, median age 48) with unilateral vestibular schwannomas operated by the retrosigmoid approach between 2010 and 2013. Patient selection was based on individual strong interest in hearing conserving surgery and independent of tumor extension (small to brainstem compressive) or hearing quality. ECochG was recorded simultaneously to ABR using a ball electrode placed at the tympanum to enable visualization not only of component I, but also III and V in most cases. ABR and ECochG recordings were retrospectively analyzed, assessed by a scoring system (Class 1: normal ABR, 2: latency delay, 3: wave III lost, 4: only wave I or V present, 5: loss of all waves) and correlated with pre- and postoperative auditory function.

Results: Preoperative ABR and ECochG classes show a quite similar distribution and positive correlation with the preoperative hearing class. But postoperative ECochG was less reliable than ABR in correlation with postoperative hearing quality: 23 of 38 patients with postoperative deafness had a preserved cochlea potential in the ECochG (class 4). Analysis of ABR and ECochG Classes identifies a strong positive correlation (P<0.01, Kendall-Tau-b preop 0.834, postop 0.726) especially for Classes 1 to 3, where wave V is still present (P<0.01 Kendall-Tau-b preop 0.740, postop 1.000). Here both methods behave almost linearly and both are highly reliable in predicting postoperative function. Only in poorer ABR Classes stronger differences become apperant where either of both techniques may be less advantageous.

Conclusions: Recording of auditory function by ECochG may be applied beyond the control of cochlea function. Auditory monitoring in vestibular schwannoma surgery is as well possible by extra-tympanic ECochG as by conventional ABR, if component V can be registered. Due to the fact that ECochG provides larger and faster responses of all components, it is especially useful in technically difficult phases. Thus, the dual mode of monitoring by both techniques provides a significant advantage over the conventional stand-alone ABR monitoring.