gms | German Medical Science

Objective: In surgical resection of vestibular schwannomas preservation of hearing is still 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 study was the characterisation of the electro-physiological and auditory functions in the early post-operative phase and their relation to relevant intra-operative changes.

Methods: In a prospective study from April 2010 to November 2011 35 patients (mean age 47 years, 19 male, 16 female) with pre-operative hearing (up to 80 dB hearing loss, > 10% speech discrimination) were investigated by continuous intra-operative and by intermittent post-operative ABR monitoring at defined intervals at the day of surgery and on first, second and fifth day thereafter. ABR quality was graduated according to presence of components I, III, V and patients' postsurgical ABR development was classified into Class A (stable recordings), Class B (intermittent fluctuations with return to baseline) and Class C (repeated fluctuations with permanent change of ABR quality). These data were correlated with intra-operative findings and post-operative hearing outcome.

Results: In 13 out of 35 patients (37%) auditory function was preserved 7 days postoperatively and in the long-term. In 21 patients post-operative ABR monitoring was indicated and 7 patients (33%) belonged to Class A, 6 (29%) to Class B and 8 patients (38%) to Class C. In cases of critical ABR changes (Classes B and C), those started 2 to 4 hours after surgery were correlated with severe intra-operative fluctuation and delayed ABR recovery. Most critical surgical periods were first 30% and last 30% of tumour reduction, intrameatal tumour dissection and meatus drilling. Local and systemic application of vasodilatative and hemodynamic agents seemed to improve recovery in certain conditions. In Class C patients ABR development to complete loss as well as recovery to good ABR quality could be observed in single cases up to day five with correct correlation to hearing function.

Conclusions: Different to previous assumptions this study documents that so-called false-positive ABR may constitute a temporary correct recording. At the end of surgery the definite status of the auditory pathway and of the ABR are not yet reached but both undergo further changes that may lead to recovery or degeneration. The critical post-surgical phase lasts up to five days and is useful to be monitored for ongoing changes.