gms | German Medical Science

Objective: For intraoperative monitoring of cochlear nerve nerve function in vestibular schwannoma surgery, auditory evoked potentials (BAEP) have been the method of choice for many years. However, BAEP provide only limited information regarding hearing quality. In contrast, intraoperative audiometry can detect the remaining but qualitatively insufficient hearing and enable adaption of the surgical procedure. The “Auditory Steady State Response” (ASSR), provides an objective audiogram. Clinical studies investigating ASSR under total intravenous anesthesia (TIVA) have not been performed. The presented study aims to evaluate the detectability of ASSR under TIVA in humans.

Methods: During craniotomy in 7 normal hearing patients under TIVA, the ASSR was recorded with a two channel EEG construction (right/left mastoid vs. vertex). As auditory stimuli, 1 minute long 1000Hz sinus tones were used (60, 70, and 80dBSPL) which were amplitude-modulated with either 40, 90 or 110Hz. Stimulation was applied monauraly to the left and to the right ear without masking of the contralateral ear, as well as binauraly. Frequency spectra were calculated from the evoked EEG. ASSR responses at 40/90/110Hz were evaluated and compared to neighboring frequency ranges using the statistical procedure described by John et al, 2001.

Results: In all patients, statistically significant ASSR responses (p<0.05; test according to John et al., 2001) could be observed at 90 and 110Hz, however not at 40Hz. Sound pressure level of the stimuli were significantly correlated to the ASSR response amplitude (left side: p<0.05, r=0.48; right side: p<0.01, r=0.52; pearson correlation). Furthermore, laterality of responses correlated strongly with the side of stimulation (left, right, binaural; p<0.0001, r=0.67; spearman's rho).

Conclusions: Our preliminary results show that 90/110Hz ASSR responses which are present in humans under TIVA are in line with results from previous studies in awake patients. Further work on frequency specificity and comparison to extraoperative pure tone audiometry may enable application of ASSR for intraoperative qualitative hearing monitoring of the cochlear nerve.