Frequency-specific ASSR monitoring during anesthesia: Preliminary results

Introduction: During surgery in the cerebellopontine angle, e.g. for acoustic neuroma (AN), there is a risk of damaging the cochlear nerve. For intraoperative monitoring, different methods are applied since several years, e.g. auditory evoked potentials (AEP). However, these are not frequency-specific and thus only provide limited information about hearing quality. In contrast, the possibility to measure an audiogram intraoperatively could suggest preserved but qualitatively insufficient hearing and allow tailoring of surgery. Since the 80ies, it is known that the „Auditory Steady State Response“ (ASSR)^1,2 can be utilized to measure an audiogram objectively, mostly independent of vigilance when high-frequency stimuli are used. Investigations regarding ASSR during TIVA (total intravenous anesthesia) have been conducted only in animals³. The presented study now investigates the application of ASSR during TIVA in humans.

Material and Methods: During surgery of a female AN-patient with normal hearing, ASSR were recorded during TIVA using a 2-channel EEG montage (right/left mastoid vs. vertex). As stimuli, 1000Hz („carrier frequency“) sinus tones (1min duration, 90dBSPL) were used, of which amplitudes were modulated with 90, respectively 110Hz („envelope frequency“). Stimulation was applied monauraly to the left and to the right without masking, as well as binauraly. Frequency spectra were calculated from the respectively registered EEG data, evoked activity at 90/110Hz was evaluated.

Results: Spectra of EEG data showed clear increases at 90, respectively 110Hz. In comparison to EEG data without stimulation, signal-to-noise-ratios of up to 2.62 were obtained; response was maximal when binaural stimulation was applied. Clear responses could be recorded with as short as 15s of data.

Conclusions: Our results demonstrate, that 90/110Hz ASSR are in principle detectable in humans during TIVA. Even under conditions of the operating room with unavoidable noise, clear responses with good SNR could be obtained. In comparison to stimulation durations of e.g. AEP, clear responses were identifiable much quicker. Therefore, application of ASSR could enable intraoperative frequency-specific information and shorter result latencies. Further studies will investigate optimal parameters for intraoperative detection of qualitative hearing loss.

References: [1] Picton TW, John MS, Dimitrijevic A, Purcell D. (2003) Human auditory steady-state responses. Int J Audiol 42:177–219.

[2] Stapells DR, Linden D, Suffield JB, Hamel G, Picton TW. (1984) Human auditory steady state potentials. Ear Hear 5:105–113.

[3] Szalda K, Burkard R. (2005) The effects of nembutal anesthesia on the auditory steady-state response (ASSR) from the inferior colliculus and auditory cortex of the chinchilla. Hear Res 203:32–44.