Continuous fast tracking of auditory threshold during middle ear surgery using the auditory steady state response (ASSR)

OBJECTIVE: To continuously track auditory threshold during middle ear surgery, using the auditory brainstem response as an indirect measure of cochlear sensitivity, thereby providing surgeons with timely feedback to help avoid iatrogenic hearing loss. APPROACH: We improved the signal-to-noise ratio (SNR) of the electrophysiological measurement of auditory brainstem responses to repetitive bone-conducted chirps (logarithmic up-chirps from 2kHz to 16kHz, with 5ms duration at 80/s). This was achieved using optimised electrode type and placement, improved bone conductor and bio-amplifier design, and a streamlined placement and retention of the electrodes and bone conductor with an over-the-head elastic strap. We masked the contralateral ear's response with wideband noise to ensure responses were from the operated ear only. Using custom-written software, we automatically adjusted chirp intensity to maintain the brainstem's auditory steady state response (ASSR) near a fixed amplitude ("the ASSR target"), set to an amplitude just above our system's measurement noise floor. The chirp intensity required to maintain this target amplitude was defined as the patient's "ASSR threshold". MAIN RESULTS: Our improvements allowed us to rapidly track hearing threshold continuously for the entire duration of middle ear surgery in fully anaesthetised patients, with the ASSR threshold initially within 10dB of psychoacoustic threshold just before surgery. The system can track threshold changes rapidly, but can be limited by background acoustic masking noise in the operating room. SIGNIFICANCE: Our system could provide surgeons with a rapid and sensitive display of cochlear sensitivity during middle ear procedures, and help identify and possibly avoid traumatic manipulations that cause iatrogenic hearing loss.