Intraoperative Estimation of Hearing Classes Using Auditory Steady-State Response

Objective: For intraoperative monitoring of the cochlear nerve, brainstem auditory evoked responses (BAEP) have been used since many years. However, intraoperative BAEP yield only limited information about the expected hearing quality, for example, regarding language understanding. In contrast, the so-called Auditory Steady-State Response (ASSR) enables objective audiometry during anesthesia, allowing evaluation of a patient's hearing at any point of the surgical procedure. Clinical studies, which evaluate the application of ASSR during total intravenous anesthesia (TIVA), are sparse. The presented study thus evaluates the viability to estimate hearing classes intraoperatively using ASSR.

Methods: In 40 patients with vestibular schwannoma, ASSR was recorded at the beginning and end of the surgical procedure. ASSR tones with 5-minute duration, 80 dB (SPL), 90 Hz amplitude modulation, and different carrier frequencies were used (500, 1,000, and 2,000 Hz). The stimulation was performed monaurally and ipsilateral to the side of surgery. Evoked responses were recorded using a two-channel EEG (left/right mastoid vs. vertex) and evaluated using a frequency-based analysis. ASSR amplitudes and pre-/postoperative hearing classes were compared using receiver-operating characteristic (ROC) statistics.

Results: ASSR with 500, 1,000, and 2,000 Hz showed AUC (area under the curve) values of 0.67, 0.73, and 0.85 for differentiating normal hearing (Gardner-Robertson [GR] class 1) and 0.72, 0.77 and 0.83 for serviceable hearing (GR 1–2). Preserved hearing of any degree (GR 1–4) was detected with AUC values of 0.71, 0.77, and 0.78. A shortened stimulation time of 1 minute showed only little impact at 2,000 Hz (AUC 0.82 GR 1, 0.86 GR 1–2, and 0.87 GR 1–4). Integration of the different carrier frequencies by evaluation of the maximum amplitude yielded constantly high AUC values (0.82–0.85).

Conclusion: Intraoperative ASSR enables robust differentiation of difference hearing ranges. Best results were achieved using 2,000 Hz or the integration of all carrier frequencies.