Cochlear implantation (CI) following vestibular schwannoma (VS) resection was first reported by Arriaga and Mark in 1996 in a patient undergoing a translabyrinthine (TL) approach (Arriaga and Marks, 1995). The rationale of this procedure relies on preserving the neural architecture of the cochlear nerve at the time of the resection. The preserved nerve cannot provide the normal auditory signal to the patient, but is able to transit the electrical stimuli to the brainstem and upper auditory pathways providing different degrees of hearing to the patient. Most of the initial CI research focused on neurofibromatosis type 2 related schwannomatosis (NF-2-SWN) patients with bilateral VS. In these cases, CI appeared as an alternative to the auditory brainstem implant (ABI), which usually provides sound detection and lip-reading enhancement, open set discrimination only achieved in a small subset of cases (Dhanasingh and Hochmair, 2021a).

In recent years, there have been numerous reports of CI in patients with unilateral or bilateral VS, comprising new settings such as CI in observed and irradiated tumors (Dornhoffer et al., 2023; Smith et al., 2022; West et al., 2019; Young et al., 2023). Most patients have hearing sensation with the cochlear implant, many of them achieving closed-set or open-set discrimination. Due to the considerable variation in audiological outcomes and the unclear nature of prognostic factors, the advent of intraoperative electrophysiological testing has facilitated the decision-making process regarding the placement of cochlear implants in such cases.

Different methods have been described to evaluate the functionality of the cochlear nerve, mainly by analyzing subjective or objective responses elicited by intracochlear or extracochlear stimulation.

In the early days of CI, the promontory test was used as a preoperative prognostic factor. This subjective method consists of placing an electrode on the cochlear surface in an awake patient, who is capable of perceiving sound through the application of an electrical stimulus (Kuo and Gibson, 2002). Aside from its limited accuracy, it is unsuitable as an intraoperative test for making decisions regarding implantation in VS surgery.

In a recent study by Polterauer et al. (Polterauer et al., 2023), electrical auditory brainstem response (EABR) promontory stimulation was outlined in a pre-operative scenario. This involved employing a golf-club-shaped stimulator electrode positioned close to the round window (RW) under local anesthesia. To our knowledge this tool has not been reported in an intraoperative setting.

The established approach of intracochlear electric stimulation using a CI electrode coupled with the recording of EABR through surface electrodes, is a well-recognized method for assessing the functionality of the auditory pathways (Dhanasingh and Hochmair, 2021b). This method has been used following TL VS resection, providing information about the cochlear nerve status (Patel et al., 2020). The main limitation of using the actual cochlear implant to elicit EABR is that a negative electrical response may not prompt explantation even if no hearing sensation is obtained. Using a non-operational cochlear implant carries various radiological, economic, and satisfaction-related consequences.

In order to have a disposable intraoperative tool to assess the functionality of the cochlear nerve, an intracochlear electrode array was developed by MED-EL, Auditory Nerve Test System. Lassaletta et al. (Lassaletta et al., 2017) found no significant differences in latency or amplitude between ABR wave-forms generated between the MED-EL cochlear test electrode and the cochlear implant proving the viability of this technique. In January 2020, the intracochlear test electrode received CE marking with the product designation ANTS (Auditory Nerve Test System, MED-EL Corporation, Innsbruck, Austria), approved for use in the European Union (EU) and in nations recognizing CE marking. While the ANTS has been applied in various settings, including inner ear malformations, cochlear nerve deficiency, and different scenarios with VS patients, controversies persist regarding its utility, specificity, and sensitivity.

The purpose of this paper was to explore the experiences of a tertiary center with the application of the ANTS in various scenarios of patients with VS, who were candidates to receive a cochlear implant in recent years. A review of the literature about the usefulness of the ANTS to monitor the cochlear nerve especially focusing in patients with VS was also performed.