In most cases, whether hearing loss was caused by the vestibular schwannoma itself or by an intervention, wearing a hearing aid is the easiest way to compensate for it [17]. However, the effect of hearing compensation by a hearing aid on vestibular schwannomas is limited because the hearing loss caused by vestibular schwannomas is a retro-cochlear type sensory neural hearing loss. Moreover, compensation with a hearing aid is less effective in patients with severe or profound hearing loss.

Another way to compensate for vestibular schwannomas in patients with severe-to-profound hearing loss is surgical intervention with tumor resection and simultaneous implantation of implantable auditory devices. Thus, among the available hearing devices, ABI and CI have been used in such situations.

Compared with CI, ABI has a unique advantage in vestibular schwannoma cases: ABI can bypass the cochlear nerve and restore hearing. Therefore, preservation of the cochlear nerve during tumor resection in patients with vestibular schwannomas is unnecessary. However, the hearing quality of the ABI is generally limited compared to that of the CI [18, 19] because it is difficult for the ABI to reproduce the audiological tonotopy of the sound that the CI can reproduce. CI is another possible hearing compensation method for patients with vestibular schwannomas who require resection. It can also be used in patients with severe or profound hearing loss caused by vestibular schwannomas. While it has the advantage of regaining a relatively high performance compared to the ABI, cochlear nerve preservation during surgery is essential for successful implantation of CIs with vestibular schwannoma surgery.

In patients with severe bilateral hearing loss accompanying vestibular schwannoma, ipsilateral CI is indicated at the same time as the removal of the vestibular schwannoma because non-concomitant CI surgery would be more difficult with intracochlear degeneration and fibrosis induced by surgical stimulation [20]. However, the need to preserve the cochlear nerve during removal of the vestibular schwannoma is a barrier to the simultaneous implantation of CIs. In cases of severe hearing loss on the operative side, the ABR response to the usual click sound stimulation is often lost or diminished. Despite the reliable preservation of inner ear nerve function, intraoperative cochlear nerve monitoring is highly challenging.

In the present study, we developed a novel monitoring method in addition to conventional intraoperative ABR [11, 12]. This method combines the ANTS (MED-EL) with DNAP monitoring. In this system, ANTS, an intracochlear electrical stimulation system using electrodes inserted into the cochlea was used, instead of sound stimulation, to perform continuous intraoperative monitoring of EABR and E-DNAP. Thus, the DNAP electrode can continuously evaluate action potentials by placing an electrode near the dorsal cochlear nerve nucleus and performing intracochlear electrical stimulation using an electrode inserted into the cochlea, instead of sound stimulation.

In this study, reliable E-DNAP was developed with relatively slow 10.1 Hz stimulation, while it has been reported that vestibular schwannoma causes the deterioration of evoking the action potential with high-rate stimulation [21, 22]. Under these conditions, we obtained reliable and continuous ANTS-mediated E-DNAP every two seconds (10.1 Hz stimulation, averaging 20 times), making real-time monitoring during tumor resection possible. The developed E-DNAP was quantitatively measured during surgery by comparing the initial amplitude prior to tumor resection. Moreover, it is compatible with FREMAP monitoring without any electrical contamination.

We report a case in which an auditory nerve tumor was removed using a translabyrinthine approach. Ipsilateral implantation of CI was performed using this monitoring system and a good postoperative CI effect was obtained. This is thought to expand the range of possibilities for simultaneous cochlear implantation with the removal of vestibular schwannomas. Moreover, the possibility of ipsilateral CI and modification of the approach to cochlear nerve preservation in vestibular schwannoma surgery in patients with severe hearing loss may expand if the cochlear nerve can be preserved with high accuracy.

This report demonstrates the successful use of a combination of DNAP monitoring and ANTS. However, the system has several limitations. First, this system requires the insertion of ANTS stimulation electrodes into the cochlea; therefore, this system would not be applicable to patients with intracochlear vestibular schwannomas. Second, the approach involves placing DNAP electrodes on the brainstem; thus, a vestibular schwannoma with a large cistern portion of > 30 mm is not preferable. Finally, cases with relatively large vestibular schwannomas that should be approached other than the translabyrinthine approach are not suitable. Therefore, the present system is more suitable for vestibular schwannoma cases with CI application, with spared intracochlear spaces for electrode insertion, and a relatively small cistern portion of the vestibular schwannoma (< 30 mm) that can be removed using the translabyrinthine approach.

Another limitation of this study is that only a single case could be included because of the limitation of the national insurance of application for cochlea implantation; principally, only bilateral severe hearing loss cases are covered in Japan, and no cochlear implantation is permitted for single-sided hearing loss, which is the most of the vestibular schwannoma cases. Therefore, the findings of this study should be replicated in other patients and at other centers in future studies.