1. IntroductionSurgical procedures for vestibular schwannoma (VS) commonly involve three approaches: retrosigmoid, middle cranial fossa, and translabyrinthine approaches. The first two approaches have been used routinely over the last 50 years. These have the potential to preserve hearing, and their indications and effectiveness are well known [1]. In recent years, the introduction of endoscopic ear surgery has led to improved anatomical knowledge from the external auditory canal (EAC) to the internal auditory canal (IAC) [2]. This allowed the development of transcanal transpromontorial surgery, in which the EAC is used as a natural corridor to remove VS involving the IAC with or without minimal extension to the cerebellopontine angle (CPA).In 2013, an exclusive endoscopic transcanal transpromontorial approach was proposed, which over the years was proven to be successful in the removal of small VS (stage I–II according to Koos classification), attributed to its better optics and direct visualisation of all the structures in the IAC and CPA [3,4]. After this encouraging experience, the transcanal corridor was used frequently, enlarging the surgical window to the CPA and extending the indications to VS of Koos stage II–III (expanded transcanal transpromontorial approach) [5,6,7,8]. Based on the low morbidity rate and good facial nerve preservation, this approach was described as very promising but not recommended if patients required a simultaneous cochlear implant (CI) placement, as proposed in the translabyrinthine approach. Marchioni and colleagues reported that in Koos stage I and II VS, complete removal of the cochlea is not necessary; rather, it is possible to partially preserve the basal turn of the cochlea and completely preserve the middle and apical turns [9]. The preservation of these anatomical structures, therefore, led to the description of the transcanal infrapromontorial approach, which allows fitting a CI to restore auditory function, especially in those patients in whom the function of the contralateral ear is hearing-impaired. This innovative approach is an excellent alternative for patients with small intracanalicular VS with limited extension (≤5 mm) into the CPA, where there is the intention to fit a CI on the same side [10].

The purpose of this study is to present our preliminary experience and to evaluate the audiologic results of the transcanal infrapromontorial approach for small VS with simultaneous placement of a CI.

3. Results

In the Otolaryngology Departments of Modena and Verona hospitals between 2015 and 2021, a total of 92 VS removals were performed using a transcanal approach: 53 transcanal transpromontorial, 32 expanded transcanal transpromontorial and 7 transcanal infrapromontorial approaches. Among those who underwent the last approach listed above, only in three cases (one in Modena and two in Verona) was it possible to preserve the cochlear nerve and therefore perform CI placement at the same time.

All were suffering from sporadic left-sided intracanalicular VS (Koos stage I) with a mean size of 12.33 mm (S.D. ± 0.58 mm). None had a facial dysfunction, and in all of them, the onset was characterised by progressive ipsilateral hearing loss coupled with tinnitus.

In patient No. 3, the symptoms occurred at the same time, whereas in patient No. 2, the tinnitus occurred two years before. In contrast, patient No. 1 initially presented with hearing loss followed by the onset of tinnitus and dizziness two and three years later, respectively. According to the Committee on Hearing and Equilibrium of the American Academy of Otolaryngology-Head and Neck Surgery (AAO-HNS), all participants presented a class C residual hearing function on the affected side [18]. On the normal side, hearing function was class B in patient No. 3 and class A in other patients. Due to the development of dizziness, in addition, patient No. 1 underwent otoneurological examinations with bithermal caloric tests according to Fitzgerald and Hallpike’s protocol. Parameters that were evaluated were the reduced vestibular response (RVR) and the directional preponderance (DP) using Jongkees’ formula and the peak of slow-phase velocity (SPV) [19]. Patients presented a slight bilateral hypofunction with peak SPV smaller on the left side. Other details are reported in Table 1.All participants were candidates for surgery according to contrast-enhanced MRI (Figure 2A,B). In two cases, surgery followed a short wait-and-scan period and was due to evidence of rapid growth, while in patient No. 2, surgery was scheduled at the first documentation of the lesion. A review of the surgical videos showed that, in all cases, the cochlear nerve was anatomically preserved and the facial nerve was amenable to proper stimulation. Nevertheless, in patient No. 1, the vascularity of the surface of the cochlear nerve did not appear to be maintained; rather, the nerve appeared swollen and prone to bleeding. A clear wave V was identified in eABRs in all cases; hence, a state-of-the-art CI device was placed with the array gently positioned into the scala tympani. Patient No. 1 and patient No. 3 received a MED-EL® Synchrony 2 Flex28 (MED-EL Medical Electronics, Innsbruck, Austria), while patient No. 2 received an Advanced Bionics® HiRes Ultra 3D HiFocus Mid-Scala (Sonova Holding AG, Stäfa, Switzerland). Therefore, Maestro 9.0 and SoundWave Professional Fitting software were used to perform intraoperative electrophysiological measurements, respectively. For all patients, impedances were in the normal range, except for electrode 6 of patient No. 2, whose values were consistent with an OC. ECAP was observed in patient No. 1 from electrodes 1 to 8, in patient No. 2 on all electrodes tested and, finally, in patient No. 3 on 10 of the total 12 electrodes. On the postoperative CT scans, patients No. 2 and No. 3 presented with regular surgical findings. In patient No. 1, a hypodense areola was observed at the level of the superolateral side of the left cerebellar hemisphere with a density similar to CSF, consistent with a suspected focal dilatation of the subarachnoid space (Figure 2D). This finding was unchanged on the follow-up CT scan (Figure 2E). In all three patients, the postoperative CT scan confirmed correct electrode placement. Other postoperative details are reported in Table 2.

For all patients, CI activation was performed one month after surgery. None of them presented residual hearing function in the surgically treated ear. Impedances were stable for patient No. 1, decreased for patient No. 2 and increased for patient No. 3. ECAP was absent along the entire array in patient No. 2 and preserved on three and ten electrodes in patients No. 1 and No. 3, respectively.

In clinical terms, patient No. 1 complained only of residual dizziness, while for all patients, the level of facial dysfunction was improved. In detail, patient No. 1 presented a regional facial dysfunction of the marginalis nerve (I HB; 86/100 SGS), while patient No. 2 and patient No. 3 presented II and III HB facial dysfunction, respectively. In contrast, during the audiological follow-up, the course of the three participants was very different.

Patient No. 1 used the CI continuously for the first three months for about 7 h per day; however, the CI was never able to evoke real sound perceptions. Impedances gradually increased up to the first month and then remained stable, whereas ECAP was detectable but of low amplitude only on electrodes 4, 8 and 11.

Patient No. 2 discontinued the follow-up due to worsening comorbidities and never used CI continuously. At the fitting session six months after surgery, impedances were slightly increased, while ECAP was absent along the entire array.

Patient No. 3 used the CI continuously for about 14 h per day. Impedances remained stable while ECAP was evident on 11 of the total 12 electrodes. At the audiometric evaluations, the tone threshold was present even if the threshold of verbal intelligibility was not reached (pure tone average at 500, 1000, 2000 and 4000 Hz around 45 dB and 50% signal recognition threshold reached at 30 dB; class B according to AAO-HNS). Impedances, after a slight increase, with values above the range only on some middle and basal electrodes, decreased and remained stable. Only for electrode 10, values compatible with OC were observed in the last fitting session, and it was, therefore, shut down out of caution. The impedance trend in the three participants is illustrated in Figure 3.

Patient No. 3 was the only one to complete the 24-month follow-up, while patient No. 1 and patient No. 2 were lost after six and nine months, respectively. At the last fitting session, 36 months after surgery, patient No. 3 presented a stable auditory and clinical situation with facial dysfunction III HB and residual hearing function class B, while impedances remained stable and ECAP was evident on all 11 active electrodes. At the last visit, patient No. 1 was stable with no facial dysfunction (I HB; 100/100 SGS) or dizziness. Currently, respective departments continue to follow patient No. 1 and patient No. 3 with periodical clinical and audiological visits.

4. DiscussionSeveral studies over the last ten years reported that cochlear implantation could safely be performed simultaneously with VS resection if the cochlear nerve was anatomically preserved and no overly aggressive manoeuvres were applied to remove the tumour. In addition to nerve preservation, audiological outcomes depended on tumour size in CPA and residual hearing function on the affected side [20,21]. Although a long-standing debate, an additional role may be played by a better contralateral hearing ear. In fact, good preoperative contralateral hearing status seemed to be a negative prognostic factor for CI performance on open-set discrimination, since a 28-fold higher risk of nonperformance with CI at 24-month follow-up was observed in the case of preoperative hearing classes A or B on the contralateral side of CI placement [22]. In contrast, in the case of normal contralateral hearing, satisfactory results were observed in terms of binaural benefits, such as sound localisation and improved speech perception, when the signal and noise came from different positions (squelch effect). Despite the slight improvement in tone and speech perception in these patients, the speech-in-noise perception and the subjective auditory benefit appear satisfactory [23]. Therefore, the good preintervention contralateral hearing function of the three participants should not be considered a limitation (Table 1).Several approaches of surgery for VS were proposed, among which the transcanal infrapromontorial approach was proposed as an excellent alternative for patients with small intracanalicular VS with limited extension into the CPA. Given the removal of only the most posterior portion of the cochlear basal turn, CI could represent a feasible option [10]. In this study, a case series of three consecutive patients who underwent cochlear implantation after transcanal infrapromontorial resection of VS at the Otolaryngology units of Verona and Modena hospitals was reported. All were suffering from sporadic left intracanalicular Koos stage I VS, with a mean size of 12.33 mm (S.D. ± 0.58 mm). All received a state-of-the-art CI device, but the outcomes of the three participants were different, since only patient No. 3 demonstrated an auditory benefit. Intraoperatively, although the cochlear nerve appeared anatomically preserved and an eABR was present for all patients, patient No. 1 had a nerve that was swollen and prone to bleeding, with reduced vascularity on the surface of the cochlear nerve. In fact, visualisation of cochlear nerve continuity after tumour dissection is definitely the main factor in assessing whether to proceed with cochlear implantation [24]. Recently, it was observed that the integrity of nerve fibres along the entire course of the nerve, the whitish colour of the nerve after saline washing and the presence of vascularity on its surface could be predictors of a good outcome in more than half the patients undergoing CI after VS resection [25]. In addition to the aforementioned factors, involvement of the IAC fundus and tumour dissection technique (en bloc or piecemeal excision) have been observed as predictors of hearing outcomes [24].Only patients No. 1 and No. 3 adequately and continuously used CI after activation, since patient No. 2 was not interested in using the CI and continuing follow-up due to worsening concomitant tumour disease, as well as no perceived benefit. Nevertheless, electrophysiological measures changed consensually throughout the course in all participants. A negative association between impedance and residual hearing was already reported in the literature. This could be due to a traumatic insertion of the array, an inflammatory response resulting in scar tissue formation and the influence of the array on the mechanical properties of the cochlea and residual hearing thresholds. Wimmer and colleagues observed higher impedances measured one month after surgery and their gradual decrement over time. Electrode contacts inserted more apically exhibited higher impedances, but basal electrodes maintained higher impedance for longer [26]. A similar trend was recorded in this study in patients No. 1 and No. 2 (Figure 3). In particular, two events are significant: the progressive increase in impedances in patient No. 1 and some values beyond the normal ranges for the basal electrodes in patient No. 3. The former was not uncommon in the postoperative period and already described as related to inner ear events leading to a loss of residual hearing and dizziness [26]. The second could be related to possible dilatation or scar tissue formation due to the removal of the posterior part of the cochlear basal turn. Finally, although its correlation with future implant performance remains uncertain, ECAP was absent since the activation session on the entire array for patient No. 2, present in three electrodes for patient No. 1 and present on 10–11 of the total 12 electrodes for patient No. 3, concurrently with the hearing outcome.The involvement of a multidisciplinary team and following participants over quite a long period were the strengths of this study. In fact, while patients No. 1 and No. 2 experienced no benefit and were followed up until the device was used, patient No. 3 was followed up until 36 months after surgery. Otherwise, this case series presented several limitations, primarily the nonassessment of the self-perceived benefit of quality of life and auditory function. While its validation in the Italian language by our group is in press, a disease-specific index for quality-of-life assessment in VS patients exists and was already used [27]. A plethora of instruments assessing self-perceived speech perception, hearing disability and hearing quality are available. Moreover, although reliable tests assessing speech-in-noise perception as well as sound localisation and binaural cues were already used in clinical settings, assessment of these important aspects was omitted. Other considerable limitations were the lack of adequate evaluation of tinnitus and vestibular outcomes. The former appeared to have a considerable impact on VS patients, although a significant improvement in tinnitus after cochlear implantation was recently reported [28]. The latter is an issue often omitted in assessing such patients.Finally, the small No. of patients may represent a methodological limitation, but it was consistent with the fact that this was a novel surgical technique indicated in limited cases, as demonstrated by there being only seven patients who underwent this procedure between 2015 and 2021 in the Otolaryngology units of Modena and Verona hospitals. Based on these considerations, after discussing VS patients in multidisciplinary tumour boards, when surgery is indicated, it should be performed in a high-volume centre and by an experienced surgeon because surgical experience influences outcomes [29]. 5. Conclusions

Cochlear implantation following VS resection through the transcanal infrapromontorial approach is a reliable alternative in patients with intracanal Koos I stage VS, and good contralateral hearing does not represent a contraindication to operation. The decision whether to proceed with the implantation should be weighed considering the endoscopic features of the cochlear nerve such as the integrity of nerve fibres along its entire course, its whitish colour after saline washing and the presence of vascularity on its surface. Subsequently, telemetric electrophysiological measurements may represent a valid predictor of the outcome of auditory function during the follow-up. Their assessment is easy and quickly performed, does not require the active participation of the patient and is independent of middle ear status.

However, not much information is available regarding outcomes such as speech-in-noise perception, sound localisation, binaural cues, self-perceived quality of life, vestibular outcomes and the burden of tinnitus after the intervention.

Given the limited indications and its recent introduction to the surgical portfolio, cochlear implantation surgery following VS resection through the transcanal infrapromontorial approach should be reserved for high-volume centres and experienced surgeons.