Cochlear Implant Insertion Routes and Intra-operative Electrophysiological Measurements: A Retrospective Analysis at a Tertiary Care Centre

Abstract

Background: Cochlear implant surgeries are performed by different surgical routes, Round window (RW), Extended round window (Ext RW) and Cochleostomy (C). Optimum intracochlear electrode placement is important to achieve a successful outcome. Intra-operative electrophysiological testing (Impedance and ECAP) is crucial to assess the device function and appropriate electrode placement. The variability of portal of electrode insertion might affect the neural response and its characteristics. Objective: To compare the intraoperative electrophysiological measurements (Impedance and ECAP) for possible differences on auditory nerve stimulation across the three electrode insertion routes. Materials and Methods: This is a retrospective data analysis of 47 cochlear implant recipients at a tertiary care centre in Central India over a period of 4 years. They were broadly divided into two groups depending on the electrode design as full banded and half banded. Intraoperative impedances were recorded for different modes of stimulation and ECAP measured at E5, E10, E15, E20 electrodes in both the groups across the three insertion routes. Results: In the half-banded group, the impedance values in different modes CG, MP1, MP2, MP1 + 2 did not differ significantly among the surgical routes at all four electrodes. While in the full banded group, for CG mode impedance values at E5 differ significantly. Rest for other modes in full banded group, there was no statistically significant difference across the three routes. Conclusion: The present study supports that there is no statistically significant difference in the intraoperative impedance and ECAP measurements in both, the full-banded and half- banded electrodes across the three surgical routes implying that, all the three surgical approaches provide equal stimulation of the auditory nerve. The CI surgeon can select the electrode insertion portal based on the surgical anatomy, the implant type and individual preferences.

Introduction

Cochlear implantation (CI) has now become a popular procedure in the treatment of sensorineural deafness. For individuals with severe to profound sensorineural hearing losses due to disease or damage to the inner ear, acoustic stimulation via hearing aids may not provide sufficient information for adequate speech perception. In such cases direct stimulation of auditory nerve by surgically implanted electrodes has been beneficial in restoring useful hearing. The cochlear implant improves hearing quality as well as speech perception and production, resulting in increasing quality-of-life advantageous in several areas, notably self-sufficiency and in socializing [1–4]. The implant transforms the sound signal into electrical stimulation, which is subsequently used to activate auditory nerve fibres, resulting in hearing [5, 6]. Patient results are influenced by a variety of parameters, including the cause of deafness, age at implantation, duration of deafness, degree of inner ear malformation and coding method [7–12]. The survival of enough neural structures in the cochlear nerve is essential for the transfer of electric inputs to the cerebral cortex. With the expansion of the candidacy criteria, there is more emphasis on preserving residual hearing and achieve maximum stimulation of the residual dendrites and spiral ganglion cells.

The standard trans-mastoid facial recess approach is widely used for cochlear implantation. There are various routes of electrode insertion, viz. round window, Extended round window and Cochleostomy. There is considerable debate amongst surgeons as to which route is superior. Cochleostomy allows for a broader space for electrode insertion and eliminates the hook region of the basal turn. Adunka et al. [13], Eshraghi [14] and colleagues have addressed prior techniques of cochlear implantation employing the RW insertion approach. To preserve residual hearing, an electrode insertion should be as non-traumatic as feasible to minimize injury to functioning cochlear structures of the inner ear and subsequent neural tissue degeneration [14]. With the introduction of new electrode designs and a greater emphasis on residual hearing preservation, there has been increased interest in using the round window as a gateway for electrode insertion [13, 15, 16]. When compared to the standard promontory cochleostomy, round window insertion requires substantially less drilling for electrode placement, lowering the risk of acoustic trauma [17] and minimizing perilymph loss and bone dust entry into the scala tympani. This study was undertaken to identify the possible effect of variability of portal of electrode insertion on the neural response and its characteristics.

Materials and Methods

This retrospective study was carried out at a tertiary care centre from 2018 to 2021(4 years). Data records of 47 recipients were retrieved. Of the 47 participants, 24 patients received full banded electrode while in 23 patients, half banded electrode was used. All the surgeries were performed by the standard posterior tympanotomy approach. Three routes of implant insertion were selected for optimal intracochlear electrode placement based on round window visibility from the posterior tympanotomy window, Round window (RW), Extended round window (ExtRW) and Cochleostomy (C). All the patients were implanted with straight electrode array of the same company and were operated by the same surgeon. The multichannel cochlear implants used in this study had 22 electrodes placed in the cochlea. The electrodes are numbered from one to twenty-two, first being the most basal and 22nd electrode placed more apically.

The Surgical Technique

1. Anesthesia: General anesthesia was administered in all patients.

2. Retroauricular Incision: A small incision, approximately three centimeters long, was made behind the ear (retroauricular). This incision provided access to the surgical site.

3. Dissection of subcutaneous tissue and muscle : To reach the bone beneath.

4. Posterosuperiorly based musculoperiosteal flap to cover the receiver-stimulator unit. Drilling the well and channel for the receiver stimulator package.

5. Mastoidectomy.

6. Posterior Tympanotomy.

7. Depending on the round window membrane visibility and the RWM angulation cochleostomy was performed.

8. Insert Electrode Array: The electrode array was carefully inserted into the cochlea through one of the three routes, RW, ExtRW or Cochleostomy.

9. After inserting the electrode array, impedance and ECAP’s were measured to assess the device function and ensure that it is stimulating the auditory nerve adequately.

10. The position of the inserted electrode array was confirmed with X-ray image taken via C-arm machine intraoperatively.

11. Closure of muscle and skin flap using 3 − 0 vicryl was done.

All the implanted patients were kept on intravenous antibiotics and had a compression mastoid dressing for 3 days. They were discharged after removing the dressing and reconfirming the implant position by taking X-ray mastoid stenver’s view. The device was activated 2–3 weeks postoperatively when the surgical wound had healed completely.

Intraoperative Electrophysiological Testing

Impedance and neural response telemetry are measured using company specific software during cochlear implant surgery. The implanted device can send functional information to the speech processor and software for clinical interpretation. Impedance is a function of the array itself and measures the resistance offered by the fluid or tissue surrounding each electrode. Neural response telemetry involves sending electrical pulses and recording the neural response to confirm that the implant is stimulating the auditory nerve appropriately. The software displays impedance and neural response data in real-time, enabling surgeons to fine tune electrode placement and settings for optimal hearing outcomes. This feedback helps ensure the effectiveness and safety of the cochlear implant.

Intraoperative impedance and ECAP measurements were recorded at E5, E10, E15, E20 electrodes in both the groups across the three insertion routes- round window, extended round window and cochleostomy.

Results

The patients submitted to cochlear implantation were aged between 1 and 5 years and had a mean age of 3.56 ± 1.13 years. Of the 47 participants, 24 patients received full banded electrode while in 23, half banded electrode was used. Depending on the surgical anatomy and degree of round window membrane visibility the appropriate electrode insertion route was selected. In 32% cases RW approach was selected as the electrode insertion portal while in 27.6% Ext RW and in 40.4% cochleostomy was done (Fig. 1).

Fig. 1.

Distribution as per route of electrode insertion

Impedance Measurement

In the full banded group(N = 24), for the CG mode, impedance values at electrodes E10, E15 and E20 did not differ significantly across round window, extended round window and cochleostomy routes. However statistically significant difference was noted for the parameter at E5 electrode with P = 0.047 (Significant). For the MP1, MP2, MP1 + 2 mode the impedance values at E5, E10, E15, E20 did not show any significant difference amongst the three surgical routes (Fig. 2).

Fig. 2.

Impedance values (kOhm) at various electrodes across the three insertion routes for Full banded electrode (N = 24)

In the half-banded group(N = 23), the impedance values in different modes CG, MP1, MP2, MP1 + 2 did not differ significantly across the surgical routes at all four electrodes (Fig. 3).

Fig. 3.

Impedance values at various electrodes across three insertion routes for Half banded electrode (N = 23)

Neural Response Telemetry (NRT)

The mean NRT measurement values at the four electrode contacts across the different surgical portals were statistically non-significant in both the groups (Tables 1 and 2).

Table 1.

Intraoperative neural response telemetry in full banded (CI 24 Rest) electrodes (N = 24)

Insertion route	E5 Mean ± SD(CL)	E10 Mean ± SD(CL)	E15 Mean ± SD(CL)	E20 Mean ± SD(CL)
Round window	209.67 ± 10.97	202 ± 5.2	181.67 ± 26.86	184.33 ± 10.02
Extended round window	188.25 ± 13.66	189.63 ± 14.75	189.25 ± 19.43	179.75 ± 32.47
Cochleostomy	198.64 ± 14.34	195.67 ± 14.34	187.33 ± 12.78	172.9 ± 14.78
F value	3.11	1.02	0.22	0.41
P value	0.065, NS	0.375, NS	0.806, NS	0.669, NS

One way ANOVA: S-Significant, NS-Nonsignificant

Table 2.

Intraoperative neural response telemetry in half banded (CI 422) electrodes (N = 23)

Insertion route	E5 Mean ± SD(CL)	E10 Mean ± SD(CL)	E15 Mean ± SD(CL)	E20 Mean ± SD(CL)
Round window	185.67 ± 14.99	188.67 ± 12.01	171.50 ± 13.51	171.67 ± 12.25
Extended round window	201.25 ± 21.06	195.50 ± 8.54	178.75 ± 13.30	180.25 ± 15.26
Cochleostomy	192.29 ± 15.07	191.83 ± 14.96	179.29 ± 20.69	167.71 ± 14.74
F value	1.75	0.42	0.71	0.89
P value	0.197, NS	0.662, NS	0.504, NS	0.422, NS

One way ANOVA: S-Significant, NS-Nonsignificant

One way ANOVA test was applied. The data analyzed showed that there is no statistically significant difference in the intraoperative impedance and ECAP measurements in both the full banded and half banded multichannel electrodes across the different surgical routes implying that, all the three surgical approaches provide equal stimulation of the auditory nerve.

Discussion

There are various routes of electrode insertion in cochlear implantation, RW, ExtRW and Cochleostomy. With the increased interest in preserving residual hearing and minimizing intracochlear trauma during cochlear implantation, increased attention must be paid to the cochleostomy site. There is considerable debate amongst surgeons as to which route is superior.

With the integration of electrical and acoustic stimulation in cochlear implantation for patients with remaining hearing ability, preserving structural tissue and auditory function has gained great significance [13]. The decline in residual hearing is a multifaceted outcome influenced by factors such as the cochleostomy approach employed, the neuronal response to electrode stimulation and the precise location of the cochleostomy site [18]. In recent years, with the advent of new electrodes and increased emphasis given to residual hearing, there is preference of RW approach over cochleostomy route to achieve atraumatic electrode insertion and preserve residual hearing. Compared to cochleostomy via the promontory, the placement through the round window notably diminishes the incidence of perforation events during electrode insertion, thereby mitigating the substantial risks associated with trauma and perilymph loss, while also reducing the likelihood of bone powder entering the tympanic scale [18]. Also, the round window provides an additional 2 mm of basal part of osseous spiral lamina for stimulation [19].

However, elevations along the periphery of the round window can present challenges during implant placement, often necessitating the drilling of its anterior lower border for smoother insertion when it is called the extended round window approach. Caution must be exercised when drilling in this region due to its close proximity to the cochlear aqueduct opening [18].

The complex anatomic course of osseous spiral lamina and basilar membrane indicates that a cochleostomy positioned anterior to the RW, not only places the basilar membrane and spiral ligament at risk of injury during insertion, but also increases the possibility of insertion of the array into the scala vestibule [20]. Therefore, cochleostomy is safer to be done inferior to RW.

Optimum auditory nerve response to electrical stimulus is an important factor for predicting successful outcome. The electrically evoked compound action potential (ECAP) measured intraoperatively is one of the first markers of the auditory nerve function after cochlear implant surgery. The electrophysiological measurements (Impedance and NRT) are used intraoperatively to confirm the auditory nerve function, device integrity and electrode functionality. Various parameters can be employed during and after implantation to assess the cochlear implant device integrity and operation, in which impedance and neural response telemetry (NRT) are the most used parameters. Impedance represents the condition of the cochlea’s tissue-to-electrode contact and surrounding environment [21, 22] and is highly suggestive of electrode integrity and function, therefore it is used to monitor and troubleshoot the cochlear implant. Neural response telemetry (NRT) measures electrically evoked compound action potentials (ECAP’s) during surgery or postoperatively in the implanted patients. This is a critical test for precisely monitoring external and internal hardware function as well as assessing cochlear stimulation via neural responses [23].

This study was aimed at comparing impedance and neural response telemetry in the intraoperative period in full banded and half banded electrodes placed through the RW approach, Extended RW approach or cochleostomy and to verify whether the choice of implantation routes produces differences in auditory nerve stimulation. Intraoperative impedances were recorded for different modes of stimulation and ECAP measured at E5, E10, E15, E20 electrodes in both the groups across the three insertion routes.

ECAP indicates the synchronized activation of cochlear nerve fibers. The NRT is a highly sensitive method to directly measure ECAP in implanted patients during and after surgery. It serves as a valuable tool for confirming internal device integrity, objectively selecting electrodes for mapping, optimizing stimulation frequencies and speech encoding strategies, estimating T levels (current for hearing perception), and determining C levels (maximum patient-perceived sensation intensity), a clinically significant parameter [24].

Studies done by Roland [25] and Karatas [26] showed better auditory function with round window approach, whereas Hamerschmidt [27] and Chang [28] showed no difference across the approaches. The data analyzed in our study supports that there is no statistically significant difference in the intraoperative impedance and ECAP measurements in both the full banded and half, banded multichannel electrodes across the different surgical routes implying that, all the three surgical approaches provide equal stimulation of the auditory nerve.

Conclusion

The cochlear implant placement routes RW, Ext RW and Cochleostomy are well established, round window being the most preferred. There is no statistically significant difference in the intraoperative impedance and ECAP measurements across these surgical routes and all the three approaches provide equal stimulation of the auditory nerve. The CI surgeon can select the electrode insertion portal based on the surgical anatomy, the implant type and individual preferences.

Funding

No funding sources.

Declarations

Conflict of Interest

The authors declare that they have no conflict of interest.

Disclosure

This work has never been published and is not currently under evaluation in any other peer reviewed publication.