Round Window Visibility in Cochlear Implantation : Pre-operative Prediction Using Various Radiological Parameters

Abstract

Purpose

The round window approach has become the most preferred route for electrode insertion in cochlear implant surgery; however, it is not possible at times due to difficult round window membrane (RWM) visibility. Our study aims to investigate the relationship between preoperative radiological parameters and the surgical visibility of the RWM in Cochlear implant patients.

Methodology

A prospective cross-sectional study of 31 patients, age < 6 years, with bilateral severe to profound sensorineural hearing loss was conducted at a tertiary care hospital. The preoperative HRCT temporal bone scan was studied, and the parameters evaluated were facial nerve location, facial recess width, and RWM visibility prediction. All patients were operated on via the posterior tympanotomy. The surgical RWM visibility was done after optimal drilling of the posterior tympanotomy recess. The relationship between the radiological parameters and surgical visibility of RWM was evaluated.

Results

The difference in the facial nerve location as per the type of RWM was found to be significant (p value < 0.05). However, the facial recess width was not significantly associated with RWM visibility. The radiological prediction of RWM visibility by tracing the prediction line over RWM was significantly associated with intraoperative RWM visibility.

Conclusion

The goal to look for preoperative scans is to predict the ease or difficulty of RWM visibility during surgery. The difficult visualization of the RWM, can result in dire intraoperative consequences. A comprehensive understanding of preoperative radiological parameters, coupled with meticulous surgical planning, is crucial to address these challenges effectively by focusing on enhancing RWM visualization.

Introduction

Cochlear implant is the treatment of choice for hearing rehabilitation in patients with bilateral severe-to-profound sensori-neural hearing loss [1]. Cochlear implant are an effective way of providing auditory stimulation in comparison to hearing aids because they bypass the damaged part of the ear and use electrical stimulation rather than amplification [2]. The classical surgical route of cochlear implantation is performed via a retroauricular approach with a mastoidectomy-facial recess technique [3].

For the insertion of electrodes, either the round window (RW) approach or the promontory cochleostomy (PC) approach can be utilized [4]. However, the PC approach can lead to acoustic trauma by requiring prolonged drilling over the promontory bone [5]. On the other hand, RW itself is used as a route for electrode insertion in the RW approach, such that less drilling is required, which causes less mechanical trauma [6, 7]. In addition, less drilling accounts for the least possibility of any bony fragments entering the Scala Tympani [8].

However, for proper insertion of the electrodes into the cochlea, clear visualization of the RW is an obligatory step, so understanding the apparent morphology and anatomical differences before cochlear implantation is a necessary step. The RW approach provides the easiest way to access the Scala Tympani and acts as a conduit for the placement of electrode arrays. Some previous studies reported that in two-thirds of the patients, the RW approach can be used, while the PC approach is utilized in cases of difficult RW exposure, as seen in the remaining one-third of cases [9, 10].

At times, due to substantial variability in the shape and size of the RW, identification of the round window niche (RWN) may be difficult even with maximum surgical effort, perhaps attributable to its complicated embryological basis. Also, the anatomy of the inner ear is very delicately intertwined around the facial nerve. Thus, the possibility of visualizing the RW by posterior tympanotomy is variable during cochlear implant surgery.

These considerations emphasize the importance of the preoperative radiological evaluation of RWN visibility and accessibility. Our study meticulously analyzed the various radiological parameters using high-resolution computed tomography (HRCT) imaging of the temporal bone. The objective was to establish an association between these radiological parameters and the intraoperative visbility of the RWM. Notably, we employ the St. Thomas classification, a valuable tool introduced by Leong et al., to guide our assessment and categorization [10].

Materials and Methods

Study Design

A hospital-based observational cross-sectional study during the period of 1 year.

Study Area

Department of Otorhinolaryngology and Head and Neck Surgery, SMS Medical College, Jaipur, Rajasthan, India.

Sample Size

31 patients undergoing cochlear implantation were included in this study. The study was carried out after institutional ethics committee approval.

Inclusion Criteria

• Age: < 6 years.

• Bilateral severe to profound sensori-neural hearing loss.

• No improvement or insufficient improvement in hearing after hearing aid trial.

• Written informed consent for the procedure.

• Patient fit for surgery.

Exclusion Criteria

• Any revision case.

• Associated cochlear malformed ear.

• Patient > 6 years of age.

• Patient implanted via alternative techniques (e.g., trans-canal route).

• Patient not fit for surgery.

• Active ear infection.

Radiological Parameters Measurement

The patients underwent the pre-requisite radiological assessment done on axial cuts of HRCT temporal bone images (0.6 mm slice thickness). The images were obtained parallel to the supra-orbito-meatal line using a GE HI Speed NX/I Dual CT scanner at 120 kV and 70 mA. The scans were visualized in the standard bone window setting in RadiAnt DICOM software (Version 2021.1), with axial cuts having maximum visibility of RWN.

The following reference lines were drawn, which are described as:

1. “EAC line” - line drawn by connecting the bony cartilaginous junction of the posterior external auditory canal wall (EAC) and the tympanic annulus (when the EAC line could not be drawn on a selected slice, it was drawn on the nearest slice and transposed to the original image).

2. “Basal turn line” - line drawn by passing through the center of the basal turn.

3. “Prediction line” - line drawn from the posterior margin of the RWN along the anterolateral part of the facial nerve (FN).

The distinct parameters measured from the above-drawn lines are described as follows:

• A)Width of the Facial Recess - measured as the vertical distance from the anterolateral portion of the FN between the two reference line i.e. the EAC line and the prediction line (Fig. 1).

Fig. 1.

The figure illustrates the measurement of the width of the facial recess (shown in red line) as a perpendicular distance between the two reference line - the EAC line (yellow line) and the prediction line (green line)

• B)The relative location of the facial nerve - measured as the vertical distance from the anterolateral part of the FN between the prediction line and the cochlear basal turn line. (Fig. 2)

Fig. 2.

The figure signifies the relative facial nerve location ( marked as red line) measurement, which is the vertical distance from the anterolateral part of the FN between the two reference line i.e. the prediction line (marked as yellow line) and the cochlear basal turn line (green line)

• C)Prediction of RWN visibility through the facial recess - This is determined by the position of the prediction line, which is drawn parallel to the EAC line and along the anterolateral aspect of the facial nerve. The RWM tracing is conducted in an anteroposterior direction. The resulting intersection point is categorized into three distinct zones: anterior (20% of the RWM), middle (60% of the RWM), or posterior (20% of the RWM), depending on how the prediction line aligns with the RWM. The intersection points are visually illustrated in Fig. 3.

Fig. 3.

In this figure, the process for assessing the prediction of RWN visibility is visually represented. Here, the red line indicates anterior intersection point of the prediction line on the RWM, the yellow line signifies middle intersection point and the green line denotes posterior intersection point

Intraoperative RWM Visibility Assessment

The intraoperative visibility of the RWM was evaluated with the help of the St Thomas’ Hospital classification. The grading of the RWM visibilty was done after performing an ‘‘optimal’’ posterior tympanotomy and adequate removal of the RWN bony overhang without breaching the RWM. The RWM visibility is graded into four types: -.

● Type I – Full exposure of the RWM.

● Type II - Partial exposure of the RWM which is sub-divided into Types IIa and IIb;

 Type IIa - More than 50% but less than 100% exposure.

 Type IIb - Less than 50% but more than 0% exposure.

● Type III – RWM is not visible.

The measurements of the three radiological parameters along with the intraoperative findings related to RWM visibility were systemically noted in an Excel spreadsheet and the analysis of the results was carried out by applying various statistical tests.

Statistical Analysis

Quantitative data were summarized in the form of a mean and a standard deviation.

The difference in mean was analyzed using a student t-test and an ANOVA test in group analysis.

Qualitative data were expressed in the form of proportions.

The difference in proportions was analyzed using the chi-square test.

The level of significance was kept at 95% for all statistical analysis.

Results

There were a total of 31 candidates enrolled in this study, with 20 being male and 11 being female. The male to female ratio was 1.82. 12 of these patients were below or up to the age of 3 years when implanted, and the rest were older than 3 years. The rest of the results can be summarized in the Tables 1, 2, 3 and 4 as given below.

Table 1.

The above table shows the intraoperative RWM exposure of our patients. 22 candidates have more than 50% RWM exposure

Intraoperative RWM classification	Number	Percentage
Type 1	22	71.0
Type 2a	8	25.8
Type 2b	1	3.2

Table 2.

The above table shows the width of the facial recess and its association with the intraoperative RWM exposure. The difference in the facial recess width as per the type of RWM was found to be statistically insignificant (p value = 0.320)

Facial recess width (mm)	Intraoperative RWM
	Type 1	Type 2a	Type 2b
Number	22	8	1
Range	3.32–6.63	3.76–5.53
Mean ± Standard Deviation	4.85 ± 0.69 mm	4.75 ± 0.61 mm	3.8

Table 3.

The above table shows the association of the intraoperative RWM exposure with the facial nerve location (mm). The difference in the facial nerve location as per the type of RWM was found to be statistically significant (p value < 0.05)

Facial nerve location (mm)	Intraoperative RWM
	Type 1	Type 2a	Type 2b
Number	22	8	1
Range	0.5–2.8	0.81–2.54
Mean ± Standard Deviation	1.65 ± 0.39	1.56 ± 0.57	0.2

Table 4.

The above table shows the association between the prediction of RWN visibility and intraoperative RWM exposure. A statistically significant association was found to be present between the two components. The only candidate with an anterior type of intersection had type IIb RWM exposure intraoperatively

Prediction line intersection over RWM	Intraoperative RWM			Total
	Type 1	Type 2a	Type 2b
Anterior			1(100)	1(100)
Middle	4(66.67)	2(33.33)		6(100)
Posterior	18(75)	6(25)		24(100)

Discussion

The introduction of cochlear implants has revolutionized the management of severe to profound hearing loss. The preferred route adopted for a cochlear implant is the posterior tympanotomy site, which is bounded medially by the facial nerve, laterally by the chorda tympani nerve, and superiorly by the fossa incudis. RW identification is the keystone step for the insertion of electrodes in cochlear implant surgery. Nowadays, cochlear implant insertion is preferred through the RW approach over the traditional cochleostomy as it might be less traumatic [7, 11].

The two bony overhangs overlie the RWN, one anterio-inferiorly and the other posteriorly, called the anterior and posterior pillars, respectively. The RWN is hyperbolic paraboloid in shape and about 2–3 mm long and 0.48–2.7 mm in width (mean 1.66 mm), located in the medial wall of the middle ear. The RWN ossification starts in the 16th fetal week and is completed at birth. The inferior wall of the RWN is formed by the process of the otic capsule, called the cartilage bar [12]. The intramembranous ossification forms the anterior and superior walls of the niche, whereas the posterior and inferior walls are predominantly formed by enchondral ossification. Thus, the shape of the RWN entrance can be altered by the uneven growth of different walls of the RWN [13–15].

However, it is not always possible to proceed with the RW approach, especially because of difficult RWM visualization. In an attempt to improve intraoperative visibility of RWM, there is an increased chance of intraoperative consequences to important structures namely the chorda tympani nerve, fallopian canal, posterior canal wall, or tympanic membrane. To avoid ending in these situations, an assessment of RWM visibility before surgery is beneficial.

Preoperatively, RWN can be assessed on the HRCT Temporal Bone to guide the surgeon for the implant trajectory through the facial recess into the RW. Meanwhile, preoperative imaging also alerts the surgeon to any anatomical variation of the facial nerve or in the shape and size of the RWN or the position of the posterior EAC wall that at times requires surgical approach modification, thus necessitating the exigency of preoperative radiographic assessment in all CI surgeries.

Our study clearly outlined a statistically significant (p value < 0.05) relationship between the prediction of the RWN visibility and the intraoperative RWM visibility, wherein only candidates with anterior intersections had intraoperative type IIb RWM. One such study by Kashio et al. (2015) in the past attempted to utilize HRCT temporal bone to predict the visibility of the RWN preoperatively. The results of our study were similar to those reported by Kashio et al. (2015) in their study of 70 candidates. In their study, all 15 patients with anterior intersection had invisible or nearly invisible RWN [8]. It was evident that difficult intraoperative RWM visualization was anticipated with the anterior intersection point of the prediction line.

Jwair et al. (2021) utilized the same prediction line, which was drawn from the anterior part of the mastoid segment of the facial nerve and towards the lower side of the basal turn of the cochlea. They categorized the intersection point between the RWM and prediction line as being either anterolateral (above the middle of the RWM) or posteromedial (below the middle of the RWM). They concluded that the outcome of the prediction line also depends on the angle of rotation of the RWM and the course of the FN, confirming their importance in evaluating the viewing angle of the RWM [16].

In our study, a significant difference in the facial nerve location level between types I, IIa, and IIb was associated with intraoperative RWM visibility. Analogous result with Kashio et al. (2015) study, 0.0 ± 0.8 mm for invisible or nearly invisible RWN, 0.7 ± 0.9 mm for partially visible RWN, and 1.1 ± 0.7 mm for fully visible RWN, respectively, and surpassingly, association of the FN location with intraoperative RWM visibility was established [8].

On the contrary, the difference in the facial recess width as per the type of intraoperative RWM was not significant, coinciding with the Kashio et al. (2015) study results [8]. Another study by Jwair et al. (2021) calculated the distance between the facial-chorda tympani nerves. They reported difficult RWM visualization with a smaller distance between the facial and chorda tympani nerves [16].

However, there was a discrepancy in the results of the Lee et al. (2012) study, where a correlation of the facial recess width with intraoperative RWM visibility was reported. This disparity could be attributed to the difference in target structures in both studies. Ours dealt with the RWN visibility, while Lee et al. (2012) focused on the stapes, which is located anterior to the RWN, directly contributing to its visibility through the facial recess width [17]. While in our study, the RWN visibility would be largely limited regardless of a wide facial recess opening attributed to the posterior location of the RWN. Therefore, during cochlear implantation, relationships between visual field structures carry more importance than the extent of the facial recess enlargement.

The findings of our study showed that intraoperative RWN visibility was associated with preoperative HRCT-based measurements of prediction of RWN visibility and the FN location, which could be used as a preoperative screening tool in cochlear implant surgery planning.

Conclusion

Preoperative imaging of the cochlear implant candidates acts as an enlightening tool for the working field of the surgeon in the exquisite, delicate-to-handle structure of the inner ear. Care must be taken to leave the important surrounding structures like the facial nerve, chorda tympani nerve, and posterior canal wall intact in an attempt to improve RWM visibility. The chances of any intraoperative event are thereby increased with the closer facial nerve location and anterior intersection point of the prediction line on the RWM. In conclusion, the integration of preoperative radiological imaging in evaluation of cochlear implant trajectory offers a proactive strategy to optimize surgical outcomes by identifying potential challenges, planning alternative approaches, and allocating experienced surgeons to complex cases. This approach not only enhances patient safety but also contributes to the overall success of surgeries. The ultimate goal is to enhance surgical outcomes and provide valuable insights for improving surgical techniques.

Abbreviations

EAC

External auditory canal

FN

Facial nerve

HRCT

High resolution computed tomography

PC

Promontory Cochleostomy

RW

Round window

RWM

Round window membrane

RWN

Round window niche

Funding

This research did not receive any specific grant from funding agencies in the public, commercial, or not for profit sectors.

Declarations

Ethical Approval

All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Informed Consent

Informed written consent was obtained from all individual participants included in the study.

Conflict of Interest

All the authors declare that they have no conflict of Interest.