A clinical study of cochlear implant electrode placement via posterior tympanotomy approach in cases with varying types of round window visibility

INTRODUCTION

Cochlear implantation is the treatment of choice for patients with bilateral profound hearing loss with no benefit with hearing aids.¹ There are various approaches for Cochlear implantation.² In most of the Cochlear implant centres all over the world, the commonly performed standard approach is through posterior tympanotomy. Electrode insertion into the cochlea is done by either Round window insertion (RWI) or by cochleostomy. Currently, there is a paradigm shift of electrode insertion from conventional cochleostomy to round window insertion because of decreased acoustic trauma, less chances of perilymph leak, decreased risk of infections and availability of wider stimulation of surface area length by 2mm.^3,4 The RWI may offer several advantages over a cochleostomy, and it is a reliable, safe, and effective technique for cochlear implantation with today's cochlear implant electrodes.⁵

Appearance of round window through posterior tympanotomy varies depending on its anatomy of bony covering over round window known as round window niche. For complete exposure of round window, the bony overhang in the antero-superior, superior and posterior parts of the Round window niche (RWN) may have to be removed. There are various grading systems depending on the exposure of round window membrane seen through the posterior tympanotomy. We have followed Saint Thomas Hospital (STH) classification (Table 1) of round window membrane after adequate removal of the RWN, which is the most popularly accepted classification worldwide.⁶

METHODS

Retrospective descriptive study was carried out in a tertiary Cochlear implant centre in South India. Ethical approval with the permission of Institutional Review Board was taken in Jan 2024. Data was extracted from hospital medical records department and there were no conflict of interests or funding involved in the study.

Study design

The study data was collected from December 2018 to December 2023. Retrospective data of children aged 1 to 6 years with bilateral profound hearing loss who underwent CI at this centre was collected from the institution’s database and were included in the study. Children with malformations of middle ear, inner ear on imaging, syndromic features, and who underwent revision surgeries were excluded from the study.

The total number of patients in the study were 476; out of which 263 (55%) were male and 213 (45%) were female. The mean age of the children undergoing the Cochlear implant was 3.6 years. 441 (93%) children underwent unilateral cochlear implant, and 35 (7%) children underwent bilateral Cochlear implant (35x2=70). So total number of Cochlear implant surgeries n=511 performed in paediatric population, who met the inclusion, exclusion criteria were analysed retrospectively, and the descriptive study was done for the type of round window as per STH classification and method of electrode insertion in each case that was already documented in the database of the Institute.

Statistics

All the demographic data and operative findings collected from the institutional database for the study were analysed by a biostatistician. Appropriate statistical analysis was performed by STATA 11.2 (College Station TX USA). Chi square test for goodness of fit was used to measure the types of RWM, methods of insertion and the results are expressed in frequency and percentages.

RESULTS

With STH classification for grading of RWM, 282/511 (55.2%) cases were found to have Type IIA followed by 96/511 (18.8%) cases had Type IIB. The most common type of round window found was Type IIA. Type I RWM was found in 69/511 (13.5%) and Type III in 64/511 (12.5%) cases respectively (Figure 1).

Out of 511 Cochlear implants, 351 (68.7%) cases had direct round window insertion, 96 (18.8%) cases had extended round window insertion, 64 (12.5%) cases had cochleostomy for insertion of electrode (Figure 2). In all Type I and Type IIA cases, direct round window insertion of electrodes was possible.

In Type IIB, all cases required extended round window exposure technique for insertion of electrode. Type III cases required cochleostomy for insertion of electrode. The result of the study is shown in Table 2.

DISCUSSION

The soft surgery technique for hearing preservation in Cochlear implantation has been the topic of discussion for several years. The round window approach has been advocated as a method to reduce insertion trauma and preserve residual hearing.⁷ However, accessing the round window membrane could be difficult due to the bony overhang of the round window niche.⁸ Additionally, the variable size and orientation of the round window may make electrode insertion difficult and challenging.⁹ In our study, all cases underwent wide optimal posterior tympanotomy and removal of bony overhang around the RW. Grading of RWM was done as per the STH classification-type I is 100% visualization of RWM, type IIA is greater than 50% and less than 100% visualization of RWM, type IIb is less than 50% visualization of RWM and type III is RWM not visualized (Table 1).⁶ In a study conducted by Panda et al RWM grading was done through posterior tympanotomy before drilling the RWN which showed grade III (less than 25% visualization of RWM) as the commonest type of RWM.¹⁰ The grade III RWM of their classification corresponds to the Type IIA STH classification after drilling of the RWN since drilling of the RWN or bony overhang increases the visualization of RWM up to three times.¹¹ As per STH classification in our study, we found that Type IIA (55.2%) is the most common type of RWM and Type III (12.5%) is the least common variety. Endoscopic view of middle ear cavity through posterior tympanotomy marking the boundaries of RWN is shown in Figure 1. Types of RWN as per STH classification is shown in Figure 2.

Direct round window insertion of the electrode array is when electrodes are inserted without any further drilling around the RWM after removal of the round window niche (only posterior pillar, tegmen rotunda are drilled). Extended RWM insertion requires drilling around the exposed part of RWM, particularly the anterior inferior part of bony ridge (removal of crista fenestrae). Extended RWM insertion is done when RWM is not seen adequately even after the removal of RWN. Cochleostomy is performed by drilling on the promontory antero inferiorly at the level of RW when RWM is not seen, that is 2 mm inferior to oval window on the promontory in anteroinferior direction.

In our study we found that all type I (13.5%) and type IIA (55.2%) cases had direct RWM insertions. All Type III (12.5%) cases had conventional bony cochleostomy while the type IIB (18.2%) cases had extended round window insertion. A similar study done by Iqbal et al with STH classification also found that type IIA (47%) is the commonest RWM in the paediatric population.¹² All type I and type III cases in their study had direct RWM insertions and cochleostomy respectively which is analogous to our study. Most of the type IIA cases and type IIB cases in their study had direct RWI and ERWI respectively which closely resembles the results of our study.

Till date, we found that though there are different grading systems for types of RWM, there is no definite criterion for the method of insertion of electrodes depending on the various types of RWM. Based on the observations and the results of our study, we have derived a criterion for method of electrode insertion in various types of round window membrane (Table 3). All type I and type IIA cases, where exposure of RWM is more than 50% to 100%, method of insertion of electrode is by direct round window.

All type IIB cases require extended round window technique by removing the bony covering of round window in anterior inferior part (crista fenestrae) for electrode insertion. All type III cases are usually associated with the need for cochleostomy. The authors found the STH classification as the best guide for the surgeon to assess the types of RWM and propose the criteria for various routes of insertion depending on types of RWM. This study result has assisted in bringing out an algorithm for various methods of electrode insertions in different types of RWM (Table 4). A similar prospective study in future with a multicentric cohort will be more useful since the current study is retrospective.

Limitations

One of the limitations of the study is subjective nature of STH classification for types of RWM and absence of CT correlation with intra-operative finding. Optimal posterior tympanotomy anatomical limits to expose RWM is not defined. Complications and outcomes were not included in our study.

CONCLUSION

This study has currently analysed the types of round window through posterior tympanotomy in the Indian paediatric population with normal temporal bone anatomy and our study revealed that Type IIA is the most common RWM that requires direct round membrane insertion, type III is the least common variant that mandates cochleostomy. STH classification aids the surgeon not only for grading of RWM but also helps the surgeon to decide the method of electrode insertion depending on the type of RWM. A rational approach in performing cochlear implantation is vital and the proposed criteria can facilitate the surgeon to decide on the appropriate method of electrode insertion depending on the type of round window membrane visibility.

ACKNOWLEDGEMENTS

The authors would like to express their sincere gratitude to the Department of Otorhinolaryngology and Head and Neck Surgery for providing the facilities and clinical material necessary for conducting this study. The authors are thankful to the faculty members and senior consultants for their guidance, valuable suggestions, and continuous support throughout the conduct of this research.

Acknowledgement is also extended to the contributions of the operating room staff, audiology team, and nursing staff for their assistance during the surgical procedures, perioperative care, and data collection. Their cooperation was essential for the successful completion of this study. Finally, the authors are grateful to all the patients and their families who are part of this study.

Funding: No funding sources

Conflict of interest: None declared

Ethical approval: The study was approved by the Institutional Review Board

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