Indian Journal of Otology

ORIGINAL ARTICLE
Year
: 2021  |  Volume : 27  |  Issue : 1  |  Page : 7--10

Anatomical variations of round window in different age groups and surgical difficulties associated with them during cochlear implantation


Rabindra Bhakta Pradhananga, Bigyan Raj Gyawali, Pabina Rayamajhi, Bebek Bhattarai 
 Department of ENT-HNS, Institute of Medicine, Tribhuvan University Teaching Hospital, Kathmandu, Nepal

Correspondence Address:
Dr. Bigyan Raj Gyawali
Department of ENT-HNS, Institute of Medicine, Tribhuvan University Teaching Hospital, Maharajgunj Road, Kathmandu 44600
Nepal

Abstract

Introduction: Round window (RW) insertion of the electrodes during cochlear implantation is the most favorable route considered by many surgeons. The objective of this study was to review the anatomy of the RW based on visibility and accessibility of RW niche (RWN) and RW membrane (RWM) and to assess their implications in surgical difficulties during cochlear implantation in different age groups. Materials and Methods: This was a retrospective observational study conducted at the Department of ear, nose, and throat-Head and Neck Surgery, Institute of Medicine, Kathmandu, Nepal. We analyzed the record data of all the cochlear implants from January 2015 to December 2019 for the visibility of RWM, RWN, and associated surgical difficulties in different age groups. RWM visibility was classified as; Grade I (>50% of RWM is visible), Grade II (25%–50% of RWM is visible), Grade III (<25% of RWM is visible), and Grade IV (RWM is not visible). Similarly, RWN visibility was classified as; Type A (difficult to visualize), Type B (partially visible), and Type C (fully visible). Our final sample size was 81. Results: The most common variant of RWN and RWM was Type C (37 cases) and Grade IV (37 cases), respectively, in the age group of <15 years. In cases >15 years, Type B (eight cases) was the most common variant of RWN and Type II (six cases) was the most common variant of RWM. There was a statistically significant association between the visibility of the RWN and RWM and the visibility of RWN and different levels of surgical difficulty (P < 0.05). Conclusions: Type C RWN and Type IV RWM were the most common variants found, mostly seen in the pediatric population. With poorer visibility of RWN, RWM visibility decreases, increasing the difficulty level of the surgery.



How to cite this article:
Pradhananga RB, Gyawali BR, Rayamajhi P, Bhattarai B. Anatomical variations of round window in different age groups and surgical difficulties associated with them during cochlear implantation.Indian J Otol 2021;27:7-10


How to cite this URL:
Pradhananga RB, Gyawali BR, Rayamajhi P, Bhattarai B. Anatomical variations of round window in different age groups and surgical difficulties associated with them during cochlear implantation. Indian J Otol [serial online] 2021 [cited 2021 Nov 27 ];27:7-10
Available from: https://www.indianjotol.org/text.asp?2021/27/1/7/329093


Full Text



 Introduction



Cochlear implantation has become one of the frequently done otological surgeries in today's scenario, which can be technically challenging sometimes. Starting from the earlier days of implantation until now, there has been a huge advancement in this field. Although sophisticated implants are coming up, the surgical aspect has not changed quite a lot, considering the limited anatomical routes to access the cochlea where the electrodes are designed to be placed. With feasible anatomy, less surgical trauma, and more chances of preservation of residual hearing, the round window (RW) approach for implant insertion is the preferred technique by most of the cochlear implant surgeons.[1]

Access to the RW can be achieved via various approaches, with the facial recess approach being the most practiced one.[2] Other approaches are suprameatal approach, transcanal approach, and pericanal approach.[3] In cases where routine mastoidectomy and facial recess approach is not feasible, the middle cranial fossa approach has also been used for cochlear implantation in recent times.[4] Whichever may be the surgical approach, a proper delineation of RW niche (RWN) and then the RW membrane (RWM) is a very crucial step, and the difficulty level of the surgery is hugely determined by the visibility of RWN.[5] A wide range of studies has been done so far, focusing on the anatomy of the RW. Various classifications have been proposed as well.[6],[7] Considering the complex anatomy of the temporal bone and wide physical variations among the patients of various ethnic groups and races, the results of these studies cannot be generalized to all the patients. The objective of our study was to review the anatomy of the RW based on the visibility and accessibility of RWN and RWM and to assess their implications in surgical difficulties during cochlear implantation.

 Materials and Methods



This was a retrospective observational study conducted in the Department of ear, nose, and throat-Head and Neck Surgery of Institute of Medicine, Kathmandu, Nepal. Approval from the Institutional Review Committee was taken. Cases of all the age groups who had undergone cochlear implantation from January 2015 to December 2019 in this institution were included. Cases who had undergone revision surgery were excluded from the study. Our total sample size was 81, after excluding three cases who had undergone revision surgery. We used the posterior tympanotomy approach to access the RW. Membranous insertion through RW was the preferred technique. However, the extended RW approach or cochleostomy had to be done in cases where access to the RW was very difficult.

RWM visibility in our study was classified as below:[6]

Grade I: >50% of RWM is visibleGrade II: 25%–50% of RWM is visibleGrade III: <25% of RWM is visibleGrade IV: RWM is not visible.

Similarly, RWN visibility was classified as below:[7]

Type A: Difficult to visualizeType B: Partially visibleType C: Fully visible.

Surgical difficulties during implantation were defined as below:

Not difficult

Easy accessibility of the insertion site via posterior tympanotomy or,Easy and complete insertion of electrodes.

Moderately difficult

Easy accessibility of the insertion site through extended posterior tympanotomy or,Complete insertion of electrodes with difficulty.

Highly difficult

Poor accessibility of the insertion site even through extended posterior tympanotomy or,Partial insertion of electrodes with difficulty.

Statistical analysis was performed using SPSS software version 25 (IBM Corp., New York, USA). We used the Chi-square test and Fisher's exact test to evaluate the association between the visibility of RWM and RWN and also between the level of difficulty and visibility of RWN and RWM. A value of P < 0.05 was considered statistically significant.

 Results



Of all the 81 cases, the majority (70) was from the pediatric age group and only 11 cases were adults. The M: F ratio was roughly 2:1.

Round window niche visibility

Type C was the most common variant of RWN seen in cases <15 years. In cases >15 years, Type B was the most common [Table 1].{Table 1}

Round window membrane visibility

In the cases <15 years, RWM with the visibility of Grade IV was the most common, followed by Grade II and Grade I. RWM with the visibility of Grade III was seen in only four cases below 5 years and five cases of age group 5–15 years. It was not seen in any cases >15 years [Table 2].{Table 2}

Association between round window niche and round window membrane visibility

All the cases having Type A RWN had a RWM visibility of Grade IV. The majority of the cases (17) with Type B RWN had Grade IV RWM visibility. Of the 39 cases having Type C RWN, 15 had Grade II, nine had Grade IV, nine had grade I, and six had Grade III RWM visibility. Fisher's exact test showed a statistically significant association between these two variables (P < 0.05) [Table 3], i.e., with the poorer visibility of RWN, visibility if RWM also decreases.{Table 3}

Round window niche visibility and the associated difficulties

The majority of the cases with Type C (32) RWN didn't encounter any difficulties. In cases with Type B RWN, there was no difficulty in 11 cases and moderate difficulty in 12 cases during surgery. Eight cases having Type A RWN encountered high difficulty during surgery. There was a statistically significant association between these two variables (P < 0.05) on using Fisher's exact test [Table 4]. This means poorer visibility of RWN is associated with more surgical difficulties.{Table 4}

Round window membrane visibility and the associated difficulties

In most of the cases (nine) with Grade I RWM, no difficulty was encountered. Fourteen cases with Grade II RWM had no surgical difficulty, and the rest of the eight cases had moderate to high difficulty. Similar were the results for the cases with Grade III RWM. The majority of the cases with Grade IV RWM encountered surgical difficulties (12 had moderate and 11 had high difficulty). There was, however, no statistically significant association between these two variables (P < 0.05) on using Fisher's exact test [Table 5].{Table 5}

 Discussion



The posterior tympanotomy approach is the widely practiced approach all over the world to access the RW for cochlear implantation.[3] Having very complex anatomy, the surgeon needs to be familiar with the morphology of the RW. RWN and RWM visibility greatly affect the surgical difficulty and, thus, the surgical outcomes. In this study, we reviewed the anatomic variations of the RWN and RWM and their implications in the surgical difficulties.

The commonest variant of RWN was Type C (fully visible) in the pediatric age group and Type B (partially visible) in the adults. In the majority of cases with poorer visibility of RWN, RWM visibility was also reduced. However, this was not the case with fully visible RWN (Type C), where there was a wide variation in RWM visibility. The bony overhang of RW greatly determines the visibility of RWM, and it is not always necessary for RWM to be visible in a fully visible RWN. On the other hand, RWN visibility through posterior tympanotomy is dependent on the position and size of RW. The more posterior and inferior location with a smaller size of RW, the less is its visibility.[8]

The RW has a complex anatomy. RWN comprises two bony overhangs also called the pillars, which are directed anteroinferiorly and posteriorly and in one-third of the cases, it may be covered with a false membrane.[9] The opening may be directed posteriorly, posteroinferiorly or inferiorly and it is usually not possible to visualize the RWM always. Several studies have been done so far on variations of RWM and RWN. In a study by Sarafraz et al. in 53 cases, Type C RWN was the most common followed by Type A and then Type B.[7] Kashio et al., on examining 70 ears, found Type C RWN being the most common and Type A being the least.[10] In contrast to our study, their study showed improved RWN visibility in adults (68%) compared to children (31.1%). Similar results were shown in the study by Dalmia and Behera where a majority of pediatric cases (60%) had Type IV RWN (25%–49% RWM not visible) and only 5% cases had Type I RWN (completely visible RWM).[11] Panda et al., in their two institutional reviews, found Type III RWN the most common; however, variations in different age groups were not mentioned.[6]

In our study, we defined the levels of difficulty based on the requirement for the extension of posterior tympanotomy and difficulty in the insertion of the electrodes. Although the requirement for the extension of posterior tympanotomy is dependent on the extent of visibility of RW, the insertion of electrodes may not be solely dependent on visibility as other anatomical factors of the inner ear may affect it. Majority of the cases with Type C (fully visible) RWN and Grade I (fully visible) RWM did not encounter any difficulties. While most of the cases with Type A (difficult to visualize) RWN encountered high difficulty during surgery, a significant number of cases with Grade IV RWM (not visible RWM) did not encounter high difficulty. This conclusion could be justified with the fact that once the RWN is visible, RWM can be easily accessed by drilling the bony overhangs. Our finding is supported by the study of Jain et al. who have concluded in their study that the extent of posterior and inferior orientation and the size of RW significantly determines the difficulty in its accessibility rather than the type of bony overhang.[8] This finding was in accordance with ours as we encountered more difficulty in cases with Type A RWN compared to cases with Grade IV RWM.

In this study, cochleostomy had to be done in 11 cases due to poor visualization of RWN and RWM. In nine cases, RWN was Type A (difficult to visualize), and in two cases, it was Type B (partially visible). All of these cases had Grade IV (not visible) RWM. Similarly, an extended RW approach was done in seven cases, of which four had Type A RWN and three had Type B RWN. Only one case had Grade III RWM, while all other cases had Grade IV RWM. In cases with poor visibility of RWN and RWM, the surgeon should bear in mind the possibility of the need for an extended RW approach or cochleostomy. Our finding is also supported by the study of Panda et al. where around 34% of cases with Type IV RWM required cochleostomy following a failed membranous insertion.[6] Similarly, Leong et al. had cochleostomy to be done in all cases with Type III (not visible) RWM and 71% of cases with Type IIb (<50% visible) RWM. Their classification of RWM visibility was based on St. Thomas classification.[12]

Limitations of our study included a limited number of the adult population, and thus, our findings cannot be generalized. Furthermore, the perception of difficulties encountered during surgery was subjective to some degree. However, we minimized this bias by involving a single surgeon. A well-designed study, including an equal proportion of adult and pediatric population, is further required to strengthen our results.

 Conclusions



Type C RWN and Type IV RWM were the commonest variants found, mostly seen in the pediatric population. With a very insignificant number of adult cases, we cannot, however, conclude this result in the adult population. With poorer visibility of RWN, RWM visibility decreases, increasing the difficulty level of surgery.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.

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