|Year : 2020 | Volume
| Issue : 1 | Page : 7-12
Assessment of the preoperative classification for computed tomography predictability of round window niche visibility through posterior tympanotomy during cochlear implant surgery
Ahmed Galal1, Omneya G Eldin2, Fatthi Baki1, Mario Sanna3
1 Department of Otorhinolaryngology, Faculty of Medicine, Alexandria University, Alexandria, Egypt
2 Department of Radiodiagnosis, Faculty of Medicine, Alexandria University, Alexandria, Egypt
3 Department of Otorhinolaryngology and Neurootology, Gruppo Otologico, Piacenza, Rome, Italy
|Date of Submission||13-Jan-2019|
|Date of Decision||13-Jan-2019|
|Date of Acceptance||28-Jan-2019|
|Date of Web Publication||26-Jun-2020|
MBBch, MSc, PhD Ahmed Galal
Department of Otorhinolaryngology, Alexandria University, Alexandria, 21523
Source of Support: None, Conflict of Interest: None
Background Transmastoid facial recess is the classical approach for cochlear implant. Many methods have been reported in the literature to detect round window niche (RWN) visibility through posterior tympanotomy during that approach. Many had unclear methodology or complex software needed for their application.
Objective The aim of the present study was to assess the preoperative predictability of multislice computed tomography for RWN visibility posterior tympanotomy.
Patients and methods Computed tomography scans of 32 pediatric cochlear implant patients with otherwise normal temporal bone anatomy were studied for RWN visibility through posterior tympanotomy, using two methods. The first was a modification of a method by Kashio and colleagues and another by Mandour and colleagues. The visibility of the RWN was assessed intraoperatively after performing the posterior tympanotomy. Statistical analysis was then performed.
Results The method of Kashio and colleagues was of statistical significance (P=0.002), whereas the method of Mandour and colleagues was statistically insignificant (P=0.465).
Conclusion Kashio and colleagues is an easy and accurate method to preoperatively predict the visibility of the RWN through posterior tympanometry.
Keywords: cochlear implant, computed tomography, round window niche
|How to cite this article:|
Galal A, Eldin OG, Baki F, Sanna M. Assessment of the preoperative classification for computed tomography predictability of round window niche visibility through posterior tympanotomy during cochlear implant surgery. Alex J Pediatr 2020;33:7-12
|How to cite this URL:|
Galal A, Eldin OG, Baki F, Sanna M. Assessment of the preoperative classification for computed tomography predictability of round window niche visibility through posterior tympanotomy during cochlear implant surgery. Alex J Pediatr [serial online] 2020 [cited 2020 Jul 7];33:7-12. Available from: http://www.ajp.eg.net/text.asp?2020/33/1/7/287729
| Introduction|| |
Cochlear implantation (CI) is the established treatment for pediatric patients with bilateral and sometimes unilateral severe to profound hearing loss ,. The classical transmastoid facial recess approach stood the test of time for the most commonly performed approach for CI till the present day ,.
The round window (RW) is the gate connecting middle and inner ear . Different parts of the round window niche (RWN) do not grow at the neither same rate nor the same time, resulting in variable phenotypes of RWN anatomy .
The walls of the RWN consist of first, superiorly the tegmen; second, inferiorly the fustis and area concamerata; third, anteriorly the sustentaculum (finiculus) and anterior pillar; and fourth, posteriorly the posterior pillar and the subiculum.
It is very difficult to compare between the different types of cochleostomy. CI started initially with RW insertion. Then there was a shift to promontory cochleostomy because of the need of a straighter course. Later on, there was a shift back toward RW insertion ,.
The role of imaging
Surgeons have to be familiar with the anatomy and imaging, and the correlation between them .
Multislice computed tomography (MSCT) is known to expose patients and especially children to radiation, to which they are more sensitive . To overcome this, many authors perform MRI as their routine preoperative assessment for CI, and CT is used on as-needed basis ,,.
However, CT is still used in many centers. It serves as a surgical road map, for instance, dural height and sigmoid sinus position, for reaching the final destination of round window membrane (RWM) and also its status itself. It helps the decision of which ear to implant, cochlear patency, and types of inner ear anomalies. Even in normally looking temporal bones, subtle anomalies can be detected by various measurement and non-measurement methods ,,. These might affect technical decisions and improve preoperative counseling, especially regarding hearing preservation . Postoperatively, CT can confirm correct insertion of the electrode and its angular depth of insertion .
Cone beam computed tomography (CBCT) is a relatively new tool in the assessment of the middle and inner ear. Some authors suggest a comparable resolution for CBCT in relation to MSCT, with significantly less radiation dose ,,.
The visibility of RWN through posterior tympanotomy (PT) has been previously assessed and classified in the literature and correlated with multiple methods to preoperative imaging ,,,.
| Aim|| |
The aim of the present study was to assess the preoperative predictability of MSCT for RWN visibility through PT intraoperatively comparing two of these reported methods ,.
| Patients and methods|| |
A total of 32 patients who underwent CI at the Alexandria University Hospital in the period from February 2017 to February 2018 were included. The following were the inclusion criteria:
- Age below 16 years.
- No sex limit.
- No specific audiologic criteria.
- RW intended implantation.
The following were the inclusion criteria:
- Previous middle ear surgeries.
- Middle ear diseases such as chronic suppurative otitis media and cholesteatoma.
- Congenital anomalies within the middle ear or cochlear duct on MSCT.
- Gross anomalies within the course of facial nerve (FN) on MSCT.
- Major trauma or fractures to skull.
| Methods|| |
Preoperative radiological assessment with 0.5 mm cut MSCT to predict the accessibility of the RWN through PT (two methods)
Measurements on CT were assessed by our radiologist preoperatively, so she was blinded to the operative findings.
Kashio et al. method .
- Draw a line passing through two points, which are tympanic annulus and bony cartilaginous junction at the nearest cut to the basal turn (BT) of cochlea (line A).
- Draw a line passing through the antero-lateral border of the mastoid segment of FN at the level of maximum round RWM exposure and should parallel line A, and this will be called line B.
- The result was recorded depending on where line B passes, whether anterior, middle, or posterior to RWM ([Figure 1]).
|Figure 1 (a) Mandour line parallel to EAC, and (b) Mandour line crossing EAC. Right ear: (a) Kashio’s line posterior and (b) round window niche fully visible.|
Click here to view
Right CT petrous bone, showing lines A and B and that line B passes posterior to RWM.
Mandour et al. method
This was accomplished by drawing a line joining the edge of the RWN and the antero-lateral part of the mastoid segment of the FN in the axial cut with the widest BT of cochlea. Then, assessment is done of whether this line passes parallel ([Figure 2]a) or crossing ([Figure 2]b) to line A . To optimize comparison, we regrouped our patients like the original work into visible and partially visible in one group and invisible in the other group.
|Figure 2 Left ear: (a) anterior Kashio’s prediction line, and (b) invisible round window niche (even after incus bridge and incus removal.|
Click here to view
A transmastoid facial recess was performed. RWN was detected through PT. Any pseudomembranes or mucosal folds covering the RWN were removed. Then, visibility was classified into RWN fully visible, partially visible, or not visible through PT. Intraoperative assessment was done by one surgeon (F.A.), who was blinded to preoperative CT measurements.
This study was approved by Medical Research Ethics Committee at Alexandria Faculty of Medicine (020802–26/11/2015) and an informed consent was obtained from children guardians.
Both methods were compared to intraoperative findings and were statistically analyzed.
| Results|| |
We statistically analyzed the predictability of each of these two methods to the intraoperative classification using the two methods ([Table 1] and [Table 2]).
|Table 1 Distribution of the studied cases according to age and sex (n=32)|
Click here to view
Kashio’s prediction line used in the present study had a statistically significant predictability of RWN visibility through PT (P<0.002) ([Table 3]).
We regrouped our patients into two groups as the original work to optimize the comparison, namely, this was visible/partially visible as a group and invisible in the other. It did not yield statistical significance concerning visibility of the RWN through PT (P=0.465) ([Table 4]).
| Discussion|| |
Optimum visibility and accessibility of the RWN through PT is imperative for a smooth and successful CI. This is valid regardless whether the intent of the site of insertion is cochleostomy, RWM, or juxta RWM , though it seems more essential in RWM-intended insertions .
Visibility of RWN through opening of the PT is not always straightforward and is variable from one patient to the other . These variations could be predicted by preoperative radiological assessment. Consequently, it could help to reduce injury to both FN and CTN and improve chance of the array insertion to be atraumatic, in the scala tympani and with maximum number of electrodes inserted .
Leong et al. used the Saint Thomas Hospital classification to assess the visibility of RWM through PT. Nevertheless, this classification had no radiological correlation, as they performed only preoperative MRI for their patients. Secondly, they assessed it after drilling the RWN. So, it is an intraoperative decision and after changing the anatomy. They classified it into four categories according to visibility of RWM into type I (100% visible RWM), type IIa (˃50% visible RWM), type IIb (˂50% visible RWM), and type III (0% visible RWM). The most common type was type I, with 46% children . It was clinically hard at least in our hands to determine the fine line between above and below 50%. We believe what is most significant to assess would be the RWN visibility through PT before drilling, as this is the first structure we look for after opening the FR. It is the landmark for the RWM.
We followed the categorization of Pendem et al. and Kashio et al. ,. We classified visibility of RWN through PT into fully visible, partially visible (with no subclasses), and invisible.
To preoperatively estimate the predictability of CT for the aforementioned classification, we applied a simple but efficient method by Kashio et al.  It assesses where a line parallel to external auditory canal (EAC) and passing through mastoid segment of FN passed in relation to RWM, which is done in the axial cut on CT with the widest BT of cochlea. This line was classified as passing anterior, through, or posterior to the RWM.
In the present study, we identified the mastoid part of FN by simply following it on the work station from the superior cuts with the tympanic segment intact. Then followed it into the mastoid segment till we reached the cut the widest BT of cochlea.
Kashio et al.  reported anterior, middle, and posterior lines in 20, 60, and 20% of their patients, respectively. This method gave in that study good intraoperative predictability. We applied this method in a prospective manner, unlike in the original study, where it was assessed retrospectively from videos of surgeries, which according to the authors themselves may underestimate RWN visibility through PT. The original study was also applied on 70 ears (45 children and 25 adults), whereas in the present study, we applied it on 32 pediatric ears with clinical comparison. We reported 12.5, 34.2, and 53.1% for anterior, middle, and posterior, respectively. Unlike the original report, the posterior line was the most common not the middle one. Yet it was statistically significant in predicting RWN visibility through PT, with P of 0.002.
Another method applied by Mandour and colleagues depended on the same line of EAC by Kashio but in relation to a line through FN and posterior edge of RWN. They classified it into parallel and crossing and correlated it to RWN, which was classified into only two groups namely visible and invisible . Assessment was done retrospectively on surgical videos. The classification was too broad containing only two possibilities, namely, visible and nonvisible . Unfortunately, in our hands, this method was not statistically significant (P=0.465).
Fouad et al.  suggested two measurements, namely, the angle between coronal plane and a line passing through FN and BT, with a cutoff point of 44° or less denoting poor visibility. The other method was measuring the distance between the coronal plane and the RW, with a cutoff point of 4 mm or less denoting low visibility. It was not defined exactly how to obtain the coronal plane. We agree with them in that identifying landmarks of EAC on CT is difficult in children, but we do not agree that this affects the accuracy of the results. Even when the bony cartilaginous junction and the annulus did not coincide in the same cut, we transposed one point till it reached the level of the other, in the present study. Then the line could be drawn accurately. In the present study, the prediction line of Kashio was significant (n=32) (P <0.002), unlike the reproduction of the Kashio method by Fouad and colleagues, who found the method to be statistically insignificant. However, it should be mentioned that they correlated to Saint Thomas classification where there were 4 grades of visibility not 3 as the present study and the original by Kashio. This would fragment the data and increase the possibility of statistical insignificance. The intraoperative data were extracted from videos, which in our opinion underestimates the visibility of the RWN through PT. Additionally, strangely in that study, Kashio’s prediction line was reported to pass anteriorly in 6%and posteriorly in 94%, and none was reported as passing through the middle , which was 60% in the Kashio’s study .
Another study tried to estimate FN effect on visibility through PT by using a tangent to the BT of cochlea. This in our opinion is not accurate, as we believe that rotation of the cochlea is different from one ear to the other. It is worth mentioning that this study had the incudostapedial joint access as the primary concern not the RWN, possibly assessing the accessibility for middle ear implants. The radiological findings of this work were not correlated with intraoperative findings .
A study used relation to the mastoid antrum and thickness of FN bony canal on CT. This method was intraoperatively correlated. Unfortunately, it focused on difficulty of performing PT and risk of FN injury not visibility of the RWN. This study also included pathological ears .
In case of invisible RWN through PT, additional measures were taken to visualize the RWN, for example, removing incus and incus bridge, cutting the CTN.
The weakness about this study was that manually drawn lines on CT might have inter and intraobserver variability and also would be affected by the thickness of cuts during the acquisition of the scan from one device and setting to the other. Moreover, our assessment was focused on RWM-intended insertions not cochleostomy, so wider exposure was needed.
The research was supported by Alexandria Faculty of Medicine.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4]