The Effectiveness of Cochlear Implants in Patients with Inner Ear Malformation

Cochlear implant (CI) is a surgical solution for patients who suffer from unilateral or bilateral severe to profound sensorineural hearing loss (SNHL) and is one of the most challenging ear surgeries as it needs a highly professional surgeon to perform it. This is especially the case for patients who have inner ear malformations (IEM), facial nerve abnormalities, cerebrospinal fluid leakage (CSF), gusher, or difficulty in finding the cochlea itself. Surgeons during the operation must be ready for any difficulties and then are required to modify the surgical approach or choose special electrodes according to the IEM. Several clinical studies have shown a significant improvement in auditory and speech perception performance after performing CI surgery for pediatric and adult patients who suffer from severe to profound SNHL in one ear or both sides of the ears. Abstract


Introduction
Cochlear implant (CI) is a surgical solution for patients who suffer from unilateral or bilateral severe to profound sensorineural hearing loss (SNHL) and is one of the most challenging ear surgeries as it needs a highly professional surgeon to perform it. This is especially the case for patients who have inner ear malformations (IEM), facial nerve abnormalities, cerebrospinal fluid leakage (CSF), gusher, or difficulty in finding the cochlea itself. Surgeons during the operation must be ready for any difficulties and then are required to modify the surgical approach or choose special electrodes according to the IEM. Several clinical studies have shown a significant improvement in auditory and speech perception performance after performing CI surgery for pediatric and adult patients who suffer from severe to profound SNHL in one ear or both sides of the ears.
In 1791, an Italian physician and anatomist described the first inner ear malformation of an eight-year-old deaf boy. From that time "Mondini dysplasia" has been used to describe any inner ear anomalies [1]. Jackler et al. [2] was the first one to propose a classification of IEM based on embryonic arrested. Sennaroglu & Saatci [3] have proposed a similar classification of IEM but with a distinction between incomplete partition type I (IP-I) and type II (IP-II). In 2015, Jeong & Kim [4] introduced a new classification of cochleovestibular malformation (CVM) based on the morphology of the cochlea and the modiolus. In 2017, a new classification and management of IEM was proposed by Sennaroğlu & Bajin [5]. In 1983, Mangabeira Albernaz reported the first CI for a patient with a malformed cochlea [6]. The most common classification that has been used to determine the IEM is Jackler et al. who classified the IEM into five categories and Sennaroğlu & Bajin [5] who classified the inner ear anomalies into eight categories (Table 1).   -Enlarged vestibular aqueduct (EVA): the midpoint between the posterior labyrinthine and operculum is equal or more than 1.5 mm with normal cochlea, vestibule, and SCCs.
-Cochlear aperture abnormalities: CA is hypoplastic when the width less than 1.4 mm, and aplastic when there is no canal or replaced by bony.

Materials and Methods
This study was divided into two sections; the first consisting of a literature review which provides an analysis of previous studies. These studies were made up of scholarly journals and approaches (using database parameters such as PubMed, Google scholar, NCPI, Europe PMC, Academia, Balkan medical and SAGE Journals as well as using Research Gate to find articles in related fields. The collection of articles is from the period between 1987 till 2020. The second section was about a retrospective study: After the ethical approval of research committee of King Abdulaziz University Faculty of Medicine, and informed consent was signed from all participants, data were retrospectively compiled for this study consisting of thirty-two participants, including twenty-three children and nine adults who underwent forty CI surgeries at an average age of 3 years and 8 months in children, and 28 years and 4 months in adults (range between 2 to 50 years), sixteen patients were excluded from this study due to missing data in the postoperative aided hearing threshold and speech tests. The current study included sixteen patients, nine children and seven adults who have severe to profound SNHL with IEM. Out of twenty CI's in total, four patients with bilateral and twelve with unilateral hearing loss, were implanted with (MED-EL Innsbruck, Austria), between a period from 2012 to 2020.
The inner ear anomalies were divided into: (n=1) incomplete partition type I, (n=12) incomplete partition type II (IP II), (n=6) enlarged vestibular aqueduct (EVA), and (n=1) common cavity (CC). The CI outcome has been evaluated by using a free filed (FF) test with adults and play audiometry or visual reinforcement audiometry (VRA) with children, In addition, the speech recognition test (SRT) and speech discrimination score (SDS) were evaluated by using the standardized Arabic speech materials lists and the patients were also evaluated with an Arabic version of categories of auditory performance II (CAP II), a computerized tomography (CT) scan and magnetic resonance imaging (MRI) was performed before the CI surgery to identify the type of inner ear anomalies. A postoperative CT scan was also performed to check the place of the inserted electrode inside the cochlea.

Literature review
The first section of this study is analyzing the outcome of auditory and speech performance of previously published studies between 1987 until 2020 for patients who were diagnosed severe Global Journal of Otolaryngology to profound sensorineural hearing loss, including IEM, and were implanted with a cochlear implant. The engine search showed a total of 799 related studies, out of which we found sixty-four studies that fulfill the inclusion criteria and are related to the outcome of a CI with IEM. The studies have included a total of 17636 CI subjects including 2836 (16%) cases who were diagnosed with IEM with this being categorized into: (n=947) enlarged vestibular aqueduct at 33%, (n=153) common cavity at 5%, (n=135) cochlear hypoplasia at 5%, (n=102) incomplete partition "unclassified" at 4%, (n=156) incomplete partition type I at 6%, (n=680) incomplete partition type II "Mondini" at 24%, (n=26) incomplete partition type III at 1%, (n=637) other malformations at 22% including single or multi malformations.
Additionally, other studies reported a variable CI surgery outcome among patients based on types of inner ear anomalies [2,4,[23][24][25][26][27][28][29][30][31][32][33][34][35][36]. Several authors reported that patients who have mild anomalies such as EVA and IP II have a better outcome than patients who have severe malformations such as a common cavity [4,[24][25][26][27][28]31]. Nevertheless, Tay et al. has concluded that the outcome of patients who have an absent cochlear nerve, electrode folding and underlying neurological disorders is poor [37]. Xia et al. reported that twenty-one patients with a common cavity had benefited from a cochlear implant with the average performance being lower than patients with normal cochlea [38]. Three studies reported in a poor outcome after CI in patients who have a narrowing internal auditory canal [4,34,37].
Busi et al. concluded that it is possible to get a good performance in the cochlear implant in children with ear or brain malformation abnormalities [39]. They also noticed the presence of central nervous system anomalies which can be an indication of a worse outcome. Aside from a negative expectation of common cavity and stenosis of an internal auditory canal for less than 2mm. Papsin concluded that care should be taken when opting for implanting patients with a narrow internal auditory canal [32]. Rachovitsas et al. concluded that children with inner ear malformation performed much better than patients with inner ear dysplastic because of their disabilities (such as CHARGE syndrome, and mental retardation). Bilingualism can also be considered as one of the factors that can affect the outcome of inner ear anomalies.
It is recommended to evaluate the cognitive and developmental delay before performing CI surgery and for counselling the parents about the expected outcome and habilitation [35].
Szudek et al. [40] reported a worse outcome from children and adults who were affecting by these factors; late of implantation, presence of gusher, and incomplete electrode insertion. Kim et al. [34] observed a poor cochlear implant outcome induced from cochlear nerve hypoplasia. Incesulu et al. had reported that they cannot accept CI surgery for inner ear anomalies except cochlear or cochleovestibular nerve agenesis due to cochlear implant contraindication [16]. Umashankar and Jayachandran have also shown a slow cochlear implant outcome of an individual with Goldenhar Syndrome associated with IEM [41]. Table 2 shows the demographic data of the sixteen subjects who were diagnosed with severe to profound SNHL, the inner ear anomalies were divided into: (n=1) incomplete partition type I, (n=12) incomplete partition type II (IP II), (n=6) enlarged vestibular aqueduct (EVA), and (n=1) common cavity (CC). There are twelve patients with unilateral CI (seven on the right side, and five on the left side), and four patients with a bilateral cochlear implant. All patients were implanted with MED-EL devices between 2012 until 2020. The types of the internal implant were distributed as follows; four of SONATAAti100, seven of CONCERTO, seven of SYNCHRONY and two of SYNCHRONY-P. It seems that most patients underwent CI surgery with a short electrode in size such as compressed, medium, form 24, Form 19, and Flex 24. The post CT scan demonstrated that fourteen CI had full insertion electrodes except three, due to fault in selecting the appropriate size of the electrode.  Table 3 shows that all the patients have a relative improvement in the post-implantation performance of aided hearing threshold and speech tests. Among the twenty CI surgeries, it has been seen that the average score of each frequency is between 31 to 36dB (Figure 1), the progression of speech performance after CI surgery was high as the mean average of the SRT was 31 ( Figure  2), the score of SDS was 72% ( Figure 3) and all the participants reaching a CAP II score between 5 to 9 had an average score of 7 out of 9 ( Figure 4) indicating that they can understand common phrases, converse with a familiar person without lip reading, use the telephone with a person they are familiar with, follow a conversation with a group in a noisy environment and are able to use the telephone with unknown people in an unpredictable context. There were four patients having had bilateral CI that benefited from their CI after implantation.  Left  55  55  45  55  55  60  53  50  76%  6   S13  Right  35  30  30  30  30  30  31  30  70%  7   S14  Right  25  30  30  35  35  35  32  35  90%  9   S15  Left  40  30  40  35  30  30  34  40  70%  8   S16  Left  40  35  35  30  30  35  34  35  60%  7 PTA4: Average Pure Tone Audiometry of (500Hz, 1,2,4KHz), SRT: Speech Recognition Threshold, SDS: Speech Discrimination Score, CAP-II: Categories of Auditory Performance II.     Table 4 shows the average outcome of each anomaly, twenty cases having a good average at the aided hearing threshold and speech performance after performing CI. Six cases with enlarged vestibular aqueduct had a significant improvement in auditory and speech tests, twelve cases with incomplete partition type II had a good outcome after implantation, one patient with an incomplete partition type I and one patient with a common cavity showed an improvement but less than the average of EVA and IP type II. Among the twenty patients with IEMs, six patients with EVA achieved the highest performance scores with approximately 76% in the SDS and 8 in the CAP-II, twelve patients with IP II achieved 71% in the SDS and 7 in the CAP-II, one patient with IP I achieved 60% in the SDS and 6 in the CAP-II, and one patient with CC had the lowest scores with approximately 52% in the SDS and 5 in the CAP-II.

Discussion
A cochlear implant is a worldwide solution for patients who have severe to profound sensorineural hearing loss. This study has resulted in a considerable benefit in the aided hearing threshold and speech tests in children and adults with inner ear anomalies. This is especially in minor malformations like an enlarged vestibular aqueduct and incomplete partition type II. In addition, bilateral cochlear implantation for patients with IEM is effective. These results are in line with previously published works. Grover M. et al. had reported all subjects improved performance after cochlear implantation, especially patients with enlarged vestibular aqueduct [25]. Bille et al. [22] has studied 28 patients with incomplete partition type II and resulted that the outcome of patients with malformed cochlear is comparable to patients with normal cochlear anatomy. Qi et al. has studied 108 IEM with Mondini dysplasia out of 700 patients and concluded that the post-operative outcome of children with Mondini is equal to children with the radiological normal inner ear [14].
Arnoldner et al. concluded that the auditory response of speech for patients with IEM is like those in children with normal cochlea factoring in the success of implantation such as a preoperative radiological examination, a well-performed surgery, and an individually tailored postoperative rehabilitation program [18]. Van Wermeskerken et al. concluded that speech perception in children with inner ear anomalies is like that of other congenitally deaf children after an average of 2 years followup [19]. Chadha et al. stated that bilateral cochlear implantations with inner ear anomalies are effective and safe [42]. Pradhananga Global Journal of Otolaryngology et al. has studied three cases with isolated EVA and concluded that a cochlear implant with a patient who has large vestibular aqueduct syndrome is effective and favorable [43].
Ozkan et al. reported that the outcome of cochlear implantation is acceptable in inner ear anomalies with patients with a visible cochlear nerve on magnetic resonance imaging. It is of fundamental importance to take the anatomical differences into account (especially after implantation during each visit), the rehabilitation sessions and to deal with each CI patient according to their needs [44]. The limitation of this study needs to be taken into account for any future research related to this topic: More research is required to evaluate the outcome of CI in patients with inner ear anomalies by using an Arabic standardized speech perception test in a different culture. It is also necessary to have a high number of patients with IEM with all types of anomalies, especially the minor and major inner ear malformation.

Conclusion
A cochlear implant is effective for children and adults who have severe to profound sensorineural hearing loss with inner ear malformations despite other factors which may influence the outcome. These can consist of the age at implantation, syndromes, pre-and post-lingual, duration of deafness, preoperative radiological examination, intraoperative challenging, the proper candidate electrode selection, postoperative complications, a well-performed surgery, an individually tailored postoperative rehabilitation program and family support. The outcome of the minor inner ear malformation such as EVA and IP-II is higher than the performance of the major anomalies like a common cavity. The most frequent inner ear anomaly and candidate for the CI with a considerable improvement after implantation is enlarged vestibular aqueduct.