Introduction

Membranous malformations account for 80% of congenital hearing loss cases. The bony abnormalities of the temporal bone can be radiologically demonstrated in 20% of cases. This group with bony anomalies present surgical challenges and also affect decision making in the management. SNHL can result from a combination of Temporal bone abnormalities that may involve the labyrinth, internal auditory canal, cerebellopontine angle, brain stem, or auditory pathways.

Cochleovestibular anomalies are a variety of congenital anomalies, the severity of which depends on the time at which an insult occurs during embryogenesis. These anomalies have been classified by Jackler1 et al. [1] in 1987 which was modified by Sennaroglu’s in [2] (Table 1). Starting at the 3rd week of gestation, an insult during each subsequent week, up to the end of the 7th week, results in a distinct inner ear abnormality. Total labyrinthine aplasia (Michel) occurs if the embryogenesis is insulted as early as the 3rd week and Incomplete partition (IP) type 2 (Mondini) is seen if it is at the 7th week. The common cavity has been described as one of the common malformations which results if the embryogenesis is halted in the 4th week. Incomplete partition anomalies represent a group of cochlear malformations, where there is a clear differentiation between cochlea and vestibule. The most common Incomplete partition type is Type 2 (Classical Mondini), IP type 1 is a more severe anomaly resulting in a cystic cochleovestibular malformation. IP type III cochlear malformation is the type of anomaly present in X-linked deafness, which was described by Nance et al. Sennaroglu and Bajin [3] added Isolated Enlarged Vestibular Aqueduct (EVA) and Cochlear aperture abnormalities to the above types of Inner Ear Malformations.

Table 1 Modified sennaroglu’s classification
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Isolated EVA with normal cochleovestibular structures in prospective cochlear implant candidates is less common, but EVA is found associated with other Cochleovestibular anomalies like in Mondini. Cochlear aperture abnormalities are usually seen with Internal auditory canal malformations. Narrow IAC (midpoint less than 2.5 mm) is seen on HRCT should be evaluated with a thickness of the 8th nerve on MRI. The hypoplastic or aplastic cochlear nerve on MRI has an important bearing on the management of congenital deafness cases. CT and MR imaging often play a complimentary role in its evaluation and preoperative assessment. Imaging has an important role in deciding candidacy for the feasibility of implant, providing realistic preoperative counselling and predicting postoperative outcomes.

Methods and Materials

The study aims to determine the prevalence of inner ear anomalies and the frequency of different anomaly types among preoperative cases for cochlear implantation.

This is a Prospective observational study of all patients who presented with severe to profound SNHL between June 2017 to May 2019 in KIMS HOSPITALS SECUNDERABAD. All subjects underwent combined CT of the temporal bone and MRI. CT scan was done on a 128-slice scanner. CT was done using a multi-detector scanner, 0.625 mm axial scans of the temporal bone were acquired using a high-resolution bone technique. The scans were retrospectively targeted for the right and left sides, and subsequently reconstructed in the coronal plane, again targeting the right and left sides individually, as well as the entire skull base. MRI was performed on a 3 Tesla MR system. MRI technique was done with selected sequences. Both axial and coronal images were reviewed in the bone window and 0.8 mm thin slices on HRCT of the temporal bone. 3D volume-rendered images of the inner ear structures were also assessed by post-processing on the workstation.

Mixed, moderate hearing loss cases were excluded. Patients with acquired causes of SNHL like meningitis leading to labyrinthitis ossificans were excluded.

Results

Total 76 cases during the study were reviewed. Maximum patients in our study belonged to 1–3 year age group constituting 70% of the total sample and 87% of the study population was less than 5 years of age (Table 2). Overall study population constituted 47 (62%) males and 29 (38%) females.

Table 2 Age-wise distribution
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Malformations were identified in 11 cases (Fig. 1). 5 cases had Narrow IAC which was the most common anomaly followed by IP-2 and Globular vestibule (3). The EVA was seen in 2 patients, 2 had cochlear dysplasia, 1 had IP type 1and 1 patient had common cavity. Michel deformity was found in one case (Fig. 2). Some of these patients had multiple anomalies, so the total number of malformations was 18 (Figs. 3, 4, 5, 6, 7).

Fig. 1
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Percentage of number of cases with congenital inner ear malformations

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Fig. 2
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Congenital inner ear malformations

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Fig. 3
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A case of 4 years old male child with bilateral profound SNHL with right narrow IAC, associated with Hypoplastic auditory nerve

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Fig. 4
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8 year old female with bilateral profound SNHL showing Globular vestibule on the left side

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Fig. 5
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8 year old female with bilateral severe SNHL showing IP type 2 anomaly. CT is showing a cystic apex with an enlarged vestibular aqueduct and 1.5 turns of cochlea

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Fig. 6
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18-year-old female with bilateral profound SNHL, a case of Labyrinthine aplasia (Michel deformity) on the left side. The patient was advised to go for ABI since the Cochlear Implant was contraindicated

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Fig. 7
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A 2 year old male child with bilateral profound SNHL, case of Common cavity on the left side, there is no partition between vestibule and cochlea

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7 of these 11 patients with malformations had cochlear implant surgery in our hospital.

Discussion

Cochleovestibular and other temporal bone anomalies are not uncommon. The prevalence of these anomalies has been reported by various studies to be ranging from 3 to 40%. The individual frequencies of various malformations have also been widely different in published literature. In our study, the incidence among prospective cochlear implant candidates with bilateral severe to profound SNHL has been 14.47% and the Narrow IAC is found to be the most common followed by globular vestibule and Incomplete partition type 2.

In the present series, 47 males and 29 females indicated no significant gender bias associated with Temporal bone anomaly. Most of our subjects were less than 5 years as shown by age distribution since the majority of our implantees were young children. The youngest was 1 year old and the oldest was 67 with only 10 cases above 6 years of age.

International literature shows a highly variable prevalence of Temporal bone anomalies. Mocan et al. [4] from Turkey in a study involving 481 cases found malformations in 187 (38%) with Incomplete partition being the most common (39%) followed by cochlear hypoplasia (8%). Aldhafeeri and Alnasi [5] in a paper published in 2016 noted 24 anomalies in 316 Cochlear implant candidates with EVA in 8 (33.3%) and Mondini deformity in 7 (29.1%), cochlear hypoplasia was seen in only 1 (4.1%).

In an article published in 2019, Klarov et al. [6] from Russia identified Inner ear malformations in 16 of 165 patients (9.7%). IP type 2 (34.3%) was common followed by IP type 3 (18.7%) and Internal auditory canal abnormalities (12.5%). Ahmed et al. [7] noted 10% as the frequency of Inner ear malformations (48/481) in cases with bilateral SNHL. In their series, complete labyrinthine aplasia was commonest followed by cochlear hypoplasia.

Higher prevalence of temporal bone malformation has been reported by Masuda and Usui [8] in 2019 (24.3%), Chinese study by Sun et al. [9] (30.69%).

An Indian study by Agarwal et al. [10] reported a prevalence of 14.28% which is very similar to the present series. In their series, Incomplete partition was the most common anomaly. whereas in the present study Narrow IAC was the commonest followed by IP type 2.S Masuda et al. [11] in 2013 reported a Narrow IAC, 32 (46.4%) as the most common anomaly like in the present study.

Many researchers focussed on cochleovestibular anomalies which did not include Internal auditory canal and nerve malformations, which explains the low reporting of Internal auditory canal anomalies. There is a wide variation in total prevalence (3–40%) as well as frequencies of individual malformations (Table 3).

Table 3 Different studies and incidence of inner ear malformations
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Conclusion

CT and MRI play an important role in the pre-op assessment of Cochlear Implant candidates. This helps in candidate selection, predicting intra op problems, choosing appropriate surgical techniques and electrode array. It also helps to counsel parents regarding prognosis.

The incidence of Temporal bone malformation in the present study is 14.47% with the Narrow IAC as the most common deformity followed by Incomplete partition type 2 and Globular vestibule. The reported prevalence of temporal bone malformation and frequency of individual anomalies is widely variable. Further studies with a larger sample size are needed to establish the prevalence of Temporal bone malformations in our population.