Management of Unilateral Hearing Loss in a 14-Year-Old with Internal Auditory Canal Duplication Using a Bonebridge Bone Conduction Implant

Introduction

Duplication of the internal auditory canal (dIAC), in which there are separate canals for the cochlear and the facial nerve, is a very rare congenital anomaly that arises during embryonic development. To date, approximately 70 cases of dIAC have been described in the literature [1–20] and it is estimated that this anomaly accounts for 0.019% of patients with profound hearing loss [21]. Morphologically, a duplicated internal auditory canal is divided into 2 canals by a complete or incomplete bony septum: an anteroinferior canal, which most commonly contains the facial nerve, and a posterior-inferior canal, which contains the vestibulocochlear nerve [21]. dIAC is a pathognomonic sign of cochlear nerve deficiency (CND) – severe cochlear nerve hypoplasia or aplasia – which results in severe or profound sensorineural hearing loss. This reduces the chance of successful cochlear implantation, but does not rule it out [7,18,21,22].

Another alternative treatment method for unilateral aplasia or severe hypoplasia of the cochlear nerve with preserved normal hearing in the other ear is a bone-conductive implant (eg, the Bonebridge implant) using the contralateral routing of signal (CROS) phenomenon. This phenomenon works by transferring sound from the deaf ear to the normal hearing ear, resulting in awareness of sound on the deaf ear side, especially when the noise source is on the hearing ear side [23].

Bonebridge is a semi-implantable device comprising a sound processor and a coil that generates bone vibrations. These vibrations are transmitted via screws attached to the mastoid process, so osseointegration is not required [24–27].

The device is intended for patients with conductive or mixed hearing loss with a threshold for bone conduction not exceeding 45 dB HL, or as a contralateral routing device in unilateral deafness (single-sided deafness, SSD) [27,28], where the hearing threshold for air conduction in the healthy ear should not exceed 20 dB at frequencies of 0.5, 1, 2 and 4 kHz, and should be lower than 70 dB HL in the diseased ear because the interaural damping phenomenon is about 50 dB [25,27,29,30].

This is the first case report of a patient with unilateral internal auditory canal duplication and cochlear nerve aplasia, single-sided deafness (SSD) [23], who received a Bonebridge implant in the CROS configuration [25,27].

The Bonebridge implant has been successfully used to treat single-sided deafness (SSD) of various etiologies. In a prospective multicenter study, Kim et al described 30 patients with SSD who were fitted with the bone conduction implant 602 (BCI602) active transcutaneous bone conduction implant device [27]. The study showed significant improvements in all domains of the Abbreviated Profile of Hearing Aid Benefit questionnaire (APHAB), and the researchers concluded that the BCI device can provide functional hearing improvement without causing any adverse effects. They also determined that the BCI device is a viable option for patients with acquired SSD and long-term deafness [27].

This report is of a 14-year-old girl with unilateral SNHL and vestibulocochlear nerve (VIII cranial nerve) aplasia due to dIAC who was treated with a Bonebridge bone conduction implant.

Case Report

The Institute of Physiology and Pathology of Hearing – World Hearing Center in Kajetany was visited by a 14-year-old girl who was diagnosed with unilateral hearing loss at the age of 13 during testing at school. Until then, the girl had not noticed any hearing loss, reporting only a reduction in speech understanding in noisy environments. Her hearing screening at birth was normal, and her perinatal and neonatal history was unremarkable. On otoscopic examination, the tympanic membranes were bilaterally normal, preserved, with reflexes, and the presence of a small preauricular exostosis was noted on the left side. There was no history of recurrent otitis media, and speech development was normal. Her tonal audiometry revealed unilateral deafness of the right ear (Figure 1). Impedance audiometry showed type A tympanograms bilaterally, stirrup muscle reflexes IPSI absent in the right ear, present in the left ear, and CONTRA present in the right ear, absent in the left ear. Verbal audiometry showed a speech discrimination rate of 0% in the right ear and 100% in the left ear.

Due to the uncharacteristic history, this patient with late-onset hearing loss was referred for imaging studies to expand the diagnosis.

A computed tomography (CT) scan showed asymmetry of the internal auditory canals (IAC): the right canal was duplicated, divided into a separate canal for the facial nerve (2.2 mm) and the vestibulocochlear nerve, which was reduced to a diameter of <1 mm in the middle section. There was also stenosis and sclerosis of the right cochlear aperture without malformation or sclerosis of the bony labyrinth; the results showed intermediate features of aplasia or severe hypoplasia of the right cochlear nerve. In comparison, the left internal auditory canal reached a diameter of 4.4 mm (Figures 2–4).

Magnetic resonance imaging (MRI) confirmed a right bipartite canal with severe stenosis of the canal to n. VIII. On the right, a single nerve was identified entering the facial part of the internal auditory canal, while no nerve entering the vestibulocochlear part was visualized. On the left side, no pathology was identified in the VII and VIII nerve complex. The configuration of the membranous labyrinth was also normal bilaterally (Figures 5, 6). After obtaining radiological images, unilateral aplasia or severe hypoplasia of nerve VIII on the right side was suspected.

In view of the test results obtained, she was referred for diagnosis with a bone implant as a CROS, using the phenomenon of bone conduction of sound. In a free-field verbal audiometry test, the degree of speech discrimination with a classic air conduction hearing aid in the right ear with active masking of the opposite ear at a level of 65 dB was 0%.

During the simulation, a test of speech understanding in noise was carried out using a bone hearing aid mounted on a soft band. The Polish Matrix Sentence Test consists of 20 sentences, each sentence containing 5 words: name, verb, number, adjective, and noun. The test is used to determine the speech recognition threshold (SRT). The test uses an adaptive procedure, ±2 dB, then ±1 dB. For the simulation, an Oticon Ponto 5 processor mounted on a soft band was used, adjusted based on the PTA test result. During the simulation, the patient achieved MATRIX test results (in SSD configuration) with SRT device= −0.4 dB SNR, without device= +3.5 dB SNR. Subjectively, the patient noticed a marked improvement in her hearing with the bone conduction hearing aid.

At the age of 14, the patient received a Bonebridge 602 implant in the right ear in a CROS configuration. During implant activation 1 month after device implantation, she achieved SRT= −10.3 dB SNR values in the SSD configuration in the Bonebridge Implant Matrix test.

To assess satisfaction with daily use of the Bonebridge implant, she completed the Abbreviated Profile of Hearing Aid Benefit (APHAB) questionnaire before surgery and 1 month after processor activation. Subjectively, the patient rated the effect of the solution as very good. The results of the APHAB questionnaire indicate improvements in hearing in 3 subscales: ease of communication (EC), communication in background noise (BN), and communication in reverberation (RV). The largest gains were recorded for situations in the third subscale. The fourth category, concerning the acceptance of unpleasant sounds, remained unchanged. An analysis of the results of the APHAB questionnaire is presented in Figure 7.

Discussion

This case study emphasizes the importance of high-resolution imaging in pediatric SNHL cases, and presents the initial findings of the Bonebridge implant as a CROS device for patients with dIAC.

Ear canal duplication is a very rare congenital defect, with approximately 70 cases described in the literature to date according to the PubMed article database. The incidence of IAC duplication in patients with sensorineural hearing loss is 0.019% [21]. Duplication of the internal auditory canal was first described by Clemens and Sandsrom in 1975 [8]. dIAC is also associated with pontine tegmental cap dysplasia (PTCD), a rare neurological syndrome that results in a hypoplastic ventral pons, tegmental cap at the dorsal pons, and cranial nerve dysfunction [31].

The structure of the inner ear of the described patient did not show any abnormalities, therefore it was possible to record acoustic otoemissions in the screening test after birth despite the absence of the cochlear nerve, resulting in a late diagnosis of unilateral deafness.

The embryological development of the inner ear is locally controlled, does not depend on any neural stimulus, and occurs independently of the development of the inner ear canal with the cochlear and facial nerves running through it. In contrast, the development, migration, growth, differentiation, and survival of immature cochlear and vestibular neurons is stimulated by a number of neurotrophic factors, including brain-derived neurotrophic factor (BDNF), nerve growth factor (NGF), neurotrophin-3 (NF-3), and neurotrophin-4/5 (NF-4/5) produced by the otic vesicle [32–36]. The IAC eventually develops by 5 months of fetal life and it has been hypothesized that its diameter depends on the volume of the migrating cochlear nerve fibers [36,37].

These processes may explain the isolated IAC anomaly and cochlear nerve deficiency found in this patient, without a defect in the cochlea itself, as a disturbance in the release of trophic factors through the ear areola at very early stages of development [32].

The bony constrictions of the internal auditory canal are best visualized on a CT scan of the temporal bone – the bony window – while the cochlear and vestibular nerves can be visualized on T2-weighted MRI. The course of the cochlear nerve should be followed along the entire length of the internal auditory canal, from the bottom of the exit in the cochlear field to the entrance in the brainstem at the cerebellopontine angle [36].

Cochlear nerve deficiency (CND) refers to a deficiency of cochlear nerve fibers in the form of hypoplasia (underdevelopment) or aplasia (non-development) as assessed by MRI. However, this is not a histopathological definition, as the cochlear nerve fibers may run together with the vestibular or facial nerves, and activation of the primary auditory cortex has been reported on functional MRI in a patient with nerve aplasia, and limited benefit from cochlear implantation has been observed in some cases. It is therefore important to interpret imaging findings in the context of clinical and audiological studies. CND is estimated to occur in 18–21% of cochlear implant (CI) recipients, although there were large differences in outcomes for hypoplasia and aplasia, so it is extremely important to differentiate between them and assess the degree of CND [36].

According to Casselman’s classification, the cochlear nerve can be considered as not deviating from the norm if its diameter is comparable to the contralateral cochlear nerve and its diameter is greater than or equal to the ipsilateral facial nerve. Hypoplasia of the nerve occurs if the cochlear nerve does not meet the above criteria, and aplasia occurs if it is completely absent in the sagittal sections of the IAC [38].

The classification by Gouverts et al considers the degree of cochlear nerve deficit together with an assessment of inner ear morphology: type I – complete absence of the cochlear-vestibular nerve; type IIa – cochlear nerve branch absent or hypoplastic, vestibular nerve present and associated vagus dysplasia; and type IIb – cochlear nerve branch absent or hypoplastic, vestibular nerve present but normal labyrinth morphology [39].

The new IAC nerve classification system according to Birman et al is based on the number of nerves visualized: grade 0 – no nerves; grade I – 1 nerve; grade II – 2 nerves; grade III – 3 nerves; grade IV – 4 nerves, 1 of which is hypoplastic; and grade V – 4 normal nerves [40].

When assessing the cochlear nerve for possible abnormalities, the following should be assessed: the spindle and its degree of sclerosis, the cochlear field at the base of the cochlea (considered constricted when it reaches <1.2 mm) [41], and the internal auditory canal (normal 2–8 mm, average 4 mm) [4].

According to Sennaroglu and Bajin’s classification, when assessing a possible cochlear nerve defect, consideration should be given to where the nerve exits the cochlea [42]. The cochlear nerve should be assessed on MRI, while sclerosis of the cochlear spindle and narrowing of the cochlear field on CT may indirectly indicate cochlear nerve pathology.

The above changes can occur in isolation or may be associated with inner ear, vestibular and facial nerve abnormalities [4], or other external or middle ear defects [3]. Individuals with CND are more likely to have labyrinthine defects than those without cochlear nerve hypoplasia or aplasia [36].

Defects of the internal auditory canal (such as atresia, stenosis, hypoplasia, and aplasia) account for approximately 12% of congenital malformations of the temporal bone [43,44].

Approximately 2% of cases of congenital profound deafness are thought to be due to an abnormality involving the cochlear nerve, with recent studies reporting that cochlear nerve deficits affect 18–21% of CI recipients [36].

Internal auditory canal duplication occurs when a bony septum (complete or partial [21]) divides the internal auditory canal into 2 compartments. The duplication may be unilateral or bilateral and the vestibulocochlear nerve may be aplastic or hypoplastic [45].

It seems that a more accurate term for the morphological picture of dIAC would be ‘bifurcation’ of the internal auditory canal, as the structures contained within the IAC are not duplicated but separated, but the English-language literature refers to a similar phenomenon as duplication.

In most cases of dIAC, the facial nerve and its function are preserved, but a few cases of n.VII hypoplasia or its paresis have been described [13,19,21,22,46].

Hearing loss has been reported in most cases of dIAC: severe, profound, or total deafness associated with hypoplasia or aplasia of the cochlear nerve [22]. The degree of hearing loss depends on the amount of preserved cochlear nerve fibers, which can be indirectly assessed by the diameter of the IAC and the appearance of the bony septum separating the 2 nerves – if the septum extends to the proximal part of the IAC, a greater degree of hearing loss can be expected (complete septum); the more distal the bony septum ends, the greater the degree of hearing and preservation of the cochlear nerve (partial septum) [45,46].

Hearing loss is most often present from birth, which was probably the case in our patient, while she developed normal speech due to compensation with the hearing ear, so that the hearing loss was not detected until the teenage years.

According to Wang et al, patients with IAC stenosis have a dysplastic vestibulocochlear nerve and may benefit from cochlear implantation, whereas patients with IAC duplication have an aplastic or hypoplastic VIII nerve, making CI contraindicated [21].

However, several cases of successful cochlear implantation have been described and benefits have been observed [2,7,18,22,47]. The effects of implantation depend on the degree of preservation of the cochlear nerve, the age at detection of hearing loss, the age at cochlear implantation, and the hearing status of the fellow ear, and the choice of treatment should be individualized for patients with dIAC [7,18].

Due to the late detection of hearing loss, probable cochlear nerve aplasia on MRI, normal hearing in the fellow ear, and satisfactory results of bone implant simulation, our patient was referred for bone conduction implantation as a CROS.

The phenomenon of bone conduction has been known since ancient times and involves the perception of cranial vibrations as an auditory sensation. Two pathways contributing to bone conduction hearing have been identified: the first involves fluid inertia in the cochlear lumen, cochlear wall compression and changes in cerebrospinal fluid pressure, and the second involves the middle or outer ear. Both mechanisms induce a fluid-pressure gradient on either side of the cochlear basement membrane, causing it to vibrate [48].

The contralateral routing of signal (CROS) phenomenon works by transferring sound from the deaf ear to the properly hearing ear, providing awareness of sound on the deaf ear side, especially when the sound source is on the hearing ear side [23].

The Bonebridge implant consists of an internal part consisting of a transducer (BC-FMT or bone-conductive floating mass transducer) fixed in the mastoid bone with 2 screws that transmit vibrations to the bone, a demodulator and a coil, and an external sound processor held on the skin surface by a magnet [25,26]. The device is intended for patients with conductive or mixed hearing loss with a threshold for bone conduction not exceeding 45 dB HL, or as a contralateral routing device in unilateral deafness (SSD) [27], where the hearing threshold for air conduction in the healthy ear should not exceed 20 dB at frequencies of 0.5, 1, 2, and 4 kHz, and should be lower than 70 dB HL in the diseased ear because the interaural damping phenomenon is about 50 dB [25,29,30].

The Polish Matrix Sentence Test measures speech comprehension in noise. The test consists of 5 columns of 10 names, 10 verbs, 10 numbers, 10 adjectives, and 10 nouns, which can be randomly combined to produce 100 000 unique sentences of a particular grammatical structure. The lower the speech reception threshold (SRT), the higher the noise level at which 50% speech intelligibility is achieved – meaning that half of the correct responses at different speech signal-to-noise-ratio (SNR) – the better the speech understanding [49].

The APHAB questionnaire involves a patient’s self-assessment of difficulties in communicating with others under different acoustic conditions or in recognizing sounds in different situations. The questionnaire consists of 24 questions to assess communication difficulties in 4 subscales: communication in favorable conditions (ease of communication, EC), communication in the presence of background noise (BN), communication in the presence of reverberation (RV) and ability to tolerate unpleasant sounds (aversiveness, AV). Scores are calculated as the arithmetic mean of the frequency of problem occurrence for all statements in a given subscale. A lower score indicates fewer communication problems and greater benefit from the device used. After using the Bonebridge implant in the CROS configuration, our patient scored lower on the first 3 subscales of the APHAB questionnaire, indicating fewer problems with communication in different acoustic conditions [50].

Kim et al report that, in a group of 30 patients with SSD following BCI implantation, all APHAB questionnaire scores showed significant improvement, including those relating to aversiveness [27]. However, aversiveness did not improve in our patient. Congenital SSD was a significant factor influencing subjective benefit [27].

Our patient achieved a score of SRT= −10.3 dB SNR on the Matrix test with the Bonebridge implant, SSD configuration, with improved speech perception and reduced head shadow effect [23].

In cases where cochlear implantation is not possible, classical CROS systems or implants with bone conduction in CROS configuration can be used to eliminate the head shadow effect and improve speech understanding, especially in noisy situations [51]. The implantable CROS system uses bone conduction to transmit vibrations through the bones of the skull directly to the healthy ear. Normal hearing in the other ear is a prerequisite for the use of this solution. The advantage of the Bonebridge implant as a CROS over the classic CROS system is primarily the comfort of wearing it, the absence of a possible occlusion effect, and better sound quality. The limitation of classical and implantable CROS systems in patients with SSD is the inability to restore binaural hearing and the unresolved problem of sound source localization [52].

Conclusions

Duplication of the internal auditory canal is pathognomonic for severe cochlear nerve hypoplasia or aplasia, which can have a significant impact on the choice of treatment and implant. It is essential to perform an imaging study before deciding on implantation. In the case of an isolated ductal anomaly without a cochlear defect, hearing screening at birth may not detect a hearing loss. In this defect, it is possible to record acoustic otoemissions in an unaltered cochlea with impaired conduction through the hypo- or aplastic auditory nerve, because the embryogenesis of the inner ear and the inner ear canal, including n.VIII, proceed independently. If unilateral hearing loss is present from birth, it may go unnoticed by the child and others due to good adaptation of the other ear where normal hearing is maintained. In the case of a unilateral anomaly with no hearing impairment on the opposite side, bone conduction implantation as a CROS should be considered. The patient may experience a reduction in the “head shadow” phenomenon and an improvement in speech understanding.