|Year : 2018 | Volume
| Issue : 1 | Page : 71-73
Congenital sensorineural hearing loss with anomalous vestibular nerve complex: A rare radiological finding
Asma Binti Abdullah1, Faizah Binti Mohd Zaki2, Luqman Bin Rosla1
1 Department of Otorhinolaryngology, University Kebangsaan Malaysia, Kuala Lumpur, Malaysia
2 Department of Radiology, University Kebangsaan Malaysia, Kuala Lumpur, Malaysia
|Date of Web Publication||24-May-2018|
Dr. Luqman Bin Rosla
Department of Otorhinolaryngology, Faculty of Medicine, University Kebangsaan Malaysia, Jalan Yaacob Latif, 56000 Cheras, Kuala Lumpur
Source of Support: None, Conflict of Interest: None
Radiological assessment is an essential tool for preoperative assessment of cochlear implant (CI) candidates with congenital sensorineural hearing loss (SNHL). It gives crucial information of the inner ear, vestibulocochlear nerve, and brain. Hypoplasia of the cochlear nerve bony canal is commonly associated with profound SNHL, whereby 88% of this abnormality was found in 95% of CI candidates. This finding may be associated with internal auditory canal hypoplasia or other cranial nerve abnormalities. We describe a rare case of a patient diagnosed with congenital SNHL exhibiting an interesting radiological finding of an anomalous vestibular nerve course alongside hypoplasia of the cochlear bony canal which could be postulated by the complex embryological development of the inner ear.
Keywords: Anomalous vestibular nerve, cochlear implant, computed tomography, magnetic resonance imaging, sensorineural hearing loss
|How to cite this article:|
Abdullah AB, Mohd Zaki FB, Rosla LB. Congenital sensorineural hearing loss with anomalous vestibular nerve complex: A rare radiological finding. Indian J Otol 2018;24:71-3
|How to cite this URL:|
Abdullah AB, Mohd Zaki FB, Rosla LB. Congenital sensorineural hearing loss with anomalous vestibular nerve complex: A rare radiological finding. Indian J Otol [serial online] 2018 [cited 2021 Apr 11];24:71-3. Available from: https://www.indianjotol.org/text.asp?2018/24/1/71/233124
| Introduction|| |
Sensorineural hearing loss (SNHL) in the pediatric age group is a condition with multiple etiologies, whereby 50% are genetic, 25% are acquired, and 25% are of unknown etiology. Worldwide, the prevalence of bilateral SNHL is 1 per 1000 live births. Therefore, early diagnosis of SNHL is vital as better developmental outcome can be achieved with early intervention.
Radiological assessment is essential for the diagnosis of congenital SNHL by providing crucial anatomical information of the inner ear, vestibulocochlear nerve, and brain. Hence, both computed tomography (CT) and magnetic resonance imaging (MRI) are required for preoperative assessment., Inner ear malformation may involve both the osseous and membranous labyrinth. It is not unusual for cranial nerve deficiency to cause congenital hearing loss. Hypoplasia of the cochlear nerve (CN) bony canal is commonly associated with profound SNHL, whereby 88% of this abnormality was found in 95% of cochlear implant (CI) candidates. This finding is known to be associated with internal auditory canal (IAC) hypoplasia or other cranial nerve abnormalities. In our case, we highlighted a case of bilateral congenital SNHL with cochlear hypoplasia with a rare, interesting finding of anomalous course of the vestibular and facial nerve.
| Case Report|| |
A 3-year-old female child who was diagnosed with bilateral SNHL presented with speech delay and showed inconsistent responses toward sound at the age of 2 years. There was no history of ear discharge or trauma. She was delivered full term via uneventful vacuum-assisted vaginal delivery. She had mild neonatal jaundice that did not require intervention. There was no family history of hearing disorders. Her immunization status was up to date and pneumococcal vaccine was also obtained.
On examination, no dysmorphism was noted, and otoscopy revealed normal tympanic membranes. The seventh cranial nerves were clinically intact. Nose, throat, and neck examination were unremarkable. Developmentally, her speech was delayed. Other developmental milestones are in keeping with chronological date.
Her first auditory assessment was done at age 2 years, 6 months. Auditory steady-state response examination reported severe-to-profound right ear hearing loss and profound hearing loss on the left. Visual response audiometry demonstrated responses only at 250 Hz (35 dBHL) and 500 Hz (55 dBHL). On acoustic brain response examination, right ear wave V was documented at 100 dBnHL while left ear showed no response at 100 dBnHL and absent cochlear microphonic bilaterally. Distortion product otoacoustic emission established weak emission. Bilateral tympanometry showed Type A findings. She was fitted with bilateral hearing aid, which however, proved futile. Therefore, she was chosen as a candidate for CI.
Her high-resolution computed tomography (HRCT) of the temporal bone demonstrated two bony canals originating from the IAC, with one representing labyrinthine segment of facial nerve anteriorly and another positioned more posteriorly (thick short arrow) hypothesized to be the bony canal of vestibular nerve. This postulation was made, considering that there was no bony canal entering the vestibule [Figure 1]. Furthermore, there was stenosis of CN canal [Figure 2]. MRI of the IAC revealed only two nerve bundles in the IAC to represent the facial and vestibular nerve with no demonstrable CN [Figure 3].
|Figure 1: Axial high-resolution computed tomography temporal bone in our patient (a) and normal patient (b). (a) There are two bony canals originating from the internal auditory canal represents labyrinthine segment of facial nerve (thin long arrow) with another bony canal more posteriorly (thick short arrow) postulated to be bony canal of vestibular nerve. There is no bony canal entering vestibule (asterisk). (b) The more posterior bony canal (thick short arrow) in normal population enters vestibule which represents bony canal superior vestibule nerve or singular canal|
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|Figure 2: Axial high-resolution computed tomography temporal bone shows absence of the cochlear nerve canal or cochlear aperture (thin long arrow) that allows cochlear nerve to enter cochlea from internal auditory canal. The thick short arrow represents basal turn of cochlea|
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|Figure 3: High-resolution magnetic resonance imaging internal auditory canal constructive interference in steady-state three-dimensional in sagittal oblique to obtain cross-section of the internal auditory canal of our patient (a and b) and of normal patient (age matched) as reference (c and d). a and c are in the same level at mid-internal auditory canal and c and d are at the level of fundus of internal auditory canal. (a) There are only two nerve bundles in the internal auditory canal represents facial nerve (thin white arrow) at superior anterior part of internal auditory canal and vestibular nerve at posterior part (thick white arrow). There is no cochlear nerve at the expected location (thin yellow arrow) as compared to normal cochlear nerve in c. In fact, the vestibular nerve is more rounded than the normal hourglass shape vestibular nerve. (b) At the level of fundus internal auditory canal, the facial nerve enters labyrinthine bony canal (thin white arrow) while the vestibular nerve is more anterior than the normal location (thick white arrow). Asterisk represents cochlea. Patient's internal auditory canal is smaller than the normal|
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| Discussion|| |
The embryology of the vestibulocochlear nerve and facial nerve is complex which also explains the intricacy of anatomical variation of these nerves in the IAC. The facial and vestibulocochlear nerves enter the IAC canal as two well-separated structures originating from the pons. The vestibulocochlear nerve originates posterior to facial nerve, and only diverge as the cochlear and vestibular nerves nearing the lateral aspect of the IAC, that is toward the IAC fundus. The vestibular nerve subsequently resumes an hourglass configuration representing superior and inferior vestibular nerve, and their separation is distinguishable at the fundus of the canal. The superior vestibular nerve supplies the superior and lateral semicircular canals as well as the utricle, while the inferior vestibular nerve supplies the posterior canal and the saccule.
Previous literature describing anatomy of the vestibulocochlear nerve in cadavers reported that the superior vestibular nerve attained a position where it could not be separated from the facial nerve in approximately one-third of the specimens. Limited data suggest symmetry of the anatomy of the canals within individuals despite the anatomy within the IAC being different from individual to individual. Bony neural canals identified in previous studies of cadaveric HRCT temporal scans noted four main bony neural canals in IAC, namely the CN, bony canal of facial nerve, bony canal superior vestibular nerve (BCSVN), and inferior vestibular nerve (singular canal [SC]) that are present at the fundus of the IAC. The BCSVN was identified in all studies (100%). This canal with its distal portion extends into the vestibule. The BCSVN had the largest variation in size and appearance. Meanwhile, the bony canal for inferior vestibular nerve, known as singular nerve, was identified in 93% of CT studies. This canal is the longest and narrowest of all four canals located at the fundus of the IAC and originates more proximal than the rest of the bony neural canals. Similar to the BCSVN, the SC terminates in the vestibule.
Our patient does not demonstrate the hour glass shaped vestibular nerve and the Superior Vestibular nerve that is expected to enter the vestibule from IAC. Instead, an anomalous bony canal was seen at the medial wall of the middle ear cavity where there should have been only one canal for the labyrinthine segment of facial nerve for it to become the first genu of facial nerve which contains the geniculate ganglion. This anomalous bony canal is more anterior than the expected BCSVN and is located lateral than the already existing bony canal for the labyrinthine facial nerve. One can argue that other differentials would include a duplicated facial nerve from the IAC. This is when MRI becomes crucial as it can depict the anatomy of the neural tissue in IAC as a separate entity from their bony canals. The nerve can be followed from the origin of brainstem to the fundus of IAC before they enter their respective bony canals. Using MRI three-dimensional constructive interference in steady-state technique, high spatial resolution and excellent contrast resolution can be obtained in evaluating small structures within cerebrospinal fluid such as nerves and vessels. Her MRI showed that the vestibular nerve did not demonstrate normal hourglass shape in the IAC and is more anterior than the expected vestibular nerve when it reached the IAC fundus, suggesting that the course is following the aforementioned anomalous bony canal.
There were few reports on anomalous course of the vestibulocochlear complex as compared to the well-known anomalous course of the facial nerve. Amongst the rarity described was vestibulocochlear complex went toward the Meckel's cave of the trigeminal nerve. The exact etiology is unknown but embryologically. Fatterpekar et al. speculated that the cause of anomalous vestibulocochlear nerve lies in the anomalous development of the ottic vesicle which inhibits the normal production of nerve growth factor. This may result in excessive neuronal degradation and prevent the normal growth of the nerve and IAC itself.,
Apart from the vestibular nerve anomaly, the cochlear nerve(CN) was also absent, hence it support the postulation of complex embryology of vestibulocochlear nerve itself. Further vestibular examination is needed in this patient for assessment of vestibular nerve function.
| Conclusion|| |
In view of the complexity of the embryology of ottic capsule in giving rise to the bony IAC as well as the intracanalicular nerves of facial nerve and vestibulocochlear nerves, the presence of anomaly of the nerve commonly the CN should trigger a clinician and a radiologist to scrutinize any anomaly of the accompanying nerves within the IAC. This needs to be correlated with clinical and other investigations related to nerve stimulation study to ensure optimum preoperative assessment and planning for rehabilitation after cochlear implantation.
Declaration of patient consent
The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Figure 1], [Figure 2], [Figure 3]