Duplication of internal auditory canal is a very rare congenital disorder of temporal bone which is usually bilateral, associated with cochlear and inferior vestibular nerve hypoplasia. Only 14 cases have been reported so far. Congenital sensorineural hearing loss associated with inner ear anomalies are 20% more common with internal auditory meatus stenosis. The normal diameter of internal auditory meatus is 2–8 mm and in stenosis, it is <2 mm. The duplicated internal auditory canal is frequently misdiagnosed as internal auditory canal stenosis. Patients with stenotic internal auditory canal have dysplastic cochlear and vestibular nerve, whereas duplicated internal auditory canal will have aplasia of cochlear and inferior vestibular nerve. The management of hypoplastic cochlear nerve would be a cochlear implant, but in case of an aplastic nerve cochlear, implant would not be beneficial. This case is of a unilateral double-barreled internal auditory canal with severe sensorineural hearing loss, which has not been reported in the literature so far. The inferior contents of internal auditory canal, the cochlear, and the inferior vestibular nerve were hypoplastic due to the duplication of internal auditory canal.
An 11-year-old boy presented with a right-sided hearing loss for 6 months. He noticed right ear hard of hearing while using his headphones. He had no history of ear discharge, dizziness, or facial asymmetry. His birth and developmental history were normal. He had no significant family history of hearing loss. Otoscopic examination showed bilateral tympanic membranes intact. The tuning fork test showed a false-negative Rinne’s on right side and positive on left side with the weber lateralized to left ear.
Radiological evaluation: High-resolution computed tomography of temporal bone in sagittal view [Figure 1] uncovers the right double-barreled internal auditory canal and normal internal auditory canal on the left side [Figure 2]. Magnetic resonance imaging [Figure 3] showed right cochlear nerve and inferior vestibular nerve extremely thinned out with normal findings on the left side [Figure 4]. A three-dimensional reconstruction of bilateral internal auditory canal elicited the same findings [Figure 5].
Audiological evaluation: A pure-tone audiogram showed right severe sensorineural hearing loss with left side normal hearing. Tympanometry showed “A” type curve on both the ears. Otoacoustic emission was absent on the right ear. A brain stem evoked potential [Figure 6] showed severe sensorineural hearing loss in the right ear with normal hearing in the left ear. Cervical vestibular evoked myogenic potentials showed P13, N23 peak and were observed within normal limits, and the peak amplitude was reduced in the right ear [Figure 7]. The ocular vestibular evoked myogenic potentials N10, P15 peaks were observed within normal limits, but the peak amplitude was enhanced in both the ears [Figure 8]. The patient found an improvement in his hearing with an air-conduction hearing aid.
Double-barreled internal auditory canal is a very rare congenital disorder of temporal bone which is usually bilateral, associated with cochlear and inferior vestibular nerve hypoplasia. Previous studies have shown that 20% of such cases were associated with inner ear anomalies, whereas our case being an exception was unilateral and without anomalies. There is a case report of internal auditory canal stenosis with facial palsy in 3 years old. The cervical and ocular vestibular evoked myogenic potentials showed minimal peak amplitude on the right side and enhanced peak amplitudes on both the sides, respectively, even though brain stem evoked potential was absent on the right side. In previous case reports, the cervical and ocular vestibular evoked myogenic potentials were normal with absent response on brain stem evoked potential.
Internal auditory canal anomalies are atresia, stenosis, and hypoplasia. It is believed that there are two theories for internal auditory canal stenosis. First, the most accepted theory states that embryonic cochlea induces the growth of the vestibulocochlear nerve and the bony canal develops around the eighth nerve through the mesoderm chondrification and ossification. If the eighth nerve is hypoplastic or aplastic, the internal auditory canal development is also impaired. The second hypothesis states that bony defect inhibits the growth of eighth cranial nerve by narrowing of the internal auditory canal. The second theory fails to explain about the preservation of facial nerve. High-resolution computerized tomography (CT) of temporal bone is highly sensitive and specific for identification of bony abnormalities and helps in detection of inner ear abnormalities. However, in case of internal auditory canal duplication, it fails to identify the neural structures in the internal auditory canal.
This is the first case in the literature of unilateral internal auditory canal duplication with severe sensorineural hearing loss where an air-conduction hearing aid was helpful, as the 14 cases which have been reported so far involve bilateral duplication/stenosis of internal auditory canal with profound hearing loss.
The approach for any case of unilateral sensorineural hearing loss in an adolescent age group should be a complete history; physical examination, audiometric tests, and high-resolution CT are preferred in case of unilateral significant hearing loss than an magnetic resonance imaging.
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