Unilateral internal auditory canal stenosis with progressive facial weakness

Internal auditory canal (IAC) stenosis is a rare cause of sensorineural hearing loss. Patient may present with symptoms of progressive facial nerve palsy, hearing loss, tinnitus and giddiness. High resolution temporal bone CT-scan and magnetic resonance imaging (MRI) are the important tools for diagnosis. No specific management has been devised. Here is presentation of a case of unilateral (left) IAC stenosis with profound hearing loss and progressive House Brackmann Grade III-IV facial weakness. The diameter of the IAC was less than 2 mm on high resolution temporal bone computed tomography (HRCT) scan. It was managed by facial nerve decompression by translabyrinthine approach in an attempt to prevent further deterioration of facial palsy.


Introduction:
Internal auditory meatus (IAM) stenosis is defined as a loss of 3mm or more in the vertical diameter of the internal acoustic meatus, or even as a meatus smaller than 2mm 1 .Inner ear abnormalities may occur in about 20% of the cases of patients who have sensorineural hearing loss 2 .
In most cases, it occurs as an isolated congenital manifestation, and in others it may be a part of systemic abnormalities.The major alteration happens due to a constriction caused by impaired bone growth compressing on the nerve bundle.The stenosis may also develop secondary to alterations in the VIII cranial nerve (vestibulocochlear nerve aplasia or hypoplasia) 3 .Similar etiology can explain progressive facial nerve palsy.The embryologic events involved in fetal growth between the 4th and the 8th weeks are crucial for bone growth, and may cause such disease 4 .The labyrinth may be aplastic or deformed or an incomplete cavity.It is not uncommon to have a com-pletely normal labyrinth though 1-3 .An acquired bone disease may also result in stenosis of the internal acoustic meatus (osteomas, osteopetrosis, Paget's disease, etc).
HRCT of temporal bone showing a narrow internal acoustic meatus is the diagnostic tool of choice.MRI scans are helpful in visualization of the IAC structures such as the VIII cranial nerve, which may be aplastic 5 .

Case Report:
A-13-year old girl presented to us with history of left ear hard of hearing noticed about one year ago of unknown onset.She also had progressive left facial nerve weakness (grade III-IV HB) of 6 months duration.There was no history of tinnitus or any history suggestive of vestibular disturbance.Antenatal, natal and postnatal history was uneventful.Hence in order to prevent further progression of facial weakness, it was decided to decompress the facial nerve at the IAM by translabyrinthine approach.After all the necessary preoperative workup, the patient was taken for surgery under general anaesthesia.

Discussion:
Facial nerve palsy and sensorineural hearing loss (SNHL) due to internal auditory canal (IAC) stenosis with hypoplasia of the cochleovestibular nerve is a rare disorder.The physician should investigate the patient's obstetric, natal and post-natal history in order to rule out malformations during this period 5 .

The diagnosis of the IAC stenosis requires both high resolution computed tomography scan (HRCT) and magnetic resonance imaging (MRI). An internal auditory canal measuring less than 2 mm in vertical dimension by high resolution CT temporal bone is called stenotic.
Approximately 12% of patients with congenital sensorineural hearing loss have radiographic evidence of inner ear   abnormalities including IAC stenosis.IAC stenosis without any other abnormalities is extremely rare.In addition to primary causes of IAC narrowing, an osseous lesion such as an exostosis or an osteoma can cause secondary narrowing of IAC.Also Paget's disease, otosclerosis and fibrous dysplasia can cause narrowing of the canal [6][7][8] .
Internal auditory canal stenosis can be an important cause of sensorineural hearing loss, facial nerve palsy, and vestibular dysfunction.In a study 4 out of 7 patients had vestibular function loss 1 .But this patient with acquired IAM stenosis showed no features suggestive of vestibular dysfunction.Surgery of the internal auditory canal occupies an increasingly important place each year and it would therefore; seem useful to review the main indications for this surgical approach as they are today.They may be classified as follows: 1.(a) surgery for the internal auditory canal stenosis itself, (b) surgery of facial nerve, (c) surgery of the vestibular nerve (neurectomy for vertigo), (d) Cochlear neurectomy for subtotal deafness with severe symptoms such as tinnitus or intolerable distortion.2. Opening the internal auditory canal may also be necessary during the course of a more extensive operation as on (a) the petrous temporal bone, (b) the posterior cranial fossa, and (c) the middle cranial fossa 8 .
As the case was of a young lady with progressive facial palsy still in its initial stages, facial nerve decompression via translabyrinthine approach was planned for.As hearing loss was unilateral with completely normal hearing in the opposite ear, it was decided to manage the patient conservatively with periodical audiological follow up.

Conclusion:
Internal acoustic canal stenosis is a rare disorder.It may present as facial neuropathy, progressive hearing loss or vestibular dysfunction.Imaging plays an invaluable role in the diagnosis.
Clinical examination showed normal otoscopic findings with absence of ipsilateral wrinkling of forehead (grade IV).Tuning fork tests were suggestive of unilateral (left-sided) sensorineural hearing loss.Audiological tests included pure tone audiogram which revealed profound hearing loss in the left ear and normal hearing in the right ear.Impedance showed 'A' type tympanogram bilaterally.Otoacoustic emissions were absent on the left side.Electrophysiological studies showed no indication of retrocochlear pathology.Videonystagmography was normal except for the caloric test which showed hypoactive left labyrinth.dimensions on the right side.No other congenital anomalies of inner ear were found.MRI showed hypoplastic left vestibulocochlear nerve.Facial nerve conduction study showed reduced CMAP (compound myogenic action potential) on the left side suggestive of left facial neuropathy.

Figure 2 :
Figure 2: CT scan of temporal bone showing narrowing of left internal acoustic canal.

Figure 6 :
Figure 6: Intra-operative facial nerve monitoring-Mastoid cavity with IAM opened up with probe tip touching the facial nerve.Inset: Facial nerve monitor