Internal auditory canal atresia, also known as internal auditory canal stenosis, is characterized by partial or complete bony atresia of the internal auditory canal, often in association with hypoplasia/aplasia of the vestibulocochlear nerve.
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Terminology
Internal auditory meatus is commonly used as a synonym for the internal auditory canal, so therefore internal auditory meatus atresia or stenosis are synonyms for this condition.
Epidemiology
This is a very rare entity with few cases reported in the literature 1.
Clinical presentation
Congenital internal auditory canal stenosis can be an important cause of sensorineural hearing loss, facial nerve palsy, and vestibular dysfunction.
Pathology
The normal growth of the cochleovestibular nerve is necessary for the IAC to form. The internal auditory meatus is formed by the transformation of mesoderm, which surrounds the eighth nerve, into cartilage and bone. Stenosis, atresia, or IAC duplication are caused by altered eighth nerve development.
Abnormal development of the eighth nerve may also occur in isolation within a normal-sized IAC, either as aplasia or hypoplasia of the cochlear branch.
Associations
Internal auditory canal stenosis commonly occurs in association with hypoplasia/aplasia of the vestibulocochlear nerve.
Internal auditory canal (IAC) stenosis frequently coexists with other inner ear abnormalities. Isolated congenital IAC stenosis without other inner ear anomalies has very rarely been reported 1.
Most IAC stenoses have normal facial nerve function, for it is assumed that the development of the facial nerve is separate from that of the vestibulocochlear nerve. However, many cases with associated facial nerve palsy were reported. It was suggested that facial palsy might develop after inflammation, compression, or ischemia of the nerve within the stenosed IAC2.
Radiographic features
High resolution bony CT reformats is the modality of choice for assessing the internal auditory canal.
IAC stenosis is diagnosed when the IAC diameter is <2 mm in vertical diameter on MRI or CT 3.
Treatment and prognosis
IAC stenosis is an important consideration in the differential diagnosis of sensorineural hearing loss, as it is a relative contraindication for cochlear implant placement. Brainstem implantation may be considered.