Normative size evaluation of IAC with MRI: Review of 3786 patients

Background: A narrow internal auditory canal (IAC) is significantly associated with congenital sensorineural hearing loss. It would therefore seem likely that any patient with an IAC measured radiographically to be under the normal range represents an abnormality and probable IAC stenosis. If narrow IAC is diagnosed with routine magnetic resonance imaging (MRI), then the cochlear nerve may be evaluated with special MRI studies. However, there is no consensus in the literature on the normal measurements of the IAC or on what parameters should be used to determine narrow IAC using MRI. In this study, we aimed to assess the normative size of IAC in normal-hearing ears and to determine whether canal size varies with age and gender using MRI.Material and methods: A retrospective review was undertaken from 2010 to 2012. A total of 7572 normal-hearing ears of 3786 patients were assessed, who had MRI due to various reasons except hearing loss. Patients under 20 years old and over 60 years old were excluded, and the subjects were divided into 4 groups at 10-year intervals. All subjects were divided by gender also. Anteroposterior (AP) and craniocaudal (CC) measurements were obtained in the middle of the IAC on axial and coronal images of 1.5-T MRI.Results: The mean age was 42 years (range 20–60 years). The mean IAC diameters were 5.93 mm with a standard deviation of 0.25 mm (max 6.99 mm, min 4.73 mm) on AP measurements and were 5.70 mm with a standard deviation of 0.26 mm (max 6.82 mm, min 4.71 mm) on CC measurements. There were no differences in the IAC diameters between males and females or with age groups.Conclusions: These measurements should provide a normative reference for comparison in radiographic assessment of any patient with suspected IAC stenosis. This measurement can help the diagnosis of narrow IAC. To our knowledge, this is the first study using MRI with a large group of patients in the literature