Currently, there is no broad consensus on how to assess and grade the IAC size. The first definition of a normal IAC was presented in 1964 and together with subsequent studies, a diameter less than 2 mm has become the most widely accepted criterion for a narrow IAC. Yet, the relation between different types of IEMs, IAC hypoplasia and cochlear nerve abnormalities are not well-understood. However, among candidates for CI surgery, patients with IEMs not only pose significant anatomical challenges to CI surgery, but also have been shown to have a worse outcome regarding hearing performance. While it has been previously suggested that development of a normal cochlea depends on the innervation by a normally developed cochlear nerve, recent temporal bone studies revealed that the cochlea undergoes a normal development even in absence of the cochlear nerve. Moreover, previous studies have reported that a narrow IAC can be associated with both a normal inner ear as well as inner ear malformations (IEMs). While cochlear nerve aplasia is a contraindication to cochlear implantation, cochlear nerve hypoplasia is associated with worse hearing outcomes in CI patients. First, a narrow IAC may contain a rudimentary or absent cochlear nerve. Ī narrow IAC can lead to an inferior CI outcome due to various reasons. Furthermore, IAC hypoplasia may be associated with an aberrant facial nerve, which renders the surgical approach to the cochlea more challenging and the risk for intraoperative facial nerve injury is increased. Among other factors, a narrow bony internal auditory canal (IAC) may be associated with poorer hearing performances after cochlear implantation as a narrow IAC commonly contains a hypoplastic cochlear nerve. Since a hypoplastic IAC can be associated with a hypoplastic cochlear nerve and sensorineural hearing loss, radiologic assessment of the IAC is crucial in patients with severe sensorineural hearing loss undergoing cochlear implantation.Ĭochlear implant (CI) surgery is the current gold-standard treatment in profound sensorineural hearing loss. Volumetric measurement of IAC in cases of IEMs reduces measurement variability and may add to classifying IEMs. Inter-rater reliability was higher in IAC volume than in IAC diameter (intraclass correlation coefficient 0.92 vs. In controls, the mean IAC diameter was 5.5 mm (SD 1.1 mm) and the mean IAC volume was 175.3 mm 3 (SD 52.6 mm 3).
The IAC diameter was measured in the axial plane and the IAC volume was measured by semi-automatic segmentation and three-dimensional reconstruction. In this multicentric study, we included high-resolution CT (HRCT) scans of 128 temporal bones (85 with IEMs: cochlear aplasia, n = 11 common cavity, n = 2 cochlear hypoplasia type, n = 19 incomplete partition type I/III, n = 8/8 Mondini malformation, n = 16 enlarged vestibular aqueduct syndrome, n = 19 45 controls). We aimed to refine the definition of a narrow IAC by determining IAC volume in both control patients and patients with inner ear malformations (IEMs). However, definitions for a narrow IAC vary widely and commonly, qualitative grading or two-dimensional measures are used to characterize a narrow IAC. A narrow bony internal auditory canal (IAC) may be associated with a hypoplastic cochlear nerve and poorer hearing performances after cochlear implantation.
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