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Purpose A narrow bony internal auditory canal (IAC) may be associated with a hypoplastic cochlear nerve and poorer hearing performances after cochlear implantation. However, definitions for a narrow IAC vary widely and commonly, qualitative grading or two-dimensional measures are used to characterize a narrow IAC. 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). Methods 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). The IAC diameter was measured in the axial plane and the IAC volume was measured by semi-automatic segmentation and three-dimensional reconstruction. Results 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 ). Statistically significant differences in IAC volumes were found in cochlear aplasia (68.3 mm 3 , p  < 0.0001), IPI (107.4 mm 3 , p  = 0.04), and IPIII (277.5 mm 3 , p  = 0.0004 mm 3 ). Inter-rater reliability was higher in IAC volume than in IAC diameter (intraclass correlation coefficient 0.92 vs. 0.77). Conclusions Volumetric measurement of IAC in cases of IEMs reduces measurement variability and may add to classifying IEMs. 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.

Objectives Enlarged vestibular aqueduct (EVA) is a common finding associated with inner ear malformations (IEM). However, uniform radiologic definitions for EVA are missing and various 2D-measurement methods to define EVA have been reported. This study evaluates VA volume in different types of IEM and compares 3D-reconstructed VA volume to 2D-measurements. Methods A total of 98 high-resolution CT (HRCT) data sets from temporal bones were analyzed (56 with IEM; [cochlear hypoplasia (CH; n  = 18), incomplete partition type I (IPI; n  = 12) and type II (IPII; n  = 11) and EVA ( n  = 15)]; 42 controls). VA diameter was measured in axial images. VA volume was analyzed by software-based, semi-automatic segmentation and 3D-reconstruction. Differences in VA volume between the groups and associations between VA volume and VA diameter were assessed. Inter-rater-reliability (IRR) was assessed using the intra-class-correlation-coefficient (ICC). Results Larger VA volumes were found in IEM compared to controls. Significant differences in VA volume between patients with EVA and controls ( p  < 0.001) as well as between IPII and controls ( p  < 0.001) were found. VA diameter at the midpoint (VA midpoint) and at the operculum (VA operculum) correlated to VA volume in IPI (VA midpoint: r  = 0.78, VA operculum: r  = 0.91), in CH (VA midpoint: r  = 0.59, VA operculum: r  = 0.61), in EVA (VA midpoint: r  = 0.55, VA operculum: r  = 0.66) and in controls (VA midpoint: r  = 0.36, VA operculum: r  = 0.42). The highest IRR was found for VA volume (ICC = 0.90). Conclusions The VA diameter may be an insufficient estimate of VA volume, since (1) measurement of VA diameter does not reliably correlate with VA volume and (2) VA diameter shows a lower IRR than VA volume. 3D-reconstruction and VA volumetry may add information in diagnosing EVA in cases with or without additional IEM.