Cochlear and vestibular volumes in inner ear malformation

Inner ear malformations (IEM) represent a diagnostic challenge. A correct diagnosis can be decisive for therapy and thus of high clinical relevance. The diagnostic standard includes high-resolution computed tomography (hrCT). Different classification systems for IEM diagnosis on hrCT have been established in the past. The aim of this study was to evaluate the utility and reliability of 3D reconstruction and volumetry of inner ear structures in the diagnosis of IEM. In total, 99 temporal bones were examined (cochlear hypoplasia [CH]: n=18, incomplete partition [IP] type I: n=6 and type III: n=6, Mondini malformation: n=11, enlarged vestibular aqueduct syndrome: n=15, control: n=43). Diagnosis was performed by experienced neuroradiologists. Semiautomated segmentation was used to obtain the volume of particular inner ear structures. Using axial hrCT, common 2D measurements were aditionally performed. 2D and 3D measurements were evaluated in terms of correlation and interinvestigator reliability. IEM showed significant differences in cochlear and vestibular volumes depending on malformation type compared with the control group. Compared to the control group (mean 78.0 mm3), only CH showed a significantly smaller volume of the cochlea (mean 30.2 mm3; p<0.0001). The 3D measurements showed a good interrater-reliability. The obtained volumes showed good correlations with previously existing 2D measurement parameters. To classify the vestibular organ as hypoplastic, we propose a cut-off value of less than 80mm3, as dilated one of greater than 135mm3. A cut-off value of less than 60mm3 may indicate a hypoplastic cochlea. Reference values for volumes of inner ear structures may be helpful in diagnosing IEM.