CT Evaluation of Labyrinth Structures in Patients With Incomplete Partition Type II Anomaly.

Objective: Incomplete partition type II (IP-II) is one of the most common inner ear malformations that necessitates cochlear implantation. It is characterized by a cystic cochlear apex resulting from the coalescence of middle and apical turns due to modiolar hypoplasia and distal interscalar septal defect. Radiologic diagnosis of IP-II anomaly could be difficult with equivocal changes in tiny labyrinthine structures, especially with inexperienced observers. In our study, we aimed to determine pertinent quantitative radioanatomical measurements of inner ear structures (lateral interscalar notch angle, lateral interscalar notch depth, and lateral height) that may contribute to the diagnosis of IP-II anomaly using temporal bone high-resolution computed tomography (HRCT).

Methods: Our study was a retrospective case-control study approved by the institutional review board. We retrospectively reviewed temporal bone HRCT examinations for the preoperative assessment of cochlear implantation of patients with sensorineural hearing loss (SNHL), using the diagnosis code of IP-II anomaly. Patients with SNHL and a radiologic diagnosis of IP-II anomaly according to the Sennaroglu classification were recruited. Thirty-four SNHL patients with IP-II anomaly (66 ears) and 24 patients without SNHL (48 ears) who underwent temporal CT imaging for inflammatory conditions or trauma not involving labyrinthine structures were included in the study. The lateral interscalar notch angle, lateral interscalar notch depth, and lateral height were independently measured by 2 observers (a senior head and neck radiologist and a novice radiologist). The parameters measured in patient and control groups were compared using the Mann-Whitney U test, and interobserver reliability was calculated.

Results: Lateral interscalar notch angle and lateral height values were found to be significantly larger, while lateral interscalar notch depth value was significantly smaller in the IP-II group (P<0.05). The agreement between the observers in all measurements, evaluated separately in IP-II and control groups, was excellent (P<0.05). The cutoff values for determining the IP-II anomaly were found to be consistent between the 2 observers, with high sensitivity and specificity.

Conclusions: CT measurements of inner ear structures in SNHL could help to diagnose IP-II anomalies, especially in patients with equivocal audiological and radiologic findings. This study validates CT measurements in the evaluation of IP-II anomaly conducted by histologic specimens and MRI in previous studies. Lateral cochlear measurements with an angle of >130 degrees, a depth of ≤0.35 mm, and a height of >3.15 mm could support the diagnosis of the IP-II anomaly. These measurements are also reproducible.