Objectives/Hypothesis: To determine the frequency of the incomplete partition type III anomaly and the genetic and clinical features associated with POU3F4 mutations in children with hearing loss. Study Design: Retrospective case series from 2000 to 2014 at the National Hospital Organization Tokyo Medical Center and collaborating hospitals. Methods: A total of 1,004 patients (from 938 families) who had hearing loss by 10 years of age and had undergone computed tomography scanning of their temporal bones were enrolled in this genetic, clinical, and radiological study. Results: The incomplete partition type III anomaly was identified in six patients (0.6%), each of whom had an enlargement of the vestibular aqueduct at the end close to the vestibule. The six patients also had POU3F4 variants, and a genetic analysis revealed frameshift deletions in three patients, a missense variant in two patients of the same family, and a large deletion in one patient. Three of the six patients with POU3F4 variants were sporadic cases, and in one patient the genetic mutation occurred de novo. Conclusions: It was indicated that POU3F4 mutations can be predicted by incomplete partition type III anomaly by radiological examination of the inner ear. All six of the patients showed mixed hearing loss, but none showed fluctuations in hearing, which may be related to the lack of vestibular aqueduct enlargement at the operculum. Level of Evidence: 4 Laryngoscope, 127:1663–1669, 2017.
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