The cochlear basal turn as a very preserved region in cochlear hypoplasias: radiological and embryological considerations from a cohort of 125 patients

Purpose: A distinct form of cochlear hypoplasia, characterized by the preservation of the first half of the basal turn with hypoplastic and anteriorly displaced upper turns, was historically associated with branchio-oto-renal (BOR) syndrome, but can also occur in other genetic, syndromic and non-syndromic causes of hearing loss. This study aims to describe this phenotype with relative preservation of the basal turn, particularly its first half, in a significant proportion of cochlear hypoplasia cases due to different causes. Methods: We retrospectively reviewed temporal bone imaging from 125 patients (250 ears) with cochlear malformations from a tertiary pediatric center, focusing on cases where the basal turn was partially or completely preserved. Temporal bone CT and internal auditory meatus MRI were assessed for cochlear morphology and associated anomalies and genetic, clinical and syndromic associations described. Results: Fifty-eight patients exhibited a preserved basal turn with different degrees of hypoplasia of the upper turns. These cases were grouped into five etiological clusters: branchio-oto-renal (BOR), CHARGE, Walker-Warburg (WWS) syndromes, other genetic cases and likely non-genetic cases (including syndromic conditions without a genetic cause identified such as oculo-auriculo-vertebral spectrum - OAVS). Genetic cases may show bilateral and symmetrical appearances, aberrant facial nerve courses were observed in 30 patients. Conclusions: Preservation of the first half of the basal turn suggests developmental arrest between 50 and 54 days of gestation, and is common across genetic and non-genetic conditions of cochlear hypoplasia. Frequent facial nerve anomalies may complicate cochlear implantation. Integrating imaging with embryological insights supports the need for refined, developmentally-based classification systems.