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

Abstract

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.

Fig. 1

(A) Hypoplastic anterior off-set cochlea with preserved first half of the basal turn shown on an axial petrous bone CT in a patient with Walker Warburg syndrome due to a POMT1 gene mutation (A); the cochlea demonstrates a normal first half of the basal turn (thick arrow) whilst its second half and the upper turns are extremely hypoplastic and anteriorly off-set (thin arrow). (B) Axial 3D CISS in a patient with EYA1-BOR showing a normal first half of the basal turn (thick arrow), and moderately hypoplastic and anterior–off set second half and upper turns (thin arrow). (C) A patient with Goldenhar phenotype/oculo-auriculo-vertebral spectrum (OAVS), shows a hypoplastic cochlea with normal length of the basal turn (thick arrow) and hypoplastic and slightly anterior offset upper portions of the cochlea (thin arrow). (D) Finally, the mildest phenotype of cochlear hypoplasia with preserved basal turn is shown in a CHARGE patient with confirmed CHD7 mutation, where the first half of the basal turn is relatively preserved (thick arrow) but also the second half (arrowhead) is present and fully developed (although slightly rotated anteriorly); conversely, the upper turns are small and dysplastic (thin arrow). In cases A, B, and D the cochlear findings were symmetrical, while in case C the contralateral cochlea was nearly normal with only subtle hypoplasia of the modiolus

Fig. 2

Ex-vivo reformats of the inner ear between 44 and 60 days (approximately 7 th to 10 th week of gestation) of developmental age (from 3 dembryoatlas.com) showing the basal turn development. This time window seems to be critical, since the described phenotype of cochlear hypoplasia appears very similar to the ex-vivo reformats between the 44th and 54th day. Purple: cochlea. Grey-blue: semicircular canals. Light blue: vestibular aqueduct