7Cochlear hypoplasia, incomplete partition & EVA
Among the implantable malformations, the incomplete partitions and the enlarged vestibular aqueduct are the ones the surgeon most needs the radiologist to pin down, because they carry a specific and sometimes dramatic hazard: the perilymph gusher. The distinction turns on two features visible on a good scan — whether the modiolus is present and whether the interscalar septa are formed — which sort the cochlea into hypoplasia (too small) or incomplete partition types I, II and III, each with its own gusher risk, from a mild ooze to a near-certain torrent that can even sweep the electrode into the internal auditory canal. The enlarged vestibular aqueduct, measured against three named criteria and linked to Pendred syndrome, completes the picture. This module reads these in detail so the operation is prepared, not surprised.
Imaging note
Representative CT and MRI images for this chapter are being added soon. The interactive figures here are original schematic teaching diagrams; to respect copyright we do not reproduce third-party radiographs.
CCochlear hypoplasia
Cochlear hypoplasia is a small cochlea — cochlear duct length <25 mm — in four types: CH1 (a bud from the IAC, no modiolus/septa), CH2 (cystic hypoplastic), CH3 (normal shape, fewer turns, short modiolus), CH4 (normal basal turn but hypoplastic, anteriorly-placed middle/apical turns with an anomalous facial nerve). All use smaller/slimmer arrays.
CThe incomplete partitions
Incomplete partitions have a differentiated cochlea-vestibule and duct length >25 mm, graded by modiolus and septa: IP1 (modiolus and septa absent, cystic, dilated vestibule); IP2 (Mondini) (apical modiolus and septa deficient, cystic apex); IP3 (modiolus absent but septa present, X-linked, bulbous IAC fundus with absent lamina cribrosa).[2002]
CGusher risk & preparation
Gusher risk tracks the IAC–cochlea partition: IP1 ~50%, IP2 a mild perilymph ooze, IP3 100% — and in IP3 the array can migrate into the dilated IAC, mandating an intra-operative fluoroscopic cross-check (Module 14). Imaging forewarns the surgeon to prepare a larger cochleostomy, a conical-stopper array, tissue glue, fascia packing and, if needed, a lumbar drain (Surgery chapter).
CThe enlarged vestibular aqueduct
IP2 is classically part of the triad IP2 + EVA + dilated vestibule (the Mondini complex). The enlarged vestibular aqueduct is one of the commonest inner-ear malformations, measured against three criteria — Valvassori–Clemis(>1.5 mm at the midpoint, most accepted), Cincinnati and Wilson — and linked to SLC26A4 (PDS) mutations and Pendred syndrome (Genetics chapter). It carries gusher risk, yet implant outcomes match non-malformed ears.[1978]
What should be anticipated?
What distinguishes cochlear hypoplasia from incomplete partition?
Which incomplete partition carries ~100% gusher risk and IAC-migration risk?