Three-Dimensional Histopathologic Correlates of Cochlear Hypoplasia Type II Versus Incomplete Partition Type II

HYPOTHESIS: Cochlear hypoplasia type-II (CH-II) and incomplete partition type-II (IP-II) may display measurable differences in modiolar: cochlear area ratios and interscalar ridge angles correlating with spiral ganglion neuron (SGN) counts. BACKGROUND: Inner ear malformations (IEM) can cause variable sensorineural hearing loss and cochlear implant outcomes. CH-II and IP-II share overlapping imaging features. Subtle differences in bony cochlear architecture may underlie distinct SGN patterns. METHODS: IP-II (n=11), CH-II (n=5), and normal controls (n=16) were identified from a digitised human temporal bone histologic collection. Radiologically relevant assessments of midmodiolar 2-dimensional (2D) morphology, including interscalar ridge angles and modiolar: cochlear area ratio, were compared with controls. SGNs were quantified. Digital 3-dimensional (3D) reconstructions were created to appreciate the difference in anatomy further. RESULTS: On midmodiolar sections, interscalar septum deficiency, indicating fusion of perilymph-containing scalae, occurred between upper basal and upper middle turns (CH-II, IP-II) and lower middle and apical turns (IP-II only) of the cochlea. Compared with normal cochleae, both CH-II and IP-II cases had a smaller modiolar: cochlear area ratio (P<0.01), and flattened interscalar ridge angles (attachment of interscalar septum to inner cochlea lumen; P<0.01). In CH-II, SGN counts were markedly reduced, ranging from 0% to 20% (mean 8%) of SGN counts in age-matched normally formed cochleae (normal hearing ears); IP-II ranged from 5% to 95% (mean 57%) of age-matched normal cochleae. CONCLUSIONS: Given the reduced SGN counts in CH-II compared with normal and IP-II, accurate radiologic subclassification of CH-II versus IP-II is necessary to optimise candidacy selection and counselling for cochlear implantation. A predictive algorithm is suggested based on histologic criteria that might help distinguish between IP-II and CH-II cochleae.