The incomplete partition type II (IP-II) anomaly of the cochlea is one of the most frequently imaging-detected congenital causes of sensorineural hearing loss. Certain imaging features of IP-II have been previously described on CT, including a lack of bony partitions within the cochlea and flattening of the anchor point or interscalar ridge between the distal basal and middle turns of the cochlea.¹ Our previous research demonstrated that IP-II anomalies can be overlooked on MR imaging assessment due to the apparent segmentation at the distal/upper middle turn of the cochlea, which, in actuality, is not due to a normal interscalar septum.

When correlated histologically, this apparent segmentation was shown to represent neurosensory elements comprised of abnormally formed neural tissue, including spiral ganglion neurons continuing through the osseous spiral lamina with a malformed organ of Corti. This neurosensory tissue manifests as a bandlike area of low T2 signal on cisternographic sequences and has a similar appearance to a malpositioned interscalar septum.²

Because of this potentially confounding finding, we were interested in determining whether there was a quantitative tool to more accurately diagnose the IP-II anomaly on MRI.

Our study demonstrates that when using a standard imaging plane, parallel to the lateral semicircular canal, a measurement from the basilar membrane of the distal basal turn of the cochlea to the next bandlike area of low T2 signal (either the malformed neurosensory element in the setting of IP-II or the interscalar septum in individuals with normal hearing) can help differentiate IP-II from a normal cochlea.

A measurement of ≥1.2 mm suggests an IP-II anomaly, and measurement values at or above 1.2 mm are not seen in individuals with normal hearing. We hope that this measurement technique may help radiologists more confidently diagnose subtle cases of IP-II.

We are interested in expanding our research to other temporal bone anomalies. Our group works in close collaboration with the National Institute on Deafness and Other Communication Disorders National Temporal Bone Registry, and we hope to use the imaging findings of histopathologic specimens of patients with hearing loss to help us better understand and diagnose mechanisms of hearing loss in vivo.

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References

1. Makary C, Shin J, Caruso P, et al. A histological study of scala communis with radiological implications. Audiol Neurootol 2010;15:383–93, 10.1159/000307345.
2. Leung KJ, Quesnel AM, Juliano AF, et al. Correlation of CT, MR, and histopathology in incomplete partition-II cochlear anomaly. Otol Neurotol 2016;37:434–37, 10.1097/MAO.0000000000001027.