Cochlear incomplete partition with lateral semicircular canal dysplasia

Case contributed by Dr Varun Babu

Presentation

Bilateral profound sensorineural hearing loss, presenting for pre-cochlear implant imaging

Patient Data

Age: 9 months
Gender: Female

The case shows bilateral symmetric inner ear malformations:

  • The external contour of the cochlea is normal in size and the cochlea is distinct from the vestibule, but the internal architecture of the cochlea is abnormal. The cochlear apex has a cystic appearance due to absence of the modiolus and interscalar septa at the level of the middle and upper turns. However, the basal turn is, for the most part, partitioned by the interscalar septum. The cochlear aperture/fossette is not enlarged.
  • The vestibule is fused with the broad lateral semicircular canal in a single globular cavity. The lateral semicircular canal bony island is absent. The posterior and superior semicircular canals are normally formed.
  • The vestibular aqueduct is not enlarged. 
  • The internal auditory canal is normal in size.
  • The facial nerve course is normal.
  • The middle ear and mastoid are normally developed.

Heavy T2 weighted images clearly delineate the presence of both vestibulocochlear nerves. The cochlear nerves appear thinned out but intact. The cochlea and vestibular apparatus show the changes identified in CT. The internal auditory meati are normal. No cerebellopontine angle mass lesions. 

Few other incidental neuroparenchymal findings identified include

  • parallel oriented dilated lateral ventricles with thinned out / partially absent posterior corpus callosum
  • absent septum pellucidum
  • prominent extra-axial CSF space in the posterior fossa with mild cerebellar/vermian hypoplasia

Case Discussion

Cochlear implant preoperative imaging needs a structured approach. High resolution CT of the temporal bones is the initial modality of choice to identify ear anomalies. The addition of MRI (mainly heavily T2-weighted sequences such as CISS or FIESTA) enables additional evaluation of the vestibulocochlear neural anatomy and morphology.

The main imaging findings in this case are (1) absence of the modiolus and interscalar septa between middle and apical turns of the normally-sized cochlea, and (2) a single globular cavity in place of the lateral semicircular canal and vestibule. The cochlear abnormality is consistent with an incomplete partition. The vestibular abnormality is a semicircular canal dysplasia known as persistent lateral semicircular canal anlage.

This case does not fit neatly into the classic descriptions of incomplete partition subtypes. In contrast to typical incomplete partition type I (cystic cochleovestibular malformation), this case has a partitioned basal turn and the cochlear aperture/cribriform area is not widely defective. In contrast to typical incomplete partition type II with the Mondini malformation triad, this case does not feature an enlarged vestibular aqueduct and would therefore not fit into the pathophysiological mechanism that Sennaroglu proposed for this cochlear anomaly (related to CSF pressure transmitted through the vestibular aqueduct) 1. Both diagnoses include a dilated vestibule (grossly in type I, minimally in type II) but do not specifically capture the semicircular canal anlage anomaly.

From a descriptive (rather than pathogenetic) standpoint, this case may be categorized as an "atypical incomplete partition type II." Kontorinis et al. proposed the designation "atypical IP-IIb" to describe the combination of cystic cochlear apex without large vestibular aqueduct but with semicircular canal dysplasia 2. This nomenclature indicates that incomplete partition type II should be based on the cochlear findings rather than a combination of cochlear and vestibular findings. Other authors (including Sennaroglu) have since identified cases of incomplete partition type II without enlarged vestibular aqueduct 3,4.

Regardless of the classification, a few imaging features are clinically relevant and should be emphasized. Because the patient has a cochlea (albeit mildly dysplastic) and cochlear nerve, she is a candidate for cochlear implantation. During cochlear implantation surgery, the risk of cerebrospinal fluid leak (CSF gusher) is likely lower than in typical incomplete partition type I, as the cribriform area does not appear widely defective. CSF pulsations would also not be expected to be observed at the round window, as in typical incomplete partition type II with Mondini malformation, because there is no third window of the enlarged vestibular aqueduct. Importantly, the bilateral symmetric nature of the findings, as well as associated brain anomalies, raises the likelihood of a genetic/syndromic etiology, for which further workup and counseling is warranted. For example, incomplete partition type II without enlarged vestibular aqueduct and with lateral semicircular canal malformations have been described in 22q11.2 deletion syndromes 4. Follow-up information was not available in the current case.

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