Cochlear hypoplasia

Case contributed by Leanne Chin
Diagnosis certain


Profound unilateral sensorineural hearing loss.

Patient Data

Age: 10 years
Gender: Male

MRI internal auditory meatus


Small rudimentary right cochlear bud with no visible modiolus or interscalar septum. Right vestibule is also malformed and dilated vestibule. Right semicircular canals are relatively normal. Right cochlear nerve is not well seen, likely absent. The right internal auditory canal is normal (or slightly narrowed) in size. No abnormal enlargement of vestibular aqueduct. 

Case Discussion

Cochlear hypoplasia (CH) is due to developmental interruption during the 6th week of pregnancy. As the name suggests, the cochlea is smaller than normal with <2 turns.

It is a generic term with four subtypes described:

1. CH-1: most severe form, bud-like cochlea arising from IAC and non-visualization of modiolus and interscalar septa. Cochlear nerve is absent or hypoplastic.

2. CH-2: small cochlea but normal external outline and variable modiolus and interscalar septa (ISS) formation. Cochlear nerve and cochlear nerve canal can be normal. In cases of underdeveloped modiolus, there is wide connection with the IAC increasing surgical risk for electrode misplacement and post-surgical "CSF gushing".

3. CH-3: "small cochlea with less than two turns" yet both external outline and internal architecture preserved. Variable degrees of vestibular and SCC hypoplasia. 

4. CH-4: "cochlea with hypoplastic middle and apical turns"; in contrast to CH-3, the basal turn is normal with hypoplastic middle and apical turns contributing to a small cochlea.

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