Incomplete partition type III

A 2-year-old male twin child underwent evaluation for complaints of delay in speech along with hearing impairment. BERA (Brain Evoked Response Audiometry) suggested moderate hearing loss in right ear and severe hearing loss in the left ear

On imaging, there is widening of fundus of the internal auditory canal on both sides with absent lamina cribrosa resulting in incomplete separation of the internal auditory canal and basal turn of the cochlea. The modiolus is absent and inter-scalar septae are preserved with both cochlea showing a characteristic corkscrew appearance. The rest of the inner ear structure including vestibule and semi-circular canals appear normal bilaterally.

Both lips of the hypothalamus appear dysmorphic and show abnormal protruding grey matter signal intensity lesions. These are seen anterior to the mamillary bodies and project into interpeduncular cistern suggestive of hypothalamic hamartomas.

Background

Incomplete partition type III (IP-III) is an X-linked non-syndromic deafness (OMIM entry: #304400), also known as DFNX2, is a genetic disease with X-linked mode of inheritance due to a mutation in POU3F4 gene (POU domain, class 3, transcription factor 4), located on chromosome Xq21. POU3F4 (also known as Brn-4) encodes a DNA transcription factor involved in the development of the nervous system, hypothalamus, pituitary gland, and inner ear and may explain the coexistence of hypothalamic and inner ear malformations. [1] It is associated with mixed conductive and sensorineural hearing loss. The diagnosis is made primarily on the basis of imaging due to its pathognomonic imaging features of inner ear and hypothalamic malformations.

Clinical Perspective

Patients present with hearing loss which can be progressive and is predominantly sensorineural with or without conductive hearing loss. Vestibular function may be impaired in affected patients.

Audiologic findings are extremely variable, however, progressive severe to profound mixed sensorineural hearing loss appears to be the most frequently encountered. [2] MR and CT imaging help to identify the inner ear malformations which can be utilised for diagnosis and selection of treatment plan.

Imaging Perspective

Diagnosis of Incomplete partition type III (IP-III) can be achieved by imaging due to the pathognomonic appearance of the otic capsule on MRI and CT. In these subjects, external dimensions of the cochlea are usually preserved and inter-scalar septae are present. However, modiolus is absent with absent bony partition between the fundus of the internal acoustic meatus and the basal turn of the cochlea. The internal acoustic meatus is dilated. Patients also show hypoplasia at the cochlear base or enlargement of the cochlear nerve canal and thinning of the otic capsule. [3] [4]

This rare genetic anomaly also shows characteristic hypothalamic malformations associated with it, ranging from thickened and moderately dysmorphic appearance of hypothalamus to hamartoma-like enlargement identified on MRI. [1]

Outcome

Literature suggests that cochlear implantation is a suitable option for treatment of patients with IP-III with use of circumferential stimulator electrode devices. Although long term audiological benefits haven’t been studied yet due to its rare occurrence. [5]

Take-Home Message / Teaching Points

Incomplete partition type III, also referred to as X-linked non-syndromic deafness, is a rare genetic inner ear malformation, however, the diagnosis can be established with typical imaging features as described. When the imaging features suggest corkscrew cochlea, it is important to also look for malformations in the hypothalamus. Radiologists should be familiar to these imaging features, thereby providing appropriate intervention and proper genetic counselling.