24th May 2021

ICRI CASE OF THE WEEK

Contributed By : Dr. Abhinav C Bhagat
Dr Jyoti Kumar

Copyright of the Case belongs to : Dr Jyoti Kumar
Professor, Department of Radiology
MAMC, New Delhi

Case History

• 8 year old girl with congenital deaf-mutism

• Uneventful antenatal history, full term vaginal delivery

• No history of hereditary hearing loss was present,
however patient had solitary left kidney with anorectal
malformation

• Otoscopy revealed normal bilateral EAC and tympanic
membrane; motor function of bilateral facial nerves was
normal; BERA suggested profound bilateral
sensorineural hearing loss.

HRCT Temporal Bone : Right ear axial (Fig 1)

HRCT Temporal Bone : Left ear axial (Fig 2)

HRCT Temporal Bone : Coronal (Fig 3)

MRI CISS sequence axial (Fig 4) and oblique sagittal (Fig 5)

WHAT ARE THE FINDINGS?

HRCT Temporal
Bone :Right ear
axial (Fig 1)

• Blue arrows- bony canal entering the temporal bone and continuing as the
labyrinthine segment, 1st genu and tympanic segment of the facial nerve canal.
• Red arrows- Postero-inferior to this canal, another thin bony canal is seen traversing
the temporal bone and reaching the inner ear labyrinth.
• Yellow arrow- Dysplastic vestibule fused with lateral semicircular canal is also seen

HRCT Temporal
Bone
:Left ear axial
(Fig 2)

Facial canal is seen anteriorly, charting its own course separate
from the bony canal to labyrinth on the left side as well.

HRCT Temporal Bone : Coronal (Fig 3)

On coronal image (Fig. 3) two bony canals are seen running parallel
to each other in the petrous temporal bone bilaterally, the superior
one is continuous with facial nerve canal and the inferior canal
reaching the inner ear labyrinth.

MRI CISS sequence axial (Fig 4) and oblique sagittal (Fig 5)

MRI CISS sequences reveal facial nerve on both sides (Fig.4a), however no
neural element is visualised in the expected location of vestibulo-cochlear
nerve bilaterally (Fig. 4b and 5).

WHAT IS THE DIAGNOSIS?

Diagnosis

Bilateral duplication anomaly of internal
auditory canal with -
• associated vestibulo-cochlear nerve agenesis
• right sided vestibule-lateral semicircular canal
dysplasia

Discussion

• Normal IAC is a bony canal , 2-8mm, 60-65 degree angle
with posterior petrous ridge

• A normal vestibulo-cochlear nerve has a trophic
influence on normal development of IAC, however
normal IAC doesn’t rule out nerve abnormalities

• IAC anomalies are uncommon (12% of temporal bone
anomalies1) and duplication of IAC is a rare entity
characterised by sensorineural hearing loss due to
frequently associated aplasia or hypoplasia of vestibulo-
cochlear nerve

Discussion

• This congenital anomaly is thought to result from early or
abnormal division of the facio-acoustic primordium2
during embryonic stage of development, resulting in
migration of facial nerve fibers away from vestibulo-
cochlear nerve.

• This leads to development of two separate canals around
each nerve. It is often associated with IAC stenosis
resulting from aplasia or hypoplasia of vestibulo-cochlear
nerve.

• Facial nerve function in these cases is usually preserved3.

Role of Imaging

• Imaging is helpful in assessing the candidacy for
cochlear implantation

• HRCT will depict structural anatomy of bony canal while
MRI helps in visualisation of neural elements

• Patients diagnosed as having vestibulo-cochlear nerve
hypoplasia or aplasia on imaging and showing
detectable responses to electrical or acoustical stimuli
may still benefit from cochlear implantation by virtue of
improved sound detection thresholds and awareness of
sound4.

Summary

• Duplication of IAC is rare.

• It is often associated with concomitant inner ear
malformations or systemic anomalies

• Facial nerve function is usually intact in these cases

References

1. Kesser BW, Raghavan P, Mukherjee S, Carfrae M, Essig G, Hashisaki GT.
Duplication of the Internal Auditory Canal: Radiographic Imaging Case of
the Month. Otol Neurotol 2010; 31: 1352-53

2. Weissman JL, Arriaga M, Curtin HD, Hirsch B. Duplication Anomaly of the
Internal Auditory Canal. AJNR 1991; 12: 867-69

3. Bakar TG, Karadag D, Calisir C, Adapinar B. Bilateral narrow duplicated
internal auditory canal. Eur Arch Otorhinolaryngol 2008; 265: 999-1001

4. Ehrmann-Müller D, Kühn H, Matthies C, Hagen R, Shehata-Dieler W.
Outcomes after cochlear implant provision in children with cochlear nerve
hypoplasia or aplasia. Int J Pediatr Otorhinolaryngol. 2018; 112:132-140.
doi: 10.1016/j.ijporl.2018.06.038. Epub 2018 Jun 23. PMID: 30055722.