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Implications of Duplicated Internal Auditory Canal on Cochlear Implantation Candidacy
Jonathan Overdevest, MD, PhD1,2; Christine Glastonbury, MD1,3; Anna Meyer, MD1,2
1University of California, San Francisco 2Otolaryngology – Head & Neck Surgery, 2Radiology & Biomedical Imaging
INTRODUCTION DISCUSSION CASE REPORT ABSTRACT
FIGURE 3
CONCLUSIONS
REFERENCES
CONTACT
Title: Implications of duplicated internal
auditory canal on cochlear implantation
candidacy
Objectives: To describe a patient with
bilateral duplicated internal auditory
canals (IACs) and their impact on
cochlear implantation candidacy.
Study Design: Retrospective case
review.
Methods: Retrospective review of
electronic medical record, including MRI
and CT imaging.
Results: An 11 year-old boy presented
for cochlear implantation evaluation with a
history of progressive bilateral
asymmetric hearing loss. At presentation,
right hearing was consistent with a
profound sensorineural loss (SNHL), but
with responses in the aided condition. Left
hearing was normal low frequency sloping
to profound SNHL with 48% word
recognition. Computed tomography and
magnetic resonance imaging revealed a
right duplicated IAC, with two nerve
structures in the superior canal, an empty
inferior canal, and an absent cochlear
canal. The left IAC was reduced in caliber
with a partial superior bony partition, two
nerve structures, and a narrow cochlear
canal. The cochleae were normal
bilaterally. Given his significant left
residual hearing and confirmed right
cochlear nerve responses despite an
absent cochlear canal, the patient was
offered right cochlear implantation.
Conclusions: Patients with IAC
duplications are rare and their candidacy
for cochlear implantation may be difficult
to determine when the status of the
cochlear nerve is uncertain.
Educational Objective: To discuss the
indications and implications of cochlear
implantation in cases of internal auditory
canal duplication where cochlear nerve
status is uncertain.
A 10-year old boy was referred to our practice for
evaluation of progressive bilateral asymmetric hearing
loss. His parents first noticed his hearing deficit when
he was 3 years of age; daycare officials corroborated
their concerns. He was subsequently diagnosed with
bilateral asymmetric SNHL, right greater than left. He
had no symptoms of vertigo. His past medical history
was significant for chronic otitis media with effusion
requiring placement of two sets of tympanostomy tubes
beginning at age 6.
His gestational and birth history were unremarkable,
other than an emergent delivery via cesarean section
after prolonged labor. On physical exam, he was found
to be mildly dysmorphic with hypertelorism, a broad
nasal bridge, prominent ears, and a long philthrum.
Tympanic membranes were thickened with evidence of
myringosclerosis, but without perforations.
Audiometry revealed left-sided mild hearing loss from
250-1000 Hz, precipitously sloping to severe-profound
sensorineural hearing loss from 1500-8000 Hz and
word recognition score of 40%. Right-sided
assessment was consistent with profound
sensorineural hearing loss from 125-8000 Hz, with
responses obtained at 125 Hz and 500 Hz that were
possibly consistent with cross-over. He demonstrated
absent ipsilateral and contralateral acoustic reflexes.
Tympanometry was consistent with normal compliance
and significant negative middle ear pressure.
Multidetector computed tomography (CT) imaging
revealed a clear duplication of the right internal auditory
canal, with an anterior superior canal containing the
labyrinthine segment of the facial nerve, and a narrow
posterior inferior canal containing a hypoplastic
vestibulocochlear nerve. The modiolus of the right
cochlea appeared hypodense, with subtle stenosis of
right oval window. Left-sided IAC had reduced caliber.
MRI showed two canals extending from the right
cerebropontine angle (CPA) into the petrous temporal
bone, where sagittal and axial images demonstrate two
nerves within the CPA extending towards these canals.
Two nerves extend into the anterior superior canal,
while a tiny nerve appears to branch from the distal
CPA component and enters what appears
morphologically to be the true IAC. On the left, 2 nerves
are seen within the CP angle extending into the IAC,
where a tiny branch from what appears to be the
vestibulocochlear nerve extends towards the tiny
cochlear canal.
Initial management included fitting with a Phonak Naída
BiCROS system. Subsequent audiometric evaluation
revealed improved right-sided aided hearing,
suggesting retained nerve function in the right ear.
Given this aid-improved hearing, he was evaluated for
cochlear implantation. He was subsequently implanted
with a Cochlear Nucleus CI422. Follow-up audiometric
exam resulted in appropriate mapping of the cochlear
device, with verification of audible perception at
medium loudness stimuli and without tactile sensation.
Although hearing loss is a common birth defect -- affecting
approximately 3-4 infants of every 1,000 newborns1 -- duplicated
narrow IAC is a rare congenital abnormality. IAC anomalies have been
described as a component of a number of syndromes, including
Michel, Mondini, Bing-Sieberman, Scheibe and Klippel-Feil17, however
these presentations rarely include IAC duplication. Indeed, an
exhaustive literature review reveals only 24 previously reported cases
(Table 1).
Multiple hypotheses have been postulated as to the etiology of IAC
stenosis and possible duplication, of which two theories are most
widely accepted. The first depicts a primary bony defect, where bony
stenosis impedes neural growth and results in aplasia or hypoplasia of
vestibulocochlear nerve (VCN). Critics of this theory argue that the
early development of the VCN by gestational week 5 is prior to IAC
chondrification and posterior ossification at 8 weeks and 5 months,
respectively, would prevent the inhibition of an established nerve.
Moreover, this theory is unlikely to account for the relatively normal
facial nerve development in cases of IAC stenosis17. An alternative
theory describes a primary nerve hypoplasia or aplasia, where the
embryonic cochlea and vestibules promote chemotactic development
of the VCN, and the absence of the VCN results in lack of inhibitory
factors to limit the chondrification process. This theory appears to be
supported by the literature demonstrating a reduction in IAC diameter
in cases of congenital VCN aplasia and no cases of IAC stenosis with
normal caliber22. However, an article from Adunka et al, reports 11
cases of VCN aplasia within normal IAC, and 9 cases of narrow IACs
that had some degree of nerve deficiency1. Additionally, it is interesting
to note that of the 21 cases of duplicated narrow IAC with available
laterality data, there appears to be a predilection toward right ear
malformation, with 18 of those cases involving the right ear (Table 1).
Previous reports have detailed the importance of appropriate imaging
in the diagnosis of narrow IAC duplication as the etiology for
congenital SNHL. Particularly, high-resolution computed tomography is
the study of choice for imaging structural inner ear abnormalities,
whereas magnetic resonance imaging is useful in depicting neural
anatomy 22–24. Nevertheless, even appropriate imaging cannot serve
as a definitive predictor of successful auditory response following
cochlear device implantation.
Many reports suggest that IAC stenosis is a relative contraindication to
cochlear implantation, as absence of the cochlear nerve would prevent
proper electrical stimulation from a cochlear device 1. The available
literature appears to support cautious optimism for implantation in the
setting of IAC stenosis. Case reports detail positive results in setting of
VCN hypoplasia1, residual hearing despite VCN hypoplasia 25, and
improved hearing after implantation in a patient with aplasia of the
cochlear nerve 9. Naturally, reports of failed implantation in the setting
of IAC stenosis and neural aplasia also exist 26–28.
To date, our case represents the first patient with duplicated narrow
IAC to undergo cochlear implantation. The positive audiometric results
provide preliminary support for the implantation of future cases of
SNHL secondary to duplicated narrow IAC.
Duplicated narrow IACs are a rare finding as a cause of
congenital SNHL. A multimodal imaging approach
should be pursued for appropriate evaluation of a
patient’s SNHL to address potential issues with both
bony and neural anatomy. Although hypoplasia or
aplasia of the VCN should induce caution in a surgeon
attempting cochlear implantation, numerous studies
demonstrate uncoupling of anatomical deficiencies and
improved hearing following cochlear implantation. Our
case of duplicated narrow IAC serves as an important
example of successful cochlear implantation despite an
underdeveloped vestibular cochlear nerve with
unknown integrity.
Hearing loss is a common birth defect1, where nearly
20% of patients with congenital sensorineural hearing
loss (SNHL) have structural abnormalities of the inner
ear2. Malformations of the internal auditory canal (IAC)
comprise a majority of these defects, often manifested
as stenosis, hypoplasia, or atresia. The normal
diameter of the IAC ranges from 2-8mm, with an
average width of 4mm3. Canals are considered stenotic
when measuring less than 2mm, a finding often
correlated with SNHL, although this relationship is
clearly inconsistent3.
Despite the prevalence of IAC anomalies, duplication of
narrow IACs remains an exceedingly rare finding with a
limited number of reported cases. Indeed, an
exhaustive literature review yields only 24 reported
cases since the seminal case presented by Everberg in
19634–21. These cases include a mix of unilateral and
bilateral IAC duplications with concomitant SNHL,
where the most describe identifiable facial nerves and
hypoplastic/aplastic vestibulocochlear nerves on MRI
and CT imaging. In this report, we present a unique
patient with unilateral IAC duplication and progressive
bilateral SNHL whose case provides a platform for
discussing the indications and implications of cochlear
implantation candidacy when cochlear nerve status is
uncertain.
Jonathan Overdevest, MD, PhD Department of Otolaryngology – HNS University of California, San Francisco [email protected]
Curtin et al, Am. J. Otol. 7, 275–281 (1986)
1. Adunka, O. F. et al. Internal Auditory Canal Morphology in Children with Cochlear Nerve Deficiency: Otol. Neurotol. 27, 793–801 (2006).
2. Jackler, R. K., et al. Congenital malformations of the inner ear: A classification based on embryogenesis. The Laryngoscope 97, 2–14 (1987).
3. Som, P. M. & Curtin, H. D. Head and neck imaging. (Mosby Elsevier, 2011).
4. Everberg, G., Ratjen, E. & Sørensen, H. Unilateral Atresia of the Internal Auditory Meatus, confirmed by Radiography. Br. J. Radiol. 36, 568–573 (1963).
5. Clemens, F. & Sandström, J. Double-Barreled Hypoplastic Internal Auditory Canal in Unilateral Deafness. Acta Radiol. Diagn. Swed. 16, 342–346 (1975).
6. Curtin, H. & May, M. Double internal auditory canal associated with progressive facial weakness. Am. J. Otol. 7, 275–281 (1986).
7. Weissman, J. L., Arriaga, M., Curtin, H. D. & Hirsch, B. Duplication anomaly of the internal auditory canal. Am. J. Neuroradiol. 12, 867–869 (1991).
8. Vilain, J., Pigeolet, Y. & Casselman, J. W. Narrow and vacant internal auditory canal. Acta Otorhinolaryngol. Belg. 53, 67–71 (1999).
9. Casselman, J. W. et al. Aplasia and hypoplasia of the vestibulocochlear nerve: diagnosis with MR imaging. Radiology 202, 773–781 (1997).
10. Cho, Y. S., et al. Narrow internal auditory canal syndrome: parasaggital reconstruction. J. Laryngol. Otol. 114, 392–394 (2000).
11. Ferreira, T., Shayestehfar, B. & Lufkin, R. Narrow, duplicated internal auditory canal. Neuroradiology 45, 308–310 (2003).
12. Demir, Ö. İ., et al. S. Narrow duplicated internal auditory canal: radiological findings and review of the literature. Pediatr. Radiol. 35, 1220–1223 (2005).
13. Weon, Y. C., Kim, J. H., Choi, S. K. & Koo, J.-W. Bilateral duplication of the internal auditory canal. Pediatr. Radiol. 37, 1047–1049 (2007).
14. Baik, H. W., et al. A Narrow Internal Auditory Canal with Duplication in a Patient with Congenital SNHL. Korean J. Radiol. 9, S22–S25 (2008).
15. Bakar, T. G., Karadag, D., Calisir, C. & Adapinar, B. Bilateral narrow duplicated internal auditory canal. Eur. Arch. Otorhinolaryngol. 265, 999–1001 (2008).
16. Kono T, et al. Narrow duplicated internal auditory canal: A rare inner ear malformation with SNHL. Arch. Otolaryngol. Neck Surg. 135, 1048–1051 (2009).
17. Lee, S. Y. et al. Narrow Duplicated or Triplicated Internal Auditory Canal: Separated Narrow Internal Auditory Canal as the Presence of VCN Fibers?
JCAT. July 33, 565–570 (2009).
18. Wang, L. et al. [Imaging features of duplication of the internal auditory canal]. Zhonghua Er Bi Yan Hou Tou Jing Wai Ke Za Zhi 45, 481–485 (2010).
19. Coelho, L. O. M., et al. Bilateral narrow duplication of the internal auditory canal. J. Laryngol. Otol. 124, 1003–1006 (2010).
20. Kesser, B. W. et al. Duplication of the internal auditory canal: radiographic imaging case of the month. Otol. Neurotol. 31, 1352–1353 (2010).
21. Natili, A. Novel Case & Review of Duplicated Internal Auditory Canals. (2012).
22. Glastonbury, C. M. et al. Imaging Findings of Cochlear Nerve Deficiency. Am. J. Neuroradiol. 23, 635–643 (2002).
23. Buchman, C. A. et al. Auditory Neuropathy Characteristics in Children with Cochlear Nerve Deficiency: Ear Hear. 27, 399–408 (2006).
24. Adunka, O. F., et al. A. Value of Computed Tomography in the Evaluation of Children With Cochlear Nerve Deficiency: Otol. Neurotol. 28, 597–604 (2007).
25. Bamiou, D. E., et al. Useful residual hearing despite radiological findings suggestive of anacusis. J. Laryngol. Otol. 113, 714–716 (1999).
26. Gray, R. F. et al. Cochlear implant failure due to unexpected absence of the eighth nerve--a cautionary tale. J. Laryngol. Otol. 112, 646–649 (1998).
27. Maxwell, A. P., Mason, S. M. & O’Donoghue, G. M. Cochlear nerve aplasia: its importance in cochlear implantation. Am. J. Otol. 20, 335–337 (1999).
28. Shelton, C., et al. The narrow internal auditory canal in children: a contraindication to cochlear implants. Otolaryngol.--Head Neck Surg.
FIGURE 1
FIGURE 2
CT of the R & L Temporal bone
MRI of Duplicated IAC
Sketch of Duplicated IAC
TABLE 1
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