Role of imaging in preoperative assement of cochlear implantation

1. PREOPERATIVE ASSESMENT IN
COCHLEAR IMPLANTATION
Dr. D.Sunil Kumar

2. Objectives of Imaging
• To review the findings that influence surgeon
on the accurate patient selection, type of
device and surgical approach.
• To detect any absolute contraindication, and
incidental anatomical abnormalities.

3. Imaging Techniques
• HRCT Temporal Bone:
– detailed evaluation of the osseous anatomy of the
inner and middle ear
– assessment of mastoid pneumatization
– degree of middle-ear aeration.
Disadvantages:
 Ionising Radiation
 Does not directly depict the cochlear nerve,shows
only the bone channel that contains it.

4. • Magnetic resonance (MR)
imaging
• High-resolution heavily T2-
weighted 3D imaging techniques
– direct visualization of the cisternal
and intracanalicular
vestibulocochlear nerve bundles.
– detailed information about the
fluid-filled labyrinth.
• Oblique sagittal images obtained
perpendicular to VIIth-VIIIth
nerve complexes through the
internal auditory canal help
distinguish the cochlear division
of the eighth nerve.

5. Normal anatomy
• Inner Ear
• The inner ear consists of the fluid-
filled membranous labyrinth, which
is enclosed by the bone of the
osseous labyrinth.
• The cochlea is the primary organ of
hearing.It is a spiral structure
having 2.5–2.75 turns around a
central column of bone (the
modiolus).
• A spiral osseous lamina projects
from the modiolus and divides the
cochlear canal into upper (scala
vestibuli) and lower (scala tympani)
compartments; the scala tympani is
the implantation destination.

6. • The vestibular system, which is located
posterolateral to the cochlea, is formed by the
vestibule and three semicircular canals:
superior, posterior, and lateral .
• These structures play a minor role in hearing,
but abnormalities often occur in conjunction
with cochlear malformations.

10. Technique for Cochlear Implantation
• The standard surgical technique for electrode
placement is
– Mastoidectomy
– opening of the facial recess to gain access to the middle-
ear cavity in the vicinity of the round window niche
– Traditionally, a cochleostomy was created that was
anterior-inferior to the round window membrane, with
the electrode array then inserted into the scala tympani of
the basal turn through the cochleostomy .
• However, round window insertion is becoming more
common in patients with substantial residual hearing,
and the occurrence of fewer traumas to the cochlea is
thought to be advantageous in preserving existing
function.

11. • Mastoid and Middle Ear
Anatomy
• Access to the cochlea from
the mastoid by partial
mastoidectomy. is central
to placing the cochlear
implant.
• Important bone landmarks,
such as the facial nerve
recess and round window
niche, are used by the
surgeon to gain access to
the cochlea for placement
of the cochlear implant
electrode array.

12. VIII th nerve complex
• The anatomy of the facial and vestibulocochlear nerve
varies on their way through the cerebellopontine angle
(CPA) and IAM.
• In the CPA the facial nerve has an anterior and slightly
superior put (which remains in this position in its entire
length) and the vestibulocochlear nerve has a
posteroinferior position.
• At the middle third of internal auditory canal, the
arrangement of the facial, cochlear, and superior and
inferior vestibular nerves is in the following
manner:anterosuperior, anteroinferior, posterosuperior,
posteroinferior.
• Cochlear nerve is larger than either the superior or inferior
vestibular nerves in 90% of normal cases and it is of almost
the same size or larger than the facial nerve in 64% of
cases.

14. • Are There Cochleovestibular Anomalies that
Preclude Implantation?
• Is There Evidence of Luminal Obstruction?
• Are There Additional Findings that May
Complicate the Surgery or Subsequent Patient
Management?

15. • Are There Cochleovestibular Anomalies that
Preclude Implantation?

16. IAC and Cochlear Nerve Anomalies
• IAC – Normal – 2-8mm
Average – 4mm
Stenotic – 2 mm
• The IAC may also be atretic or may have a bony septum
that partitions it into two or more separate canals.
• The morphologic characteristics and size of the IAC are not
reliable indicators of the integrity of the cochlear nerve.
• High-resolution MR imaging is the preferred modality for
accurate assessment of the cochlear nerve
• Sagittal oblique images obtained in a plane perpendicular
to the long axis of the IAC provide the best depiction of the
four major nerves of the IAC

17. Axial 0.8-mm-thick SSFP MR image shows the parallel courses of the facial
(black arrowheads) and superior vestibular (white arrowheads) nerves as they cross
the cerebellopontine angle to enter the internal auditory canal through the porus
acusticus (double arrow).

18. Sagittal image shows the four nerves within the IAC: the
facial nerve (straight arrow), cochlear nerve (curved arrow),
and superior and inferior vestibular nerves (arrowheads).

19. • Three types of cochlear nerve anomalies have been
described
• Type 1 cochlear nerve anomaly - stenotic IAC is seen
with an absent eighth nerve.

20. • Type 2 anomaly, a common vestibulocochlear nerve
is found, with hypoplasia or aplasia of its cochlear
branch.
– Type 2a – associated innerear malformations
– Type 2b - isolated

21. Cochlear nerve aplasia in a 4-year-old girl with congenital hearing loss who was
under consideration for cochlear implantation. left (a) and right (b) internal auditory canals,
show the main branches of the facial and vestibulocochlear nerves in cross section. The
anterior (A), posterior (P), superior (S), and inferior (I) aspects of the canals are labeled for
ease of orientation. Absent cochlear nerve noted on right side

22. • At CT, stenosis of cochlear aperture(small canal at the fundus of the
IAC, through which the cochlear nerve passes to enter the cochlea)
indirectly indicates hypoplasia or absence of the cochlear nerve.
• Although the criteria for a normal aperture size are not well defined
in the literature, an aperture with a diameter smaller than 2 mm is
usually considered abnormal.

23. • Implications for Cochlear implantation:
• A small nerve is only a relative contraindication to
cochlear implantation. An appreciably thin cochlear
nerve, may still effectively transmit impulses to allow
hearing.
• At one time, evidence of an absent or severely
dysplastic cochlear nerve was considered an absolute
contraindication to cochlear implantation in any
patient.
• More recently, implantation in children with an absent
or deficient eighth nerve was performed with varying
results.
• Outcomes are more difficult to predict and poorer than
expected compared with those in patients with normal
nerve anatomy but some young children have achieved
substantial word recognition and spoken language skills
.(Young NM, Kim FM, Ryan ME, Tournis E, Yaras S. Pediatric cochlear
implantation of children with eighth nerve deficiency. Int J Pediatr
Otorhinolaryngol 2012;76(10):1442–1448. )

24. • A potential alternative treatment for children
with cochlear nerve deficiency is auditory
brainstem implantation.
• Auditory brainstem implantation is currently
approved by the FDA only for use in
individuals who are deafened by
neurofibromatosis type II and are aged 12
years and older.

25. Cochleovestibular Abnormalities
• Jackler et al. hypothesised that dysplasias result
from a developmental arrest during varying
stages of inner ear organogenesis and
identified the following groups:
– labyrinthine aplasia
– cochlear aplasia
– cochlear hypoplasia
– common cavity malformation
– incomplete partitioning
• cystic cochleovestibular dysplasia (incomplete
partitioning type I)
• incomplete partitioning type II (true Mondini
malformation)

26. • Michel aplasia
Michel aplasia, also known as complete labyrinthine aplasia
characterised by complete absence of cochlea and vestibular
structures, is a rare congenital inner ear abnormality, accounting for
approximately 1% of cochlear bony malformations.
The internal auditory canals (IACs) may be small in size or
absent.Cochlear nerve deficiency will be seen.

27. Complete labyrinthine aplasia. (a) Axial high-resolution CT image shows the absence of inner
ear structures. Note the flat medial wall of the middle ear cavity (arrow) and the hypoplastic
petrous bone (*). (b) Coronal CT image shows an atretic IAC (arrow). (c) Sagittal MR image
obtained in a plane perpendicular to the IAC depicts a single nerve (arrow) within the small
IAC. Combined with the findings in a and b, this feature is suggestive of absence of the eighth
cranial nerve.

28. • Cochlear aplasia
– Cochlea – absent with dense otic bone at the site of cochlea.
– Vestibular system- normal,dilated or hypoplastic

29. • Implications for cochlear implantation:
• Complete labyrinthine and cochlear aplasia
are rare, constituting less than 1% and 3%,
respectively, of cochlear bone abnormalities.
• Cochlear and labyrinthine aplasia, are
considered to be absolute contraindications;
in spite of reports questioning it.

30. • Common cavity
– single cavity that represents the undifferentiated
cochlea and vestibule with no internal archictecture
– The semicircular canals are frequently malformed but
occasionally normal

31. • The modiolus is absent, and the opening of
the internal auditory canal into the common
cavity malformation is abnormal, which may
account for the increased presence of CSF in
these malformations (in contrast to perilymph
fluid in a normal labyrinth)
• Hence there is increased incidence of
spontaneous CSF leakage into the middle ear
and recurrent meningitis in these anomalies

32. • A cochlear implant can be successfully placed into a
common cavity malformation, but the procedure may
be technically challenging.
• If CSF is already present within the cochlea, a brisk leak
of CSF may be encountered after cochleostomy is
performed.
• The leak must be effectively sealed to minimize the risk
for meningitis.
• Even if there is initially no CSF within the common
cavity malformation, the electrode array may be
wrongly positioned into the internal auditory canal,
resulting in a CSF leak
• CSF leakage may be avoided by controlling the tip of
the array at the time of insertion; thus, identifying this
type of malformation may influence the choice of
electrode array and cochleostomy technique.

33. • Cochlear aplasia vs Common cavity:
• Look beneath the cochlear promontry!!
• cochlear aplasia - dense sclerotic bone
• Common cavity – deformed cavity beneath
promontry

34. Incomplete partition I(cystic cochleovestibular
Malformation)
vestibule is distinguishable from the cochlea
• Cochlea – no bony modiolus, resulting in an empty
cystic cochlea
• Vestibule – cystic dilatation
• Figure of 8 appearance
• The cribriform area between the cochlea and IAC is
often defective, and all patients have a large IAC,
predisposing them to increased risks for meningitis and
for a perilymphatic gusher in the event of surgery

35. Type I incomplete partition. (a) Axial CT image shows a cystic
cochlea without any internal architecture, accompanied by a
dilated vestibule that forms a figure eight (straight arrow). Note
the coexistent mucosal disease in the mastoid air cells and
middle ear (curved arrow). (b, c) Axial CT (b) and MR (c) images
obtained in another patient show a cystic cochlea (white
arrowhead) that is separated from a dilated IAC (curved arrow)
by a partially dehiscent cribriform plate (black arrowhead).
Straight arrow in b = opacification of the middle ear and
mastoid air cells.

36. • Cochlear Hypoplasia
• The cochlea and vestibule can be
differentiated from each other but the size of
the cochlea is smaller than normal.
• Cochlea – small with one or partial turn
• Vestibule and semicircular canals –
malformed, may be normal
• IAC - small

39. • Incomplete partition II
(Mondini deformity) :
• most common type more than
50% of all cochlear
deformities.
• Cochlea – Basal turn normal,
middle and apical turns
coalesce to form a cystic apex.
• Modiolus present in basal turn
associated with a large
endolymphatic duct and sac
and an enlarged vestibular
aqueduct
• Vestibule – dilated with
normal semicircular canals.

40. Type II incomplete partition. (a) Axial CT image
shows the absence of the modiolus from a cystic
cochlear apex (straight arrow) formed by
coalescent apical and middle turns. Dilatation of
the vestibular aqueduct (arrowhead) and
vestibule (curved arrow) and a coexistent middle
ear effusion (*) also are seen. (b) Axial CT image
obtained at a slightly lower level shows a normal
basal turn of the cochlea (arrow). (c) Coronal CT
image shows the cystic cochlear apex (arrow).

41. • In incomplete partition and cochlear hypoplasia,
the cochlea is present but does not form the
normal basal, middle, and apical turns.
• The surgical importance of these abnormalities is
primarily determined by the degree of cochlear
malformation and the potential for
communication with CSF.
• Malformations with a dysplastic or absent
modiolus are more likely to have CSF present in
the cochlea, although an intact modiolus does
not exclude the possibility of a fistula.

42. Enlarged Vestibular Aqueduct
• The vestibular aqueduct is a bony canal that
extends from the medial wall of the vestibule,
to the endocraneal surface of the posterior
part of the petrous portion.
• It contains the endolymphatic duct, as an
extension of the membranous labyrinth, which
ends in cul-de-sac between the layers of the
duramater
• An enlarged vestibular aqueduct, either by
itself or in association with mild cystic cochlear
dysplasia, has been reported to be the most
common inner-ear malformation associated
with SNHL
• Enlarged vestibular aqueduct (EVA) is
commonly defined as having a width larger
than 1.5 mm, measured at the midpoint of the
common crus and external aperture

44. • Clinically, children with an enlarged vestibular
aqueduct have hearing that ranges from normal to
profoundly lost.
• The natural history is that of progressive but
unpredictable hearing loss.
• Although the cause is unknown, one theory is that
trauma may affect it by causing hemorrhage into the
endolymphatic sac.
• To minimize the risk for a sudden decline in hearing,
patients are generally counseled to wear a helmet and
refraining from activities in which high-impact and
unprotected head trauma are likely to occur.
• Once individuals with an enlarged vestibular aqueduct
lose hearing to the degree that hearing aids are no
longer sufficient, cochlear implantation is the only
effective treatment option.

45. Vestibule and Semi- circular Canal
Abnormalities
• Isolated anomalies of the vestibule or
semicircular canal typically do not affect surgical
planning.
• However absence of the semicircular canals is
strongly associated with CHARGE syndrome.
• CHARGE syndrome is associated with cochlear
dysplasias, dysplastic middle-ear cavity and an
anomalous facial nerve course, which may
complicate the surgical approach and
cochleostomy

46. • Is There Evidence of Luminal Obstruction?

47. Cochlear Patency
• One of the most important causes of postnatal
acquired SNHL is bacterial meningitis, which
may have profound effects on cochlear
patency that limit or preclude cochlear
implantation.
• Deafness secondary to bacterial meningitis is
often associated with labyrinthitis ossificans, a
progressive process that may obstruct the
cochlea.

48. • Labyrinthitis ossificans
• Progressive condition characterised by fibrosis
of membranous labyrinth progressing to
ossification.
• due to complication of meningitis, otosclerosis
and trauma.
• commonly bilateral but may be asymmetrical.
• Changes typically begin at the basal cochlear
turn and proceed apically

49. • Early implantation before the onset of substantial
obstruction is necessary to achieve optimal electrode
insertion and avoid the need for complex alternative
surgical approaches, which have more variable outcomes.
• Early fibrotic changes of labyrinthitis ossificans are seen at
MR imaging as a loss of the normal hyperintensity of fluid
on T2-weighted images of the membranous labyrinth
• Labyrinthine enhancement may also accompany or precede
fibrotic changes at contrast material–enhanced MR imaging

50. • In later stages ossification of membranous labyrinth are
better depicted in CT than MRI.
• A completely mineralized labyrinth may mimic congenital
absence of the cochlea and vestibule; however, in contrast
to patients with hypoplasia, those with labyrinthitis
ossificans should have a normal-sized internal auditory
canal, petrous apex, and cochlear aperture.

51. • Cochlear ossification does not contraindicate
the placement of cochlear implant, but it
might complicate the surgical procedure.
• Preoperatively, is important to know the
presence and extent of pathology to select the
surgical approach and type of electrode

52. • Are There Additional Findings that May
Complicate the Surgery or Subsequent
Patient Management?

53. Other Anatomic Considerations
• Facial Nerve
Anomalies
– hypoplastic or
dehiscent facial nerve
canal

54. • Vascular Anomalies
– high-riding or
dehiscent jugular bulb
– large occipital
transosseous veins
may affect the
placement of the
postauricular incision
– an aberrant carotid
artery

55. • Middle-Ear and Mastoid Variants
• Absence or dysplasia of the round
window niche, which typically
serves as a bone landmark for
cochleostomy, necessitates the use
of other structures to guide entry
into the basal cochlear turn
• Assessing the configuration and
pneumatization of mastoid air cells
and the middle-ear cavity may also
aid surgical planning
• Middle-ear opacification also limits
visualization during surgery, and
acute otomastoiditis requires
treatment before implantation to
decrease the risk for infection and
meningitis.

56. Conclusions
• Cochlear implantation is the first medical treatment to
enable deaf children to learn to talk and develop age-
appropriate language and literacy.
• Imaging is an important part of the preoperative
workup in these children.
• A greater understanding of the process for evaluating
patients who are eligible for cochlear implantation, the
surgical procedure, and the implant device itself
enables the radiologist to provide more useful
information to referring clinicians and ultimately
improves the quality of care these children receive.