Review of Cochlear Implant
R3 施偉勛 Aug 20, 2003
History
1) Volta, 1790, inserted a metal rod in each ear with 50 volts of
electricity
2) Djourno and Eyries, 1957, first stimulate the acoustic n by an
electrode
3) House, 1972, first single-channel implant, stimulate nerve via scala
tympani
4) FDA approval for use in adults in 1984, in children in 1990
5) The 1990’s improvement in speech processor design
Component of CI
1) Microphone: receive and transduce sound into an electrical
representation by analog (continuous) fashion
2) External speech processor and signal-transfer hardware:
Amplification
Compression: the dynamic range between their absolute threshold and
painful sound (most comfortable level): 5 dB in high
tone, 10-25 dB in low tone of profound SNHL p’t
dynamic range correlated with spiral ganglion cell count
Filtering: the frequency for understanding speech, 100 to 4000 Hz
Shaping
3) Transmitter (outer coil): send processed signal to the receiver via
radiofrequency
4) Receiver: receive the signal and send electrical energy to electrodes
5) Electrode array: discharge the neural components of the auditory
system
Type of CI
1) encoding sound information
a. analog coding: electrodes are continuously stimulated
b. digital coding: electrodes are stimulated in a pulse fashion
2) stimulation system
a. monopolar system: only one ground electrode for all the others
The electrical fields may interfere with others can’t stimulate >
1 or too closer
b. bipolar system: the ground for each electrode is much closer
( adjacent to, or a few electrodes away) smaller field with less
interference, more discrete
more spatially selectively activation of the spiral ganglion without
better speech perception
Speech Processing Strategies
1) spectral peak (SPEAK):
 filter sound (200-10000Hz) into 20 different bands
 stimulus rate: the lowest frequency of speech (180-300 cycles/s)
 speech frequency ( 280-1000 Hz) stimulated the apical electrode
800-4000 Hz stimulated the basal electrode
 Three high frequency filter (2000-2800 Hz, 2800-4000 Hz, > 4000 Hz)
stimulated apical electrodes due to greater incidence of ganglion cell
survival at the apex
2) continuous interleaved sampling (CIS) strategy:
 filter the speech into 8 bands
 the bands with highest amplitude compressed stimulated electrode
 stimulus rate: high-rate pulsatile stimuli to capture the fine details of
speech
3) advanced combined encoder (ACE) strategy:
 filter the speech into a set number of channels
 select the highest envelope signals for each cycle of stimulation
 stimulus rate: in a very rapid fashion
4) simultaneous analog strategy (SAS):
 closely mimics the normal ear
 all sound compressed and filtered into 8 channels
 simultaneously and continuously to the tonotopic electrode
 no effort to select for speech frequency
SPEAK and CIS: relative success, SAS: limited success
Etiology for bilateral severe SNHL
Idiopathic, Congenital anomaly, Meningitis, Meniere’s disease, Sudden
deafness, Ototoxicity, hemosiderosis, noise-induced HL
Meningitis and CI
1) early implantation is indicated if evidence of ossification is noted after
meningitis
2) followed with CT/MRI until the implantable age
3) treat with steroid to avoid HL
4) observe for 6 months before implantation, for regaining the hearing
5) ossification is incomplete, then drill forward along the basal coil for
4-5 mm to scala tympani or placed into scala vestibuli
Indications for CI
FDA approval
1) Adult criteria (≧ 18 y/o)
a. bilateral severe-to-profound SNHL with 70 dB PTA with
postlingually deaf
b. little or no benefit from hearing aids ( attempt binaural HA for at
least 6 mo)
c. psychological suitability
d. word discrimination scores < 40% in the best aided condition
e. no anatomical deformity to preclude implantation success
f. no general anesthetic contraindication
2) Pediatric criteria (12 m/o to 17 years 11 months)
a. bilateral severe-to-profound SNHL with >90 dB PTA in the better
ear,
b. no benefit with HA
c. open-set sentence recognition tests < 30% under best-aided
condition
d. must tolerate wearing HA for a period
e. parents be highly motivated and have reasonable expectations
3) Consideration of CI in children < 2 y/o
a. electrical stimulation prevents further neural degeneration
b. maximum plasticity and better result
c. the effects of head growth
d. the possibility of inner ear complication from otitis media, esp in
this age
e. the effects of electrical stimulation on the developing nervous
system
4) Prelingually deafened adult:
 improved sound awareness, but minimal improvement in speech
recognition skills
Contraindication for CI
Absolute
1) The absence of the cochlea, and cochlear n atresia ( small IAC)
2) active middle ear disease, ex. Cholesteatoma under 2 stage op
3) neurofibromatosis II
4) mental retardation, psychosis, organic brain dysfunction, unrealistic
expectations
Relative
1) Labyrinthitis ossificans ( obliteration of the basal cochlear duct):
2) Other forms of inner ear malformation: ex: common cavity, incomplete
partition of the cochlea, membranous anomaly.
Work-up for patients seeking CI
1) auditory behavioral responses, ABR, stapedial reflex test, DPOAE
2) HRCT/MRI of temporal bone
 auditory threshold (+), then CT is sufficient
 auditory threshold (-), narrow IAC in CT, then MRI
3) psychological and medical evaluation
4) trial of appropriate HA with intensive auditory training
Informed consent
1) Parents must understand that CI do not restore normal hearing and that
auditory and speech outcomes are highly variable and unpredictable
2) The importance of long-term habilitation must be stressed
Surgical Procedure
1) implanting ear: better or worse-hearing side
 better-hearing ear had higher spiral ganglion cell counts postmortem
 no long-term advantage to implant the better ear implant the worse
ear and wear an aid in the best hearing ear ( binaural sound localization)
2) extent of hair removal:
 4 fingerbreadths above and behind the ear
3) post-auricular incision:
 the distance between device and incision > 1.5 cm
 C shape: preserve superficial temporal artery, contraindicated with
preexisting post-auricular incision
 inverted U or J incision: maximize the blood supply from both
occipital and superficial temporal arteries, easily incorporate into
previous post-auricular incision
4) creation of anteriorly-based temporoparietal flap, and
subperiosteal pocket
 flap must be thinned to 6 mm to allow the magnetic attraction
5) cortical mastoidectomy with minimal saucerization
6) posterior tympanotomy for opening the facial recess
 familiarity with surgical anatomy remains the cornerstone to avoid
facial n injury
7) seat the receiver stimulator
 the anterior margin of implant should lie behind the posterior margin
of the auricle due to the postauricular microphone
 An island of bone is left in the center of the wall to protect the dura
8) cochleostomy:
 anterior/anterior-inferior to the round window
 for the anatomical hook
 hypotympanic air cells may mimic the anatomy of round window
9) deep insertion of electrode array to the apex in scala tympani
 malposition into the cochlear aqueduct, E tube and near the
labyrinthine part of facial nerve, carotid canal (0.2 mm between
cochlea and canal, 0.25-7mm)
 endoscopy of cochlea: 0.7 mm in diameter, 1600 quartz fibers, field
vision: 55° with a depth of focus of 1 to 25 mm, magnification of 60x
10) seal the opening by muscle or fascia
11) Nerve response telemetry
12) intra-operative radiograph:
 7% abnormal radiograph when procedure goes without difficulty
13) turn up 1 month after operation for mapping
CI surgery with inner ear malformation
1) cochleostomy in common cavity: approximate position of the
ampulated limb of the LSC
2) associated with round or oval window malformation or aberrant
fallopian canal facial nerve monitor
3) CSF gusher should be expected during op
Surgical Problem in CI, experience in NTUH (Mar 1994 to Sep 2001)
1) perioperative
Cochlear calcification: 4/58 (6.9%), common cavity of inner ear: 4
(6.9%), Perilymphatic leakage: 4 (6.9%), CSF gush out: 1 (1.7%)
2) early complication
wound infection: 2 (3.4%), flap necrosis with implant exposure: 1, facial
palsy: 1, vertigo: 5 (8.6%), low grade fever: 7 (12%)
3) late complication
cholesteatoma: 1, CI failure: 3 (5.1%)
Postoperative Complication (0.7-11.7%, 6.9% in NTUH)
1) Major complication: require revision surgery
# device problem: eletrode should be left in place in re-implantation
 long-term extrusion: due to contraction of fibrous tissue surrounding
the cable
 acute extrusion: by the elastic memory of the cable
 the split “ incus bridge” technique: between the short process of the
incus and the LSC, 0.7 mm for Nucleus M22 and 1.0 mm for
Mini-Med Clarion
 The distance between the tip of incus and the round window is little or
no change during the maturation of the infant skull ( Clark )
# facial palsy:
# flap breakdown
# cholesteatoma, osteomyelitis, brain infarction
2) Minor complication
# wound infection
# facial n stimulation: 3% ( turn off the responsible electrodes)
# CSF leaks
# meningitis
# bleeding
# post-operative vertigo: 75% incidence (Steenerson)
About increased incidence of the post-op meningitis, CDC suggested
1) < 2 y/o children should receive pneumococcal conjugate vaccine
2) ≧2 y/o completed conjugate series: 1 x polysaccharide vaccine
3) 24-59 m/o without any vaccination: 2 x conjugated, 1 x polysaccharide
4) ≧ 5 y/o: 1 x pneumococcal polysaccharide vaccine
Postoperative Rehabilitation
1) For the prelingually deaf p’t, auditory and speech training are
imperative
2) For the postlingually deaf one, more complex listening skills is needed
Factor affect the auditory performance of CI recipients
1) Age of onset of deafness:
a. postlingual onset better than prelingual or perilingual
b. age is relative unimportant in postlingually deafened adults
2) Period of auditory deprivation
a. the shorter the period, the faster and higher will be the achievement
of open-set speech sore
b. after the age 7, plasticity was greatly reduced
c. 55 y/o, deaf for 40 yrs less favorable than a 70 y/o, deaf for 5 years
3) Age at implantation
a. prelingually deaf children implanted before age 6 can catch up to
implanted postlingually deaf children within 2-5 years
b. 90% implanted before age 2 were integrated into mainstream
education, only 20-30% implanted after age 4 (Govaert et al)
c. implantation at 2 year of age better auditory performance than at 3
years of age
4) Residual hearing
6) Device factor: channel >4
Clinicopathological studies ( Kawano et al)
1) Threshold of electrode correlated with distance of the one from the
spiral ganglion
2) Dynamic range correlated with spiral ganglion cell count
3) Psychophysical threshold increased as pathologic change (i.e. new
bone and fibrous tissue secondary to implantation) increased
Cochlear implants in the third millennium
1) Partial insertion cochlear implantation: for high-tone SNHL
2) Bilateral implantation: one processor and receiver, implant electrode in
both ears
3) To provide better reproduction of the coding of sound
4) To further improve the perception of speech and other sounds in noise
90% sentence sore for SPEAK strategy in quiet, 60% in 10 dB noise
5) To develop a totally implantable CI
Fiberoptic lever system: detect TM vibration via the modulation of
light intensity by laser
All electronics and rechargeable battery in mastoid process
6) Use nerve GF to protect the hearing nerve from die back after deafness
a. neurotrophins may lead to better results with CI with more nerves
to stimulate
7) To reestablish auditory plasticity to achieve optimal speech perception
Neurotropin released by CI electrode reactivate the production of NT
along the auditory pathway establish the neural connections required
for coding
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