s1042368008000156.pdf

Neurosurg Clin N Am 19 (2008) 317–329

Cochlear and Brainstem Implantation

Elizabeth H. Toh, MD a , * , William M. Luxford, MD b

a Department of Otolaryngology, University of Pittsburgh, Eye & Ear Institute, Suite 500, Pittsburgh, PA 15213, USA

b Clinical Studies Department, House Ear Institute, 2100 West Third Street, Los Angeles, CA 90057, USA

Cochlear and auditory brainstem implants

oﬀer safe and eﬀective hearing habilitation and

rehabilitation for profoundly deafened adults and

children. Brainstem implant technology is cur-

rently approved for use in patients with neuroﬁ-

bromatosis type 2, who have lost integrity of

auditory nerves following vestibular schwannoma

removal. An update on implant devices, speech

processing strategies, candidacy criteria, and per-

ceptual performance are provided in this article.

This article provides an update on issues

related to cochlear implantation, including device

design, speech processing strategies, candidate

selection, surgical technique, and perceptual

performance.

Implant device

Basic components

Cochlear implants

All cochlear implant systems possess an exter-

nally worn device and an implanted internal

component ( Fig. 1 ). The external hardware con-

sists of an ear-level microphone, an ear-level or

body-worn speech processor, and a transmitter

placed behind the ear. The internal component

consists of a receiver–stimulator, linked to an in-

tracochlear electrode array via a lead wire. Some

implant devices have a second electrode which

serves to ground the stimulating electrode. Sound

received by the microphone is transduced into

electrical signals, which are ﬁltered, analyzed,

and digitized by the speech processor and for-

warded to the transmitting coil. The encoded

signals are then delivered to the implanted re-

ceiver–stimulator by radio-frequency electromag-

netic induction. This signal is reconverted to an

electrical signal, which is then delivered to the

implanted electrode within the scala tympani.

Current applied to the electrodes radiates into

the ﬂuid of the scala tympani, spreads through

the habenula perforata of the osseous cochlear

modiolus, and stimulates the auditory nerve.

Cochlear implantation is an established treat-

ment for selected individuals with bilateral severe

to profound sensorineural hearing loss (SNHL)

who derive limited beneﬁt from conventional

hearing aids. The ﬁrst cochlear implants, developed

in the early 1960s, comprised single electrodes that

were surgically placed within the scala tympani, in

an eﬀort to electrically stimulate the auditory nerve

in patients with absent or dysfunctional cochlear

hair cells. These early devices restored some degree

of sound awareness to recipients and, in many

cases, facilitated lip-reading far better than their

hearing aids had. The introduction of multichannel

devices in the early 1980s, development of ad-

vanced speech coding strategies, and reﬁnement of

candidacy criteria have led to substantial improve-

ments in postimplant performance, evidenced by

improved open-set speech understanding in both

children and adults.

This article originally appeared in Otolaryngologic

Clinics of NA: volume 35, issue 2, April 2002; p. 325–42.

It has been updated to reflect recent advances in implant

technologies.

* Corresponding author.

E-mail address: toheh@upmc.edu (E.H. Toh).

Electrode design

The design of the electrode array diﬀers in the

presently available commercial implants. Four

implant systems have been FDA approved for

doi:10.1016/j.nec.2008.02.014

neurosurgery.theclinics.com

318

TOH & LUXFORD

Fig. 1. Essential components of cochlear implant system (Cochlear Corp., Englewood, CO).

use in adults and children in the United States

(the Nucleus Freedom Implant [Cochlear Corp.,

Englewood, Colorado], the HiRes 90K Implant

[Advanced Bionics Corp., Sylmar, California],

and the PULSAR CI100 and SONATA TI100

Implants [MED-EL Corp., Innsbruck, Austria]).

There has been a trend in the past decade toward

implantation of modiolar-hugging electrode ar-

rays. The closer proximity of these electrode

arrays to spiral ganglion cells oﬀers theoretic

advantages of improved sound quality, speech

recognition, and power eciency [1] .

The Nucleus Freedom Implant system cur-

rently uses the Contour Advance electrode with

Softip ( Fig. 2 ). This consists of a 25mm long pre-

curved modiolar-hugging electrode with 22 plati-

num electrode contact plates held in a straight

position with a soft platinum wire stylet. The elec-

trode tip comprises a conical tapered silicone elas-

tomer designed to improve the insertion

characteristics of the original Contour electrode

and minimize tip fold-over during the insertion

process ( Fig. 3 ). The Contour Advance with Sof-

tip electrode inserted using the Advance Oﬀ Stylet

(AOS) technique has been shown to signiﬁcantly

reduce trauma to the intracochlear structures dur-

ing he insertion process [2] . Using the AOS tech-

nique, the electrode is inserted until its tip

reaches near the back of the basal turn of the

cochlea, then is advanced oﬀ the stylet ( Fig. 4 ).

A marker on the outer surface of the electrode

(11 mm from the tip) delineates the insertion point

at which the AOS technique should begin. As with

earlier generation Nucleus implant systems, the

Freedom Implant includes a ground or reference

electrode, allowing monopolar stimulation of all

22 electrodes in the array, thus reducing power

consumption.

Similar to the Nucleus Freedom implant, the

HiRes 90K implant is housed in a titanium case

with a removable magnet and telemetry coil

attached and encased in silastic ( Fig. 5 ). The de-

vice comes with 2 electrode options. The HiFocus

Helix electrode is 24.5 mm long and consists of 16

Fig. 2. Nucleus Freedom implant (Coclear Corp.,

Englewood, CO).

COCHLEAR AND BRAINSTEM IMPLANTATION

319

Fig. 3. Advance Contour electrode with Softip (Co-

chlear Corp., Englewood, CO).

longer, narrower, and less curved silicone elec-

trode array, and is designed to be inserted to

a depth of 25 mm.

The PULSAR CI100 and SONATA TI100

devices developed by MED-EL Corporation

house the same I100 electronics in a ceramic and

titanium casing respectively ( Fig. 6 ). The standard

electrode array for these 2 devices features 12

pairs of electrode contacts on a soft and ﬂexible

straight electrode. The design of this array allows

for the deepest insertion depth (approximately

31 mm) which then enables stimulation of a larger

number of nerve ﬁbers within the cochlea.

In addition to the standard electrode inventory

provided by all the manufacturers, modiﬁed

electrode conﬁgurations including straight, com-

pressed and split electrodes ( Fig. 7 ) are generally

available with each device to implant congenitally

malformed and ossiﬁed cochleae.

In addition to the standard electrode inventory

oﬀered with each of these cochlear implant

systems, modiﬁed electrode designs such as the

straight electrode, short electrode and split elec-

trodes, are available for implanting malformed or

ossiﬁed cochleae.

planar platinum-iridium contacts arranged along

the medial surface of the silicone electrode array.

Dielectric partitions between the electrode con-

tacts prevent channel interaction resulting from

longitudinal spread of electrical current toward

neighboring groups of nerve ﬁbers. This pre-

curved electrode comes preloaded on a stylet as-

sembly which advances the electrode array oﬀ

the stylet to an insertion depth of 21.5 mm. This

conﬁguration theoretically allows for the elec-

trodes to lie close to the cochlear modiolus, thus

providing improved sound ﬁdelity and hearing

performance. The HiFocus 1j electrode has the

same number of electrode contacts on a slightly

Speech-coding strategies and speech processors

Speech-coding strategies are software pro-

grams stored within the speech processor, which

convert pitch, loudness and timing of sound into

useful electrical signals [3] . Strategies are typically

either non-simultaneous or simultaneous.

Fig. 4. Advance oﬀ stylet technique for insertion of Advance Contour electrode with Softip (Cochlear Corp.,

Englewood, CO).

320

TOH & LUXFORD

Fig. 5. HiRes 90K implant (Advanced Bionics Corp.,

Sylmar, CA).

Another popular strategy is the continuous

interleaved sampling (CIS) strategy [5,6] . With

this strategy, each electrode receives pulses at

a rate of 600 to 1000 pulses per second. Speech

is divided into several frequency bands, and the

amplitude envelope is extracted from each band.

This information is then translated into an electri-

cal impulse that drives the electrode representing

that frequency band. In the SPEAK and CIS

strategies, the electrode pulses are generated se-

quentially so that no two electrodes are active at

the same time. This strategy avoids the problems

of electrode interactions.

The Nucleus Freedom processors ( Fig. 8 ) use

a combination of the SPEAK, ACE and CIS

strategies. The original ACE strategy allows stim-

ulation rates up to 14,400 pps (pulses per second),

whereas the more recent ACE (RE) strategy

allows for higher stimulation rates of up to

34,000 pps.

HiResolution (HiRes) Sound is available in the

HiRes Implant System from Advanced Bionics.

HiRes is designed to oﬀer a wide, programmable

dynamic range, preserve spectral and temporal

details of sound and stimulate at rates of up to

83,000 pps. In the HiRes 90K implants, the

number of sites of stimulation can be increased

beyond the number of electrode contacts.

Through simultaneous delivery of current to pairs

of adjacent electrodes, stimulation can be

‘‘steered’’ to sites between the contacts by varying

The most widely used speech coding strategy,

SPEAK, or spectral peak, samples sound approx-

imately every 4 ms and processes this information

into 20 diﬀerent frequency bands [4] . The proces-

sor selects an average of six ﬁlter bands that have

the highest energy in each 4 ms interval and pres-

ents pulses sequentially to the six corresponding

electrodes. Up to 10 maxima can be sampled in

this fashion. This results in stimulation of up to

10 electrodes every 4 ms, representing the spectral

energy levels of the sound input during that inter-

val. The ACE, or advanced combined encoder,

strategy is similar to the SPEAK strategy but

uses a much higher rate of stimulation.

Fig. 6. MED-EL Pulsar CI100 and Sonata TI100 implants (MED-EL Corp., Innsbruck, Austria).

COCHLEAR AND BRAINSTEM IMPLANTATION

321

MED-EL coding strategies provide high stim-

ulation rates up to 50,700 pps and individual

current sources for each channel. With the in-

troduction of the I100 electronics platform and

the OPUS speech processors, MED-EL developed

the FSP (Fixed Place strategy). The timing of

stimulation is used to code the temporal structure

of the sound signal in the low and mid frequency

range by using channel-speciﬁc sampling se-

quences [7] .

Telemetry

Neural response telemetry is a method that

enables direct measurement of auditory nerve

action potentials from cochlear implant patients.

This technology is currently available for all four

implant devices [7] . Initial recordings are obtained

intraoperatively once the implant electrode has

been inserted into the scala tympani and the

receiver-stimulator has been secured in place.

The information obtained is useful for trou-

bleshooting device failures and optimizing param-

eters for speech-processing strategies. This is

particularly useful in mapping cochlear implants

for younger pediatric patients who lack auditory

experience.

Fig. 7. Nucleus Freedom implant with split electrode

(Cochlear Corp., Englewood, CO).

Patient selection

the proportion of current delivered to each

electrode of the pairs. The Advanced Bionics

strategies currently in use are the Hi-Res-P

(Pulsatile), HiRes-S (Simultaneous) and HiRes

Fidelity 120.

Candidate selection for cochlear implantation

has evolved as the devices and patient perfor-

mance evolved. In general, adults and children

with bilateral severe to profound SNHL, who

receive little or no beneﬁt from conventional

hearing aids, are in good physical and mental

Fig. 8. Nucleus Freedom (A) BTE sound processor, (B) Bodyworn sound processor (Cochlear Corp., Englewood, CO).

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