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tion may not provide substantial benefit. These individuals
are often able to hear speech but not understand it as the
important mid-high frequency information is not audible.
Treatment options for this population are limited. It is often
difficult to provide the large variation in gain required at
different frequencies using a conventional HA and some
individuals may not even require any amplification in the
low frequencies. Frequency transposition hearing aids,
which compress a wide input frequency range into the
(audible) low frequencies, might be expected to provide
substantial benefit in this population, but to date clinical
outcomes with currently available devices have been disap-
pointing (4,5).
Cochlear implantation has become a routine treatment for
severe – profound hearing loss over the past 30 years and
listening performance has consistently been shown to be
improved in individuals with no preoperative hearing or
those who cannot benefit significantly from conventional
HAs. However, while high levels of performance are often
reported in favourable listening situations, CI users typi-
cally have substantial difficulty in segregating competing
speakers or in background noise conditions. This difficulty is
believed to be largely due to relatively poor representation
of the low frequency “fine structure” of the acoustic signal
(i.e. the voicing fundamental frequency range) by electrical
stimulation (6,7).
Candidacy criteria for cochlear implantation typically
involve specified levels of preoperative speech under-
standing (using HAs where appropriate) and unaided audio-
metric thresholds (8). Individuals with severely sloping high
frequency hearing losses often fall within these criteria, but
may be reluctant to proceed with implantation due to fears
of losing their residual natural hearing. During the early
years of cochlear implantation it was assumed that any
residual hearing would be lost following surgery, but later
experience has shown that hearing loss is not inevitable,
particularly when “soft surgery” techniques are employed.
Individuals with steeply sloping hearing loss have repre-
sented a particularly interesting population in the field of
cochlear implantation in recent years. As a CI electrode
is usually inserted via the basal (high frequency) region
of the cochlea it was postulated that combined electrical
and acoustic stimulation might provide a feasible treat-
ment option for this population. In principle, a CI electrode
inserted into the basal region of the cochlea could provide
high frequency information by electrical stimulation
and possibly preserve the residual apical (low frequency)
cochlear function which could be provided with acoustic
amplification if required. Such a combination might be
more effective than either acoustic or electrical stimulation
in isolation. In this article, we aim to provide an overview
of existing clinical experience relating to “electro-acoustic
stimulation” (EAS) together with hardware options avail-
able from Cochlear Ltd and with an update of recent clinical
outcomes.
Principles and implementation of EAS
Much of the early work on EAS included animal studies into
the physiology of combined electrical and acoustic stimu-
lation (9), in an attempt to clarify whether the two modal-
ities could provide effective synergistic stimulation of the
spiral ganglion cells. This was considered important as the
firing patterns produced by electrical and acoustic stimula-
tion differ considerably. However, progress with the clinical
application of EAS has arguably been more directly influ-
enced by parallel clinical studies. As outcomes from CI have
generally improved over the years, individuals with greater
levels of residual hearing have been implanted. When
there is some level of useful (aidable) hearing, individuals
are often implanted in the poorer ear in order to avoid any
risk of poorer outcomes post-implantation. Many studies
have reported that such CI recipients can benefit from the
combination of electrical stimulation in the implanted ear
and acoustic input on the opposite side, i.e. “bimodal stimu-
lation” (10,11). This demonstrates that the central auditory
system is able to effectively combine the neural responses
to electrical and acoustic stimulation.
Many of the early trials with combined electrical and
acoustic stimulation in the same ear used relatively short
electrode arrays in the anticipation that these would facil-
itate better preservation of low frequency acoustic hearing
than conventional full length arrays. Cochlear Ltd produced
two commercial devices based around the CI24RE Freedom
implantable cochlear stimulator. The Hybrid S8 device used
a 10mm electrode array with 6 active electrode contacts.
A multicentre trial in the US reported useable preserved
low frequency hearing in 80% of subjects after 1 year, and
significant improvement in speech understanding from the
addition of acoustic input was demonstrated in 82.5% of
subjects (12). However, a minority of subjects appear to
lose residual hearing at surgery or some time later (13),
and in this situation a very short electrode array usually
provides less hearing benefit than a conventional array (14).
For this reason, an alternative array, the “Hybrid L24” was
subsequently produced by Cochlear. The Hybrid L24 has
22 contacts spaced over 17mm, and typically extends to
around 270o from the round window, i.e. to the 2000 Hz
region of the cochlea (15). High levels of hearing preserva-
[Electro-Acoustic Stimulation - an option when hearing aids are not enough - Herbert Mauch Biomed Eng. et al.]