Électronique du Mazet ECHODIA ELIOS, Руководство пользователя

Страница: 39 / 151

User guide
ELI O S
ECH001XN111-A4 – 07/2022
38
Introduction and test setup 3.6 Otoacoustic emission (Shift-OAE, DP-gram and TEOAE)
3.6 Otoacoustic emission (Shift-OAE, DP-gram and TEOAE)
The cochlea, the peripheral organ of hearing, is capable of emitting low-amplitude sounds in response or not to acous-
tic stimulation. These sounds are easily recorded in the external auditory canal using a sensitive miniaturized microphone.
The genesis of these sounds coming from the cochlea, called otoacoustic emission, depends on the proper functioning of
specific cells in the cochlea: the outer hair cells (OHC). In addition, the integrity of the eardrum and the ossicular chain
is also necessary for the transmission of the acoustic stimulation wave and for the propagation of the physiological re-
sponse from the cochlea to the eardrum.
3.6.1 Shit
-OAE
Shit-OAE: Distorsion Product Otoacoustic Emission Phase Shift
When distortion products are being recorded, a bi-tone acoustic stimulation is applied (simultaneous presentation of
two pure sounds). These 2 stimulating sounds, called primary sounds, have respective frequencies f1 and f2 and will
generate the emission of a characteristic distortion product in humans at a frequency of 2f1-f2.
This measurement, accomplished only by ECHODIA records an acoustic response (derived from outer hair cell) of
the cochlea following a double acoustic stimulation. "Monitored" over a given time, the cochlear physiological parameter
allows real-time monitoring of the intra-cochlear pressure change.
In a healthy subject like a subject affected by endolymphatic hydrops, a postural test (changing from the standing
positions to laying down) increases the intra-cochlear pressure (exacerbated as part of Ménière's disease). Indeed, during
the postural test, there is a change in the distribution of the cerebrospinal fluid (CSF) which propagates as far as the
cochlea via the cochlear aqueduct.
The repetition of an identical acoustic stimulation is followed by identical responses by the outer hair cell. However,
any environmental change in the OHC, and more particularly any change in the intra-cochlear pressure will constrain
them and slow down their responses. The frequency and amplitude characteristics of the OHC are more or less identical
when the intra-cochlear pressure is increased. However, the pressure stress will slow down the emission of the PMC or
DPOAE by inducing a measurable delay, a phase shift.
3.6.2 DP-gram
DP-gram : Otoacoustic Emission Distorsion Product Phase Shift Graphic
During the recording of the distortions product otoacoustic emission (DPOEA), a bi-tone acoustic stimulation is ap-
plied. This particular stimulation brings two specific regions close to the cochlea into action and leads to the excitation
of a third cochlear region. The OHC's excited in this way, because of their contraction property, will put the basilar
membrane into movement, driving the endolymphatic fluid and finally connecting the ossicles to the eardrum. Put into
vibration, the eardrum will emit a low amplitude sound (1: 10000) which can easily be recorded and identified.
These two stimulating sounds, called primary sounds, have respective frequencies of f1 and f2 and will generate the
emission of a characteristic distortion product for humans at a frequency of 2f1-f2. For instance, with the presentation of
2 primaries f1=1000Hz and f2=1200Hz, the expected distortion product will be 2f1-f2 = 800Hz. The generated distortion
product is lower in frequency and smaller in amplitude than the primaries. It is the amplitude of the distortion product
that will be considered as a criterion for evaluating the cochlear function and more specifically the OHCs of the emitting
region (at a frequency of 800 Hz in this example). Accordingly, a distortion product having an amplitude of more than 7
dB with respect to the background noise will be the signature of the presence and functionality of the OHCs in the emitting
region.
By varying the frequencies of the 2 primaries f1 and f2, it is possible to gather together the different distortion products
and establish a curve known as DP-gram (a graphic of the distortion products, similar to an audiogram). By observing
the cochlear spectrum, from 1000 Hz to 5kHz, we can estimate the impairment severity of the OHCs and thus estimate
the level of deafness.
This makes the DP-gram a simple and fast test which can be reproduced and which, above all, is not invasive. The
presence of PDAs allows us to affirm (when there is no transmissional deafness) the cochlear functionality of the OHCs.
The
DP-gram study is recommended for the early screening of deafness at the maternity stage, following up infants in
neonatology intensive care units, in pediatric audiological evaluation, in following up abrupt deafness, professional and
toxic deafness.