ABSTRACT
Objective: To compare cochlear outer hair cells function in normally
hearing patients with and without tinnitus.
Methods: Twenty four tinnitus patients
were selected as a study group based on having normal hearing threshold levels
in the frequency range between 750 to 6000 kHz. The control group consisted of
24 otologically normal subjects who have normal hearing threshold levels
without tinnitus and were selected to match the study group by age and gender.
Results: Significant differences in distortion
product otoacoustic emissions amplitude at P< 0.05 were found between the
study group and the control group in the frequency range between 750 to 6000 Hz.
Conclusions: There is an association between tinnitus and reduced
distortion product otoacoustic emission amplitudes indicating reduced cochlear
outer hair cells function.
Key words: Cochlear
outer hair cells activity, Normal hearing, Otoacoustic
emission, Tinnitus
JRMS
June 2010; 17(2): 27-31
Introduction
Tinnitus is a common, yet poorly
understood disorder defined as a sound perception in the absence of external
stimulus. This symptom is highly associated with hearing loss.(1)
The association between tinnitus and hearing loss has been well described.
According to different reports, 85 to 96% of patients with tinnitus present with
some levels of hearing loss and only 8 to 10% have normal hearing. In this last
group, the isolated presence of tinnitus suggests that it may be a primary
symptom of diseases that are only diagnosed after the onset of hearing loss. The
origin of tinnitus in this group is still more obscure than in those with
concomitant hearing loss.(2)
Recent advances in the study of the
cochlear mechanical process suggest that one source of tinnitus may be related
to cochlear mechanical activity; damage to outer hair cells of the cochlea is
believed to be one of principal mechanism of tinnitus. If the outer hair cells
of the human cochlea are to be involved in the generation of tinnitus, testing
of otoacoustic emissions could provide a reliable means of recording outer hair
cells dysfunction.(3)
Otoacoustic emission (OAE) is a
phenomenon that may occur in a large group of normal hearing subjects. This
emission may be spontaneous or may be a response to given auditory stimuli
transiently evoked otoacoustic emissions (TEOAE) and distortion product
otoacoustic emissions (DPOAE). The primary purpose of otoacoustic emission test
is to determine cochlear status, especially hair cell functioning. DPOAE is a
type of otoacoustic emission in which the stimulus consist of two different
pure tones at two different frequencies (i.e. F1 and F2; F2>F1) and two
intensity levels (i.e. L1 and L2; L1>L2) the relationship between L1/L2 and
F1/F2 dictates the frequency response. DPOAEs can be recorded with better reliability
at higher frequencies than TEOAE. Therefore, it is useful for detection of
cochlear damage.(4)
Patients with tinnitus and normal hearing
constitute an uncommon group and there
is rare literature on tinnitus cases with normal hearing, just a few papers
studied tinnitus in normal hearing patients usually involving less than 20
cases.(5)
Shiomi et al. investigated the
cochlear activity in nine tinnitus patients with normal hearing and 55 tinnitus
patients with hearing loss using DPOAEs. Significant decreases in DPOAEs
amplitude over a limited frequency range were observed in 93.3% of 14 ears of
the normal hearing tinnitus group and in 96% of the 15 ears of the hearing
impaired tinnitus group. The averaged DP-Gram of the normal hearing tinnitus
group was significantly different from that of the normal subjects.(3)
Igna et al. studied the
otoacoustic emission in patients with complaints of tinnitus and normal hearing,
only 10 patients of the evaluated 104 patients with tinnitus met the inclusion criteria
that were to have thresholds of 20 dB or less in each frequency at tonal audiometry.
The results of their study showed that 60% had otoacoustic emission absent or
lowered in one or more of the tested frequencies.(6)
Favero et al. studied the medial olivocochlear
bundle effect in a group of 44 patients with tinnitus and 44 patients without
tinnitus using DPOAEs, the results observed among subjects with and without
tinnitus showed an association between absence of contra lateral suppression
and presence of tinnitus at all tested frequencies studied. They concluded that
there was a clear association between diminished effectiveness of the medial
olivocochlear bundle and the presence of tinnitus.(7)
Vicky et al. studied a group of
306 tinnitus ears with and with out hearing loss; they found that DP-Gram
amplitude in 94.8% of tinnitus ears with sensory neural hearing loss were lower
than normal amplitude or was absent within a frequency range associated with
elevated pure tone threshold. In 59% of cases with normal hearing and tinnitus,
the amplitude of DPOAEs at frequencies near the tinnitus frequencies were
decreased and there were no detectable SOAEs.(8)
Gouveris et al. compared
the cochlear outer hair cell function in 32 ears with acute tonal tinnitus and normal
hearing threshold, with 17 ears of normal hearing with out tinnitus; they found
that tinnitus ears exhibited relatively increased amplitude of DPOAEs at higher
frequencies (4-6.3kHz) when compared with the group of normal ears and
relatively decreased DPOAEs amplitudes at middle frequencies (1650-2400 Hz).(9)
Granjeiro et al. compared the
transient and distortion product otoacoustic emissions in patients with and
without tinnitus and normal hearing, they found that DPOAEs amplitudes were
smaller in 68.4% of the tinnitus group than in the control group in the
frequency range 3000 and 4000 Hz.(10)
From the previous studies, it is clear
that most of the studies compared tinnitus in groups of patients with and without
hearing loss using different types of otoacoustic emission at limited frequencies
(not all frequencies).
The present study is different from that as
it is comparing cochlear outer hair cells function in normally hearing patients
with and without tinnitus using diagnostic distortion product otoacoustic
emission protocol at all audiometric testing frequencies ranged from 750-6000 Hz,
in order to test the hypothesis that cochlear outer hair cells’ function may be
involved in the generation of tinnitus.
Methods
This is a
retrospective study. Two groups of otologically normal subjects
with no history of ototoxic, no otropic drug intake, no exposure to noise or
any history of ear surgery, were included in the present study. The study group consisted of 24
patients with tinnitus and normal hearing threshold levels of both genders with
an age range from 20 to 56 years. The
control group consisted of 24 patients who have normal hearing threshold levels
without tinnitus with an age range from 20 to 56 years to match the study
group. Hearing threshold levels were
considered to be within normal if they were less than 20 dB HL at all tested
frequencies.
At the beginning of the study random
selection of 60 patients with tinnitus of different ages groups and both sexes who
were referred to the audiology department at King Hussein Medical Centre for
audiological assessments was done. Assessments were
as follows:
1.
Otoscopic examination: otoscopic investigation of the
external and middle ear were carried out on each ear of 60 patients by Ear Nose
and Throat specialist doctors to evaluate any disorders; of the 60 patients
otoscopic examination revealed that there were 14 patients who had middle ear
disorders. These disorders were otitis media with effusion in six patients;
perforated tympanic membrane in eight patients. Those 14 patients were excluded
from the rest of the assessments. The rest of the assessments were limited to
the 48 patients who have had normal otoscopic examination.
2.
Pure tone audiometery:
hearing threshold levels measurements in the region of 750-6000Hz were
carried out on each ear of the 48 patients using descending of 10dB step size
and 5dB step size to determine the lowest hearing threshold levels at all
tested frequencies. All measurements were carried out by a qualified audiologist
in an isolated test booth using clinical diagnostic audiometer (Interacoustic
type AC 40). Of the 48 patients, normal
hearing threshold levels were found in 24 patients bilaterally defined as
hearing threshold levels 20 dB HL or less; conductive hearing loss was found in
four patients bilaterally and in six patients unilaterally; sensorineural
hearing loss was found in four patients bilaterally and in two patients
unilaterally; and high frequency hearing loss was found in eigh patients. Those
24 patients who showed different types of hearing loss were excluded from the
rest of the assessments; therefore the rest of the assessments were limited to
the 24 patients who have normal hearing threshold levels with tinnitus. The control group of 24 subjects included in
the present study was otologically normal without tinnitus; never had any ear
infection; nor exposed to noise or had any ear surgery. This control group
underwent all assessments carried out on
the study group including otoscopic examination, pure tone audiometry,
tympanometry and distortion product otoacoustic emissions
3.
Tympanometry: test of the middle ear function was carried
out on each ear of the 24 patients using middle ear analyzer (Interacoustic);
the findings revealed no abnormalities that may interfere with the accuracy of DPOAEs.
4.
DPOAEs protocol DP-gram in the
frequency range 750-6000Hz using Escourt system were recorded from each ear of
the selected 24 patients with tinnitus (the study group) and 24 patients
without tinnitus (the control group) who
showed normal audiological findings. Recording were repeated twice to ensure
the reliability of the results.
Results
Table I shows the mean hearing threshold
levels and standard deviation of both groups, the study group with tinnitus (n=24)
and the control group without tinnitus (n=20).
There are some differences in the mean hearing threshold levels between
the study and the control groups. The differences being in the size of 5 dB HL
at all tested frequencies which are not significantly different at P< 0.05.
Table II shows the mean and standard
deviation of distortion product otoacoustic emissions amplitude in dB of both
groups. It is apparent that there are differences
between the mean DPOAEs amplitudes of both groups, these differences being in
the size of 10 dB at all tested frequencies.
Inferential
Statistics
One way ANOVA test was used to calculate
the significant differences at P<0.05 between the mean hearing threshold
levels and the mean DPOAEs amplitude of the study and the control groups.
No significant differences in the mean
hearing threshold levels between the two groups were noticed at all tested
frequencies.
Significant differences in the mean
DPOAEs amplitude at P<0.05 were noticed at all tested frequencies between the
two groups, these differences being in the size of 10 dB.
Discussion
The results of the present study revealed
significant decreases in the DPOAEs amplitude over the tested frequencies range
in all tinnitus patients with normal hearing compared to non tinnitus patients
with normal hearing. Hearing threshold
levels did not significantly differ between tinnitus and non tinnitus normal
hearing patients, but DPOAEs amplitudes of tinnitus patients were significantly lower than normal patients, which implies that decrease of DPOAEs amplitudes may be related to the presence of tinnitus.
Table I. Mean and
standard deviation of hearing threshold levels of the study group and control
group
|
Frequency(Hz)
|
Mean and SD
of hearing threshold levels (dB) of the study group with tinnitus
|
Mean and SD
of hearing threshold levels (dB) of the control group without tinnitus
|
Differences
in the mean hearing threshold levels (dB) between the study and control group
|
|
|
Mean
|
SD
|
Mean
|
SD
|
|
|
750
|
18.37
|
4.44
|
13.25
|
5.83
|
5.12
|
|
1000
|
17.
|
4.38
|
11.25
|
4.77
|
6.25
|
|
2000
|
16.37
|
4.41
|
10.75
|
4.16
|
5.62
|
|
3000
|
19.5
|
3.54
|
15.75
|
4.16
|
3.75
|
|
4000
|
22.5
|
3.58
|
15.37
|
5.30
|
7.13
|
|
6000
|
19.12
|
4.15
|
13.25
|
5.83
|
5.87
|
Table II. Mean and standard eviation of distortion product otoacoustic emissions
amplitudes (dB ) of the study and control group
|
L1
dB
|
L2 dB
|
F1(Hz)
|
F2(Hz)
|
Gm(Hz)
|
DP-NF of the study group
|
DP-NF of the control group
|
Differences in DP means between control
and study group
|
|
|
|
|
|
|
Mean
|
SD
|
Mean
|
SD
|
|
|
60.8
|
55.4
|
4686
|
5623
|
5133
|
4
|
0.7
|
16.2
|
0.14
|
12.2
|
|
65.4
|
55.3
|
3327
|
3983
|
3640
|
2.4
|
0.28
|
12.4
|
0.54
|
10
|
|
56.7
|
55.4
|
2343
|
2811
|
2566
|
4.8
|
0.14
|
13.2
|
0.56
|
8.4
|
|
65.9
|
55.4
|
1640
|
1968
|
1797
|
4.9
|
0.42
|
13.2
|
0.56
|
8.2
|
|
52.4
|
56.0
|
1171
|
1406
|
1283
|
6.4
|
0.28
|
16.2
|
0.14
|
9.8
|
|
51.0
|
47.3
|
843
|
1031
|
932
|
-6
|
0.28
|
4.2
|
0.28
|
10.2
|
|
61.4
|
54.9
|
609
|
750
|
676
|
-2.2
|
0.28
|
8.5
|
0.41
|
10.7
|
L1:
intensity level of stimulus 1
L2: intensity level of stimulus 2 F1: frequency of
stimulus 1
F2: frequency of stimulus 2 GM: frequency of distortion
product DP: distortion
product amplitude (dB)
NF: noise floor
Our
results are consistent with the results obtained by Shiomi et al.,(3) Igna et al.,(6) Fvaero et al.,(7) Vicky et al.,(8) and Granjeiro et al.(9) that there was a significant decrease in DPOAEs
amplitudes over a limited frequency range observed in the normal hearing
tinnitus group when compared to the non tinnitus normal hearing group. Even most of the previous studies showed that
the decreased amplitudes of the DPOAEs were just in limited frequencies. This
may be due to the protocol of testing used in each study, as some of the
protocols may just screen for specific frequencies either at mid or high
frequencies, whereas in our study we used the diagnostic protocol from 750-6000Hz.
In contrast our results do not agree with
the results obtained by Gouveris et al.(9) stating
that tinnitus ears exhibited relatively increased amplitude of DPOAEs at higher
frequencies (4-6.3kHz) when compared with the group of healthy ears and relatively
decreased DPOAEs amplitudes at middle frequencies. The discrepancy between our
results and Gouveris et al. results may be due to the existence
of an acute progressive lesion of the cochlea such as recruitment which is the
abnormal growth of loudness as the intensity of sound increased, given that all
in Gouveris et al.’s study group had acute symptoms, or could be that
the increased amplitude of the DPOAEs in this precise frequency region stems
from the fact that most of their patients perceived tinnitus at the 4-6kHz
frequencies and hence that a primary lesion of the cochlea exists at this
precise region, or any damage to the inner hair cells of the cochlea may result
in the increased DPOAEs amplitudes.
Decreases
in the DPOAEs amplitudes indicate dysfunction of the outer hair cells. This
dysfunction may be due to the reduction in the outer hair cells activity which
results from the imbalance between the external hair cells and internal hair
cells. The external hair cells are more prone to damage and when it occurs,
they fail to perform the inhibition over the inner hair cell function. This
loss of inner hair cells inhibition results then in tinnitus. Tinnitus in turn,
which is an unpleasant sound, may act as a noise which resulted in a decrease
of the DPOAEs amplitudes due to increase
internal ear noises heard by the patients as ringing noises. This noise is one
of the factors that result either in absence or decrease in DPOAEs amplitudes.
In the presence of these results, tinnitus masker could be a good solution for
the relief of annoyance resulted from tinnitus.
Conclusion and Recommendation
It can be
concluded that there is an association between tinnitus and reduced cochlear outer hair
cells activity which indicate that the outer hair cells of the cochlea are
involved in the generation of tinnitus. Further research needs to be carried
out on the effectiveness of tinnitus maskers used by this group of patients.
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