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ORIGINAL ARTICLE
Distortion product otoacoustic emission
fine‑structure: An insight into the ear
asymmetries
Bhamini Sharma
Audiologist, Hong Kong
Abstract
Context: Use of distortion product otoacoustic emission (DPOAE) as a measure of hearing sensitivity is
common in clinical practice. However, use of a more informative DPOAE fine‑structure has been limited due to
non‑reliability of DPOAE fine‑structure. Aim: The current study was aimed at testing the interaural differences
between the DPOAE fine‑structure across three age groups. Settings and Design: Acoustically treated room
with a calibrated dual channel audiometer (Orbiter 922) along with TDH‑39 headphones and B‑71 bone
vibrator. GSI Tympstar was used for tympanometry and acoustic reflex measurements while ‘ILO V6’ OAE
analyzer was used for recording of DPOAE and DPOAE fine‑structure. Material and Methods: A total of 98
subjects with normal peripheral hearing sensitivity were tested for DPOAE fine‑structure. The three age groups
consisted of young (8-18 years; n = 50), middle aged (30–40 years; n = 30), and elderly (50-60 years;
n = 18). DPOAE fine‑structure was studied at 8 points per octave on a total of 25 frequencies from 1000
to 8000 Hz. Statistical Analysis Used: Repeated measure analysis of variance. Results: There was
a significant difference (P < 0.05) in the amplitudes at frequencies between 2000 and 3000 Hz. This
was evident irrespective of the age groups. There was also a decrease in DPOAE amplitude in elderly
group. Conclusions: Interaural asymmetry can be attributed as a reason to these findings and it occurred
mostly in the speech perception frequencies. Reduced amplitude of DPAOE in the elderly group can be
attributed to presbycusis.
Keywords: Distortion product otoacoustic emission, Fine‑structure, Interaural asymmetry
Introduction
In a normal ear, distortion product otoacoustic emissions
(DPOAE) can be recorded from the ear canal using pure
tones of certain frequencies and intensities. When the
stimulus frequencies (f1, f2) are varied in small steps, distinct
peaks and valleys in DPOAE level versus fine‑structure are
observed, which is referred to as DPOAE fine‑structure.
This is characterized by consistent maxima and minima in
dependence of frequency with depth of the notches up to
20 dB,[1‑3] which show a periodicity of 3/32 octave.[2,4]
In the past, there have been studies comparing the DPOAE
fine‑structure across age[5‑7] and gender.[7] Dhar and Abdala[5]
found that DPOAE fine‑structure has more depth and wider
spacing in newborns when compared with adults. However,
Wagner et al., [6] suggested that DPOAE fine‑structure
Address for correspondence: Ms. Bhamini Sharma,
Audiologist, Hong Kong.
E‑mail: [email protected]
56
is independent of age. Sharma and Sinha [7] studied 98
participants of different age groups and they reported a
significant difference in the DPOAE fine‑structure between
the younger and the older age group.
Currently, DPOAE is being utilized as an important measure
for predicting hearing thresholds. Relationship between
DPOAE amplitude levels and hearing thresholds has been
reported in the past. Some authors[8‑10] have documented a
good agreement between low DPOAE levels and high auditory
thresholds. This was true in a few of their subjects while in
other subjects this relationship was not that good. Gaskill and
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DOI:
10.4103/0971-7749.131866
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Sharma: DPOAE fine‑structure asymmetry
Brown[1] reported a statistically significant correlation between
DPOAE levels and auditory thresholds across the audiometric
frequency range in about 50% of the ears they tested. They
also reported the existence of 80% correspondence between
DPOAE levels and behavioral audiograms.
In contrast, studies have reported a weak correlation
between auditory thresholds and DPOAE amplitude level.
According to Kimberley et al., [11] using an otoacoustic
emission (OAE) as a predictor of an individual’s hearing
threshold may not be accurate. Martin et al.,[12] studied
subjects affected with noise‑induced hearing loss where
they found a strong negative relation between DPOAE level
and auditory threshold. Similarly, for the fine‑structure
of the DPOAE, there are equivocal findings regarding the
correlation between the puretone threshold and the DPOAE
fine‑structure, whereas few studies report a good correlation
between the fine‑structure and the hearing threshold, others
report a weak correlation between the fine‑structure of
DPOAE and the hearing threshold in children as well in
adults.[5‑7,13]
Since, there have been studies exploring the amplitude
differences in DPOAE across frequency, age, and gender, there
are very few studies that have commented on the dependence
of amplitude on interaural difference. Hence, the present study
aimed to find if there were any differences between the DPOAE
amplitudes for right versus left ear across the frequencies, age,
and gender. This would reflect the asymmetry between the
ears at finer levels.
Materials and Methods
Participant selection criteria
Participants with pure tone threshold of less than or equal to
15 dB hearing level (HL) in the octave frequency from 250 to
8000 Hz for air conduction and bone conduction thresholds
were included. They had speech identification scores (SIS)
of greater than or equal to 90% in quiet. All the participants
had normal middle ear functions as revealed by immittance
measures and ear, nose, and throat (ENT) evaluations. In
addition, all the participants had presence of DPOAE as
defined by signal‑to‑noise ratio of 6 dB or greater in the
frequencies between 1000 and 8000 Hz.
Participants
A total of 98 subjects in the age range of 8-60 years
were included in the study. According to the age, these
participants were allocated to three groups, (1) Group I:
50 (25 males and 25 females) young individuals (aged
8-18 years, mean age: 12.6 years); (2) Group II: 30 (15 males
and 15 females) middle aged individuals (aged 30-40 years,
mean age: 34.0 years); and (3) Group III: 18 (9 males and
9 females) elderly individuals (aged 50–60 years, mean age:
52.3 years).
Indian Journal of Otology | April 2014 | Vol 20 | Issue 2 |
Instrumentation
For air and bone conduction and speech identification testing,
a calibrated dual channel audiometer (Orbiter 922), was used
along with TDH‑39 headphones and B‑71 bone vibrator.
A calibrated immittance meter (GSI Tympstar) was used for
tympanometry and acoustic reflex measurements while ‘ILO
V6’ OAE analyzer was used for recording of DPOAE and
DPOAE fine‑structure.
Test environment
All the evaluations were done in an acoustically treated
two‑room situation with permissible noise levels as per ANSI
S3.1.[14]
Procedure
Detailed case history
A detailed case history the information on family history
of hearing loss, presence or absence of diabetes, exposure
to occupational noise, evident neurological or otological
problems was obtained.
Speech identification scores
SIS were measured in quiet using word lists developed
by Vandana and Yathiraj.[15] The two word lists with 20
phonetically balanced words in each list were administered.
The SIS were noted at 40 dB sensation level (SL) (w.r.t speech
recognition threshold).
Middle ear evaluation
Tympanometry was carried out using 226 Hz probe tone
frequency and both ipsilateral and contralateral acoustic
reflexes were elicited for 500, 1000, 2000, and 4000 Hz.
DPOAE
DPOAEs were measured for f2 frequencies of 1001, 2002,
4004, and 7996 Hz. The f1/f2 ratio of 1.22 was kept constant
as the transmission of 2f1‑f2 basally occurs maximally at this
ratio.[16] The intensity of the two stimuli (L1 and L2) was kept
constant as 65 and 55 dB, respectively.
DPOAE fine‑structure
DPOAE fine‑structure amplitude was obtained for 8 points
per octave at 25 frequencies where f2 frequency varied from
1001 to 7996 Hz at (1001, 1086, 1184, 1294, 1416, 1538, 1685,
1831, 2002, 2185, 2380, 2600, 2832, 3088, 3369, 3662, 4004,
4358, 4761, 5188, 5652, 6165, 6726, 7336, and 7996 Hz). The
primary tone frequency ratio (f2/f1) was kept constant at 1.22.
The presentation levels for f1 and f2 were kept at 65 and 55dB
HL, respectively.
Results
The amplitudes of the DPOAEs at the various frequencies were
compared. Statistics were carried out using SPSS (Version 20)
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Sharma: DPOAE fine‑structure asymmetry
developed by IBM corporation. Mixed analysis of
variance (ANOVA) revealed a significant main effect of ear
asymmetry (F (1, 91) =6.75, P < 0.05), frequency (F (24,
2184) =112.05, P < 0.01), and age (F (2, 91) =45.48, P < 0.01).
However, there was no significant interaction among ear and
age (F (2, 91) =0.35, P > 0.05). Repeated measures ANOVA was
done to compare the DPOAE amplitudes for the right and the
left ear for the 25 frequencies within each age group [Table 1].
Table 1 reveals that there was not much significant difference
between the right and left ears in all the age groups. DPOAE
amplitudes at almost all frequencies were similar for both the
ears. However, there were some differences between the two
ears around 2-3 kHz. This was evident in the overall, younger,
and middle age group while there was no significant difference
at any frequency in the older age group.
Discussion
Overall, in the young and middle age group, a difference in
DPOAEs can be observed between the right and the left ears
at some frequencies. However, in the old age group there is
no significant difference in the amplitude. This may be due
to the degeneration in the cochlea that occurs equally across
all the frequencies and results in reduction of asymmetry.
Table 1: Depicts the interaural comparison of DPOAE
amplitudes among the various age groups
Frequency
1001
1086
1184
1294
1416
1538
1685
1861
2002
2185
2380
2600
2832
3088
3369
3662
4004
4358
4761
5188
5652
6165
6726
7336
7996
‘P’ values for right vs left ears
All subjects Young age
Middle age
Older age
(n=98)
group (n=50) group (n=30) group (n=18)
Further, in the young and middle age groups, it can be
observed that the region between the 2000 and 3000 Hz are the
ones with maximum asymmetry. This may be due to the fact
that these frequencies are responsible for speech perception
and thus they are bound to have a differential effect with
respect to ears.
Further, it can also be observed that there is a decrease in
DPOAE amplitude in the older age groups compared with
the other two age groups. This can be well attributed to
presbycusis. There have been similar reports in the past
supporting this notion.[17,18]
The results of the present study are in consonance with
the studies of Pavlovčinová et al.,[19] and Kastanioudakis
et al.[20] While these studies were done on a smaller number
of subjects, the present study confirms their findings with a
larger database.
Conclusion
The present study found that there was largely a no significant
difference between the ears in the DPOAE amplitudes at
frequencies between 1 and 8 kHz. However, the difference
appears in the frequencies responsible for perception of
speech. This implies that there is an occurrence of asymmetry,
which is reflected in DPOAE.
References
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
<0.05
DPOE: Distortion product otoacoustic emission
58
According to literature, two ears of a person differ right from
the external ear to the inner ear. However, as in cases of hearing
loss the asymmetry may have been reduced, it is possible that
degeneration may be the cause of lack of difference in the
DPOAE amplitudes.
<0.05
<0.05
<0.01
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How to cite this article: Sharma B. Distortion product otoacoustic
emission fine-structure: An insight into the ear asymmetries. Indian
J Otol 2014;20:56-9.
Source of Support: Nil. Conflict of Interest: None declared.
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