|Year : 2014 | Volume
| Issue : 2 | Page : 56-59
Distortion product otoacoustic emission fine-structure: An insight into the ear asymmetries
Audiologist, Hong Kong
|Date of Web Publication||3-May-2014|
Source of Support: None, Conflict of Interest: None
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
|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
| 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, ,, which show a periodicity of 3/32 octave. ,
In the past, there have been studies comparing the DPOAE fine-structure across age ,, and gender.  Dhar and Abdala  found that DPOAE fine-structure has more depth and wider spacing in newborns when compared with adults. However, Wagner et al.,  suggested that DPOAE fine-structure is independent of age. Sharma and Sinha  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 ,, 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 Brown  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.,  using an otoacoustic emission (OAE) as a predictor of an individual's hearing threshold may not be accurate. Martin et al.,  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. ,,,
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.
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).
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.
All the evaluations were done in an acoustically treated two-room situation with permissible noise levels as per ANSI S3.1. 
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.  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.
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.  The intensity of the two stimuli (L1 and L2) was kept constant as 65 and 55 dB, respectively.
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) 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: Depicts the interaural comparison of DPOAE amplitudes among the various age groups|
Click here to view
[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. 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.
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. ,
The results of the present study are in consonance with the studies of Pavlovčinová et al .,  and Kastanioudakis et al.  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.
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