|Year : 2015 | Volume
| Issue : 2 | Page : 98-102
Otoacoustic emissions in noise-induced cochlear damage in artillery soldiers
Aman Pankaj, Abhijeet Bhatia
Department of ENT, Sikkim Manipal Institute of Medical Sciences, Gangtok, Sikkim, India
|Date of Web Publication||20-Apr-2015|
Department of ENT, Sikkim Manipal Institute of Medical Sciences, 5th Mile, Tadong, Gangtok - 737 102, Sikkim
Source of Support: This research was funded by Indian Council of Medical Research., Conflict of Interest: None
Aim: Study the utility of distortion product otoacoustic emissions (DPOAEs) in the detection of preclinical noise-induced hearing loss (HL) in artillery soldiers. Materials and Methods: This cross-sectional study included all male subjects aged 18-35 years from the Indian Army and general civilians. Subjects underwent pure tone audiometry and DPOAE test. DPOAE parameters of subjects with normal hearing, nonexposed (control group 1 [CG1]) and noise exposed, HL (control group 2 [CG2]) were compared with noise exposed normal hearing (study group [SG]). Results: Mean DPOAE amplitude and sound to noise ratio of SG at all frequencies were significantly more than CG2 and less than CG1. SG had lower proportion of ears with absent DPOAEs than CG2, but higher than CG1 at all frequencies. Conclusions: Hearing loss is common among artillery soldiers. Artillery soldiers with normal hearing are more likely to develop HL due to impaired DPOAEs as compared to civilians with no history of noise exposure. Adequate safeguards are necessary to prevent handicap.
Keywords: Distortion product otoacoustic emissions, Noise induced hearing loss, Pure tone audiometry
|How to cite this article:|
Pankaj A, Bhatia A. Otoacoustic emissions in noise-induced cochlear damage in artillery soldiers. Indian J Otol 2015;21:98-102
| Introduction|| |
Noise-induced hearing loss (NIHL) is one of the most common occupational hazards, accounting for about 16% of hearing loss (HL) worldwide.  Regular, prolonged exposure to noise above 85 dB may cause NIHL. The unprotected ear can be exposed to a maximum of 115 dB for 15 min/day. Noise above 140 dB is unsafe even with protection. High-intensity impulse noise might lead to hearing impairment. NIHL is common in army soldiers on account of firearms and heavy machinery noise in excess of 140 dB. Automatic guns produce 175 dB; artillery guns produce noise around 180 dB. Small arms produce >150 dB. Hearing protection devices (HPDs) are not used universally in the armed forces because they might distort and attenuate localization cues. HPDs permit sound attenuation of only up to 50 dB. ,, Hearing disabilities were the most prevalent disabilities attributable to military service in the U.S. in 2006. More than $1.2 billion was spent on rehabilitation.  It is, therefore, essential to detect HL at a preclinical stage.
Otoacoustic emissions (OAEs) testing is an important tool for an early identification of NIHL. Outer hair cells in the inner ear are vulnerable to external effects such as noise; the first signs of cochlear abnormalities may be identified by OAE testing. Pure tone audiometry (PTA) can detect HL only after it has occurred whereas OAEs have been shown to detect NIHL at a preclinical stage.  Previous studies have revealed impaired OAE parameters in soldiers with lower auditory thresholds. OAE testing is sensitive and objective and, therefore, provides valuable information for the diagnosis of NIHL and for supporting audiometry in the diagnosis and monitoring of cochlear function when exposed to noise. , Despite a large body of literature indicating utility of distortion product OAEs (DPOAEs) in the detection of noise-induced cochlear damage in military personnel, it is yet to be universally accepted for the purpose.  The other screening modalities such as high-frequency and extended high-frequency audiometry and other varieties of OAEs have also been studied extensively for the purpose. However, there continues to be a dearth of well-established screening protocols for NIHL for use on a large scale. The OAEs can be used for screening in multiple ways. It can be a tool to screen individuals who already have NIHL and those who have a history of chronic exposure to noise. Transient evoked OAEs have been shown to be more useful for this purpose. Another use of OAEs for screening purposes is to monitor the cochlear function of individuals exposed to occupational noise. DPOAEs are more useful for this purpose. It has been suggested, therefore, that TEOAEs and DPOAEs should be used in conjunction with each other. ,,
This study aims to study the utility of DPOAEs in detection of preclinical NIHL in Indian Army soldiers chronically exposed to firearm noise.
| Materials and Methods|| |
This case control study was carried out in the sound treated audiometry room in the ENT department of Central Referral Hospital, Gangtok from May to July 2013. A clearance certificate was obtained from the Sikkim Manipal Institute of Medical Sciences institutional ethics committee (reference number: IEC/129/13-07) prior to starting the study. The participants of the study were drawn from volunteers among the artillery soldiers of the Indian Army posted in Sikkim after approval from the military authorities and the general civilian population. Informed written consent was obtained from all the participants.
All Indian Army soldiers tested belonged to the same artillery unit and hence used similar weapons and practiced almost simultaneously. Only the duration of military service varied. The firearms used included hand-held weapons such as 5.56 mm INSAS rifle, MP5 K Pistol, AK 47 automatic rifles and 9 mm carbine machine gun and artillery mortar like 105 mm light field gun, 51 mm light mortar and 155 mm howitzers. The soldiers of the artillery unit included in the study had undergone firing practice 2 months prior to beginning the study.
Only male participants between 18 and 35 years of age were included in the study since artillery soldiers in the Indian Army are exclusively male. The participants were first assessed through the detailed history and ENT examination. Individuals detected to have any ear disorder except for NIHL were excluded from the study at this stage. They were then explained about the procedure to be performed by the investigators. The participants then underwent PTA. Any individual with bone conduction threshold >20 dB at any frequency was considered to be having SNHL and was grouped accordingly. The participants with conductive or mixed HL were excluded from the study at this stage. The participants were then allotted an appropriate group as detailed below:
Control group 1
Consisted of civilians with no history of chronic noise exposure and with normal hearing on PTA. Civilians having any HL were excluded from the study.
Control group 2
Consisted of Indian Army artillery soldiers with a history of firearm noise exposure >3 years at a regular interval of <6 months with NIHL confirmed by PTA. Individuals with acute acoustic trauma were excluded from the study.
consisted of Indian Army artillery soldiers with a history of firearm noise exposure for at least 3 years at an interval of <6 months with no HL observed on PTA.
Indian Army soldiers with a history of firearm noise exposure <3 years or at an interval of >6 months were excluded from the study. All subjects were then assessed with the help of DPOAE test using the Otoread DP Screener (software version 7.70) device manufactured by Interacoustics A/S, Denmark. 
Procedure for distortion product otoacoustic emission testing
After ensuring a clean external auditory canal, the appropriate sized ear probe attached to the instrument was inserted inside the external ear canal, and the DPOAE test was performed. Both the ears were tested one after the other. Sound frequencies f 1 and f 2 were given to the ear simultaneously. The default settings of the DPOAE instrument were used. Intensity of f 1 was 65 dB, and that of f 2 was 55 dB. The averaging time was 4 s. The resultant OAEs determined by the formula 2f 1 -f 2, with f 2 being centered at 2, 3, 4 and 5 kHz, were considered for analysis. DPOAE amplitude and sound to noise ratio (SNR) (difference between DPOAE amplitude and the noise level at all the measured frequencies) were analyzed. DPOAE amplitude below −5 dB or SNR <6 dB were considered as DPOAE absent at that frequency. The results of the study group (SG) were then tabulated on Excel worksheets, and the means were compared with those of the two control groups for significance by the unpaired t-test using GraphPad Prism 6 software (GraphPad Software Inc., USA). 
| Results|| |
A total of 136 subjects (272 ears) were assessed however only 100 subjects (200 ears) who passed the inclusion criteria were included in the study. The SG included 84 ears, the control group 1 (CG1) consisted of 60 ears and control group 2 (CG2) included 56 ears. Ears included in the SG had a mean age of 28.52 years (minimum: 22 years; maximum: 35 years), whereas ears included in CG1 and CG2 had mean ages of 25.26 (minimum: 19 years; maximum: 35 years) and 30.10 (minimum: 24 years; maximum: 35 years) years respectively.
Of 56 ears included in CG2, 10 ears were right ears, and 14 were left ears. A total of, 32 ears were of 16 participants with bilateral HL. Pure tone audiogram of SG ears was significantly higher than that of CG1 ears at all frequencies except 250 Hz and 500 Hz though the hearing threshold of SG ears was below 20 dB at all frequencies. The bone conduction thresholds were higher at higher frequencies in all the groups. However, the thresholds were seen to rise at 1000 Hz in SG and CG1 before again decreasing in 2000 Hz [Table 1].
The number of ears with absent DPOAEs were higher at higher frequencies. SG had intermediate values between CG1 and CG2 [Table 2].
Mean DPOAE amplitude at each frequency was reduced in SG as compared to CG1 but was higher compared to CG2 at all measured frequencies. The differences were significant at all measured frequencies. It was also observed that the DPOAE amplitude was lower at higher frequencies in all three groups. The DPOAE levels varied with the pure tone audiograms for all three groups except that the bone conduction thresholds at 1000 Hz showed a spike in the SG and CG1 [Table 3].
Mean SNR showed similar trends as of mean DPOAE amplitude at all tested frequencies. Since SNR is a measure of the difference between the noise floor level and the DPOAE amplitude, a higher value represents a more robust DPOAE response. The SNR was observed to be lower at higher frequencies in all three groups [Table 4].
Although the bone conduction thresholds of the SG were normal, the SNR values were observed to be lower at higher frequencies [Figure 1] and [Figure 2].
| Discussion|| |
In the current study, mean bone conduction thresholds of SG at each frequency were significantly more than that of CG1 at higher frequencies, though hearing thresholds were below 20 dB at all frequencies in both groups. The bone conduction threshold in SG was significantly less than CG2 at all tested frequencies. Mean DPOAE amplitudes and SNR of SG were significantly less than CG1 but significantly more than CG2. DPOAEs were absent in a greater proportion of ears in SG than CG1 but in a lesser proportion than CG2.
Even though the bone conduction thresholds of SG were below 20 dB at all frequencies, they were significantly worse than those of CG1 at higher frequencies. The bone conduction thresholds of SG were significantly less than CG2 at all tested frequencies. The decrements in bone conduction thresholds of SG cannot be deemed to be useful for screening purposes because there are no established criteria for the purpose. Possibly, testing higher frequencies as in high-frequency audiometry might warn individuals before the HL spreads to the lower frequencies and hence manifests as clinical HL. , All soldiers used small firearms along with their usual artillery weapons. Fifty-six of these were found to have unilateral HL. This may be due to their habit of practicing firearms with the same hand every time. The unilateral HL among the artillery soldiers may also depend upon the distance of the injured ear from causal firearm, the number of shots as well as on the usage of HPDs. Just 1-year of military service might be sufficient to result in decreased DPOAE amplitudes. 
In the current study, mean DPOAE amplitude, as well as SNR of SG, were significantly less than that of CG1 at all tested frequencies. The decrements were significant even in the lower frequencies, where PTA of SG was observed to be unaffected in comparison with SG. The parameters were lower at higher frequencies in all three groups. The proportion of SG participants with absent DPOAEs was also greater than those of CG1. Previous studies have demonstrated similar outcomes. , It has been demonstrated that reduced DPOAE parameters at frequencies with normal audiometric thresholds.  A study on personal listening device users reported reduction of DPOAEs and high-frequency audiometric thresholds in spite of normal audiograms.  The test-retest variability of DPOAEs has also been observed to be lower than PTA. 
On analyzing the correlation between mean SNR and bone conduction thresholds, it was observed that though there is minimal worsening of bone conduction thresholds with frequency in SG and CG1, the fall in mean SNR is steeper, even in CG1 [Figure 1] and [Figure 2]. The fall in SNR of SG and CG2 is even steeper. This represents a greater decrease in SNR for every decibel loss observed in bone conduction thresholds. This observation implies that minimal damage to the outer hair cells can show discernible changes in DPOAEs. A longitudinal study reported that while the decrements in hearing thresholds among noise exposed subjects were negligible, the decrements in DPOAEs were significant but small.  A prospective study on ship engine room workers reported no correlation between DPOAE and PTA in early NIHL. 
The ultimate goal of such studies is to determine a gold standard modality for assessment of cochlear function in noise exposed individuals for earliest possible reliable evidence of an impending HL. For this purpose, prospective comparative studies involving TEOAEs, DPOAEs, and high-frequency audiometry are necessary. They should also be able to establish the variation of decrease in OAEs high-frequency audiometric thresholds with the quantum and duration of HL. Proven ability to diagnose impending HL with certainty would justify institutional hearing screening protocols, given the financial and legal implications on the employers of adverse outcomes. Till the time such gold standard investigation emerges, standard PTA would continue to be regarded as the gold standard with which the other modalities of audiometric assessment can be compared. ,
| Acknowledgments|| |
The authors acknowledge the contribution of Indian Council of Medical Research (ICMR) in partially funding this study (Reference ID: 2013-00939). This study was conducted as a part of ICMR short-term studentship project. We are grateful to Indian military authorities in Sikkim for their assistance in conducting this study. We offer our sincere gratitude to all the subjects who volunteered for participation in the study. The identity of individual military personnel or the involved units cannot be revealed on account of security reasons.
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[Figure 1], [Figure 2]
[Table 1], [Table 2], [Table 3], [Table 4]