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Background: Acoustic shock injury has been described as a permanent injury to the auditory system either due to daily noise dose of in excess of 85 decibels or very loud impulse sound reputed to be in excess of 120 decibels and acoustic incidents.
13.Acoustic shock injury (ASI) MYRIAM WESTCOTT Dineen and Westcott Audiologists, Heidelberg, Victoria, Australia. This study was performed to compare the results of audiogram and the newer diagnostic method distortion product otoacoustic emissions (DPOAEs) parameters due to acoustic shock injury in call center professionals working in a noisy environment and prone to acoustic injury.
Evaluation of acoustic shock induced early hearing loss with audiometer and distortion product otoacoustic emissions. There is a concern regarding issues of health and safety that are unique to this new and developing industry. Effects of industrial noise exposure on distortion product otoacoustic emissions (DPOAEs) and hair cell loss of the cochlea - long term experiments in awake guinea pigs.
The relation among hearing loss, sensory cell loss and tuning characteristics in the chinchilla.
Sensitivity of transient evoked and distortion product otoacoustic emissions to the direct effectsof noise on the human cochlea.
Monitoring noise susceptibility: Sensitivity of otoacoustic emissions and subjective audiometry. Materials and Methods: Hearing functions of 340 subjects were first assessed with pure tone audiometry and then DPOAEs results were compared among acoustic shock exposed subjects with normal audiogram and those with abnormal audiogram. Results: Out of 340 acoustic shock exposed subjects 304 were normal on audiometric testing and 34 had abnormal audiograms. It has been defined as the temporary or permanent disturbance of the functioning of the ear or of the nervous system which may be caused to the user of a telephone ear phone by a sudden sound in the acoustic pressure produced by it. The DPOAE test consisted of presenting two primary tones at frequencies f1 and f2 at levels L1 and L2.
Subsequently on DPOAE testing out of 304 acoustic shock exposed subjects with normal audiogram 125 failed and 181 passed. Conclusions: This study showed that DPOAEs are more sensitive than audiometry to detect pre-symptomatic inner ear damage. These are the results of regular and repeated acoustic shock, which has damaged the outer hair cells of the inner ear that interpret sound vibrations as words, music, or other sounds.Acoustic shock-induced hearing loss is permanent and not correctable by medical or surgical treatment, but is the most preventable type of hearing impairment. The stimuli levels were held constant at L1=65 dB sound pressure level (SPL) and L2=55 dB SPL.

Acoustic shock-induced hearing loss is currently detected and monitored with pure-tone audiometry.
This method is subjective, time consuming, and not quite sensitive to small changes in pure tone thresholds. Since recently, a modern diagnostic method, otoacoustic emissions (OAEs), may be used as an accurate, objective, fast, and noninvasive tool for assessing the function of outer hair cells in clinical practice.
Also, acoustic shock differs from noise-induced hearing loss as it is believed to occur at sound pressure levels below those which present an immediate risk to hearing damage.OAEs are low-level sounds originating within the cochlea. The pure tone audiometry thresholds for 4000 Hz were significant in group with abnormal audiogram. Through reverse propagation, some of this acoustic energy leaks from the cochlea and travels through the middle ear to the external auditory meatus, where it can be recorded using a sensitive microphone.
Signal noise ratio >6 dB was regarded as an indication of a DP being present, and the subjects were assigned as a pass (normal) case. DPOAEs may be an alternative to pure-tone audiometry in monitoring cochlear changes in subjects exposed to occupational noise.
This method is subjective, time consuming, and not quite sensitive to subtle small changes in pure tone thresholds. OAEs are a diagnostic method that is used as an accurate, objective, sensitive, fast, and noninvasive tool for assessing the function of outer hair cells. Subjects had no underlying disease (diabetes mellitus, hypertension, dyslipidemia) and were selected on the basis of the lack of exposure to ototoxic agents, ear trauma or surgery, chronic ear diseases, and a family history of hearing loss. For these reasons, OAES have been proposed as an alternative method for monitoring cochlear function in cases with noise exposure. Results from several studies have demonstrated that OAES parameters are depressed following noise exposure. They had no history of upper respiratory tract infection or ear problems and were not under the influence of alcohol, so any confounding effects were avoided.
Exclusion criteria were the presence of cardiovascular events, diabetes, hyperlipidemia, and immeasurable audiometric data because of poor collaboration by the participant. Cochlear dysfunction in noise-induced hearing loss may extend beyond the frequency region suggested by the audiogram.
The inclusion criterion was exposure to a mean (SD) noise level equivalent to 85 dB (A) for 2 years and use of the same noise-protection equipment.The research proposal was approved by the ethical committee, Vydehi Institute of Medical Sciences and Research Center, and informed consents were obtained in all subjects.

Procedures The subjects completed a questionnaire regarding the history of noise exposure, ear diseases, and underlying diseases.
Conventional pure-tone audiometry and DPOAEs were conducted after otoscopic examination, which included the removal of wax from the ear canal. Results range from very slight dips at 4 and 6 kHz to a moderate degree of hearing impairment with flat pattern, similar to that noted by Beastall (1992) [11] and in those cases of cordless telephone handset injury reported by other researchers. Conventional pure-tone audiometry The conventional pure-tone audiometry was conducted in a sound-treated room.
Myriamwescott [13] attributed the acoustic shock injury to the possible neurophysiological phenomenon, in particular the tonic tensor tympani syndrome.
A cluster of symptom-attributed acoustic shock were complained by the subjects in our study too, but we could not correlate our study outcome with any middle ear pathology and neurophysiological syndrome.
The examined ear was defined as "normal" if a threshold shift larger than 25 dB was not found over the whole frequency range. 26 out of 34 subjects in the acoustic shock-exposed group with abnormal audiogram were found to have failed in the OAES test which was expected, whereas 125 out 304 subjects of the group with normal audiogram were found to have failed in the OAES test which forms significant proportion.Though overall pass percentage was more than fail percentage giving audiometry its existing due credibility what was more shocking to observe was that the majority in the acoustic shock-exposed group with normal audiogram failed in OAES leading to lower pass percentages. Lower pass rates in the acoustic shock-exposed group with normal audiograms we found out with our study will clearly indicate the importance of DPOAE testing to detect early changes due to acoustic shock. DPOAEs provided functional information about well-defined frequency-specific regions of the cochlea.It has been shown that there is little intra-subject variability in OAE measurements. Workers with abnormal DP screening results, even if audiometric results are normal, should have follow-up counseling and evaluation of hearing-protective-device effectiveness to prevent irreversible noise-induced hearing loss. However, there are no guidelines for the use of OAEs to detect and monitor noise-induced hearing loss now. Clarifying advantages of DPOAEs in terms of sensitivity to early manifestations of noise insults, or their utility in predicting future loss in hearing, will require a longitudinal follow up in which the same people are watched over time to see how their hearing changed with their OAEs.In conclusion, the presenting results are in agreement with the findings of previous studies, in that DPOAE testing was shown to be more sensitive to pre-symptomatic inner ear damage than conventional audiometry but cannot estimate hearing thresholds. DPOAE testing should be implemented as a screening and monitoring test for noise-exposed workers rather than as a replacement of audiometry.

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