|Year : 2015 | Volume
| Issue : 3 | Page : 138-140
Disability evaluation in acoustic blast trauma
Department of ENT, Occupational Disease Centre, ESIC Medical College and Post Graduate Institute of Medical Sciences and Research, Chennai, Tamil Nadu, India
|Date of Web Publication||14-Jan-2016|
4, Saravana Apartments, 348 TTK Road, Alwarpet, Chennai - 600 018, Tamil Nadu
Source of Support: None, Conflict of Interest: None
Introduction: Acoustic blast trauma is different from Noise induced hearing loss. Blast trauma can damage the tympanic membrane, ossicles and cochlea singly or in combination. It produces immediate severe hearing loss and may be associated with tinnitus and vestibular symptoms. Hearing loss recovers spontaneously in many cases but may be permanent in 30-55% cases. Thirteen patients working in an explosive manufacturing unit in Andhra Pradesh were exposed to blast trauma at work place. All these workers complained of immediate hearing loss and were subjected to audiological investigations. Methods: Initial evaluation showed a severe sensorineural type of hearing loss 10 of the 13 cases (77%). They were referred to our Medical board for disability evaluation after 2-3 years of initial injury. Pure tone audiometry indicated severe hearing loss in 12 of 13 cases (92%) that was not correlating clinically. Re-evaluation with Acoustic reflex and ABR (BERA) tests were done and permanent disability was evaluated with the results of these investigations. Observations: No significant hearing loss was found in most patients and these patients had minimal disability. Conclusion: Objective hearing tests should be carried out after one year or more before evaluation of permanent disability.
Keywords: ABR, audiometry, blast trauma, permanent disability, sensorineural hearing loss
|How to cite this article:|
Raju G. Disability evaluation in acoustic blast trauma. Indian J Occup Environ Med 2015;19:138-40
| Introduction|| |
The term acute acoustic trauma represents injury to the ear due to blast or impulse noise to the auditory system usually caused by detonation of explosives. Impulse noise is a single pulse of about 140 db or more that fades quickly. Acoustic trauma is different from Noise induced hearing loss that develops over several years and is of insidious in onset. Blast trauma can damage the tympanic membrane, ossicles and cochlea singly or in combination and produces immediate severe hearing loss and may be associated with tinnitus and vestibular symptoms. Hearing loss recovers spontaneously in many cases but may be permanent in 30-55% cases. Suitable compensation is awarded after calculation of permanent disability which is usually based on pure tone thresholds. As pure tone audiometry is subjective, malingering for the purpose of claiming compensation needs to be excluded.
| Materials and Methods|| |
Thirteen patients working in an explosive manufacturing unit in Andhra Pradesh were exposed to blast trauma at work place. All these workers complained of immediate hearing loss and were subjected to audiological investigations. None of the workers had exposure to noise at workplace and all were using earmuffs (PPE) at workplace. Initial evaluation showed a severe sensorineural type of hearing loss in all the cases. They were referred to our Medical board for disability evaluation after 2-3 years of initial injury. All the patients were re-evaluated in our centre with clinical examination and audiological investigations comprising of Pure tone audiometry, Impedence audiometry, Acousticreflex test and ABR (BERA) test and in some cases Otoacoustic emission (OAE) test. Based on estimation of pure tone thresholds with the results of these investigations, of monaural hearing impairment of each ear was calculated by averaging pure tone thresholds at 500, 1000, 2000 and 4000 Hz and subtracting from 25 dB (lower fence) and multiplying the result by a factor of 1.5. Whole man impairment was calculated using the formula, (Better ear% multiplied by 5) + (worse ear) 6.
- Tinnitus was a feature in 11/13 cases
- Associated Tympanic membrane perforation was found in 3/13 cases
- In the immediate period (less than 1 month) 10 of the 13 cases had pure tone threshold of more than 70 db in the worse ear [Table 1]
- Audiometry 2 years after trauma showed pure tone threshold more than 60 db in 12 of the 13 cases [Table 2]
- Acoustic reflex was present in 12 of the 13 cases at 80 db and above [Table 3]
- Hearing Threshold evaluation by ABR (BERA) was less than 40 db in 12 of the 13 patients [Table 4].
| Discussion|| |
Acoustic-blast trauma is a dramatic event that and often accompanied by multiple injuries. It is caused by impulse noise which may be defined as a single-pulse sound or a burst of sound with a duration of between 0.001 and 1 second and is typified by a sound, typically equal to or greater than 140 dB, which rapidly rises to a sharp peak and then quickly fades. Examples of sources of impulse noise include commercial blasting, sonic boom, muzzle blast and backfire from an internal combustion engine.
Blast overpressure is usually distinguished from impulse noise. Movement of considerable volumes of combustion products and air often reach tens of kilopascals while in impulse noise they are usually less than 2 kPa.
Blast trauma can involve the external, middle and internal ears. In the middle ears it may Blast waves involve the lead to perforation of the ear drum and ossicular disruption. In such an event, paradoxically the damage to the inner ear is reduced as the tympanic membrane and ossicle absorb some of the damaging shock., Permanent, pure SNHL is the most prevalent type of auditory impairment in blast trauma, accounting for 35 to 54 percent of injuries.
Injury to the inner ear occurs due to mechanical shearing of the basilar membrane. This produces a hearing loss (Temporary threshold shift) and tinnitus. The initial presentation is frequently a mixed hearing loss (conductive and SNHL) due to middle ear injury. Blast injuries produce a sloping high-frequency hearing loss that often affects frequencies below 8 kHz., The hearing loss may be permanent in 30-55% cases.,
Experimental work has shown that recovery following blast trauma starts immediately; stops for 5-12 hours, then resumes till a stable level is reached. It is generally accepted that, providing there has been no further exposure to harmful noise, further deterioration in hearing is unlikely to be attributable to the initial injury a year after an episode of acute acoustic trauma. Recovery of hearing after blast trauma with stabilization of hearing has been reported by other authors also [Jagdale 3, Isha Tyagi 4].,
Acoustic trauma and noise induced hearing loss are separate entities and are caused by different pathological mechanisms in the middle and inner ears. Acoustic trauma is largely reversible whereas NIHL permanent disability.
In India NIHL is a compensable occupational disease as per Employees state Insurance Act (1948) and Workmen's Compensation Act (1923). The first compensations for NIHL were awarded only in 1996. There is an increasing awareness amongst industrial workers about the deleterious effects of noise. Industrial accidents are often associated with multiple injuries leading to temporary and permanent disabilities. Hearing loss due to acoustic trauma is a dramatic event and may tempt to claim compensation.
All the patients in the present series were victims of blast trauma and were referred for evaluation for permanent disability. Otoscopy revealed healed perforation in 3 patients Initial audiometry was done between 7 and 28 days after injury showed pure tone thresholds of >70 dB in more than 75% cases (10/13).
The gold standard of disability evaluation in case of hearing loss is pure tone audiometry which is purely subjective and can be fallacious due to malingering. Therefore objective tests such as Acoustic reflex threshold and ABR for hearing threshold estimation have become important. In these cases malingering was suspected when all the patients were able to comprehend normal speech and pure tone audiometry showed severe to profound hearing loss in almost 12 of 13 (92%) patients. Therefore other objective tests were done. Hearing threshold estimation can be done from acoustic reflex threshold. Acoustic reflex threshold is approximately 70-105 db above the pure tone threshold and can be reliably used for measuring pure tone threshold.
In this series, Acoustic reflexes were elicited between 80-95 dB in 12 of 13 patients thus estimating the hearing thresholds of <25 db in these cases. This was corroborated by ABR that estimated the hearing thresholds of 21-30 dB in 12 of the thirteen cases.
Disability calculation was done by averaging pure tone thresholds at 500, 1000, 2000 and 3000 Hz (Ref). On the basis of Pure tone Audiometry alone, 11 of the 13 cases had >75% impairment, whereas calculation with re-estimated of pure tone threshold with Acoustic reflexes and ABR showed the impairment to below 5% in 12 of the13 cases, thus avoiding significant financial liability [Table 5].
As literature on the subject suggest, recovery of hearing following blast trauma does not occur beyond one year. Hence, for proper permanent disability calculation in Acoustic blast trauma, evaluation should be done after a period of at least one year after injury. Objective tests like Acoustic reflexes, OAE and ABR play an important role besides pure tone audiometry in the evaluation of hearing threshold levels. This study also demonstrates spontaneous recovery of hearing loss in a large number of cases of Acoustic blast trauma.
| Conclusion|| |
Hearing loss in blast injuries recovers spontaneously in many cases. The initial hearing loss represent a temporary threshold shift (TTS). Objective hearing tests should be carried out after one year or more before evaluation of permanent disability.
Financial support and sponsorship
Conflicts of interest
There are no conflicts of interest.
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[Table 1], [Table 2], [Table 3], [Table 4], [Table 5]