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Cohort case studies on acoustic trauma in operation Iraqi freedom.

Background

Hearing is a critical sensor of Soldiers that is vital to both their survivability and lethality. When hearing loss is present, the ability to conduct auditory tasks is greatly diminished. Good hearing is required to perform such tasks as localizing sound, gauging auditory distance, identification of a sound source, and understanding verbal orders or radio communications. This multidimensional sense provides an indispensable amount of information on the battlefield and can mean the difference between life and death in combat. The ability to distinguish the sounds of different weapons, both friendly and enemy, is a combat-critical skill.

Poor hearing jeopardizes the unit mission and increases the likelihood of a serious mishap due to a Soldier's decreased situational understanding. Verbal communications and hand and arm signals between dismounted Soldiers remain the primary means of communication on the battlefield. Although technological advances have improved battlefield communication systems, these electronic advances cannot overcome the fact that human hearing is required to complete most communication.

Sound is often the first source of information a Soldier has before direct contact with the enemy. Unlike visual cues, information carried by sound comes to us from all directions, through darkness, and over or through many obstacles. Aggressive action produces sound the enemy cannot hide or camouflage. The ability to hear and recognize combat-relevant sounds is a vital component to situational understanding and provides a tactical advantage. Noise-induced hearing loss is a tactical risk and threatens both individual and unit combat effectiveness. Hearing loss due to noise exposure usually occurs in the high frequencies. Since speech sounds that give meaning to words (for example, consonants such as ch, th, sh, f, and p) are high-frequency sounds as well as the sounds that provide the ability to determine the signature of weapons and vehicles, high-frequency hearing loss is particularly devastating to military operations. In the heat of battle, many words can be mistaken--even more so if hearing loss is present. For example; breach and break, attack and get back, cease fire and keep firing, stay down and go around, or right car and white car. Figure 1 displays a spectrograph of the sentence "get the white car." Each speech sound from the sentence is superimposed at the location corresponding to its occurrence. The horizontal axis represents time in seconds and the vertical axis represents the frequency of the sound in Hz. The colors represent intensity. The brighter the color, the louder the sound is at that frequency. When the same sentence is filtered to H3 hearing profile levels, * the decrease or absence in intensity in the higher frequency region at the top of the spectrograph is considerable. This is a visualization of just how much speech cues are not audible in a Soldier with an H3 profile.

Outside of combat, the ability to hear still matters for safety and performance reasons. In fact, most of the 150 different enlisted jobs in the Army do not directly involve combat. Even so, most of these jobs do require combat deployments and have occupational hazards such as noise and ototoxins. These auditory hazards are compounded by 12- to 18-month deployments that have lengthy work days, no weekends, and very little free time away from work. The symptoms of noise-induced hearing loss can be deceptively subtle, usually with no obvious physical injury or wound, but the effects can be permanent, debilitating, often untreatable, and, most importantly, preventable.

LITERATURE REVIEW

During the first year of the war in Iraq, there was an average of one medical evacuation a day for hearing loss (with no other concurrent injury). Medical evacuations for hearing loss were sent to the audiology clinic at Landstuhl Regional Medical Center in Germany. McIlwain found that out of the 564 patients seen there during this time, 65% were from blast injuries. (2) Sensorineural hearing loss from friendly forces weapons systems made up approximately 25% of the injuries. The remaining 10% were balance-related or conductive type hearing loss that was predominantly unrelated to hazardous noise exposure. As a result, a military audiologist position was temporarily placed in Baghdad in 2004 to evaluate acoustic trauma patients. This provided an efficient way to determine a Soldier's hearing ability without the need for a lengthy and expensive medical evacuation for a nonlife-threatening injury. Oftentimes, Soldiers are exposed to an explosion such as an improvised explosive device or a mortar round and have no apparent injuries, but can sense their hearing has decreased and tinnitus is present. With no visible injuries, the Soldiers return to their duties. This is where the term "invisible injury" is derived.

[FIGURE 1 OMITTED]

The ability to distinguish the sounds of different weapons, both friendly and enemy, is a skill that is taught in the Army. If the sounds of weapons fire are coming from the next block of buildings, knowing whether it is enemy or friendly, small arms or automatic weapons, small caliber or large caliber, or if it is a rocket propelled grenade or an antitank weapon can be critical information that determines a Soldier's reaction. Katzel et al found that the signature sounds distinguishing a weapons system are primarily above 4 kHz. (3) The frequencies above 4 kHz are also where hazardous noise affects the cochlea the most, and where the tell-tale "noise notch" occurs. (4) Consequently, identification of noise signatures, communication, gauging auditory distances, and localization are negatively affected. Studies have shown that the ability to accomplish a unit's mission is directly proportional to its ability to communicate effectively. If effective communication drops by 30%, the ability to control the unit in order to accomplish the task drops by 30% as well. (5)

Weiner and Ross describe the resonant characteristics of the outer ear as boosting the sound pressure level of the frequencies between 2500 Hz and 3500 Hz. (6) Donahue and Ohlin describe the middle ear as frequency selective because the transfer functions of the middle ear allow the mid- to high-frequency sounds (approximately 1500 Hz through 4000 Hz) to pass through it with considerably less resistance than the low-frequency sounds. (4) The result is that the low-frequency sounds reach the cochlea at a lower intensity than when it entered the ear canal. Conversely, sounds at frequencies between 1 kHz and 3 kHz are transferred to the cochlea with significantly less resistance and greater intensity than when they entered the ear canal. Rudmose and Ward independently demonstrated that when high intensity pure tones reach the cochlea in the 1 kHz to 3 kHz frequency range, the resulting threshold shift occurs approximately a half to one whole octave above the pure tone exposure. (7,8) As the waveform increases in amplitude on the basilar membrane due to an increase in sound intensity, the vibration becomes less localized and moves toward the basal portion of the cochlea. (4) Ylikoski and Ylikoski state that this movement causes damage to loci of the cochlea that are different from the stimulus frequencies. (9) For broad-band noise with equal energy in all bandwidths, the maximum threshold shift occurs between 3000 Hz and 6000 Hz. (8)

Studies of noise-induced hearing loss in the Global War on Terror have been analyzed. Cave found that more than 50% of 258 acoustic trauma patients seen at the Walter Reed Army Medical Center from April 2005 through August 2005, had significant hearing loss, and age could not account for the change in hearing from before to after deployment. In addition, one-half of these patients reported having tinnitus. (10) Helfer data mined hearing loss associated diagnoses codes of postdeployment and nondeployed Soldiers between April 1, 2003 and March 31, 2004. He found that 68% of 806 postdeployment evaluations had been diagnosed for at least one of the following: acoustic trauma (5.6%), permanent threshold shift (29.3%), tinnitus (30.8%), eardrum perforation (1.6%), or moderately severe hearing loss or worse (15.8%). The nondeployed group had 4% of 141,050 diagnosed with the same hearing loss related codes: acoustic trauma (0.1%), permanent threshold shift (0.5%), tinnitus (1.5%), eardrum perforation (0.1%), or moderately severe hearing loss or worse (2.2%). (11) In 2007, the Veterans Administration Rehabilitative Research and Development Department reported that 839,907 veterans were identified as having service-connected hearing loss that required compensation from the Veterans Benefit Administration. In 2006, total compensation to Veterans was over $1.2 billion for hearing loss and tinnitus disabilities (12) (p3) and accounted for 17% of the total disability claims. (12) (p12) This is an increase of 18% from the previous year and a 56% increase since 2002. (12) (p12) These studies corroborate that the sounds of combat can be devastating to a Soldier's hearing readiness.

Bohne and Harding found that the cochlea undergoes 2 histopathologic stages after an acoustic trauma: degeneration of the outer hair cells and the continued degeneration of supporting cells, afferent nerve fibers, and additional hair cells. The second histopathologic stage has a delayed onset with respect to identification of threshold shifts with routine monitoring. (13) Simply put, hearing loss is progressive after an acoustic assault and therefore the actual rate of hearing loss in the Army is greatly underestimated. Multiple tours of duty in Iraq and Afghanistan will accelerate this delayed onset due to lengthy work days, no weekends, and large doses of hazardous noise exposure on a regular basis. The number of servicemen and servicewomen on disability because of hearing damage will increase no less than 15% a year under current combat conditions and disability policies. (14) The US Army Center for Health Promotion and Preventive Medicine has followed veterans' disability claims since 1969. In 2008, the disability payments from the Veterans Administration for tinnitus and hearing loss exceeded one billion dollars. Unfortunately, a Government Accountability Office investigation found that the average pending and appeal process of applying for a service connected disability in 2007 was 789 days. (15)

Even if a Soldier's hearing thresholds are within a normal tolerance, the damage may have begun. Future hazardous noise exposure will append to previous damage and lead to future hearing loss that is not within acceptable limits for Army standards. Once a Soldier's Speech reception threshold in the best ear is greater than 30 dB hearing level (measured with or without hearing aid), their ability must be evaluated for functionality and personal risk with respect to their jobs. For instance, if a helicopter pilot has a hearing loss and poor speech intelligibility; many lives are at risk if the radio communication cannot be heard. Also, the pilot risks further hearing loss to the hazardous noise of the helicopter. If the findings of the review board are negative, the Soldier is offered a medical discharge or a change to a job that does not involve hazardous noise exposure. Even if Soldiers choose to change jobs rather than take a medical discharge, the organizational knowledge and technical experience goes with them.

VARYING EXPOSURES

The following cohort case studies were observed using air conduction hearing threshold data collected during evaluations conducted in 2006 at the US Army Audiology Clinic in Baghdad, Iraq. The 2 cohort case studies presented here are the effects of acoustic trauma while wearing hearing protection and the effects of acoustic trauma while not wearing hearing protection.

Cohort Case Study No. 1

Paired data of predeployment and during deployment hearing thresholds of 50 US Army Soldiers (100 individual ears) were randomly observed among Soldiers that were exposed to acoustic trauma while wearing hearing protection. All subjects were noise-free for at least 14 hours before evaluation. Only threshold data from Soldiers with normal type A tympanograms were collected. Of this sample, 25 of the Soldiers reported exposure to explosions in combat while wearing some form of hearing protection and 25 that had not been exposed to explosions, but received hearing screenings as a part of routine physical exams. During each evaluation, predeployment audiometric thresholds were compared to the current results. One Soldier in the hearing protected acoustic trauma group had one ear with a perforated tympanic membrane, so that ear was excluded from the data set, reducing the number of ears to 49. Since data were paired, no weighting for age or gender was used. The differences in thresholds predeployment and during deployment at the individual frequencies of 500 Hz, 1 kHz, 2 kHz, 4 kHz, and 6 kHz were then compared between groups with a one-way analysis of variance (ANOVA) using Statistical Package for Social Sciences (SPSS), Version 11.0 (SPSS Inc, Chicago, Illinois). Levene's statistic was used to test for homogeneity of variance at each frequency between groups. Since there were only 2 groups, no post hoc tests were necessary.

[FIGURE 2 OMITTED]

The null hypothesis: there is no significant difference between predeployment and ongoing deployment audiometric threshold levels at the individual frequencies of 500 Hz, 1 kHz, 2 kHz, 4 kHz, and 6 kHz between routine physical exam group and hearing protected acoustic trauma group. The null hypothesis was rejected for the individual frequencies of 500 Hz, 1 kHz, and 2 kHz. There was a significant difference in hearing threshold levels at these frequencies. All frequencies passed Levene's test, except 4 kHz. Figure 2 displays the mean threshold differences and error bars for each group and frequency. The descriptive statistics are displayed in the Table.

The analysis of variance at 500 Hz revealed a highly significant difference between groups, F=9.463, p<0.05 with a medium effect size ([[eta].sub.2]) of 0.09. The analysis of variance at 1 kHz revealed a highly significant difference between groups, F=6.076, p<0.05 with a medium [[eta].sub.2] of 0.06. The analysis of variance at 2 kHz revealed a significant difference between groups, F=9.657, p<0.05 with a medium [[eta].sub.2] of 0.09. The analysis of variance at 4 kHz revealed no significant difference between groups, F=2.707, p>0.05 with a small [[eta].sub.2] of 0.03. Homogeneity of variance was violated, a=0.045, p<0.05. The analysis of variance at 6 kHz revealed no significant difference between groups, F=1.607, p>0.05 with a small [[eta].sub.2] of 0.02.

The increase in standard deviation with the increase in frequency is notable in the postdeployment thresholds, but expected in individuals exposed to hazardous noise. An analysis of men exposed to hazardous noise in the International Standards Organization 1999 database by Bovo et al showed that male workers exposed to a noise level of 100 dBA for 30 years exhibited a hearing loss at 4 kHz with a variation of 60 dB. (15) This is consistent with the findings of the hearing protected acoustic trauma group. Further, several studies attribute this variation to mechanical resonance and sound transfer function of the ear canal, the action of stapedial reflexes, and genetics. (15-17) The significance levels were least remarkable at 4 kHz and 6 kHz due to the low power and the violation of homogeneity of variance at 4 kHz. Ferguson and Tukane describe the one-way ANOVA as being robust enough to overcome violations of homogeneity of variance.(18) However, the results of 4 kHz and 6 kHz interpretation should be based on the mean and error bars in Figure 2.

The increase of hearing thresholds in the hearing protected acoustic trauma group is least remarkable at 4 kHz and 6 kHz. The attenuation characteristics of hearing protection may explain the greater protective effect of the 4 kHz and 6 kHz over the lower frequencies. Higher frequency sound energy is more easily obstructed than lower frequency sound energy in passive hearing protection. To a large extent, the wavelength of the sound is responsible for this greater attenuation in the high frequencies; the higher the frequency, the shorter the wavelength and vice versa. Generally speaking, acoustic energy is attenuated more if the earplug is greater than one-half the wavelength of the sound. Since the Soldiers in this cohort case study were wearing a variety of approved hearing protection (polyvinyl foam earplugs, combat arms earplugs, and tactical communication and protective systems), a properly sized and fitted hearing protector of any given size or style will therefore attenuate higher frequency sound with a shorter wavelength than a lower frequency sound with a longer wavelength. This is consistent with the protective effect at 4 kHz and 6 kHz in this study.

The statistical significance at 500 Hz, 1 kHz, and 2 kHz may also be attributable to the earplug preventing the acoustic reflex from occurring during the impulse noise. Fletcher found that the acoustic reflex was more effective at protecting hearing from gunfire in frequencies below 1 kHz than the single flanged earplug. However, he also found the single flanged earplug to be most effective in frequencies 2 kHz and greater. (17) This corresponds to the observed hearing thresholds of this case study, but also does not take into account bone conduction of the sound. Berger found that at 40 dB in the frequency of 2 kHz, sound reaches the cochlea via bone conduction even when hearing protection is worn. (19) If we take into account the half wavelength theory mentioned in the previous paragraph, it is expected that higher frequencies are attenuated more through the human body and therefore the lower frequency sounds are louder at the cochlea via bone conduction. This also may account for some of the difference patterns observed.

Further, Price describes the middle ear as a linear system up to 120 dB sound pressure level, and that the transfer functions of the middle ear are flat in the lower frequencies and decrease at a rate of 6 dB per octave at frequencies above 1 kHz. (20,21) Kobrak and von Bekesy found that in human cadavers' ears the stapes changed its mode of vibration at high intensities in such a way that less energy was transmitted to the cochlea. (22,23) These studies support the idea that the middle ear can peak clip high intensity impulse noise. Since the explosions could not be meticulously measured, it is not plausible to argue that the hearing protected acoustic trauma group benefited from this middle ear peak clipping, but is worth mentioning.

Cohort Case Study No. 2

Independent samples of during deployment hearing thresholds of 81 US Army Soldiers (161 individual ears) were randomly observed in two groups: routine physical exams and acoustic trauma without hearing protection. All subjects were noise-free for at least 14 hours before evaluation. Only threshold data from Soldiers with normal type A tympanograms were collected. Of this sample, 34 of the Soldiers reported acoustic trauma in combat and 47 had not been exposed to acoustic trauma, but received hearing screenings as a part of routine physical exams. One Soldier in the acoustic trauma group had one ear with a perforated tympanic membrane, so that ear was excluded from the data set reducing the number of ears to 67. All subjects were under 25 years of age, so no weighting for age or gender was used. (24) The thresholds at the individual frequencies of 500 Hz, 1 kHz, 2 kHz, 4 kHz, 6 kHz, 8 kHz, and 12 kHz were then compared between groups with a one-way ANOVA using SPSS, Version 11.0. Levene's statistic was used to test for homogeneity of variance. Figures 3 and 4 display the quartiles and outliers at each frequency. Since there were only 2 groups, no post hoc tests were necessary.

The null hypothesis: there is no significant difference between audiometric threshold levels at the individual frequencies of 500 Hz, 1 kHz, 2 kHz, 4 kHz, 6 kHz, 8 kHz, and 12 kHz between the routine physical exam group and the acoustic trauma group.

The null hypothesis was rejected for the individual frequencies of 500 Hz, 1 kHz, 2 kHz, 4 kHz, 6 kHz, 8 kHz, and 12 kHz. There was a significant difference in hearing threshold levels at these frequencies.

The analysis of variance at 500 Hz revealed a highly significant difference between groups, F=5.485, p<0.05 with a medium [[eta].sub.2] of 0.03. The analysis of variance at 1 kHz revealed a highly significant difference between groups, F=6.371, p<0.05 with a medium [[eta].sub.2] of 0.04. Homogeneity of variance was not violated, [alpha] = 0.67, p>0.05. The analysis of variance at 2 kHz revealed a significant difference between groups, F=11.661, p<0.05 with a medium [[eta].sub.2] of 0.07. Homogeneity of variance was violated, [alpha] = 0.03, p<0.05. The analysis of variance at 4 kHz revealed no significant difference between groups, F=25.017, p>0.05 with a small [[eta].sub.2] of 0.01. Homogeneity of variance was violated, [alpha] = 0.00, p<0.05. The analysis of variance at 6 kHz revealed no significant difference between groups, F=17.159, p>0.05 with a small [[eta].sub.2] of 0.01. Homogeneity of variance was violated, [alpha] = 0.00, p<0.05. The analysis of variance at 8 kHz revealed no significant difference between groups, F=27.589, p>0.05 with a large [[eta].sub.2] of 0.17. Homogeneity of variance was violated, [alpha] = 0.00, p<0.05. The analysis of variance at 12 kHz revealed no significant difference between groups, F=28.736, p>0.05 with a large [[eta].sub.2] of 0.15. Homogeneity of variance was violated, a=0.00, p<0.05.

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

The significance levels were remarkable at all frequencies. The increase in standard deviation with the increase in frequency is notable, but expected in hazardously noise exposed individuals. Several studies attribute this variation to mechanical resonance and sound transfer function of the ear canal, the action of stapedial reflexes, and genetics. (15-17)

Balatsouras evaluated extended high frequency hearing (greater than 8 kHz) in basic trainees of the Greek Army. (25) The purpose was to determine if there was value added to the inclusion of extended high frequency threshold testing with the standard audiology battery. The subjects had been exposed to acoustic trauma by small arms weapons fire. The conclusion was that extended high frequency temporary threshold shift subsided and there was no significant benefit from the added time and effort for conducting this procedure.

Hamernik identified impulse noise, specifically blast waves with very short durations (0.5 millisecond) and high peak intensities, as capable of producing a mechanical impulse which can result in extremely high shear stresses and premature failure of elastic structures. (26) He further described blast wave exposure as producing 2 fundamentally different lesion patterns: severe mechanical damage to the organ of Corti where large pieces of sensory and supporting cells were torn loose from the basilar membrane, and lesions that were more limited in extent and consisted primarily of missing or damaged sensory cells with the structural elements of the organ of Corti remaining essentially intact. This latter pattern of loss was frequently associated with damage to the tympanic membrane.

The acoustic traumas in this study were from improvised explosive devices or car bombs and the results above 8 kHz were permanent and quite large threshold shifts. This is likely due to the spectral and intensity differences in small arms fire and improvised explosive device exposure.

Improvised explosive devices (IEDs), which were used sparsely at the outset of Operation Iraqi Freedom in March 2003, now account for nearly 70% of all US casualties from hostile action in Iraq. (27) Understandably, the IED was the most common type of impulse exposure in Iraq in 2006. During this phase of Operation Iraqi Freedom, most of the IEDs were constructed out of 105mm artillery shells. Price measured the impulse and spectrum of this explosive device. At 5.64 meters, the impulse has a spectral peak at ~100 Hz with an A duration of 0.3 millisecond. The second most common impulse exposure was from the standard issue M16 rifle. At 4.24 meters, it has a spectral peak of ~600 Hz with an A duration of 0.2 millisecond. (21) Either of these, when situated where there is a reflection of the impulse, will create a second reflected impulse exposure that can be as much as 90% of the initial impulse's energy with similar spectral energy. In an urban terrain such as Baghdad, warfare often takes place in city streets where there is a great deal of reflective surfaces. The spectral peak of the 2 most common combat exposures is below 1 kHz and is another probable variable for the hearing protected acoustic trauma groups hearing post-explosion threshold configuration.

DISCUSSION

Army audiology plays a very important role in preventive medicine and the standard 3 levels of prevention are routinely used. Primary preventive measures include proper selection and use of hearing protection, annual education, and taking a baseline audiogram. Secondary preventive measures involve identification of the early stages of noise induced hearing loss and taking steps to prevent its progression through intervention, follow-up monitoring, and clinical validation of results. If primary and secondary prevention strategies do not work, tertiary services such as hearing aid fitting, aural rehabilitation, and administrative controls are used. The primary and secondary preventive measures of hearing conservation have had a tremendous impact in the reduction of the number of Soldiers with hearing loss over the past 4 decades, but current large scale combat operations have reduced the success rate of conventional hearing conservation in the Army.

Hearing conservation is a robust program in the Army.

Unfortunately, hazardous noise and its effects on hearing cannot be eradicated with a one treatment vaccination, it is an ongoing program that requires continuous efforts and leadership support. Army deployments are fluid and the environments to which Soldiers are exposed are constantly changing. For the prevention of hearing loss, this has traditionally posed a problem because hearing conservation programs were not designed with combat in mind. With asymmetric warfare (coalition forces observing different rules of engagement than insurgents) and a nonlinear battlefield (no frontlines), Soldiers are experiencing more exposure to the sounds of combat. This has forced audiologists to rethink their approach to prevention in these challenging environments.

Even though these cohort case studies were not able to control for the many factors that affect hearing in combat, they do provide a field perspective on hearing protection being used in combat and how it correlates with previous research. This article only addresses hearing thresholds of Soldiers who reported wearing hearing protection when they were exposed to an explosion. It is important to point out that for the many Soldiers were not wearing hearing protection, the hearing loss was substantial and typically involved conductive and sensorineural components. There is also some anecdotal evidence that central hearing loss was a comorbid component of traumatic brain injury. The prevalence of this type of acoustic trauma in Operations Iraq Freedom and Enduring Freedom are not yet known, but are being studied.

The vestibular system may also be damaged by hazardous noise due to its close proximity and similarity in cell structure to the cochlea. (28) Soldiers are exposed to explosions, such as improvised explosive devices, mortars, or car bombs. They are also exposed to many steady-state noises such as aircraft, track vehicles, or large electrical generators. These noise sources may cause asymptomatic damage to their vestibular system. Shupak et al did find that symmetric noise-induced hearing loss is correlated with symmetric peripheral vestibular system damage. (29) These results were corroborated by M. Hill and D. S.

McIlwain (unpublished data, 2006). The reason it is possible to be unaware of a vestibular deficit in conjunction with acoustic trauma is because of the complex relationship between the central nervous system (CNS) of the brain and the 3 primary sensory modalities critical to equilibrium (vestibular, visual, and proprioceptive systems). If an insult to the vestibular system occurs, the CNS relies heavily on information from vision and proprioception to make up for the lack of neural firing from the balance center to compensate. The CNS adapts to the different levels of neural input it receives. During this adaptation time, the individual often experiences a slight feeling of imbalance, dizziness, or even vertigo, especially in the absence of vision. Symptomatic feelings of imbalance, dizziness, and vertigo typically subside. The vestibular system, combined with the visual and proprioceptive systems, contributes to spatial orientation. It is estimated that 80% of spatial orientation is based on visual cues, but when visual cues are no longer available or are diminished, the vestibular system's role is critically elevated. Situations while flying aircraft or driving an armored personnel carrier, such as white-outs (snow) or brown-outs (sand), may lead to greatly reduced visual cues. If a pilot or driver's vestibular system is damaged, the chance of spatial disorientation occurring in low-vision environments may increase, resulting in a potentially catastrophic accident. It is also possible that this spatial disorientation could be a cause of danger for the ground troops in similar low visibility situations while weighed down with a basic combat load.

CONCLUSION

The solution is on the battlefield. Even if the Soldiers are not directly involved in combat, the common denominator of the small but significant high frequency threshold shift is a combat deployment.

Gates and Fallon recommend a more aggressive operational hearing program should be implemented with more Army audiologists deployed to meet the recommended one Army audiologist per 10,000 Soldiers. Currently, there is only one audiologist for over 160,000 deployed Soldiers in Iraq, and none in Afghanistan. (30)

Increased sensitivity for secondary intervention is also warranted. It is recommended that Soldiers with a small but significant high frequency threshold shift (average positive 10 dB threshold shift at 4 kHz and 6 kHz or a positive threshold shift of 15 dB in either 4 kHz or 6 kHz) postdeployment should receive a follow-up audiogram. Emphasis should be placed on Soldiers avoiding noise of any kind for at least 14 hours with reeducation on what constitutes hazardous noise. If a small but significant high frequency threshold shift is confirmed on the follow-up audiogram, the Soldier should receive at least a verbal acknowledgement that there has been a small change in hearing, interviewed on possible causes, and a more detailed education on the long-term personal and professional consequences of hearing loss. The small but significant high frequency threshold shift should be viewed as an early indicator of noise induced hearing loss because it places Soldiers at higher risk for clinically significant noise-induced hearing loss.

The Army spends a considerable amount of time and money training an all-volunteer force. In an instant, a Soldier can become a risk for further injury as well as put others at risk due to decreased job performance. Currently, the best solution to the age-old problem of hazardous noise in the Army is the military audiologist. These professionals are indispensable in developing solutions for unique situations such as noise abatement and the selection and use of contemporary hearing protection in combat environments.

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REFERENCES

(1.) Army Regulation 40-501: Standards of Medical Fitness. Washington, DC: US Dept of the Army; December 14, 2007:80.

(2.) McIlwain DS. CAOHC Deployed. CAOHC Update. 2004;16(3):4. Available at: http://www.caohc.org/updatearticles/fall04.pdf. Accessed April 7, 2009.

(3.) Combat Recognition Requirements. Human Engineering Report SDC 383-6-1. Office of Naval Research. April 15, 1952:20-25.

(4.) Donahue A, Ohlin D. Noise and the impairment of hearing. Occupational health: the Soldier and the industrial base. Washington, DC: Borden Institute. 1993;207-252.

(5.) Garinther GR, Peters LJ. Impact of communications on armor crew performance. Army Res Dev Acquis Bull. January-February 1990:1-5.

(6.) Weiner F, Ross D. The pressure distribution in the auditory canal in a progressive sound field. J Acoust Soc Am. 1946;18:401-408.

(7.) Rudmose W. Hearing loss resulting from noise exposure. In: Harris CM, ed. Handbook of Noise Control. 1st ed. New York: McGraw-Hill; 1975.

(8.) Ward W. Noise-induced hearing damage. Otolaryngol. 1973;2:377-390.

(9.) Ylikoski ME, Ylikoski JS. Hearing loss and handicap of professional Soldiers exposed to gunfire noise. Scand J Work Environ Health. 1994;20(2):93-100.

(10.) Cave K. Blast injury of the ear: clinical update from the global war on terror. Mil Med. 2007;172:726-730.

(11.) Helfer T, Jordan N, Lee R. Post deployment hearing loss in US Army Soldiers seen at audiology clinics from April 1, 2003 through March 31, 2004. Am J Audiol. 2005;14:161-168.

(12.) US Dept of Veterans Affairs. Annual Report: National Center for Rehabilitative Auditory Research, January 1-December 31, 2007. Portland, OR: 2008.

(13.) Bohn B, Harding G. Degeneration in the cochlea after noise damage: primary versus secondary events. Am J Otol. 2000;21(4):505-509.

(14.) Dole B. GAO Findings and Recommendations Regarding DoD and VA Disability Systems. Washington, DC: Government Accountability Office; May 25, 2007.

(15.) Bovo R, Ciorba A, Martini A. Genetic factors in noise induced hearing loss. Audiological Medicine. 2007;5:25-32.

(16.) Holmes A, Widen S, Erlandsson S, Carver C, White L. Perceived hearing status and attitudes toward noise in young adults. Am J Audiol. 2007;16(suppl):S182-S189.

(17.) Fletcher J. Comparative attenuation characteristics of the acoustic. J Acoust Soc Am. 1960;32:1524.

(18.) Ferguson GA, Takane Y. Statistical Analysis in Psychology and Education. 6th ed. Montreal, Quebec: McGraw-Hill Ryerson Limited; 2005.

(19.) Berger E, Kieper R, Gauger D. Hearing protection: surpassing the limits to attenuation imposed by the bone conduction pathways. J Acoust Soc Am. 2003;114:1955-1967.

(20.) Price R. Upper limit to stapes displacement: implications for hearing loss. J Acoust Soc Am. 1974;56:3.

(21.) Price R. Relative hazard of weapons impulse. J Acoust Soc Am. 1983;73:556-565.

(22.) Kobrak HG. The Middle Ear. Chicago, Illinois: University of Chicago Press; 1959.

(23.) Von Bekesy G. Experiments in Hearing. Wever EG, ed. New York: McGraw-Hill; 1960.

(24.) ISO 7029:2000. Acoustics-Statistical Distribution of Hearing Thresholds as a Function of Age. International Organization for Standardization: Geneva, Switzerland; 2000.

(25.) Balatsouras D, Housioglou E, Danielidous V. Extended high frequency audiometry in patients with acoustic trauma. Clin Otolaryngol. 2004;30:249-254.

(26.) Hamernik R, Turrentine G, Roberto M, Salvi R, Henderson D. Anatomical correlates of impulse noise-induced mechanical damage in the cochlea. Hear Res. 1984;13:229-247.

(27.) Depenbrock P. Tympanic membrane perforation in IED blasts. J Spec Oper Med. 2008;8,51-53.

(28.) Golz A, Westerman S, Westerman L, et al. The effects of noise on the vestibular system. Am J Otolaryngol. 2001;22:190-196.

(29.) Shupak A, Bar-El E, Podoshin L, Spitzer O, Gordon C, Ben-David J. Vestibular findings associated with chronic noise induced hearing impairment. Acta Otolaryngol. 1994;114(6):579-585.

(30.) Gates K, Fallon E. Hearing Conservation Program: Doctrine, Organization, Training, Materiel, Leadership and Education, Personnel and Facilities. Aberdeen Proving Grounds, MD: US Army Center for Health Promotion and Preventive Medicine; 2007:7.

MAJ D. Scott McIlwain, MS, USA

MAJ (Ret) Bryan Sisk, AN, USA

Melinda Hill, AuD

* H3 hearing profile is defined by the US Army Standards of Medical Fitness1 as "speech reception threshold in best ear not greater than 30 dB HL, measured with or without hearing aid; or acute or chronic ear disease."

MAJ McIlwain is an instructor and curriculum writer at the US Army Medical Department Center and School, Fort Sam Houston, Texas.

MAJ (Ret) Sisk is the Coordinator of Outlying Clinics at the Veterans Administration Hospital, Temple, Texas.

Dr Hill is a principle investigator in the Department of Acoustics, US Army Aeromedical Research Laboratory, Fort Rucker, Alabama.
Data from comparative tests of predeployment and during-deployment
audiometric thresholds between paired control and exposure groups.

                           Descriptive Statistics

                                                             Std.
                      N    Minimum   Maximum     Mean   Deviation

PE500HZ              50     -24.00     15.00   2.6000     8.64492
PE1000HZ             50     -15.00     20.00   5.1000     7.31855
PE2000HZ             50     -20.00     20.00   2.5000     7.54850
PE4000HZ             50     -30.00     25.00   2.9000     9.15234
PE6000HZ             50     -35.00     20.00    .9000    11.76687
AT500HZ              49     -15.00     20.00   7.5510     7.29743
AT1000HZ             49     -10.00     30.00   8.8776     7.92315
AT2000HZ             49     -10.00     40.00   7.9592     9.83819
RT4000HZ             49     -15.00     50.00   6.5510    12.68047
AT6000HZ             49     -25.00     55.00   4.3878    15.39862
Valid N (listwise)   49

Glossary
  PE--routine physical exam group
  AT--hearing protected acoustic trauma group

Note: PE and AT are followed by the corresponding frequency in Hz
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Author:McIlwain, D. Scott; Sisk, Bryan; Hill, Melinda
Publication:U.S. Army Medical Department Journal
Article Type:Report
Geographic Code:1USA
Date:Apr 1, 2009
Words:6071
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