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Marine Corps Breacher training study: auditory and vestibular findings.

BACKGROUND

In the tactical environment, initial breaching is an important specialty. If the breach into a potentially hostile location is not successful or the entry portal is not opened in a rapid fashion, enemy combatants will have time to react and military operatives and innocents may well be put in jeopardy. This paper is an overview of a contemporary research protocol conducted at the Marine Corps Weapons Training Battalion, Quantico, VA. This study was a comprehensive collaborative research initiative that evaluated a variety of environmental, auditory, and vestibular factors among Marines enrolled in the Breacher Training Course. The length of the course was 2 weeks and involved multiple exposures to blast overpressure and physical shock from ingress strategies used during the training. Observational data were collected pretraining, during training, and posttraining between September and June 2007. There was no change in the way the Marines conducted their training, and all data were collected based on the actual training scenario. Table 1 presents the series of evaluation methods. The primary objective of this research protocol was to determine if students in the breacher training were at risk of injury during standard training practices. In view of the specific area of the author's expertise, as well as in the interest of brevity, this article focuses solely on the auditory and vestibular affects of the breacher injury study.

AUDITORY EFFECTS

Introduction

Hearing is a critical Warrior sensor that increases 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, identifying a sound source, and understanding verbal orders and radio communications. This multidimensional sense provides an indispensable amount of information on the battlefield. Good hearing can mean the difference between life and death in combat, as well as in training.

Verbal communications, as well as hand and arm signals between dismounted Warriors, 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 communication. No matter how sophisticated the communication system, effective communication requires normal hearing.

Studies have shown that the likelihood of accomplishment of a unit's mission is directly proportional to the ability of the personnel in that unit to communicate effectively. If effective communication drops by 30%, the capability to control the unit to accomplish the task drops by 30% as well. (1) During combat, this problem is magnified by the chaotic environment, the complexity of the problems encountered, and the reaction time required. Warriors' hearing must be protected from damage caused by hazardous impact and sustained noise without compromising the ability to hear and communicate in these environments. Hearing loss is an invisible injury that is often viewed as having little or no impact on military operations. However, sound is frequently the first source of information a Warrior has before direct contact with the enemy. Unlike visual information, auditory cues come 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. The ability to distinguish the sounds of different weapons, both friendly and enemy, is a combat-critical skill. 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 whether it is a rocket propelled grenade or an antitank weapon determines a Warrior's reaction and is critical information available only with good hearing. Oftentimes, Warriors 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 Warriors return to their duties. This is where the term "invisible injury" is derived.

Hearing Threshold Analyses

Pure tone hearing thresholds, the lowest level of sounds that can be detected 50% of the time, were measured using a GSI 61 clinical grade audiometer (Grason-Stadler, Inc, Eden Prarie, MN) with E-A-RTone 3A insert earphones (Aearo Company, Indianapolis, IN). Paired data of hearing thresholds of 38 subjects (76 ears) were compared pretraining and posttraining. All subjects were wearing Department of Defense (DoD) approved hearing protection of their choice. All subjects were noise free for 14 hours before data collection and only threshold data from subjects with normal middle ear function were collected. Since the data were paired, no weighting for age or gender was used. The differences in hearing thresholds at 500 Hz, 1 kHz, 2 kHz, 3 kHz, 4 kHz, 6 kHz, and 8 kHz were evaluated before training began and after training was completed.

The breacher training resulted in:

* A slight increase in hearing threshold at 500 Hz ([bar. X] = 0.3289, SD=4.3463). This increase was not statistically significant: [t.sub.75]=-0.66, P >.01, 2-tailed.

* An increase in hearing threshold at 1 kHz ([bar.X]=1.8421, SD=4.46035). This increase was statistically significant: [t.sub.75]=-3.6, P <.01, 2-tailed.

* An increase in hearing threshold at 2 kHz ([bar.X]=1.8667, SD=4.33159). This increase was statistically significant: [t.sub.75]=-3.732, P <.01, 2-tailed.

* An increase in hearing threshold at 3 kHz ([bar.X]=1.8421, SD=3.72756). This increase was statistically significant: [t.sub.75]=-4.308, P <.01, 2-tailed.

* A slight increase in hearing threshold at 4 kHz (x=1.0526, SD=4.34479). This increase was not statistically significant: [t.sub.75]=-2.112, P >.01, 2-tailed.

* A slight increase in hearing threshold at 6 kHz ([bar.X]=1.3816, SD=5.45275). This increase was not statistically significant: [t.sub.75]=-2.209, P >.01, 2-tailed.

* A slight increase in hearing threshold at 8 kHz ([bar.X]=8.553, SD=7.67629). This increase was not statistically significant: [t.sub.75]=-0.971, P >.01, 2-tailed.

Even though there is statistically significant positive threshold shifts at 3 individual frequencies, the results only address a change in the means using very sensitive and robust analyses. Thus, it does not reflect significance with respect to the actual number of significant positive threshold shifts as defined by the DoD standard of an average shift of 10 dB in either ear at 2 kHz, 3 kHz, and 4 kHz. Therefore, another analysis must be performed using the same data set in a nonparametric delineation to if there is an increase in the number of significant threshold shifts between pretraining and posttraining.

Calculations to determine if there were any significant threshold shifts according to the DoD standard of an average shift of 10 dB in either ear at 2 kHz, 3 kHz, and 4 kHz were performed. The number of subjects (not individual ears as with the previous analysis) with a significant threshold shift and no threshold shifts were counted. A chi-square analysis was conducted on the categorical variables using SPSS version 11.0 (IBM SPSS, Chicago, IL).

The analysis showed a statistically significant increase in positive significant threshold shifts, [chi square] (1, n=38) =5.158, P<.Q5. Ironically, the subjects also showed a statistically significant increase in negative significant threshold shifts, [chi square] (l, n=38)=6.737, P<.05.

It may seem paradoxical that there were statistically significant positive threshold shifts at 1 kHz, 2 kHz, and 3 kHz in the previous analysis and there were nearly as many negative significant threshold shifts as there were positive significant threshold shifts posttraining. However, the method used to quantify whether a significant threshold shift had occurred is nonparametric and does not take into account the magnitude of any threshold shift, just whether a defined shift has occurred or not. The test for means actually takes into account the size of the threshold shifts. The negative threshold shifts that occurred were not remotely as large as the positive threshold shifts and in many cases were shifts in only one individual frequency. On the other hand, the positive significant threshold shifts were predominantly in more than one individual frequency and had much larger shifts. Figures 1 and 2 illustrate the thresholds by frequency in pretraining and posttraining with box plots. * This data set shows predominantly normal data both pretraining and posttraining with several of the students demonstrating extreme hearing threshold shifts. Therefore, the data and analyses support the conclusion that breacher training can result in permanent significant hearing loss, even with approved hearing protection. It also opens the discussion of why did some of the Marines get affected by the training and others did not. It is possible that the exposures were different among the trainees (ie, distance from blast, angle of incidence, etc) or that there were different protective factors in the approved hearing protection. However, it also is possible that some are more genetically susceptible to losing hearing than others and the subject warrants further exploration. (2)

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Distortion Product Otoacoustic Emission Analyses

Otoacoustic emissions are an excellent objective measure of cochlear health and are generally present when hearing thresholds are under 30 dB hearing level at each corresponding frequency measured. Otoacoustic emissions are frequency specific sounds that are generated within the inner ear. Otoacoustic emissions decrease or disappear at corresponding frequency loci of the cochlea after the cochlea has been damaged (ie, hearing loss at 3 kHz will correspond to decreased or absent otoacoustic emissions at 3 kHz). Paired data of distortion product otoacoustic emissions of 26 subjects (52 ears) from the same group as the hearing thresholds were taken from were compared pretraining and posttraining. The number of subjects is smaller than the hearing threshold group because some participants did not complete the posttraining measurements. All subjects were noise free for 14 hours before data collection and only threshold data from subjects with normal middle ear function were collected. Since the data were paired, no weighting for age or gender was used. The difference in distortion product otoacoustic emissions were compared at 1828 Hz, 2016 Hz, 3047 Hz, and 4124 Hz before and after training was completed. The Bio-logic Scout Sport (Bio-Logic SAS, San Carlos, CA) was used to measure the distortion product otoacoustic emissions with the parameter settings listed in Table 2.

The breacher training resulted in:

* A slight increase in the distortion product otoacoustic emission at 1828 Hz ([bar.X] =0.5077, SD=9.20903). This increase was not statistically significant: [t.sub.52]=0.398, P>M, 2-tailed.

* A slight increase in the distortion product otoacoustic emission at 2016 Hz (bar.X]=0.0692, SD = 10.13394). This increase was not statistically significant: [t.sub.52]=0.049, P>.01, 2-tailed.

* An increase in distortion product otoacoustic emission at 3047 Hz ([bar.X] = 3.2712, SD=7.18548). This increase was statistically significant: [t.sub.52]=3.283, P <.01, 2-tailed.

* A slight decrease in the distortion product otoacoustic emission at 4124 Hz (bar.X]=-1.4615, SD=8.82423). This decrease was not statistically significant: [t.sub.52]=-1.194, P >.01, 2-tailed.

Figure 3 shows box plots for the individual frequencies pretraining and posttraining. Since otoacoustic emissions decrease with declining cochlear health at the corresponding loci of the frequency response in the cochlea, it stands to reason that it may serve as an early indicator of possible hearing loss. The cochlea requires more energy than most parts of the body to function and therefore has a high metabolism. (3) Therefore, if anything goes wrong in the cochlea, it is more apparent in the measures of the outer hair cells which is the origin of the otoacoustic emission and is an extremely sensitive measure of cochlear function. Bohne and Clark estimate that hearing thresholds can reflect normal levels with up to 25% of the outer hair cells in the cochlea being permanently damaged. (4) Early indication of potential cochlear damage would afford an audiologist the opportunity to take preventive precautions before hearing loss actually manifests itself. Therefore an analysis was conducted during mid training to determine if there was any significant change in the distortion product otoacoustic emissions, with particular attention being paid at 3 kHz, since it was the only significant change in the preanalysis and postanalysis.

The breacher training resulted in:

* A slight decrease in the distortion product otoacoustic emission at 1828 Hz ([bar.X]=-0.1442. SD=4.99248). This decrease was not statistically significant: [t.sub.52]=-0.208, P >.01, 2-tailed.

* A slight increase in the distortion product otoacoustic emission at 2016 Hz (bar.X]=0.1596, SD=6.44136). This increase was not statistically significant: [t.sub.52]=-0.179, P>.01, 2-tailed.

* A slight increase in the distortion product otoacoustic emission at 3047 Hz (bar.X] = 1.0981, SD=7.40986). This increase was not statistically significant: [t.sub.52]=1.069, P>.01, 2-tailed.

* A slight decrease in the distortion product otoacoustic emission at 4124 Hz (bar.X]=-0.7038, SD=6.31472). This decrease was not statistically significant: [t.sub.52]=-0.804, P>.01, 2-tailed.

While the distortion product otoacoustic emissions did not prove useful in this population for early identification of outer hair cell damage and hearing loss, the measures did match the permanent hearing threshold shifts at posttraining measurements. One explanation is that the hazardous noise was impulse in nature and could easily have induced instantaneous damage to the outer hair cells. This would account for the inability of the distortion product otoacoustic emissions measures to detect small amounts of outer hair cell damage as in the Bohne and Clark study. (2) Hazardous steady state noise exposure typically causes gradual degeneration of the cochlea and hazardous impulse noise can cause instantaneous damage.

AUDITORY DISCUSSION

Weiner and Ross (5) describe the resonant characteristics of the outer ear as boosting the sound pressure level of the frequencies between 2500 Hz and 3500 Hz. Donahue and Ohlin (6) 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. 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 (7) and Ward (8) 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. 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. (5) Ylikoski and Ylikoski (9) state that this movement causes damage to loci of the cochlea that are different from the stimulus frequencies. For broadband noise with equal energy in all bandwidths, the maximum threshold shift occurs between 3 kHz and 6 kHz. (8)

Hazardous noise exposure causes 2 types of cochlear hearing loss: temporary and permanent. Temporary threshold shifts occur from metabolic fatigue and tend to recover within 48 hours, but is relative to the length and intensity of exposure. Restoration of hearing thresholds after a temporary threshold shift are due to the restoration of depleted metabolites and neurotransmitters, a decrease of edema in the hair cells, and healing of micro tears in the structure of the cochlea. Permanent threshold shifts are a result of the swelling and deforming of outer hair cells and alterations in endoplasmic reticulum. (10) The cochlear hair cells detect displacement of the basilar membrane and are the weakest link in the transduction of sound energy through the cochlea. The more intense a sound gets, greater the amplitude of displacement of the basilar membrane and therefore more shearing force on the hair cells. Bohne and Harding (11) 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 delayed onset with respect to identification of threshold shifts with routine monitoring. Simply put, hearing loss is progressive after an acoustic assault and therefore the actual rate of hearing loss among the participants in this study is greatly underestimated, especially when other exposures during training or multiple combat tours are taken into account. Hamernik (12) 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. 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 number of servicemen and servicewomen on disability because of hearing damage will increase no less than 15% under current combat conditions and disability policies. (13) Even if a Warrior'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 military standards. Once a service member's speech reception threshold in the best ear is greater than 30 dB hearing level (measured with or without a 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 he or she cannot hear radio communication. Also, the pilot risks further hearing loss due to the hazardous noise of the helicopter. If the findings of the review board are negative, the service member is offered a medical discharge or a change to a job that does not involve hazardous noise exposure. Even if service members choose to change jobs rather than take a medical discharge, the organizational knowledge and technical experience goes with them.

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VESTIBULAR AFFECTS

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 whiteouts (snow) or brownouts (sand), may lead to greatly reduced visual cues. If the vestibular system of a pilot or driver 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 weighted down with a basic combat load. If a relationship between hearing loss and vestibular damage can be identified, the results could lead to further studies on possible audio-vestibular screenings.

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. Mcllwain (14) found that 65% of the 564 patients seen at the audiology clinic during this time were there because of blast injuries. 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 of a lengthy and expensive medical evacuation for a non--life-threatening injury. (14) Unfortunately the average number of monthly audiology evaluations reached 175 in 2007. (15) If Warriors are experiencing hearing loss, they may also be experiencing asymptomatic vestibular damage.

Limits of Stability Assessment

The Limits of Stability test quantifies the maximum distance a person can intentionally displace their center of gravity by leaning their body in a given direction without losing balance, stepping, or reaching for assistance. These movements are referred to as dynamic balance. The measured parameters are reaction time, center of gravity movement velocity, directional control, end point excursion, and maximum excursion. For each of 8 trials, the subject maintains the center of gravity standing on a balance platform that measures movement. The subject tracks his or her movement on a computer screen that displays movement with a cursor. On command, the subject must move the center of gravity as quickly and accurately as possible towards a second target located on the screen, and then holds that position as close to the target as possible. The subject is allowed up to 8 seconds to complete the trial. The reaction time is the time in seconds between the command to move and the subject's first movement. Movement velocity is the average speed of the center of gravity movement in degrees per second. Endpoint excursion is the distance of the first movement toward the designated target, expressed as a percentage of maximum limits of stability distance. The endpoint is considered the point at which the initial movement toward the target ceases. Maximum excursion is the maximum distance achieved during the trial. Directional control is a comparison of the amount of movement in the intended direction to the amount of extraneous movement. The ability of a subject to voluntarily move the center of gravity to positions within the limits of stability is fundamental to mobility tasks such as reaching for objects, transitioning from a seated to standing position, and walking. Reaction time delays are commonly associated with difficulties in cognitive processing, motor diseases, and traumatic brain injury. Reduced movement velocities are indicative of high-level central nervous system deficits such as Parkinson's disease, age-related disorders, and traumatic brain injury. Inability to reach targets in a single movements and poor directional control are indicators of motor-control abnormalities.

This test was conducted using a NeuroCom Basic Balance Master (NeuroCom, Clackamas, Oregon). In this assessment, the researcher entered each subject's height into the computer to determine foot placement on the platform. For each of the comprehensive outputs, a 3 (group) by 3 (time) analysis of variance was conducted. In this study, the limits of stability assessment was completed twice, in succession, during the pre- and postevaluations, and a single assessment of the 8 body weight shifts were performed at the interim evaluations due to time constraints. Data from the first trial of the pre- and postevaluations and the single trial from the interim evaluation are used in the following analyses, the results of which are presented in Table 3.

While there were 3 statistically significant outcomes, the results were overall clinically unremarkable. Six subjects showed abnormal responses to limits of stability assessment. Three of these subjects scored outside normal limits for reaction time in both interim and postevaluations, as compared to preevaluation. Two subjects were outside the normal range for endpoint maximum excursion at preevaluation, and one for endpoint maximum excursion at postevaluation. Abnormal limits of stability testing are indicative of a possible functional balance deficit. Specifically, slower reaction time may indicate a central processing problem in which a person may be able to recognize a target, but have difficulty quickly moving toward the target. Deficiencies in directional control may indicate a person has difficulty maintaining their balance once they reach the target. A deficit in endpoint maximum excursion indicates that a subject had difficulty transferring his center of gravity toward the outer edge of the individual base of support, affecting gait and stance.

The modified Clinical Test of Sensory Intergration on Balance provides an objective measure of postural control and was measured with the Neurocom Basic Balance Master system. When performing this evaluation, the subjects stood with feet positioned on a set of reference marks on the force plate platform. The lateral malleolus of each foot was positioned relative to an indicator line and the outside edge of each foot was aligned perpendicular to the anterior-posterior center line. This foot position was used for 2 tests: firm surface with eyes open (to evaluate the visual contribution to balance), and firm surface with eyes closed (an evaluation of the vestibular balance component). A 4- inch thick foam pad with similar markings as the force plate platform was then placed on the force plate platform and the feet were positioned similarly. Two tests were performed with the foam pad: foam surface with eyes open (to evaluate the cognitive component of balance), and foam surface with eyes closed (an evaluation of the proprioceptive aspect of balance). The subject was instructed to stand quietly with his arms at his side for approximately 30 seconds (3 trials for 10 seconds each) for each of the 4 conditions. Sway in degrees per second were measured for each trial. All subjects completed the 4 tests for the pre- and postevaluations. An interim evaluation was completed for the subjects of the September cohort, but only the foam surface eyes-open and foam surface eyes-closed conditions were conducted at interim evaluation for the June cohort. The June cohort did not participate in the interim evaluation firm surface eyes-open and firm surface eyes-closed tests due to time constraints.

For the comprehensive score, a 3 (group) by 3 (time) analysis of variance showed significant results for the interaction effect [F.sub.2,70]=2.69, P =.04, [[eta].sup.2]=0.13, Power=0.72, main effect for group [F.sub.2,35]=4.46, P=.02, [[eta].sup.2]=0.20, Power=0.73, and main effect for time [F.sub.2,70]=23.95, P <.01, [[eta].sup.2]=0.41, Power=1.00. Follow-up analyses showed that the groups were equivalent at the pre-assessments [F.sub.2,35] = 1.81, P=.18 and were significantly different at the interim [F.sub.2,35]=4.64, P=.02 and at postevaluations [F.sub.2,35]=3.53, P=.04 with instructors and controls showing significantly poorer performance than the students at the interim and post-evaluations. This finding indicates that the breacher instructors did not experience performance decrements any greater than that associated with the control group.

To investigate the influence of breacher training on static balance, a series of t tests were performed to evaluate all subjects exposed to the breacher training environment (instructors and students) between pre-and postevaluations.

* There was no change in firm surface eyes-open condition between pre- and posttraining ([bar.X]=0.00, SD=0.01). There was no statistical significance [t.sub.31]=0.27, P >.01, 2-tailed.

* There was a slight increase in the firm surface eyes-closed condition between pre- and post-training ([bar.X]=0.01, SD=0.07). There was no statistical significance [t.sub.31] =-0.23, P>.01, 2-tailed.

* There was a slight decrease in the foam eyes-open condition between pre- and post-training ([bar.X]= -0.03, SD=-0.02). There was no statistical significance [t.sub.31] =2.5, P>.02, 2-tailed.

* There was a decrease in the foam eyes-closed condition between pre- and post-training ([bar.X]= - 0.33, SD=0.04). This decrease was statistically significant [t.sub.31]=4.25, P<.01, 2-tailed.

* There was a decrease in the comprehensive condition between pre- and post-training ([bar.X]= -0.10, SD=0.02). This decrease was statistically significant [t.sub.31]=3.36, P<.01, 2-tailed.

While there were 2 statistically significant variables in this assessment, overall the findings were clinically unremarkable.

Dynamic Visual Acuity Test

The dynamic visual acuity test is an assessment of the ability of a person to accurately identify an object that changes in size and orientation during head movement at predetermined velocities in degrees per second. It measures impairment and quantifies the impact of vestibular ocular reflex system injury or pathology on a subject's ability to maintain visual acuity while moving. In normal individuals, losses in visual acuity are minimized during head movements because the vestibular ocular reflex system maintains the direction of gaze on an external target by moving the eyes in the opposite direction of the head movement. When the vestibular ocular reflex system is injured, visual acuity degrades during head movements. The output of this evaluation is the acuity of vision while moving the head. The results are expressed in LogMAR units. LogMAR is a scale that is expressed as the logarithm of the minimum angle of resolution. This test requires rhythmic head movements in left to right and down to up planes. Once the head completes 3 consecutive cycles of movements, an optotype in the shape of the capital letter "E" is immediately presented to the subject on a computer monitor. The optotype is presented for 40 milliseconds. The size and orientation of the optotype is manipulated from trial to trial by the Neurocom software which is determined by each individual's static visual acuity. The subject responds by saying the orientation of the optotype and the researcher documents the response.

To assess time and group effects for movement plane, a 2 (head movement) by 2 (time) by 3 (group) analysis of variance (ANOVA) was performed for left and right; pre- and postevaluations; between the instructors, students, and controls. Table 4 presents the ANOVA results conducted for the horizontal plane. Two effects emerged significant. The head movement group was significant, with instructors and students showing significantly less performance degradation from pre- to postevaluation than the controls for both the left and right movement direction. A time effect was significant between pre- to postevaluation performances of the control group.

To assess time and group effects for the vertical movement plane, a 2 (head movement) by 2 (time) by 3 (group) ANOVA was performed for down and up; pre- and postevaluations; between the instructors, students, and controls. Table 5 displays the results of the ANOVA performed for vertical movement. The main effect for head movement emerged significant, dynamic visual acuity was significantly better in the up movement than the down movement direction.

While there were some statistically significant findings in this test, the overall results for perception time, gaze stabilization, and dynamic visual acuity testing was clinically remarkable.

VESTIBULAR DISCUSSION

The vestibular system may also be damaged by hazardous noise due to its close proximity and similarity in cell structure to the cochlea. (16) Warriors 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 (17) did find that symmetric noise-induced hearing loss is correlated with symmetric peripheral vestibular system damage. These results were corroborated by M. E. Hill, AuD, and D. S. Mcllwain, AuD (unpublished data, 2006). It is possible to be unaware of a vestibular deficit in conjunction with acoustic trauma because of the complex relationship between the central nervous system 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 central nervous system relies heavily on information from vision and proprioception to make up for the lack of neural firing from the balance center to compensate. The central nervous system 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. If studies such as the one presented in this article can aid in determination of how much hazardous noise effects the peripheral vestibular system, the development of screening tools may be of great importance.

CONCLUSION

In an instant, a Warrior can become "walking wounded." These types of injuries can affect the lives of our service members and their families. It also places them at risk for further injury as well as put others at risk due to decreased job performance. Research such as this are important to understanding the invisible injuries our Warriors are experiencing, such as hearing loss and mild traumatic brain injury, and is even more tantamount in developing prevention strategies and treatments.

Hearing loss was statistically and clinically significant in this study, whereas the vestibular findings were overall unremarkable. Therefore, hearing conservation practices should be reviewed by a military audiologist to determine a proper solution to prevent hearing loss in this course. It must be noted that the pressure measurements, head orientation, and air sampling are not discussed in this article, but did show some potentially serious exposures that may cause health affects over time and are addressed in other publications. It must also be noted that long term affects of this type of training were not the object of this research protocol. As with many research initiatives involving new scientific territories, some of our research questions were answered and many new ones were discovered. Therefore, a new breacher injury research protocol has been developed based on lessons learned and is currently in progress.

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ACKNOWLEDGEMENTS

The authors thank the Marine Corps Weapons Training Battalion, Quantico, VA; CDR Walter Carr, Defense Advanced Research Projects Agency; Applied Research Associates, Denver, CO; and the US Army Aeromedical Research Laboratory, Fort Rucker, AL, for their cooperation and assistance in the conduct of this study.

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(12.) 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.

(13.) GAO Findings and Recommendations Regarding DoD and VA Disability Systems. Washington, DC: US Government Accountability Office; May 25, 2007. Report GAO-07-906R Military and VA Disability Systems. Available at: http:// www.gao.gov/new.items/d07906r.pdf. Accessed April 22, 2011.

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(15.) Merkley J. Update on Army audiology in Iraq. Paper presented at: 11th Annual Force Health Protection Conference; August 9-15, 2008; Albuquerque, NM.

(16.) Golz A, Westerman S, Westerman L, et al. The effects of noise on the vestibular system. Am J Otolaryngol. 2001;22(3):190-196.

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* Box plots provide a vertical view of the data in percentiles. The boundaries of the box indicate the 25th percentile and the 75th percentile. The length of the box represents the difference between the 25th and 75th percentiles. The horizontal line inside the box represents the median. The lines drawn from the ends of the box show the largest and smallest values that are not outliers. Outlier and extreme data points are labeled as "o" (outlier) and "*" (extreme). The outliers are cases with the values between 1.5 and 3 box-lengths from the 75th percentile or 25th percentile. The extreme values are cases with the values more than 3 box-lengths from the 75th percentile or 25th percentile.

Paul St. Onge, PhD

MAJ David S. McIlwain, MS, USA

Melinda E. Hill, AuD

Timothy J Walilko, PhD

LTC(P) Lynette B. Bardolf, MS, USA

Dr St. Onge is a Principal Investigator, US Army Aeromedical Research Laboratory, Fort Rucker, Alabama.

MAJ McIlwain is the Otolaryngology Program Director, Medical Education and Training Campus, Fort Sam Houston, Texas.

Dr Hill is a Principal Investigator, US Army Aeromedical Research Laboratory, Fort Rucker, Alabama.

Dr Walilko is a Biomedical Engineer, Applied Research Associates, Denver, Colorado.

LTC(P) Bardolf is Clinical and Research Audiologist, Lyster Army Health Clinic and US Army Aeromedical Research Laboratory, Fort Rucker, Alabama.

Dr Chancey is a Principal Investigator, US Army Aeromedical Research Laboratory, Fort Rucker, Alabama.
Table 1. Categories Investigated and their Corresponding Variables

   Areas of                    Evaluation Factors
Investigation

Pressure        2 free field or interior overpressure measurements
                28 breacher overpressure measurements
                2 gauges in the helmet, 2 gauges on the vest

Head            Yaw
  Orientation   Pitch
                Roll

Air Sampling    Lead
                Copper

Auditory        Immittance
                Acoustic reflexes
                Thresholds
                Distortion product otoacoustic emissions

Vestibular      Dynamic visual acuity test
                Limits of stability
                Modified clinical test of sensory integration on
                  balance

Table 2. Distortion Product
Otoacoustic Emissions Results. *

        Parameter            Setting

Frequency begin (Hz)            1800
Frequency end (Hz)              4300
F2/F1 ratio                     1.22
Points per octave                 30
L1 level dB                       65
L2 level dB                       45
Minimum DP amplitude (dB)         -5
Noise floor (dB)                 -17
S/N ratio (dB)                     8
Point time limit (seconds)        20
Sample size                     1024
No. of tests                       1
Minimum No. of samples            50

* F1 indicates Frequency 1; F2, Frequency
2; L1, the amplitude of F1; L2 the
amplitude of F2; DP, distortion product;
and S/N, signal to noise.

Table 3. Analysis of Limits of Stability

Comprehensive
   Output         Effect         df         F          P

Reaction time   Time           2, 70       0.39       0.68
                Group          2, 34       0.64       0.53
                Interaction    2, 70       0.86       0.49

Movement        Time           2, 70       1.25       0.29
velocity        Group          2, 34       0.38       0.38
                Interaction    2, 70       2.48       0.10

Endpoint        Time           7, 70       6.77      <0.01
excursion       Group          2, 34       0.56       0.69
                Interaction    2, 70       2.42       0.10

Maximum         Time           2, 70       7.19      <0.00
excursion       Group          2, 34       1.89       0.12
                Interaction    2, 70       4.29       0.02

Directional     Time           2, 70       4.42      <0.01
control         Group          2, 34       1.63       0.18
                Interaction    2, 70       0.31       0.73

Comprehensive
   Output         Effect      [[eta].sup.2]    Power

Reaction time   Time              0.01          0.11
                Group             0.04          0.15
                Interaction       0.05          0.26

Movement        Time              0.03          0.26
velocity        Group             0.02          0.26
                Interaction       0.12          0.47

Endpoint        Time              0.16          0.91
excursion       Group             0.03          0.18
                Interaction       0.12          0.46

Maximum         Time              0.17          0.92
excursion       Group             0.10          0.54
                Interaction       0.20          0.71

Directional     Time              0.11          0.74
control         Group             0.09          0.48
                Interaction       0.02          0.10

Table 4. Analysis of Variance of the Dynamic
Visual Acuity Test in the Horizontal Plane

         Effect              F        P      [[eta].sub.2]    Power

Head movement               3.39     0.07        0.09          0.43
Head movement group         4.69     0.02        0.22          0.75
Time                        5.30     0.03        0.14          0.61
Time group                  0.86     0.43        0.05          0.19
Head movement time          0.43     0.52        0.01          0.10
Head movement time group    0.22     0.80        0.01          0.74
Group                       0.03     0.97        0.00          0.05

Table 5. Analysis of Variance of the Dynamic
Visual Acuity Test in the Vertical Plane

         Effect               F         P      [[eta].sup.2]    Power

Head movement               6.47      0.02         0.16         0.69
Head movement group         2.12      0.14         0.11         0.40
Time                        0.77      0.39         0.02         0.14
Time group                  0.25      0.78         0.01         0.09
Head movement time          1.91      0.18         0.05         0.27
Head movement time group    1.43      0.25         0.08         0.28
Group                       0.16      0.85         0.01         0.07
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Author:Onge, Paul St.; McIlwain, David S.; Hill, Melinda E.; Walilko, Timothy J.; Bardolf, Lynette B.
Publication:U.S. Army Medical Department Journal
Article Type:Report
Geographic Code:1USA
Date:Jul 1, 2011
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