Summer - United States Special Operations Command

The development of chronic NIHL progressesthrough two phases. A brief hearing loss, more commonlyreferred to as a temporary threshold shift (TTS),characterizes the first stage. It occurs after noise exposureand completely resolves after a period of rest.Often reported as auditory fatigue, most studies indicatethat it is associated with no sensory cell damage orminimal, reversible cell changes. 14,18 Eventually, afterrepeated exposure to noises intense enough to produceTTS, a permanent threshold shift (PTS) will occur.This is an irreversible increase in hearing thresholdsand defines the second stage of chronic NIHL. At thispoint, there has been irreversible hair cell damage. 5,14,18In contrast to chronic NIHL, acoustic traumarefers to a sudden permanent hearing loss caused by asingle exposure to an intense sound. It occurs whenexcessive sound energy strikes the inner ear. Exposureto noise from firearm use during military service isprobably the most frequent etiology of acute acoustictrauma worldwide; therefore, it may be regarded as aprofessional disease in military populations. 19 Thesound pressure levels capable of causing acoustictrauma vary between individuals but average around130-140dB. 14 The hearing loss is sudden, sometimespainful, and is often followed by a new onset of tinnitus.For the vast majority of patients, tinnitus presentsas the most annoying symptom, with the risk for permanenttinnitus being considered more critical for thepatient than any degree of hearing loss resulting fromacoustic trauma. 19 Although the audiogram may showthe typical 3000-6000Hz sensorineural notch seen withchronic NIHL, down-sloping or flat audiograms thataffect a broad range of frequencies are more common.2,14,20 Direct mechanical injury to the sensory cellsof the cochlea is thought to be the mechanism of injuryin acoustic trauma.Noise exposure and NIHL are the most commoncause of tinnitus. 2,5,6 The relationship betweennoise exposure, NIHL and tinnitus has been addressedin a number of articles. A review of these studies waspresented by Axelsson & Barrenas, 1991, and it wasfound that noise exposure and NIHL were by far themost common cause of tinnitus; if “acoustic trauma”was included, at least one-in-three cases were causedby noise. 6 Tinnitus may occur following a single exposureto high-intensity impulse/impact noise (a shortburst of acoustic energy which can either be a singleburst or multiple bursts of energy), long-term exposureto repetitive impulses, long-term exposure to continuousnoise, or exposure to a combination of impulsesand continuous noise. 2,4,6THE “IMPACT” OF NOISE ON THE MILITARYA staggering number of Soldiers and Marinescaught in roadside bombings and firefights in Iraq andAfghanistan are coming home with ringing in theirears. High rates of tinnitus among patients exposed togunfire and explosive detonations suggest that impulse/impactnoise is likely to precipitate tinnitus associatedwith acoustic trauma, excessive noiseexposure, and NIHL. 2,4,6,10,12 According to researchpublished in the December 2005 issue of AmericanJournal of Audiology (AJA), Soldiers sent to battlezones are over 50 times more likely to suffer NIHL lossand/or tinnitus than Soldiers who do not deploy. 21 Accordingto a report released in 2007 by the House AppropriationsSubcommittee on Military Quality of Life,as a result of ongoing combat operations, one in threepost-deploying Soldiers report acute acoustic traumaand one in four report hearing loss and/or hearing complaintsto include tinnitus. 12From World War II and well through the VietnamWar, hearing damage has been a leading disability.According to the Department of Veterans Affairs,hearing damage is the number one disability in the Waron Terror, with some experts predicting the true tollcould take decades to become clear. 13 According to theAmerican Tinnitus Association (ATA), more will bespent on veterans’ disability compensation for tinnitusand other hearing conditions over the coming yearsthan for any other medical injuries from the Iraq andAfghan wars. 4 Between 2000 and 2005, the number ofveterans with tinnitus disabilities more than doubledand the amount paid to veterans with tinnitus disabilitieswent up more than two-and-a-half times. 4,22Presently, tinnitus is the most prevalent disabilityamong new cases added to Veterans Affairs numbers;nearly 70,000 of the more than 1.3 million troops whohave served in Afghanistan and Iraq are collecting disabilityfor tinnitus. 13,23 In fact, recent studies demonstratedthat 49-50% of all Soldiers exposed to explosiveblasts in Iraq and Afghanistan had tinnitus and 60%had tinnitus, often related to hearing loss. 4,13,22,24 Thenumber of servicemembers on disability because ofhearing damage is expected to grow 18% a year, withpayments totaling $1.1 billion annually by 2011. 4,13The economic consequences to the military forhearing impairment, to include tinnitus, include losttime and decreased productivity, loss of qualifiedworkers through medical disqualification, military disabilitysettlements, retraining, and expenses related tomedical treatment such as hearing aids and audiometrictesting. 10 While the economic consequences are significant,the military implications in a combat zone canbe deadly.A study published in the Army RD&A Bulletinin 1990, concluded that those with hearing impairmentswere 36% more likely to hear the wrong command, and30% were less likely to correctly identify their target. 4Additionally, it was noted that Soldiers with hearingimpairments only hit the enemy target 41% of the time,while Soldiers without hearing impairments hit theenemy target 94% of the time. Those with hearing impairmentswere 8% more likely to take the wrong tar-Tinnitus, a Military Epidemic:Is Hyperbaric Oxygen Therapy the Answer?35