Blast Injury of the Ear: Clinical Update from the Global War on Terror

Military Medicine,  Jul 2007  by Cave, Kara M,  Cornish, Elizabeth M,  Chandler, David W

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The purpose of this study was to describe the effects of blast exposure on hearing status. This study retrospectively analyzed hearing thresholds and otologic complaints for >250 patients with blast-related injuries from the global war on terror. Of patients who received full diagnostic evaluations, 32% reported a history of tympanic membrane perforation, 49% experienced tinnitus, 26% reported otalgia (ear pain), and 15% reported dizziness. Expected hearing thresholds were computed by applying age-correction factors to hearing tests performed earlier in the service members' careers and before their most recent deployment. Expected hearing thresholds were significantly better than actual postdeployment thresholds, indicating that significant changes occurred in the patients' hearing that could not be accounted for by age. Results from this study underline the need for documentation of preand postdeployment hearing tests and prompt otologic evaluation for the blast-exposed population.

Introduction

rphls study details the otologic and audiologic consequences J. of blast exposure in those who served during the global war on terror. Current rates of blast exposure have contributed to military hazardous noise reaching the greatest levels in >30 years.1 Heifer et al.2 reported that soldiers who received hearing tests associated with postdeployment health care visits were 76 times more likely to have permanent worsening in their hearing, compared with those who had nondeployment-related visits. Gondusky and Reiter3 found that, among Marines in the 1st Light Armored Reconnaissance Battalion during Operation Iraqi Freedom (OIF) who were treated for a single injury, otologic trauma was the most common injury. From 2000 to 2005, the Department of Veterans Affairs reported a more than twofold increase in claims for hearing loss and tinnitus. In 2005, tinnitus and hearing loss were the top two awarded disabilities, with annual compensation for both disorders totaling over $1 billion.4 Because otologic injury can result in reduced hearing and can compromise a warfighter's situational awareness immediately and permanently, the auditory effects of blast are of particular interest to military health care providers. Hearing loss can hamper a service member's ability to meet mission requirements, remain in his or her military occupational specialty, and maintain fitness for duty, which must be assessed by the military health care provider.

A blast is generally the energy that results when solids or liquids are rapidly converted into gas.5 Gas molecules are rapidly heated and travel faster than the speed of sound, In a highly pressurized form.6 The pressurized gas fills the same volume as the original liquid or solid. The area of high pressure expands into the surrounding medium (i.e., air), creating a peak amount of overpressure, known as a shock wave. The shock wave is followed by a drop in atmospheric pressure, known as underpressurization, creating a superheated blast wind.7 Damage resulting from the detonation of explosive devices is likely to occur inside enclosed spaces and within rigid fixed structures, which create reverberations of the initial wave.

Injuries resulting from blasts are categorized into four groups, namely, primary, secondary, tertiary, and quaternary. Primary blast injuries are caused by over- or underpressurization from the blast and most commonly affect air-filled and fluid-encased organs within the body, such as the lungs.7·8 Primary injury to the lungs can cause air emboli, which can restrict blood flow to the brain and spinal cord, middle ear, and gastrointestinal tract.8 secondary injuries occur from flying debris, including bomb fragments, and can affect any part of the body. Tertiary injuries are the result of the individual being thrown by the blast pressure wave; this results in injuries such as fractures, blunt trauma injury, and amputations. Quaternary injuries include all blast injuries not described in the aforementioned types, including inhalation of toxic fumes, crush injuries, and burns.

Otologic injury is typically considered a primary blast injury. The overpressurization of air molecules and the resultant impulse noise (characterized by excessive peak pressure levels) can cause instantaneous sensorineural, conductive, or mixed hearing losses.9 The tympanic membrane (TM) ruptures in ~50% of adult ears. Other middle ear damage, such as disarticulation of the ossicular chain or fracture of the ossicles, can also result from blast exposure.10

Within the cochlea, the basilar membrane (BM) is the structure that is most vulnerable to the effects of acoustic trauma." Because of the extreme force exerted on the BM, blast waves can tear the inner and outer hair cells away from their support cells. This can rupture the reticular lamina that connects the hair cells to supporting structures, leading to toxicity and death of the hair cells. The hearing loss that ensues from the anatomic damage to the inner ear can be temporary or permanent and occurs most often in the 2000- to 8000-Hz region along the BM.12 Reported rates of permanent noise-induced hearing loss vary from 35% to 54% of blast injuries. 1043ThC purpose of this report is to describe the auditory consequences of blast exposure. The present study analyzes differences in pre- and postdeployment hearing levels, accounting for the effects of age, to determine whether blast exposure is related to significant changes in hearing.