Blast-related ocular trauma comprises a specialized subgroup blast injuries which cause penetrating and blunt force injuries to the eye and its structure. The incidence of ocular trauma due to blast forces has increased dramatically with the introduction of new explosives technology into modern warfare. The availability of these volatile materials, coupled with the tactics of contemporary terrorism, has caused a rise in the number of homemade bombs capable of extreme physical harm.
Military classification of improvised explosive devices
The United States Department of Defense classifies IEDs as explosive machines that are constructed exclusively (i.e., without mass production) and result in the direct physical harm of surrounding individuals. The use of these bombs by insurgents has been the number one cause of death and injury among Coalition soldiers since the start of Operation Iraqi Freedom in April 2003.Ramasamy A, Harrisson SE, Clasper JC, Stewart MP., Injuries from roadside improvised explosive devices. J Trauma 2008; 65(4):910-4. Detonation of the IED occurs remotely or as victim-induced mechanical disturbance.Weichel ED, Coyler MH. Combat ocular trauma and systemic injury. Curr Opin Ophthalmol 2008; 19(6):519-25.Wolf SJ, Bebarta MV, Bonnet CJ. Blast injuries. Lancet 2009; 374(9687):405-15. Further classification of IEDs falls under the mechanism of delivery – vehicle-based, boat-borne, animal-borne, suicide bomber – and the resultant effect upon detonation: Explosive: Bombs of this nature incorporate chemicals and substances that result in the formation of a large blast; may incorporate pyrotechnics. Often employs shrapnel to inflict harm by mechanical trauma. Incendiary: utilize highly exothermic chemical processes to initiate the rapid spread of fire and pyrotechnic damage Chemical: Bombs in this class include noxious chemical materials that may cause a patho-physiological response in individuals exposed to the blast area during and post-explosion. Biological: Much like the chemical-type, with the exception that biological bombs use vector-borne pathogens or other biohazardous materials to initiate a patho-physiological response in exposed individuals.United States Department of Defense., Joint Publication 1-02, Department of Defense Dictionary of Military and Associated Terms. 2008, amended 2011. 171.
Explosion physics
The discharge of a bomb is characterized by the near-instantaneous sublimation of solids or rapid vaporization of liquids into the gas phase. The amount of explosive materials used, the concentration and identity of secondary materials incorporated into bomb design, and the location and height of bomb placement all determine the magnitude of the explosion. The gas formed displaces the surrounding medium – usually air – and causes a dramatic increase in pressure that forms a characteristic blast wave, often referred to as the leading shock wave. Physically, this wave is characterized as a non-linear, discontinuous wave front that features infinite amplitude and accompanying acoustic pressure wave that may generate a pressure as high as 100MPa in a time as short as one microsecond. This peak pressure, or blast overpressure point, generates a positive pressure during blast wave propagation and results in the dispersal of positive pressure across the blast radius. This positive pressure phase is immediately followed by a period of negative pressure relative to pre-blast conditions; this phase may also account for injuries sustained during a blast.Leonardi ADC, Bir CA, Ritzel DV, VandeVord PJ. Intracranial pressure increases during exposure to a shock wave. J Neurotrauma. 2011. 2011. 28(1):85-94.Nakagawa A, Manley GT, Gean AD. Mechanisms of primary blast-induced traumatic brain injury: Insights from shock wave research. J Neurotrauma. 2010. Accepted for publication; available online at http://www.liebertonline.com/doi/pdfplus/10.1089/neu.2010.1442
Impact of explosion upon individuals in the blast radius The prevalence of mines in Operation Iraqi Freedom and Operation Enduring Freedom has made them the most frequent mechanism of injury behind the traumatic "signature" in modern warfare, blast-induced traumatic brain injury (bTBI). Whereas body armor has lowered the incidence of death due to collapse of gas-filled organs (the most frequent cause of blast-related deaths prior to Operation Desert Storm) healthcare providers must now develop methods for treating bTBI. Despite their frequency on the warfront, the home-produced nature of these mines makes classifying patient presentations difficult for military healthcare providers. The majority of lethal bTBIs reveal axonal shearing as the mechanism of fatality, with the greatest amount of nerve fiber and vascular shearing occurring in the frontal and temporal lobes.
Biophysicists have implicated acoustic impedance, or the ratio of acoustic pressure to particle velocity, as a factor contributing to blast damage in vivo. Wave transitions between tissues with significantly different acoustic impedances, particularly between the external environment and bone, causes focal mechanical damage as a result of wave energy dissipation. Current research has implicated the importance of a histological component in blast trauma; patients exposed to blast waves often present with elongation and/or splitting of cells due to the shear stress of a shockwave. This cellular damage often follows the direction of wave propagation. Patient distance from the epicenter, materials employed in the bomb design, and confinement of the bomb all determine the degree of trauma incurred by patients exposed to bombings. Additionally, skull size and geometry, the degree of tissue penetration by the wave, and a possible "lens effect" due to wave reflection upon incidence with the concave calvarium and/or dissipation in the gas-filled sinuses may further complicate wave transmission. Additionally, researchers have implicated both the auditory canals and the orbitals as potential routes for wave propagation into the central nervous systemNakagawa A, Manley GT, Gean AD. Mechanisms of primary blast-induced traumatic brain injury: Insights from shock wave research. J Neurotrauma. 2010. Accepted for publication; available online at http://www.liebertonline.com/doi/pdfplus/10.1089/neu.2010.1442.Chavko M, Watanabe T, Adeeb S, Lankasky J, Ahlers ST, McCarron RM. Relationship between orientation to a blast and pressure wave propagation inside the rat brain. Journal of Neuroscience Methods. 2010. 30(1):61-66.
Blast-related ocular trauma
Ocular trauma is the fourth most common injury sustained in military combat today. In a pool of 387 randomly selected soldiers injured by blast trauma in Operation Iraqi Freedom, 329 (89%) sustained ocular trauma. Emergency treatment of resulting injuries falls under the realm of emergency care and effective patient triage, often incorporating protocols for blunt and penetrating trauma. As a result, physicians have devised a concise algorithm for the treatment of patients with ocular injuries secondary to blast trauma.
Mechanism of injury Ocular trauma may result from primary blast exposure. Spallation forces arise as the blast wave displaces a dense medium across a less dense interface, and inertial forces may cause displacement of optical structures. Primary blast ocular trauma therefore comprises non-penetrating mechanical injuries such as hyphemas, ruptured globes, conjunctival hemorrhage, serous retinitis, and orbital fracture.Harlan JB, Pieramici DJ. Evaluation of patients with ocular trauma. Ophthalmol Clin North Am. 2002; 15(2):153-61. However, ocular trauma most commonly falls under the realm of secondary blast injuries, in which debris displaced by the blast overpressure and resultant blast wave causes physical trauma to the eye and/or orbital. Therefore, secondary blast ocular trauma is distinguished by penetrating- or blunt-force injury to any of structural component of the eye or orbital; open globe injuries, adnexal lacerations of the lacrimal system, eyelids, and eyebrows comprise the majority of injuries in this group.
Skull flexure Within the last two decades, researchers have reconsidered the role of the skull in bTBI. While it was originally considered that the skull remained static upon contact with the primary wave front, clinically significant skull flexure has been documented in vivo with rats exposed to blast waves and with model human heads exposed to blast conditions. In contact with a blast wave, the skull becomes elastic due to its deformable foundation – the external environment, the cerebrospinal fluid of the dura, and the brain itself. During a blast, the brain collides with the dynamic skull and rebounds in accordance with localized cranial pressure spikes. This trauma may account for the localized axonal injuries that characterize bTBI. Chavko et al. (2010) explored cranial position as a function of bTBI severity, finding that rats directly facing the blast wave front featured the highest intracranial amplitude and pressure duration periods (in comparison with rats perpendicular to the wave front and those facing away from the blast wave) Alessandra Dal Cengio Leonardi's group at Wayne State University expanded upon the skull flexure hypothesis in rat models, further correlating increased age and body mass to increases in intracranial pressure for rats in front-facing bTBI. Chavko's group remarked further on the role of Kevlar armor in fluid pressure damage to neurovasculature, finding that subcortical hemorrhage seen in bTBI patients has been linked to local pressurization rather than vascular hydrodynamics.Long JB, Bentley TL, Wessner KA, Cerone C, Sweeny S, Bauman RA. Blast overpressure in rats: Recreating a battlefield injury in the laboratory. J Neurotrauma. 2009. 26:827-40.
Assessment and treatment in the military setting The majority of blast-related ocular injuries occur in soldiers who present with other life-threatening injuries that require immediate intervention. Current Combat Support Hospital (CSH) protocol requires the surgical stabilization of any life-threatening injuries, as well as hemodynamic stability, prior to initial eye evaluation and surgical repair. Therefore, initiation of emergency ophthalmic care often occurs hours after injury. Initial examination by a military ophthalmologist begins with gross examination of each eye and orbital. 73-82% of all ocular injuries resulting from mine explosions are due to fragmentation of shrapnel upon detonation, so gross anatomical inspection by penlight may not rule out open globe injury. Harlan JB, Pieramici DJ. Evaluation of patients with ocular trauma. Ophthalmol Clin North Am. 2002; 15(2):153-61./ref> Computerized tomography (CT) may detect foreign matter and aid the clinician in determining the presence of an open-globe injury.
Closed globe injuries Current military standard employs the Birmingham Eye Trauma Terminology System (BETTS) and Ocular Trauma Classification Group to define and treat blast injuries. Trauma is further split into two distinct groups: closed globe injury and open globe trauma. Treatment of closed globe trauma begins with the division of the eye into zones, each with unique anatomical structures and injury patterns: Zone I: the conjunctiva and corneal surface; the most common injuries seen in this zone are foreign debris retention by the conjunctiva or corneal epithelium/stroma, as well as corneal abrasions. The Seidel test may be used to evaluate the status of the anterior chamber, thereby determining the presence of corneal perforation and pathological anterior chamber leakage.