Blast Injuries and Blast-Induced Neurotrauma: Overview of Pathophysiology and Experimental Knowledge Models and Findings

Explosions are physical phenomena that result in the sudden release of energy; they may be chemical, nuclear, or mechanical. This process results in a near-instantaneous pressure rise above atmospheric pressure. The positive pressure rise (“overpressure”) compresses the surrounding medium (air or water) and results in the propagation of a blast wave, which extends outward from the explosion in a radial fashion. As the front or leading edge of the blast wave expands, the positive phase is followed by a decrease in pressure and the development of a negative wave (“underpressure”) before subsequently returning to baseline. Figure 45.1 shows an idealized form of a shock wave (Friedlander wave) (Friedlander, 1955) generated by a spherical, uncased explosive in the air in free field conditions. The extent of damage from the blast wave mainly depends on five factors: (1) the peak of the initial positive-pressure wave (an overpressure of 690–1,724 kPa, for example, 100–250 psi, is considered potentially lethal) (Champion et al., 2009); (2) the duration of overpressure; (3) the medium of explosion; (4) the distance from the incident blast wave; and (5) the degree of focusing because of a confined area or walls. Intensity of an explosion pressure wave declines with the cubed root of the distance from the explosion. Thus, a person 3 m (10 ft) from an explosion experiences nine times more overpressure than a person 6 m (20 ft) away. Additionally, explosions near or within hard solid surfaces can be amplified two to nine times because of shock wave reflection (Rice and Heck, 2000). Indeed, it was observed that victims positioned between a blast and a building often suffer injuries two to three times the degree of the injury of a person in an open space. People exposed to explosion rarely experience the idealized pressure-wave form. Even in open-field conditions, the blast wave reflects from the ground, generating reflective waves that interact with the primary wave, thus changing its characteristics. In a closed environment (such as a building, an urban setting, or a vehicle), the blast wave interacts with the surrounding structures and creates multiple wave reflections, which, interacting with the primary wave and between each other, generate a complex wave (Ben-Dor et al., 2001; Mainiero and Sapko, 1996).Blast injuries are characterized by interwoven mechanisms of systemic, local, and cerebral responses to blast exposure (Cernak, 2010). When a blast generated by explosion strikes a living body, part of the shock wave is reflected and another fraction is absorbed becoming a tissue-transmitted shock wave. The transferred kinetic energy causes low-frequency stress waves that accelerate a medium from its resting state, leading to rapid physical movement, displacement, deformation, or rupture of the medium (Clemedson, 1956; Clemedson and Criborn, 1955). Thus, a militarily relevant blast injury model should be able to capture and measure these phenomena based on sufficient knowledge of shock wave physics, the characteristics of the injurious environment generated by an explosion, and the clinical manifestations and sequelae of the injuries. The purpose of this chapter is to outline the pathophysiology of blast-body/blast-brain interactions and to summarize the scientific evidence to date for the selection of appropriate experimental models for characterizing and understanding these interactions.