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In addition, recent data show that a blast wave causes the skull to dynamically deform, which creates localized regions of high and low pressure, and large pressure gradients that sweep through the brain. Several mechanisms have been proposed, including bulk acceleration of the head, transmission of loads through orifices in the skull and compression of the thorax, which generates a vascular surge to the brain. Shock tube experiments in animals confirm that blast pressures, without associated cranial impacts, can cause TBI. The mechanisms that are responsible for primary blast injury to the brain are being increasingly well understood. The reader looking for more details in this area is referred to excellent recent reviews on this topic.

However, those aspects of the topic that specifically pertain to TBI are briefly discussed here. The physical characterization of blast injury has been addressed in detail in previous publications and is well covered elsewhere in this special issue, and will not be discussed in detail. In brief, the mechanical insult from blast injury is classified as either primary, secondary, tertiary or quaternary. This has risen to prominence in the current conflicts, and represents a specific form of primary injury, with sufficiently different physical attributes (and possibly biological consequences) to be classified separately. This alternating stretch and compression of the tissues adds substantially to the tissue damage in a wound from a high-velocity missile and is particularly pertinent in penetrating brain trauma. The cavity reforms and collapses several more times with rapidly diminishing amplitude until all the energy has been dissipated. As the bullet passes, subatmospheric pressure enlarges the cavity, then, within a few microseconds, the cavity begins to collapse as a result of pressure and tissue recoil. A transient water vapour-filled cavity develops around the bullet and its track, which may be many times the diameter of the bullet. Cavitation, or cavity formation along the bullet track, results from the compression and acceleration of tissue in a direction forward and laterally away from the track of the bullet or other missile. Shock waves are generated by the compression of tissues that lie ahead of the bullet and are an effect limited to high-velocity missiles (generally exceeding 2500 feet s −1).

Angle of impact, yaw and tumbling of the bullet will also affect the extent of damage caused. Laceration and crushing are generated by the projectile displacing the tissues in its track. There are three mechanisms of tissue damage owing to bullets: laceration and crushing, shock waves and cavitation. Where m is the mass and v the velocity, so although the KE of the projectile varies linearly with weight, it increases exponentially with velocity. TBI is rarely an isolated finding in this setting, and persistent post-concussive symptoms are commonly associated with post-traumatic stress disorder and chronic pain, a constellation of findings that has been called the polytrauma clinical triad. Visual and/or auditory deficits are common, and there is a significant risk of post-traumatic epilepsy. In addition, severe early oedema has led to increasing use of decompressive craniectomy, and blast TBI may be associated with a higher incidence of vasospasm and pseudoaneurysm formation. Clinical care has much in common with civilian TBI, but intracranial pressure monitoring is not always available, and protocols need to be modified to take account of this. The efficient logistics of clinical care delivery in the field may have a role in optimizing outcome. Assessment involves schemes that are common in civilcian practice but, in common with civilian TBI, takes little account of information available from modern imaging (particularly diffusion tensor magnetic resonance imaging) and emerging biomarkers. Indeed, blast-induced mild TBI has been referred to as the signature injury of the conflicts in Iraq and Afghanistan. There is an increasing incidence of military traumatic brain injury (TBI), and similar injuries are seen in civilians in war zones or terrorist incidents.