The projectile directly crushes tissue (176). As a projectile passes through the body, tissue is temporarily (in the order of 10 ms) accelerated in a radial direction (16,176). The maximum radial extent of the "temporary cavity" is dependent on the amount of energy released from the projectile and the degree of tissue elasticity (16,142,176-178). The energy of a projectile increases exponentially based on its velocity (kinetic energy = mv2/2 where m is the projectile's mass and v is the velocity [16,176-178]). Civilian hunting rifles have velocities ranging from 600 to 1200 m/s (2000-4000 ft/s), civilian handguns less than
300 m/s (1000 ft/s), and a .22-caliber rifle 300 to 380 m/s (1000-1250 ft/s ). A bullet from a hunting rifle generates a large temporary cavity (Fig. 39; ref. 179). The temporary cavity collapses into a "permanent cavity," i.e., a grossly visible wound track generally wider than the bullet diameter (Fig. 42; refs. 16 and 177). The size and shape of a permanent cavity is not necessarily dependent on the caliber of the weapon (177). If a low-velocity firearm (e.g., handgun) is involved, tissue elasticity of tissue can cause a wound track to be smaller in diameter than the projectile (16). The size of the temporary and permanent cavities is determined by the nature of the tissue. Less elastic tissue—e.g., brain as compared with muscle and skin—undergoes a larger area of cavitation, resulting in a wider wound track (16,176). Energy dissipation is furthered when the bullet deforms and deviates along its path in the body (16,176). This can cause a track to appear more irregular (177). Bullet fragmentation is enhanced by contact with bone (176). The track may be wider close to the entrance site if the tissue has been lacerated by bone fragments from the entrance fracture or wider closer to the exit site when tumbling of the bullet has occurred (142).
Despite the localized area of tissue destruction, effects are actually more widespread grossly and microscopically, even when low-energy ammunition is involved (16,177, 180,181). For example, in cases of low-velocity fatal gunshot wounds to the head, the usual scenario is respiratory arrest, followed by cardiac arrest, within seconds to minutes (142,180,182). The permanent track in the brain appears as a line of hemorrhage and necrosis, associated with hyperchromatic shrinkage of neurons and loss of glial fibrillary astro-cytic protein and ^-amyloid precursor protein immunostaining within 2 to 4 mm (about
Fig. 33. Suicidal gunshot wound (head). Blood spatter on dorsum of right hand (firing hand).
0.125 in.) of the track (161,183). The rigid confines of the cranium enhance pressure created by temporary cavity formation (129,176). During the passage of the projectile in the brain, distant effects outside the visible wound track occur. Pressure marks (grooves) are observed on the uncal gyri and cerebellar tonsils (177). They are not necessarily caused by cerebral edema, although this has been observed within minutes of wounding (177). Brain contusions are observed in the vicinity of the entry (coup) and on the opposite side (contrecoup) of the brain, including the exit site (Fig. 43; ref. 177). Contusions may be seen on the base of the frontal, temporal, and occipital lobes, similar to blunt trauma injuries, and the cerebellar tonsils from contact against the base of the skull and falx tentorium, respectively (Fig. 44; ref. 176). Posterior fossa entrance wounds are associated with a laceration of the corpus callosum as it collides with the ventral margin of the falx. Contusions, indirect skull fractures (see Subheading 19.3.), and cerebral perivascular hemorrhages (petechiae) are signs of increased intracranial pressure, and the mechanism of death is brainstem dysfunction (176,177,182). Gross and microscopic examination can show basal ganglionic, brainstem, and cerebellar hemorrhages remote from the wound track (Fig. 44). Diffuse axonal injury at distant sites has been described (176,180). If the amount of energy dissipation by the projectile exceeds the elasticity of the organ, bursting occurs (16,179). Very destructive injuries are apparent. In contrast, the injuring capability of a very low-velocity projectile is simply the result of its penetration of a vital structure (16,72).
The assessment of firearm injuries includes the determination of how quickly an individual dies. Incapacitation is defined as the inability to perform complex and longer
lasting movements because of brain injury or massive blood loss (see Heading 14.; refs. 129 and 176). The duration of survival and level of consciousness influence whether purposeful activity was possible after the wound was inflicted (e.g., repeated weapon discharge in a case of multishot suicide; see Fig. 3 and refs. 55 and 109). The finding of two instantaneously lethal wounds in an apparent multishot suicide is suspicious of homicide, except when two guns are used (55,129).
The majority of firearm deaths are related to traumatic brain injury; for the period from 1989 to 1998, firearms were the leading cause of fatal traumatic brain injury in the United States (184). Injuries of certain areas of the brain (internal capsule, diencephalon, basal ganglia, cerebellum, brainstem, upper cervical cord) are considered immediately incapacitating and lethal (16,55,57,111,129,182,185). A higher survival rate is seen when brain injuries are limited to a single cerebral lobe compared with tracks crossing the mid-coronal and midsagittal planes (186). The sella turcica and the base of the anterior cranial fossa act as a horizontal bony barrier against radial tissue displacement and overpressure posteriorly from a frontal injury (182). Frontal lobe injury allows survival in some cases (47,67). The frontal lobes are areas of the brain not essential to purpose-
ful action or consciousness, but, depending on the caliber of the weapon, wounds at this site can be either instantaneously, rapidly, or not immediately lethal (111,129,182). Missiles, either crossing the midcoronal and midsagittal planes or present in the posterior cranial fossa, were lethal in all cases in one study (186). These sequelae occurred regardless of caliber (186).
Spinal gunshot wounds tend to fracture or shatter the vertebrae through which they pass, but incomplete involvement of the intervertebral ligaments maintains the stability of the vertebral column (187,188). The spinal cord is injured by the crushing and tearing effect of the primary track, and the effects on the spinal cord of the temporary cavity is less because the spinal cord is less tightly confined by the spinal canal. Vertebral artery injury, including propagation of bilateral vertebral artery thrombi to the basilar artery, can occur (189).
Marked secondary ischemic brain injury, brain swelling, and intracerebral hemorrhage, no different from that associated with severe blunt force injury, occurs in cases of gunshot wounds that survive for more than a few hours after the injury (142).
Bleeding from a cerebral venous sinus allows a short period of purposeful activity (182).
Wounds of the heart and aorta are rapidly lethal, allowing some voluntary movement for 10 to 15 s, although more prolonged activity has been observed (16,55, 105,129,145,176). When targets of "secondary" importance (e.g., liver, kidney, lung) are injured, bleeding is extensive, but the victim can be conscious and survive for a period of time (minutes) (129).
Nonlethal self-inflicted firearm injuries involving the lower face, mouth (palate), and frontal bone have been observed (16,22,111,182). Delayed morbidity and mortality caused by infection from firearm and air gun wounds have been described (Fig. 45; refs. 66, 70, 139, and 186).
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