Projectile and Explosive Injuries of the Central Nervous System

David Fowler

FIREARM INJURY

Biomechanics

Injuries caused by projectiles and explosives are largely kinetic energy injuries. There are certain exceptions to that statement, especially with regard to explosives, that will be described below. An injury is a disruption of the normal anatomy caused by energy to that site at a rate and intensity too great for the tissue or organ to adapt. Several basic laws of physics must be considered with regard to firearms and explosives. Energy cannot be created; however, it can be converted from one form to another, and there is always an equal and opposite reaction to any action.

When the trigger is pulled to discharge a firearm, gas pressure builds up rapidly in the chamber as the propellant burns. The bullet engages the rifling in the barrel and is propelled out of the firearm’s muzzle. The energy the bullet has is equal to the recoil energy of the firearm. The firearm is much heavier and thus has more inertia. It therefore accelerates toward the firer with less speed than the bullet traveling in the opposite direction. The firearm also has a larger surface area on the grip or butt to apply the energy to the firer’s body. The tissue at this site can adapt to the slower, larger object. The bullet, however, has a small frontal surface area and is traveling at a high speed, precluding the tissue from dispersing the energy, so injury occurs. A bullet is in effect a small, high-speed, blunt object.

There are many texts on internal ballistics (the bullet in the firearm) and external ballistics (the bullet in the air). This chapter will concentrate on terminal ballistics, which is the effect of a moving bullet in human tissue and organs.

Epidemiology

The head is the most common target of gunshot wounds, both in homicidal and suicidal shootings. A review of our experience at the Office of the Chief Medical Examiner in the State of Maryland in a 1-year period (2006) showed gunshot wounds as the cause of death in 679 cases (15.7% of all autopsies performed at the office). The manner of death was homicide in 441 cases (64.5% of gunshot wounds), suicide in 234 cases (35.5%), and undetermined in four cases (0.5%). No single case was certified as accidental. Age, sex, number of gunshot wounds, and number of gunshot wounds to the head, specified by manner of death, are listed in Table 9.1.

Neuropathology

The brain is encased in a nonexpandable, noncompressible osseous structure. Radial displacement forces from the bullet track expand and flatten the brain against the skull, with the potential for subarachnoid hemorrhage, cortical contusions, or orbital roof (contrecoup) fractures in locations away from the bullet path. Oehmichen et al. defined different zones of injury related to gunshot wounds of the brain. In concentric cylinders surrounding the permanent defect (permanent cavity) produced directly by the bullet, there was an immediately outer zone of glial, vessel, and fiber necrosis, surrounded by a zone of hemorrhage, and an outer zone of neuronal (cell bodies and axons) necrosis and degeneration.

Specific features to consider relating to gunshot wounds to the head are as follows:

The central nervous tissue lacks elasticity and offers little resistance to a penetrating projectile.
The brain reaction to tissue injury is characterized by bleeding and swelling. This sudden increase in intracranial volume, in a system with little margin for change, has potentially catastrophic repercussions, such as herniation and death.
Bone fragments produced at the site of the entrance wound may become secondary projectiles, with paths different from that of the bullet, causing additional injuries.
In the head, to reach the brain, the bullet must cross a relatively thick bone. The bullet may fragment or the bullet core and the jacket may separate and form multiple potential projectiles.
To retrieve ballistic evidence, most brains with gunshot wounds are sectioned fresh and cannot be preserved for a neuropathologic examination after fixation. Sectioning fresh a traumatized brain with bleeding and/or swelling is challenging, and subtle injuries may be easily missed.
At close range, shotgun wounds to the head have devastating “explosive-like” effects, and the determination of entrance and exit wounds and wound paths may require reconstruction of the skull and soft tissue of the head.
The internal injuries produced by rifle wounds may be as destructive as those of close range shotgun wounds, and it may not be possible to map the actual bullet path in the brain as a result of potential brain extrusion, liquefaction, or fragmentation.
Small bullets (.22 caliber) seldom exit the head, but they may ricochet inside the cranial cavity and be found in the subdural space at a distance from the injury path in the brain.
Intraoral gunshot wounds, with the end of the barrel pointing midline with an upward direction, commonly produce transection of the upper brainstem, as the bullet enters the clivus and immediately encounters the cerebral peduncles or upper pons, and result in instantaneous death.
The brain does not regenerate after an injury, thus long-term morbidity includes permanent neurologic deficits. In addition, infection (abscesses are more common than meningitis) and posttraumatic epilepsy are relatively common sequelae in survivors of gunshot wounds to the head.

TABLE 9.1 Cases with Gunshot Wounds as the Cause of Death Seen at the State of Maryland Medical Examiner Office During the Year 2006 (unpublished review)

Characteristic			Total	Homicide	Suicide	Undetermined^a
No. of cases			679	441	234	4
	Total shootings, %		100	65	34.5	0.5
	Total autopsies for each category, %		15.7	76.2	65.2	0.5
Sex
	Men (%)		624 (92)	406 (92)	214 (91.5)	4 (100)
	Women (%)		55 (8)	35 (8)	20 (8.5)	0 (0)
Age
	Mean (SD)		36.5 (17.6)	29.4 (11.2)	49.8 (19.5)	34.5 (19.3)
	Range		0-91	0-76	12-91	12-59
No. of wounds (%)
	One		402 (59.2)	165 (37.4)	234 (100)	3 (75)
		To head	286 (71.1)	83 (50.3)	200 (85.5)^b	3 (100)
	Two		73 (10.8)	72 (16.3)	0	1 (25)
		At least one to head	37 (50.7)	37 (51.4)	0	1 (100)
	Multiple (3-22)		204 (30.0)	204 (46.3)	0	0
		At least one to head (1-7)	106 (26.4)	106 (52.0)	0	0
^aThe majority of drug deaths in the State of Maryland are classified as “undetermined” in manner, making this group a significant proportion of all autopsies. ^bFifty-eight were intraoral gunshot wounds.