3 Ballistics Factors That Impact Severity of Penetrating Brain Injury

Rachel Barezinsky was driving by a haunted house with her friends one night in late August 2006, when she was shot in the back of the head. The bullet went through the right side of Rachel’s brain, piercing through the four lobes that are responsible for movement, memory, emotion and impulse control, before settling in the left side of her brain.

When she arrived at Ohio State University Medical Center, her doctors gave her a 1% chance of survival. She made it through, despite all odds, but not without complications. After surgery, Rachel found that she was paralyzed on her left side, hooked up to a ventilator and felt confused.

While several weeks in rehab helped her with some memory problems, three years later, she suffered from a grand mal seizure. She can no longer remember the four months prior to the shooting, the shooting itself or anything two years after it.

But why are some penetrating brain injuries from firearms so much worse in some patients over others? Beyond factors such as age and firearm type, the ballistic characteristic of the penetrating object, most often bullets, are a key aspect of traumatic brain injury (TBI) patient outcome.

The severity of damage to the brain is often the result of the trajectories of the bullet and bone fragments through the brain, as well as changes in intracranial pressure (ICP) at the time of impact.

The ability of an object, such as a bullet, to penetrate the brain, depends on a variety of ballistic factors including projectile velocity, kinetic energy and the angle and location of entry, among several others.

Projectile Velocity

While ballistics terms “high-velocity” and “low-velocity” often have varying definitions, according to a 2015 report published in Frontiers in Neurology, the severity and type of injury created by each is thought to be the best way to distinguish the intensity of the velocity.

Low-velocity projectiles, which can include anything from knife wounds to air rifle pellets, are thought to travel anywhere from 610 m/s to 914 m/s. They result in wounds that create localized brain tissue damage along the object’s path, depressed skull fractures and scalp lacerations.

However, most of the energy thrusted into low-velocity projectiles ends up being expended prior to contact with the skull, resulting in minimal damage. Unlike high-velocity projectiles, the size of penetrating brain injuries for low-velocity projectiles tends to correspond with the size of the projectile itself, and therefore the location of injury is limited to the direct path of the bullet through the skull. Since a much smaller area is affected, neurological defects tend to be limited, unless a critical part of the brain, such as the midbrain, is impacted.

High-velocity projectiles, which Alvis-Miranda et al. defines as those that move at a rate greater velocity than 2000 ft/sec, are more likely to cause mortality due to the principles of oscillation, rotation and fragmentation making. High-velocity projectiles create temporary and permanent cavities that causes damage to areas beyond that of the initial contact point.

In a Frontiers in Neurology report, researchers found that the ballistic factors of high-velocity penetrating brain injuries can cause three types of tissue damage to the brain:

1. 1. 1. The wound tract: where the gunshot wound caused lacerated and crushed tissues
      2. Shearing and stretching of tissues in the area adjacent to the wound tract
      3. A surrounding area in which the injury has prevented small blood vessels from filling, causing extravasation of blood

These mechanisms of damage will ultimately result in a permanent cavity. The size of the cavity is determined by both the size and the speed of the penetrator. For high-velocity penetrators, the size of the cavity ends up being larger than the diameter of the projectile, due to the principles of cavitation, tumbling and yaw (rotation).

Additionally, a temporary cavity will form in the area adjacent to the permanent cavity due to large scale cavitation that diverges from the wound tract. This temporary cavity can lead to cerebral edema, which results in increased ICP. If ICP reaches a high enough level, such as above 20 mmHg, brain herniation could prove fatal for the patient.

Kinetic Energy

Firearms that are the most fatal are those in which all of the energy is aimed directly into the target, according to ballistics principles, penetrating vital tissue without exiting the body. Therefore, measuring a firearm’s kinetic energy is considered to be the most accurate method of determining a projectile’s ability to injure, according to an article published in Archives of Otolaryngology – Head and Neck Surgery. As the velocity of the projectile doubles, the kinetic energy quadruples.

But the angle at which a projectile is fired can influence the amount of kinetic energy released into the body, which determines the amount of resulting tissue destruction. For example, if a non-tumbling bullet enters the body at a right angle, it will have minimal contact with the skin and will direct a smaller amount of kinetic energy to the surrounding tissues.

But a bullet that enters the body at an oblique angle and tumbles through the tissues will have directed a greater amount of kinetic energy into the tissues, causing more damage.

Air resistance can also cause a projectile to rapidly lose kinetic energy, which also decreases the velocity of the projectile. But both factors are often influenced by the shape of the projectile itself. The sharper the nose of the bullet is, the less decrease in velocity by air resistance. Bullets that have an irregular shape will have a slower decrease in velocity and kinetic energy.

Entry Site of Penetrating Brain Injury

Certain areas of the brain are much more likely to cause severe injury or death if they are damaged from gunshot head wounds compared to others. The American Association of Neurological Surgeons notes that the path of the bullet in a penetrating brain injury through the cranial tissue has an effect on the severity of the resulting injuries and potential complications.

If the bullet wound passes through the right frontal lobe tip towards the forehead while being above the skull base, there is likely to be only mild damage, because the bullet would have not passed through any vital brain tissue or structures.

Conversely, in a 2020 study published in the Journal of Neurosurgery that examined 179 patients that had suffered from penetrating or perforating injuries, it was found that projectile trajectories (PT) that were either bitemporal or entered through the frontal bone, traversed the midline and entered the contralateral parietal lobes were universally fatal.

Researchers also noted that patients with a bihemispheric PT had a mortality rate 27.28 times higher than for patients with any other PT.

However, if the frontal lobes are impacted – which is one of the more common areas of impact in TBI patients – this can hinder one’s ability to make plans, understand social cues and manage impulse control.

What’s more, if deep brain structures such as the amygdala or hypothalamus are damaged, it can affect the victim’s ability to create and remember memories, especially those that are particularly emotional. These symptoms tend to linger long after the patient’s injury, impacting around 5.3 million U.S. citizens.