Penetrating Brain Injury refers to the damage caused by any object that enters the brain. The damage is caused by the impact of the object itself, as well as by forcing bits of skin, hair, bone, and fragments of the object itself into the brain. The penetrating object can by a knife, bullet, or other sharp object.

Obviously, penetrating brain injuries are serious medical emergencies. An object traveling at a high rate of speed, such as a bullet, may pass entirely through the head and exit on the opposite side. This is called a perforating head injury or a through-and-through injury. A bullet can also be a low velocity projectile, and is similarly dangerous, despite its reduced speed. After entering the head, rather than exiting, a slower object can rebound within the confines of the skull, damaging a large area.

The destruction caused by firearms is considerable. Of all the causes of traumatic brain injury, firearms cause the most death. Furthermore, over 90 percent of traumatic brain injuries caused by firearms result in death. Of those who survive a penetrating brain injury, about half get late-onset epilepsy.