Penetrating head injury

A penetrating head injury, or open head injury, is a head injury in which the dura mater, the outer layer of the meninges, is breached. Penetrating injury can be caused by high-velocity projectiles or objects of lower velocity such as knives, or bone fragments from a skull fracture that are driven into the brain. A perforating head injury is one in which the object passes through the head and leaves an exit wound. Head injuries caused by penetrating trauma are serious medical emergencies and may cause permanent disability or death.

Mechanisms
In penetrating injury from high-velocity missiles, injuries may occur not only from initial laceration and crushing of brain tissue by the projectile but also from the subsequent cavitation. High-velocity objects create centrifugal forces and can create a shock wave that cause stretch injuries, forming a cavity that is three to four times greater in diameter than the missile itself. A pulsating temporary cavity is also formed by a high-speed missile and can have a diameter thirty times greater than that of the missile. Though this cavity is reduced in size once the force is over, the tissue that was compressed during cavitation remains injured. Destroyed brain tissue may either be ejected from entrance or exit wounds or packed up against the sides of the cavity formed by the missile.

Low-velocity objects usually cause penetrating injuries in the regions of the skull's temporal bones or orbital surfaces, where the bones are thinner and thus more likely to break. Damage from lower-velocity penetrating injuries is restricted to the tract of the stab wound, because the lower-velocity object does not create as much cavitation. However, low-velocity penetrating objects such as slow bullets may ricochet inside the skull, continuing to cause damage until they stop moving.

Pathophysiology
Though it is more likely to cause infection, penetrating trauma is similar to closed head injury such as cerebral contusion or intracranial hemorrhage in a number of ways. As in closed head injury, intracranial pressure is likely to increase due to swelling or bleeding, potentially crushing delicate brain tissue. Most deaths from penetrating trauma are caused by damage to blood vessels, which can lead to intracranial hematomas and ischemia, which can in turn lead to a biochemical cascade called the ischemic cascade. The injury in penetrating brain trauma is mostly focal (affects a specific area of tissue).

Sometimes in penetrating injuries, the brain releases thromboplastin, which can lead to problems with clotting.

Outcome
The highest-velocity injuries tend to have the worst associated damage. Penetrating injury from any missile such as a bullet has a mortality rate of 92%. Thus, firearms cause the most head injury-related deaths. Perforating injuries have an even worse prognosis.

Penetrating head trauma can cause loss of abilities controlled by parts of the brain that are damaged. A famous case of penetrating head trauma was that of Phineas Gage, whose personality drastically changed after he sustained a penetrating injury to his frontal lobe.

Up to 50% of patients with penetrating brain injuries get late-onset epilepsy.