The brain is protected from external injuries by hair, skin, and the skull. Damage to the brain occurs as a result of compression of the skull from impact, which compromises integrity of the bone. The amount of damage depends on the force of the injury.

Head injuries are divided into two categories: primary or direct injuries (damage is caused by the impact) and secondary injuries that result in brain swelling, infection, and hypoxia of the cerebral tissue. Head injuries are also referred to as closed or blunt trauma and open or penetrating trauma.

Mechanisms of trauma include deformation, acceleration-deceleration as seen in coup-countercoup injury, and diffuse axonal injury. The injuries may be insignificant or may lead to poor outcomes. Factors predictive of poor outcomes include intracranial hematoma, increasing age, abnormal motor signs, impaired or absent eye movements or pupil reflexes, early sustained hypotension, hypoxemia/hypercapnia, and ICP > 20 mm Hg. Scalp injuries such as abrasions, contusions, lacerations, and hematomas often bleed profusely, but most are benign and only significant if associated with meningeal tears.

Skull fractures occur with or without brain injury. The major complications of skull fractures are intracranial infections, bleeding and hematomas, meningeal tears, brain tissue damage, and increased ICP. The location of a fracture may be significant, as a basilar fracture generally crosses the paranasal sinuses in the frontal bone or the middle ear in the temporal bone, causing meningeal tears that result in cerebrospinal
fluid (CSF) fistulas, increasing the risk of infection. In addition, blood may collect around the eyes or ears.