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Traumatic brain injury (TBI) is physical injury to brain tissue that temporarily or permanently impairs brain function. Diagnosis is suspected clinically and confirmed by imaging [primarily computed tomography (CT), although magnetic resonance imaging (MRI) can be helpful later when it is logistically possible to obtain]. Initial treatment consists of ensuring a reliable airway and maintenance of adequate ventilation and blood pressure. Surgery is often needed in more severe cases to remove intracranial hematomas, provide room for the brain to swell, or place monitors to track intracranial pressure (ICP) and brain oxygenation. In the first few days after the injury, significant efforts are made to maintain adequate brain perfusion and oxygenation, and prevent complications that can result from an altered sensorium. Various periods of rehabilitation are often needed.In the United States, as in much of the world, TBI is a common cause of death and disability.1–3 Causes include motor vehicle crashes and other transportation-related causes (eg, bicycle crashes, collisions with pedestrians), falls (especially in older adults and young children), assaults, and sports activities.Head injuries can cause various types of structural damage. Structural changes may be gross or microscopic, depending on the mechanism and forces involved. Patients with less severe injuries may have no gross structural damage. Clinical manifestations vary markedly in severity and consequences. Injuries are commonly categorized as open or closed.Open injuries involve penetration of the scalp and skull (and usually the meninges and underlying brain tissue). They typically involve bullets or sharp objects, but a skull fracture with overlying laceration due to severe blunt force is also considered an open injury.Closed injuries typically occur when the head is struck, strikes an object, or is shaken violently, causing rapid brain acceleration and deceleration. Acceleration or deceleration can injure tissue at the point of impact (coup), at its opposite pole (contrecoup), or diffusely; the frontal and temporal lobes are particularly vulnerable. Axons, blood vessels, or both can be sheared or torn. Blood vessels that are disrupted leak, producing contusions, intracerebral or subarachnoid hemorrhages, and hematomas (epidural and subdural).Concussion is defined as transient and reversible posttraumatic alteration in mental status (eg, loss of consciousness or memory) lasting from seconds to minutes and, by arbitrary definition, <6 hours.4 Gross structural brain lesions and serious neurologic residua are not part of concussion, although temporary disability can occur and postconcussion symptoms such as nausea, headaches, dizziness, and memory disturbances can be considerably disabling.5,6Diffuse axonal injury (DAI) occurs when deceleration causes shear-type forces that result in generalized, widespread disruption of axonal fibers and myelin sheaths (although DAI may also result from minor head injury).7,8 Gross structural lesions are not part of DAI, but small petechial hemorrhages in the white matter are often observed on CT scan (and histopathologic examination).9 DAI is sometimes defined clinically as a loss of consciousness lasting >6 hours in the absence of a specific focal lesion. Edema from the injury often increases ICP, leading to various manifestations. DAI is typically the underlying injury in shaken baby syndrome.Contusions (bruises of the brain) can occur with open or closed injuries and can impair a wide range of brain functions, depending on contusion size and location.10 Larger contusions may cause brain edema and increased ICP. Contusions may grow and evolve into larger lesions that are referred to as intracerebral hematomas. The distinction between contusions and intracerebral hematomas is not well defined.