Traumatic brain injury (TBI) is the result of physical trauma to the head causing damage to the brain. This damage can be focal, or restricted to a single area of the brain, or diffuse, affecting more than one region of the brain. By definition, TBI requires that there be a head injury, or any physical assault to the head leading to injury of the scalp, skull, or brain. However, not all head trauma is associated with TBI.
TBI is sometimes known as acquired brain injury. The least severe and most common type of TBI is termed a concussion, which is technically defined as a brief loss of consciousness after a head injury without any physical evidence of damage on an imaging study such as a CT or MRI scan. In common parlance, concussion may refer to any minor injury to the head or brain.
Symptoms, complaints, and neurological or behavioral changes following TBI depend on the location (s) of the brain injury and on the total volume of injured brain. Usually, TBI causes focal brain injury involving a single area of the brain where the head is struck or where an object such as a bullet enters the brain. Although damage is typically worst at the point of direct impact or entry, TBI may also cause diffuse brain injury involving several other brain regions.
Closed head injury refers to TBI in which the head is hit by or strikes an object without breaking the skull. In a penetrating head injury, an object such as a bullet fractures the skull and enters brain tissue.
Diffuse brain damage associated with closed head injury may result from back-and-forth movement of the brain against the inside of the bony skull. This is sometimes called coup-contrecoup injury. "Coup," or French for "blow," refers to the brain injury directly under the point of maximum impact to the skull. "Contrecoup," or French for "against the blow," refers to the brain injury opposite the point of maximum impact.
For example, coup-contrecoup injury may occur in a rear-end collision, with high speed stops, or with violent shaking of a baby, because the brain and skull are of different densities, and therefore travel at different speeds. The impact of the collision causes the soft, gelatinous brain tissue to jar against bony prominences on the inside of the skull.
Because of the location of these prominences and the position of the brain within the skull, the frontal lobes (behind the forehead) and temporal lobes (underlying the temples) are most susceptible to this type of diffuse damage. These lobes house major brain centers involved in speech and language, so problems with communication skills often follow closed head injuries of this type.
Depending on which areas of the brain are injured, other symptoms of closed head injury may include difficulty with concentration, memory, thinking, swallowing, walking, balance, and coordination; weakness or paralysis; changes in sensation; and alteration of the sense of smell.
Consequences of TBI can be relatively subtle or completely devastating, related to the severity and mechanism of injury. Diffuse axonal injury, or shear injury, may follow contrecoup injury even if there is no damage to the skull or obvious bleeding into the brain tissue. In this type of injury, damage to the part of the nerve that communicates with other nerves degenerates and releases harmful substances that can damage neighboring nerves.
When the skull cracks or breaks, the resulting skull fracture can cause a contusion, or an area of bruising of brain tissue associated with swelling and blood leaking from broken blood vessels. A depressed skull fracture occurs when fragments of the broken skull sink down from the skull surface and press against the surface of the brain. In a penetrating skull fracture, bone fragments enter brain tissue. Either of these types of skull fracture can cause bruising of the brain tissue, called a contusion. Contrecoup injury can also lead to brain contusion.
If the physical trauma to the head ruptures a major blood vessel, the resulting bleeding into or around the brain is called a hematoma. Bleeding between the skull and the dura, the thick, outermost layer covering the brain, is termed an epidural hematoma. When blood collects in the space between the dura and the arachnoid membrane, a more fragile covering underlying the dura, it is known as a subdural hematoma. An intracerebral hematoma involves bleeding directly into the brain tissue.
All three types of hematomas can damage the brain by putting pressure on vital brain structures. Intracerebral hematomas can cause additional damage as toxic breakdown products of the blood harm brain cells, cause swelling, or interrupt the flow of cerebrospinal fluid around the brain.
Laurie Barclay, The Gale Group Inc., Gale, Detroit,