There is no set timeline for recovery from a traumatic brain injury, or TBI. Many factors influence recovery times, including the severity of injury, access to early rehabilitation, the amount of psychosocial support an individual receives, and the presence of other complicating medical conditions.
Not everyone who has a brain injury will develop dementia. However, there does appear to be an increased risk of developing dementia in individuals with more severe brain injuries, and in some individuals who have been exposed to multiple mild brain injuries and repetitive head impacts. More research is needed to identify who is at greatest risk and why.
Brain injury is caused by a force acting on the brain. This generally takes the form of contact or impact forces, when the brain is moved suddenly and, as a result, the surface of the brain is compressed against the inside of the skull, or rubs across some of the uneven surfaces of the inside of the skull. This rapid movement of the brain can also cause the nerve cells and blood vessels to be stretched and break. A different kind of injury may occur when a foreign object, such as a bullet enters the brain.
Certain parts of the brain play an important role in the circuitry of learning, attention and memory. Unfortunately, these brain regions are particularly vulnerable to damage from the biomechanical forces that are associated with brain injury. (See What causes traumatic brain injury?)