Neurobiology of Brain Injury
What happens in the brain with mild traumatic injury or concussion
So what happens to the brain during a mild traumatic injury or concussion? The primary damage in most TBI from mild to severe is a specific type of damage: diffuse axonal injury (DAI).
Every nerve cell (or neuron) in the brain has one axon. One nerve cell sends information to another by sending it signals across a synapse (the gap between neurons) from the branches at the end of its axon. A mild brain injury or concussion occurs when axons and small blood vessels are damaged, but not necessarily destroyed. This damage disrupts the nerve cells' ability to communicate with other cells. There may also be damage to the synapses through which information is sent. This damage can cause release of chemicals (neurotransmitters) and inflammation that may further impair brain function - perhaps just in the early stages after injury. Damage to small blood vessels can cause swelling that reduce blood flow in the brain. If both axons and small blood vessels are damaged, scarring (also called gliosis) can occur.
DAI is the signature pathology of TBI at all levels of severity. It is the mechanism by which TBI causes loss of consciousness - transient or prolonged.
DAI produces damage at a microscopic level, so imaging such as CT or even routine MRI may not identify injury unless there is some bleeding in the brain. Microscopic damage can, however, be seen with specialized MRI scans. These scans called susceptibility scans are very sensitive to small blood vessel damage, and scans called diffusion tensor images are very sensitive to injury to axons and to gliosis. Increases in the amount of injury increases the severity of the clinical condition.
Although MRI techniques can detect microscopic injury, there is no evidence that performing an MRI on all patients who present with a concussion or mild brain injury helps identify treatment or predict the recovery.
Approximately 3-5% of patients seen in the emergency department with apparently mild TBI - Glasgow Coma Scale 13-15 and post-traumatic amnesia resolving in less than 24 hours - have a significant brain contusion (bruise) or hemorrhage (bleeding), either outside the brain (subdural or epidural) or inside the brain in addition to DAI. The combination of mild injury to the brain's nerve cells plus a significant local injury has been labeled "complicated" mild traumatic brain injury. Recovery from this injury is usually good, but it take may take longer than a TBI with just equivalent DAI. There may be some less common residual symptoms depending on the site of the local injury.