A traumatic brain injury (TBI) is damage to the brain caused by an external force, such as a blow, bump, or jolt to the head, or an object penetrating the skull. In the United States alone, TBIs caused roughly 214,110 hospitalizations in 2020 and 69,473 deaths in 2021, and those numbers don’t include the many cases treated in emergency rooms, urgent care clinics, or never treated at all.
How TBI Severity Is Classified
TBIs fall into three categories based on how long a person loses consciousness and how long they experience memory loss (called post-traumatic amnesia) afterward. Emergency teams also use the Glasgow Coma Scale, a 15-point scoring system that tests eye opening, verbal responses, and movement, to quickly gauge severity at the scene.
Mild TBI (including concussions) involves loss of consciousness lasting less than 30 minutes, memory loss lasting up to one day, and a Glasgow Coma Scale score of 13 to 15. This is the most common type and accounts for the vast majority of cases.
Moderate TBI involves loss of consciousness lasting 30 minutes to 24 hours, memory loss lasting one to seven days, and a Glasgow Coma Scale score of 9 to 12.
Severe TBI means loss of consciousness for more than 24 hours, memory loss lasting more than seven days, and a Glasgow Coma Scale score of 3 to 8. These injuries carry the highest risk of lasting disability or death.
What Happens Inside the Brain
The damage from a TBI unfolds in two phases. The first is the immediate, physical injury: the force of impact can bruise brain tissue, tear blood vessels, and shear the long, thread-like fibers (axons) that connect brain cells to one another. This axon damage, sometimes called diffuse axonal injury, is especially common when the head accelerates or decelerates rapidly, like in a car crash. It disrupts the brain’s ability to send signals between regions and can begin breaking down within minutes.
The second phase is a cascade of chemical and metabolic problems that builds over the hours and days following the initial hit. Damaged cells release a flood of the brain’s excitatory signaling chemicals, which overstimulate surrounding neurons and cause a harmful surge of calcium into those cells. This calcium overload cripples the cells’ energy-producing machinery (mitochondria), starving them of fuel and triggering the production of toxic molecules called free radicals. These free radicals attack cell membranes, proteins, and DNA, widening the zone of damage well beyond the original injury site. Swelling and inflammation compound the problem, restricting blood flow and oxygen delivery to vulnerable tissue.
This secondary damage is a major reason why someone with a TBI can appear stable initially and then deteriorate. It’s also why rapid medical evaluation matters, even when the injury seems minor at first.
Symptoms to Recognize
TBI symptoms vary widely depending on severity and which part of the brain is affected. They generally fall into three overlapping categories.
Physical symptoms include headache, nausea or vomiting, blurred or double vision, dizziness, loss of balance, fatigue, seizures, slurred speech, and weakness in the arms, legs, or face. Two red flags that warrant immediate emergency care are unequal pupil sizes and clear fluid draining from the nose or ears, which can signal a skull fracture.
Cognitive and behavioral symptoms include confusion, disorientation, difficulty remembering or concentrating, trouble making decisions, and changes in sleep patterns. Some people sleep far more than usual, while others struggle to fall asleep at all.
Emotional and sensory symptoms include irritability, anxiety, depression, mood swings, sensitivity to light or sound, ringing in the ears, and an unexplained bad taste in the mouth. Unusual combativeness or agitation can also appear, sometimes catching family members off guard.
Symptoms in Children
Young children can’t always describe what they’re feeling, so the signs look different. Watch for persistent crying that can’t be consoled, changes in eating or nursing habits, loss of interest in favorite toys, loss of previously learned skills like toilet training, unsteady walking, vomiting, and seizures.
How TBI Is Diagnosed
A non-contrast CT scan of the head is the standard first-line imaging tool after a suspected TBI. It’s fast, widely available, and very reliable at identifying bleeding, skull fractures, and swelling that might require surgery. For mild TBIs, a normal CT scan is a strong indicator that no surgical intervention is needed.
MRI becomes useful in specific situations. If the CT looks normal but neurological symptoms persist or worsen, MRI can pick up subtler damage, particularly to axons, that CT misses. However, routine MRI isn’t currently recommended for every mild TBI. For children with mild symptoms, imaging isn’t always necessary and is typically guided by clinical judgment rather than performed automatically.
Recovery Timelines
Most people with a mild TBI or concussion recover within a few days to a few weeks with proper rest and a gradual return to normal activities. The brain needs time to heal, and pushing too hard too soon, whether physically or mentally, can delay recovery.
If symptoms haven’t improved within two to three weeks, or if they get worse once you resume regular activities, that’s a signal to follow up with a healthcare provider. When symptoms linger for months after the injury, the condition is often called post-concussive syndrome. Persistent headaches, difficulty concentrating, sleep disruption, and mood changes are the most common complaints. Recovery from moderate and severe TBI takes significantly longer and often involves structured rehabilitation programs addressing physical, cognitive, and emotional functioning.
Long-Term Risks
A single moderate or severe TBI can raise the risk of developing neurodegenerative diseases later in life. A study of World War II veterans found that those who sustained a moderate TBI in early adulthood were 2.3 times more likely to develop Alzheimer’s disease roughly 40 years later, while those with severe TBI were 4.5 times more likely compared to veterans without head injuries.
The link to Parkinson’s disease is particularly well-supported. Two of the largest population-based studies to date found that people with a history of moderate or severe TBI were at least 1.8 times more likely to develop Parkinson’s over an average follow-up of about five years. Even a single mild TBI appears to increase Parkinson’s risk, with multiple independent studies reporting about a 1.6-fold increase.
Repetitive TBIs, including hits that don’t rise to the level of a diagnosed concussion, are associated with chronic traumatic encephalopathy (CTE), a degenerative brain disease most widely discussed in contact sports. CTE can only be definitively diagnosed after death through brain tissue examination, which makes studying it in living people challenging. The relationship between repetitive head impacts and dementia or movement disorders is still being clarified, but the accumulating evidence has driven significant changes in concussion protocols across professional and youth sports.
Not every TBI leads to long-term neurological problems. Many people recover fully, especially from mild injuries. But the potential for lasting consequences underscores why preventing repeat injuries and allowing complete recovery before returning to activities that carry head-injury risk is so important.