A traumatic brain injury (TBI) happens when a sudden external force disrupts normal brain function. Falls are the single largest cause, accounting for nearly half of all TBI-related hospitalizations in the United States. Motor vehicle crashes, assaults, sports collisions, blast waves, and penetrating wounds round out the major categories. In 2020 alone, roughly 214,110 people were hospitalized for a TBI, and that number doesn’t count the many injuries treated in emergency rooms, urgent care clinics, or never treated at all.
Falls
Falls are the most common cause of TBI across all age groups, but the risk is especially concentrated at the extremes of life. In children, 55% of brain injuries result from falls off stairs, furniture, playground equipment, or bicycles. In older adults, the numbers are even more striking: people aged 75 and older account for about 32% of all TBI-related hospitalizations and 28% of TBI-related deaths, with falls being the overwhelming driver.
What makes falls so dangerous for the brain is that they often involve the head striking a hard surface at speed, even from a relatively short height. A slip on a wet bathroom floor or a tumble down two or three steps can generate enough force to bruise brain tissue, tear blood vessels, or cause bleeding between the skull and the brain. For older adults on blood-thinning medications, even a seemingly minor bump to the head can lead to a slow, life-threatening bleed.
Motor Vehicle Crashes
Car, motorcycle, and bicycle crashes are the second most common cause of TBI and are the leading cause of TBI-related death in children older than five. The forces involved are much higher than in a typical fall. In a collision, the skull stops moving abruptly while the brain continues sliding forward inside it, slamming against the front of the skull and then rebounding against the back. This creates injuries at two sites: the point of initial impact (the coup) and the opposite side of the brain (the contrecoup). The combination can damage multiple brain regions at once.
Rotational forces during a crash add another layer of damage. When the head twists or whips sideways, the brain’s nerve fibers stretch and shear. This type of injury, known as diffuse axonal injury, disrupts the communication pathways that connect different parts of the brain. It’s a major reason why severe car crash TBIs often cause widespread cognitive problems rather than a single, isolated deficit.
Sports and Recreation Injuries
Concussions are the most common form of sports-related TBI, and they happen far more often than many parents and athletes realize. In high school sports, more than two out of three concussions result from collisions between athletes. The sports with the highest concussion rates per 1,000 athletic exposures are boys’ tackle football, girls’ soccer, boys’ lacrosse, boys’ ice hockey, and boys’ wrestling.
The specific mechanism varies by sport. Tackling causes almost two out of three concussions in football. In soccer, heading the ball is the most common activity linked to concussion, with about one in three concussions among girls and one in four among boys happening during a header, usually because of a collision with another player while going for the ball. In cheerleading, nearly all concussions are linked to stunts involving tosses or lifts. In baseball, about one in four concussions come from being hit by a pitch. Wrestling concussions most commonly happen during takedowns.
These injuries are typically classified as mild TBIs, but the word “mild” refers to the initial severity, not the potential consequences. Repeated concussions, especially when the brain hasn’t fully recovered from a previous one, can cause compounding damage over time.
Violence and Assault
Assaults are a significant and often underreported cause of TBI. Physical abuse is the number one cause of TBI-related death in children aged four and younger, a sobering statistic that reflects the vulnerability of developing brains to shaking and blunt force. In infants, abusive head trauma (sometimes called shaken baby syndrome) can cause devastating injuries because the brain is still soft and the neck muscles are too weak to stabilize the head.
Among adults, interpersonal violence including domestic violence accounts for a substantial share of TBIs. Many of these injuries go undiagnosed because victims may not seek medical care or may not connect ongoing symptoms like headaches, memory problems, or mood changes to a past head injury. Firearm-related injuries represent the most lethal end of this spectrum: firearm-related suicide is the most common cause of TBI-related death in the United States overall.
Blast Injuries
Explosions cause a unique form of TBI that is especially prevalent among military service members. When a bomb or improvised explosive device detonates, it generates a pressure wave that moves through the air faster than sound. This wave of overpressure passes through the skull and into the brain, causing injury without any direct impact to the head. The brain can be damaged even when a person shows no visible wounds, which is why blast-related TBIs are often called “invisible injuries.” Standard physical exams and routine imaging frequently come back normal despite real underlying damage.
Blast injuries can also cause TBI through secondary mechanisms: flying debris striking the head, the person being thrown against a surface, or the collapse of a structure. In combat zones, it’s common for multiple mechanisms to overlap in a single event, making these injuries complex to diagnose and treat.
How the Brain Gets Damaged
Regardless of the cause, TBI comes down to mechanical forces acting on brain tissue. There are two basic patterns. In translational injuries, the head moves in a straight line and stops suddenly. This creates a pressure spike at the impact site and a zone of negative pressure on the opposite side. The positive pressure bruises tissue directly, while the negative pressure on the far side can cause tiny cavitation bubbles to form in the brain tissue. When those bubbles collapse, they inflict additional local damage.
In rotational injuries, the head twists or pivots. This generates shearing forces that stretch and tear the long nerve fibers connecting distant brain regions. These fibers are especially vulnerable because different layers of brain tissue move at different speeds during rotation. The result is widespread disruption to the brain’s wiring rather than a single bruise or bleed. Many real-world injuries involve both patterns at once: a car crash, for instance, typically produces both straight-line deceleration and rotational whipping of the head.
Penetrating Injuries
Penetrating TBI occurs when an object breaks through the skull and enters brain tissue. Gunshot wounds are the most common cause, but industrial accidents, falls onto sharp objects, and combat shrapnel can also drive material into the brain. These injuries are far less common than closed-head TBIs but are much more likely to be fatal or to cause severe, permanent disability. The damage includes not only the direct path of the object through brain tissue but also the pressure wave it creates, which can injure tissue well beyond the wound track.
Who Faces the Highest Risk
TBI can happen to anyone, but certain groups face disproportionate risk. Young children are vulnerable because of their large head-to-body ratio, thinner skulls, and developing brains. Teenagers and young adults face elevated risk from sports, reckless driving, and risk-taking behavior. Adults 75 and older have the highest rates of TBI hospitalization and death, driven almost entirely by falls compounded by age-related factors like balance problems, vision loss, medication side effects, and thinner blood vessels that bleed more easily.
Military personnel face unique exposure to blast injuries. People experiencing domestic violence face repeated head trauma that often goes unrecognized. And anyone involved in high-speed activities, from cycling to skiing to construction work, carries elevated risk simply by the nature of what they’re doing. Understanding the specific cause matters because it shapes the type of brain damage, the likely symptoms, and the path to recovery.