Traumatic brain injury (TBI) happens when an external force disrupts normal brain function. That force can come from a blow to the head, a sudden jolt, a blast wave, or even strangulation that cuts off oxygen. Falls are the single largest cause, accounting for nearly half of all TBI-related hospitalizations in the United States, followed by motor vehicle crashes, assaults, and firearm injuries.
The Most Common Causes
Falls lead the list by a wide margin. A slip on ice, a tumble down stairs, or a fall from a ladder can all deliver enough force to injure the brain, especially in older adults and young children. Motor vehicle crashes are the next major contributor, generating the kind of rapid acceleration and deceleration that is particularly destructive to brain tissue. Assaults, including punches, kicks, and being struck with objects, round out the top causes of non-fatal TBI.
Firearm-related suicide is the most common cause of TBI-related deaths in the United States, a statistic that underscores how the injury’s severity spectrum ranges from mild concussions to fatal wounds. The causes that send the most people to the hospital are not the same ones that kill the most people, which is why public health data can look different depending on whether you’re reading about hospitalizations or fatalities.
What Actually Happens Inside the Skull
When your head takes a hit or stops suddenly, two waves of damage occur. The first, called primary injury, is purely mechanical. It happens at the moment of impact. The brain can strike the inside of the skull, blood vessels can tear, and tissue can be crushed or stretched. This damage is immediate and irreversible.
The second wave, called secondary injury, unfolds over hours or even days. Swelling, bleeding, and rising pressure inside the skull reduce blood flow to brain cells. When starved of oxygen and fuel, those cells lose the ability to regulate their internal chemistry. Calcium and sodium flood in, triggering a chain reaction: cells release large amounts of excitatory chemicals (especially glutamate), which overstimulate neighboring cells and cause further breakdown. As more cells die, they release toxic byproducts that damage surrounding tissue. This cascade is why someone with a TBI can appear stable initially and then deteriorate. It’s also the window where medical intervention can make the biggest difference.
How Rotational Forces Cause Hidden Damage
Not all brain injuries come from a direct blow. Some of the most devastating damage results from the brain twisting inside the skull. When your head rotates rapidly, as it does in a car crash or a hard tackle, the brain’s long nerve fibers (axons) get stretched and sheared by the resulting forces. This is called diffuse axonal injury, and it can be widespread, tearing connections across multiple brain regions simultaneously.
These shear forces increase with higher accelerations, and the damage is often invisible on standard CT scans. A person with diffuse axonal injury may lose consciousness immediately or show confusion and cognitive problems that seem disproportionate to what the scan shows. The injury disrupts the brain’s internal communication network, which is why symptoms can include everything from memory loss to personality changes.
Sports Concussions
Contact sports produce TBI through two slightly different mechanisms depending on the type of hit. Short, sharp impacts, like helmet-to-helmet collisions in football, generate high-magnitude accelerations over roughly 15 milliseconds. The brain moves relative to the skull, stretching blood vessels and bouncing against bone. Falls to the ground in football produce a similar pattern.
Longer-duration hits, like a shoulder-to-head check in ice hockey (lasting 20 to 30 milliseconds), tend to produce lower peak accelerations but cause the brain to move with the skull, creating internal stresses and strains that spread from the center of the brain outward through the cortex. Both mechanisms cause concussions, but they stress different parts of the brain in different ways. This is one reason concussion symptoms vary so much from one athlete to another.
Blast Injuries in Military Settings
Explosions injure the brain through four distinct stages, each with its own mechanism. The primary blast injury comes from the pressure wave itself, a wall of compressed air that moves through the body and can crush tissue and organs. The secondary phase occurs when debris and fragments propelled by blast winds strike the head, causing penetrating or blunt-force injuries. In the tertiary phase, the blast wind throws the person’s entire body, producing the same kind of impact injuries seen in car crashes or falls. Finally, quaternary injuries come from heat, light, and toxic gases released by the explosion, which can cause burns and chemical damage.
A single explosion can produce all four types simultaneously, which is why blast-related TBI is often more complex than civilian head injuries and harder to diagnose clearly.
Why Older Adults Face Higher Risk
Falls that would leave a younger person with nothing more than a bruise can cause life-threatening brain bleeds in someone over 65. Part of the reason is that the brain shrinks slightly with age, creating more space between the brain and skull. This allows more movement on impact and stretches the bridging veins that connect the brain’s surface to the skull’s inner lining, making them more likely to tear.
Blood-thinning medications dramatically raise the stakes. Millions of older adults take anticoagulants to prevent strokes and blood clots, but these same drugs make it harder for the body to stop bleeding inside the skull after a fall. Even widely prescribed newer blood thinners like rivaroxaban and apixaban carry concern for higher rates of dangerous intracranial bleeding after head trauma. A ground-level fall, something as simple as tripping over a rug, is the most common scenario. This combination of age-related brain changes and anticoagulant use is why emergency departments treat elderly head injuries with particular urgency, even when the fall itself seems minor.
Intimate Partner Violence and Strangulation
Domestic violence is an underrecognized cause of TBI. Brain injuries in this context come from two main mechanisms: blows to the head and strangulation. A punch, a shove into a wall, or being slammed against a surface can cause the same concussive or more severe injuries seen in sports and falls. Strangulation adds a different kind of brain injury entirely. Pressure on the neck restricts blood flow and oxygen, causing the type of damage that comes from oxygen deprivation rather than mechanical force.
The numbers are staggering. More than two-thirds of intimate partner violence victims are strangled at least once, according to data used in federal training programs. Roughly 6.2 million women and 581,000 men in the U.S. have reported being knocked out, slammed against something, or choked by a partner. Studies estimating how many domestic violence victims have sustained brain injuries put the range anywhere from 11 to 79 percent, a gap that reflects how often these injuries go undiagnosed. Victims may not seek medical care, may not connect their symptoms to a brain injury, or may not feel safe disclosing what happened.
How Severity Is Classified
TBI is grouped into three levels based on how responsive a person is after the injury, measured by the Glasgow Coma Scale. This scale scores eye opening, verbal responses, and movement on a scale from 3 to 15. A score of 13 to 15 indicates a mild TBI, which is the clinical term for a concussion. Scores of 9 to 12 are classified as moderate, and 3 to 8 as severe.
The word “mild” is misleading. A concussion can cause weeks or months of headaches, memory problems, light sensitivity, and mood changes. The 2023 diagnostic criteria from the American Congress of Rehabilitation Medicine confirm that “concussion” and “mild TBI” are interchangeable terms when brain imaging looks normal. Diagnosis relies on the mechanism of injury combined with symptoms like confusion, brief loss of consciousness, memory gaps around the event, or abnormal neurological exam findings. You don’t have to be knocked out to have a concussion, and a normal CT scan doesn’t mean your brain wasn’t injured.