Brain injury is a serious health condition that occurs when an outside force damages the brain, affecting how a person thinks, feels, and acts. Every nine seconds, someone in the United States experiences a brain injury, making it one of the most common yet misunderstood medical emergencies. The effects can range from brief confusion to life-altering disabilities, impacting not only the person injured but also their families and communities.

Understanding Brain Injury

A brain injury happens when something strikes your head with enough force to make your brain bounce around or twist inside your skull, or when an object pierces through the skull and enters the brain tissue. This is not a simple bump on the head that resolves quickly. When the brain moves violently inside the skull, it can damage brain cells, injure blood vessels, and create chemical changes that prevent the brain from working as it should. The injury can be immediate, called primary injury, or it can develop gradually over hours, days, or even weeks after the initial trauma, known as secondary injury.

Brain injuries come in different forms. A penetrating brain injury, also called an open brain injury, occurs when something sharp like a bullet, shrapnel, or knife pierces the skull and damages a specific part of the brain. A non-penetrating brain injury, also known as a closed head injury or blunt injury, happens when an external force is strong enough to move the brain inside the skull without breaking through the bone. Some severe accidents can cause both types of injury in the same person.

Healthcare providers classify brain injuries by severity. More than 75 percent of all brain injuries are considered mild, and doctors often use the term concussion when talking about mild brain injury. However, even a mild brain injury can cause significant problems that last for weeks or months. People with mild brain injury may struggle to return to work or school. Moderate and severe brain injuries typically cause more serious, long-term health problems that can affect every aspect of a person’s life.

Epidemiology

Brain injury affects thousands of people around the world every year, representing a major public health concern. In the United States alone, the combined rates for brain injury-related emergency department visits, hospitalizations, and deaths increased during the decade from 2001 to 2010. However, the number of deaths related to brain injuries decreased over the same period, likely due to improvements in emergency medical care and treatment protocols.

In 2020, more than 214,000 people in the United States needed hospital care for a brain injury. That same year, more than 69,000 people died from issues involving brain injury. In 2021, there were over 69,000 brain injury-related deaths in the United States, which equals about 190 brain injury-related deaths every day. The numbers highlight how common and serious this condition truly is.

An estimated 5.3 million Americans are currently living with disability related to brain injury. This represents a substantial portion of the population coping with the long-term consequences of these injuries. On a global scale, brain injury is the leading cause of death and disability worldwide. In the European Union, brain injury accounts for one million hospital admissions per year.

Certain groups face higher risks than others. Men are more likely to get a brain injury than women, and they are also more likely to have serious brain injury. Adults aged 65 and older are at the greatest risk for being hospitalized and dying from a brain injury. The highest rate of injury occurs in people between the ages of 15 and 24 years. Persons under the age of five or over the age of 75 are also at higher risk.

Research shows that some populations experience greater risk of brain injury or face longer-lasting problems after injury. These include racial and ethnic minorities, service members and veterans, people experiencing homelessness, people in correctional and detention facilities, survivors of intimate partner violence, and people living in rural areas. These health disparities reflect broader inequalities in access to healthcare, safe living conditions, and support services.

Causes

Brain injuries result from various types of trauma to the head. Understanding how these injuries happen can help people recognize dangerous situations and take steps to protect themselves and their loved ones. The circumstances that lead to brain injury vary depending on whether the injury is closed or penetrating.

Motor vehicle crashes account for 50 percent of all brain injuries and represent the leading cause of brain injury among persons under the age of 65 years. These crashes include accidents involving cars, trucks, motorcycles, bicycles, and pedestrians struck by vehicles. The violent forces involved in motor vehicle accidents can cause the brain to move rapidly inside the skull, resulting in bruising, tearing of brain tissue, bleeding, and other forms of damage.

Falls are the most common cause of brain injury overall and represent the leading cause among persons aged 65 years and older. Falls can happen anywhere—at home, at work, or in public spaces. Older adults are particularly vulnerable because age-related changes in balance, vision, and bone strength make falls more likely and their consequences more severe. Falls lead to nearly half of all brain injury-related hospitalizations.

Sports-related injuries account for close to 300,000 brain injuries each year. Winter sports such as skiing and ice skating alone account for close to 20,000 brain injuries annually. Contact sports like football, hockey, boxing, and soccer carry particularly high risks, but any activity where there is potential for falls or collisions can result in brain injury.

Being struck by or against an object is another common cause. This might include being hit by falling debris at a construction site, struck by equipment during work or play, or hit with a weapon during an assault. Assaults and violence, including intimate partner violence, represent a significant source of brain injuries, particularly among certain populations.

Child abuse is the most common cause of brain injury in children under age four. Young children are especially vulnerable to brain injury because their heads are proportionally larger relative to their bodies, their neck muscles are weaker, and their brains are still developing.

Blast injuries from explosions represent a particularly complex cause of brain injury. Explosions can cause multiple types of trauma simultaneously, including pressure waves that travel through the brain, blunt trauma from being thrown by the blast, and penetrating injuries from shrapnel. Military personnel are especially at risk for blast-related brain injuries. Some extreme events such as explosions, natural disasters, or severe accidents can cause both closed and penetrating brain injury in the same person.

Penetrating injuries specifically occur when objects like bullets, shrapnel from explosions, bone fragments from skull fractures, or weapons such as hammers, knives, or baseball bats pierce the skull and enter the brain tissue. Firearm-related suicide is the most common cause of brain injury-related deaths in the United States.

Risk Factors

While anyone can sustain a brain injury, certain factors increase the likelihood of experiencing this type of trauma. Age plays a significant role in risk. Very young children under the age of five have developing brains and proportionally larger heads, making them more vulnerable to injury from falls and abuse. Teenagers and young adults between 15 and 24 years old engage in more risk-taking behaviors, including dangerous driving, contact sports, and activities that can lead to trauma. Adults aged 65 and older face the highest risk overall due to increased likelihood of falls related to balance problems, vision changes, medications that cause dizziness, and conditions affecting bone strength.

Gender affects risk, with men being significantly more likely than women to sustain a brain injury. Men are also more likely to have serious brain injuries and to die from their injuries. This difference may relate to higher rates of risk-taking behavior, participation in contact sports, dangerous occupations, and involvement in violence among men.

Certain occupations carry elevated risk. Military service exposes individuals to combat situations, blasts, and training accidents. Construction workers face dangers from falls, being struck by objects, and equipment accidents. Athletes in contact sports like football, boxing, hockey, and soccer sustain repeated head impacts. Emergency responders, police officers, and firefighters work in situations where trauma is common.

Lifestyle factors and behaviors increase risk as well. Not wearing seatbelts or helmets leaves individuals unprotected during accidents. Driving under the influence of alcohol or drugs dramatically increases the risk of motor vehicle crashes. Participating in high-risk sports or recreational activities without proper safety equipment and training creates opportunities for head trauma. Involvement in violence, whether as perpetrator or victim, raises the likelihood of sustaining brain injury.

People with certain medical conditions face higher risk. Those taking blood thinners like anticoagulants such as warfarin, rivaroxaban, and apixaban, or antiplatelet medications like clopidogrel, ticagrelor, and aspirin, have increased risk of bleeding in the brain following even minor head trauma. Individuals with balance disorders, vision problems, or conditions affecting coordination are more likely to fall. People with a history of previous brain injury face increased vulnerability to subsequent injuries, and repeated injuries can have cumulative effects.

Social and environmental factors also matter. People living in poverty may lack access to safe housing, protective equipment, and quality healthcare. Those living in areas with high crime rates face greater exposure to violence. Individuals experiencing homelessness lack safe shelter and are vulnerable to assault and exposure to dangerous conditions. Survivors of intimate partner violence face repeated risk of head trauma from assault.

Symptoms

The symptoms of brain injury vary widely depending on the severity of the injury and which parts of the brain are affected. Symptoms can appear immediately after the injury or develop gradually over hours, days, or even weeks. People don’t always make the connection between bumping their head and not feeling well, especially when symptoms appear later. Symptoms also change over time as the brain recovers from injury or as complications develop.

For mild brain injury, physical symptoms are often the first noticed. Headache is extremely common and may persist for days or weeks. Nausea or vomiting can occur, especially in the first hours after injury. Fatigue or drowsiness makes it difficult to maintain normal activity levels. Dizziness or loss of balance affects walking and coordination. Problems with speech may include slurred words or difficulty finding the right words. Clear fluids draining from the nose or ears can indicate a serious skull fracture and requires immediate medical attention.

Sensory symptoms affect how a person perceives the world. Blurred vision or tired eyes make reading and driving difficult. Ringing in the ears, called tinnitus, can be constant or intermittent. A bad taste in the mouth may persist even after eating or drinking. Changes in the ability to smell affect how food tastes and can be a warning sign even when other symptoms seem mild. Sensitivity to light or sound means that normal levels of brightness or noise become uncomfortable or even painful.

Cognitive, behavioral, and mental symptoms affect thinking and emotions. Loss of consciousness can last from a few seconds to a few minutes, though many people with mild brain injury remain conscious after the injury. Even without losing consciousness, people often experience a state of being dazed, confused, or disoriented. Memory problems make it hard to remember what happened before or after the injury, or to form new memories. Concentration problems make it difficult to focus on tasks, follow conversations, or complete work. Mood changes or mood swings cause rapid shifts between different emotional states. Feeling depressed or anxious can develop soon after injury or during recovery. Difficulty sleeping affects the ability to fall asleep, stay asleep, or sometimes results in sleeping more than usual.

Moderate to severe brain injuries include all the symptoms of mild injury plus additional, more serious problems. Loss of consciousness can last from several minutes to hours. Persistent headache or headache that worsens over time indicates increasing pressure inside the skull. Repeated vomiting or nausea that doesn’t improve suggests serious complications. Convulsions or seizures occur when abnormal electrical activity affects the brain. Larger than normal pupils, called dilation of the pupil, especially if only one pupil is dilated, indicates dangerous pressure on the brain. Slurred speech becomes more pronounced and persistent. Weakness or numbness in the arms and legs affects one or both sides of the body. Loss of coordination makes walking and performing tasks difficult. Increased confusion, restlessness, or agitation that worsens over time requires immediate medical attention.

In babies, symptoms appear differently because infants cannot describe what they feel. Issues with eating or nursing mean the baby refuses food or has trouble feeding. Inconsolable crying means nothing caregivers do helps the baby stop crying. Changes in sleep patterns or unusual drowsiness differs from the baby’s normal behavior.

Some symptoms appear specifically within the first 24 hours after injury and signal serious complications. Unequal eye pupil size indicates pressure on one side of the brain. A person who is hard to wake up or cannot be awakened needs immediate emergency care. New neurological problems such as weakness of the face, arms, or legs on one side of the body suggest stroke-like damage. Convulsions that begin after a head injury indicate the brain has been seriously damaged.

Prevention

Preventing brain injury requires awareness of risks and taking practical steps to protect yourself and others. Since falls are the leading cause of brain injury overall and the most common cause in adults over 65, fall prevention is crucial. At home, remove tripping hazards like loose rugs, electrical cords, and clutter. Install handrails on stairs and grab bars in bathrooms. Ensure adequate lighting throughout the home, especially on stairs and in hallways. Use non-slip mats in bathtubs and showers. For older adults, regular vision checks, reviewing medications that might cause dizziness with a doctor, and exercises to improve balance and strength can significantly reduce fall risk.

Motor vehicle safety prevents the leading cause of brain injury in young adults. Always wear a seatbelt when in a vehicle, whether as driver or passenger. Ensure children are properly secured in age-appropriate car seats or booster seats installed according to manufacturer instructions. Never drive under the influence of alcohol or drugs. Avoid distracted driving by putting away cell phones and other distractions. Obey speed limits and traffic laws. When riding motorcycles or bicycles, always wear a helmet that meets safety standards and fits properly.

Sports and recreational safety protects against the approximately 300,000 sports-related brain injuries that occur annually. Wear appropriate protective equipment for your activity, including helmets for contact sports, cycling, skateboarding, skiing, snowboarding, and other high-risk activities. Ensure helmets fit properly and are designed for the specific sport. Follow safety rules and practice good sportsmanship. Learn and use proper techniques for your sport to minimize collision risks. If you sustain a head injury during sports, stop playing immediately and don’t return to the activity until a healthcare provider clears you.

Workplace safety prevents injuries in occupational settings. Use required safety equipment including hard hats, face shields, and protective gear. Follow workplace safety procedures and protocols. Keep work areas clean and organized to prevent trips and falls. Use ladders and scaffolding properly. Report unsafe conditions to supervisors. For those in high-risk occupations like construction, military service, or emergency response, additional specialized training in injury prevention is essential.

Preventing violence-related brain injury requires both personal and community-level approaches. Store firearms safely, unloaded and locked away with ammunition stored separately. Support efforts to reduce intimate partner violence through education, intervention programs, and support services for victims. Teach children conflict resolution skills and create safe environments for them. Communities can work to reduce violence through improved lighting, neighborhood watch programs, and support for at-risk individuals.

For young children, preventing abuse and neglect is critical since child abuse is the most common cause of brain injury in children under four. Never shake a baby or young child, as this can cause severe brain injury. Learn about normal child development to have realistic expectations of children’s behavior. Seek help when feeling overwhelmed by parenting responsibilities. Support families through parenting education, mental health services, and practical assistance.

Managing health conditions that increase fall risk helps prevent injury. Work with healthcare providers to manage conditions affecting balance, vision, or coordination. Review medications regularly, as some cause dizziness or drowsiness. If taking blood thinners, be extra cautious about fall prevention since these medications increase bleeding risk after head trauma. Use assistive devices like canes or walkers if needed, without embarrassment, as they significantly reduce fall risk.

Pathophysiology

Understanding what happens inside the brain during and after injury helps explain why symptoms occur and why treatment is critical. When the head is struck or moved violently, the brain—which floats in cerebrospinal fluid inside the skull—can collide with the skull’s bony interior. This collision can occur at the point of impact and also on the opposite side as the brain rebounds, creating what doctors call coup and contrecoup injuries.

The immediate physical damage takes several forms. Bruising of brain tissue, called contusions, occurs when small blood vessels in the brain rupture at the site of impact. These bruised areas can swell and may need to be surgically removed if they cause dangerous pressure. Torn tissues result when the force is strong enough to rip brain structures. Blood vessels can tear, leading to bleeding in the brain called hemorrhage. Blood may accumulate between the skull and the outer covering of the brain, forming an epidural hematoma, or between the brain’s covering layers, forming a subdural hematoma. These collections of blood compress brain tissue and increase pressure inside the skull.

The brain can also twist inside the skull during rapid acceleration or deceleration, such as in car crashes or when someone is violently shaken. This twisting motion stretches and tears nerve fibers, called axons, that connect different parts of the brain. This widespread damage, known as diffuse axonal injury, disrupts communication between brain cells and can cause loss of consciousness and long-term impairment even when imaging tests don’t show obvious damage.

Skull fractures create additional problems beyond the broken bone itself. Fragments of bone can penetrate the brain in penetrating injuries. Fractures at the skull base can tear the membranes covering the brain, allowing cerebrospinal fluid to leak from the nose or ears and creating a pathway for infection. Some fractures compress or damage brain tissue directly.

Beyond the immediate mechanical damage, brain injury triggers a cascade of chemical and cellular changes that can cause additional harm over hours, days, or weeks. This secondary injury process often determines the final outcome. When brain cells are damaged, they release chemicals that can harm neighboring cells. An excessive release of neurotransmitters, the chemicals brain cells use to communicate, can overexcite nearby cells and cause them to die. This process, called excitotoxicity, spreads damage beyond the initial injury site.

Swelling is a major concern after brain injury. Unlike most body parts that can expand when swollen, the brain is confined within the rigid skull. As the brain swells, pressure inside the skull increases, a condition called intracranial hypertension. This pressure can crush brain tissue, reduce blood flow to the brain, and force parts of the brain down through openings in the skull—a life-threatening emergency called herniation.

Inflammation develops as the body’s immune system responds to injury. While inflammation helps heal most injuries, in the brain it can cause additional damage. Immune cells release substances that can harm brain cells and increase swelling. The balance between helpful and harmful inflammation continues for weeks or months after injury.

Blood flow changes affect recovery. Brain injury can damage blood vessels or disrupt the mechanisms that regulate blood flow to different brain regions. Reduced blood flow means brain cells don’t get enough oxygen and nutrients, leading to further cell death. Some areas may have too much blood flow, contributing to swelling and pressure.

The blood-brain barrier, a protective layer that normally keeps harmful substances out of the brain, can break down after injury. This allows proteins, fluid, and inflammatory molecules to enter brain tissue, worsening swelling and damage.

Metabolic changes occur in brain cells. After injury, cells may not be able to use energy efficiently, even when blood flow is adequate. This energy crisis makes cells vulnerable to additional stress and impairs their ability to repair damage. Brain cells may also have difficulty maintaining the proper balance of minerals and water, contributing to swelling and cell death.

Long-term changes can develop. Some people develop seizures months or years after brain injury because damaged brain tissue creates abnormal electrical activity. Scar tissue forms where brain cells died, potentially interfering with normal brain function. In some cases, the injury triggers ongoing processes that resemble degenerative brain diseases.

For some people, brain injury begins a chronic disease process rather than a single event that heals completely. The initial trauma sets off changes that continue to evolve over time, potentially leading to ongoing problems with thinking, behavior, emotions, and physical function. This understanding has changed how doctors view brain injury, recognizing it not just as an acute emergency but as a condition requiring long-term monitoring and support.