Traumatic Brain Injury (TBI)

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What is a traumatic brain injury (TBI)?

A traumatic brain injury (TBI) can be caused by a forceful bump, blow, or jolt to the head or body, or from an object that pierces the skull and enters the brain. Not all blows or jolts to the head result in a TBI.

Some types of TBI can cause temporary or short-term problems with normal brain function, including problems with how the person thinks, understands, moves, communicates, and acts. More serious TBI can lead to severe and permanent disability, and even death.

Some injuries are considered primary, meaning the damage is immediate. Other outcomes of TBI can be secondary, meaning they can occur gradually over the course of hours, days, or appear weeks later. These secondary brain injuries are the result of reactive processes that occur after the initial head trauma.

There are two broad types of head injuries: Penetrating and non-penetrating.

  1. Penetrating TBI (also known as open TBI) happens when an object pierces the skull (e.g., a bullet, shrapnel, bone fragment, or by a weapon such as hammer or knife) and enters the brain tissue. Penetrating TBI typically damages only part of the brain.
  2. Non-penetrating TBI (also known as closed head injury or blunt TBI) is caused by an external force strong enough to move the brain within the skull. Causes include falls, motor vehicle crashes, sports injuries, blast injury, or being struck by an object.

Some accidents such as explosions, natural disasters, or other extreme events can cause both penetrating and non- penetrating TBI in the same person.

Signs and symptoms

Seek immediate medical attention if you experience any of the following physical, cognitive/behavioral, or sensory symptoms, especially within the first 24 hours after a TBI:


  • Headache
  • Convulsions or seizures
  • Blurred or double vision
  • Unequal eye pupil size or dilation
  • Clear fluids draining from the nose or ears
  • Nausea and vomiting
  • New neurologic deficit, such as slurred speech; weakness of arms, legs, or face; loss of balance


  • Loss of or change in consciousness anywhere from a few seconds to a few hours
  • Decreased level of consciousness (e.g., hard to awaken)
  • Mild to profound confusion or disorientation
  • Problems remembering, concentrating, or making decisions
  • Changes in sleep patterns (e.g., sleeping more, difficulty falling or staying asleep); inability to waken from sleep
  • Frustration, irritability


  • Light-headedness, dizziness, vertigo, or loss of balance or coordination
  • Blurred vision
  • Hearing problems, such as ringing in the ears
  • Bad taste in the mouth
  • Sensitivity to light or sound
  • Mood changes or swings, agitation, combativeness, or other unusual behavior
  • Feeling anxious or depressed
  • Fatigue or drowsiness; a lack of energy or motivation

Headache, dizziness, confusion, and fatigue tend to start immediately after an injury but resolve over time. Emotional symptoms such as frustration and irritability tend to develop during recovery.

TBI in children

Children might be unable to let others know that they feel different following a blow to the head. A child with a TBI may display the following signs or symptoms:

  • Changes in eating or nursing habits
  • Persistent crying, irritability, or crankiness; inability to be consoled
  • Changes in ability to pay attention
  • Lack of interest in a favorite toy or activity
  • Changes in sleep patterns
  • Seizures
  • Sadness or depression
  • Loss of a skill, such as toilet training
  • Loss of balance or unsteady walking
  • Vomiting

Effects on consciousness

A TBI can cause problems with consciousness, awareness, alertness, and responsiveness. Generally, there are four abnormal states that can result from a severe TBI:

  1. Minimally conscious state—People with severely altered consciousness who still display some evidence of self-awareness or awareness of one's environment (such as following simple commands, yes/no responses).
  2. Vegetative state—A result of widespread damage to the brain, people in a vegetative state are unconscious and unaware of their surroundings. However, they can have periods of unresponsive alertness and may groan, move, or show reflex responses. If this state lasts longer than a few weeks, it is referred to as a persistent vegetative state.
  3. Coma—A person in a coma is unconscious, unaware, and unable to respond to external stimuli such as pain or light. Coma generally lasts a few days or weeks after which the person may regain consciousness, die, or move into a vegetative state. 
  4. Brain death—The lack of measurable brain function and activity after an extended period of time is called brain death and may be confirmed by studies that show no blood flow to the brain.

How TBI affects the brain

TBI-related damage can be confined to one area of the brain, known as a focal injury, or it can occur over a more widespread area, known as a diffuse injury. The type of injury also affects how the brain is damaged.

Primary effects on the brain include various types of bleeding and tearing forces that injure nerve fibers and cause inflammation, metabolic changes, and brain swelling.

  • Diffuse axonal injury (DAI), one of the most common types of brain injuries, refers to widespread damage to the brain's white matter. White matter is composed of bundles of axons (the projections of nerve cells that carry electrical impulses and connect various areas of the brain to one another). DAI usually results from rotational forces (twisting) or sudden forceful stopping that stretches or tears these axon bundles. This damage commonly occurs in auto accidents, falls, or sports injuries. DAI can disrupt and break down communication among nerve cells (neurons) in the brain. It also leads to the release of brain chemicals that can cause further damage. Brain damage may be temporary or permanent and recovery can be prolonged.
  • Concussion is a type of mild TBI that may be considered a temporary injury to the brain but could take minutes to several months to heal. Concussion can be caused by a number of things including a bump, blow, or jolt to the head, sports injury or fall, motor vehicle accident, weapons blast, or a rapid acceleration or deceleration of the brain within the skull, such as the person having been violently shaken. The individual either suddenly loses consciousness or has sudden altered state of consciousness or awareness. A second concussion closely following the first one causes further damage to the brain—the so-called “second hit” phenomenon—and can lead to permanent damage or even death in some instances. Post-concussion syndrome involves symptoms that last for weeks or longer.
  • Hematomas are bleeding in and around the brain caused by a rupture to a blood vessel. Different types of hematomas form depending on where the blood collects relative to the meninges, the protective membranes surrounding the brain, which consist of three layers: dura mater (outermost), arachnoid mater (middle), and pia mater (innermost).
    • Epidural hematomas involve bleeding into the area between the skull and the dura mater. These can occur within minutes to hours after damage to a brain vessel under the skull and are particularly dangerous.
    • Subdural hematomas involve bleeding between the dura and the arachnoid mater, and, like epidural hematomas, exert pressure on the outside of the brain. They are very common in the elderly after a fall.
    • Subarachnoid hemorrhage is bleeding between the arachnoid mater and the pia mater.
    • Bleeding into the brain itself is called an intracerebral hematoma and damages the surrounding tissue.
  • Contusions are a bruising or swelling of the brain that occurs when very small blood vessels bleed into brain tissue. Contusions can occur directly under the impact site (a coup injury) or, more often, on the complete opposite side of the brain from the impact (a contrecoup injury). They can appear after a delay of hours to a day. Coup and contrecoup lesions generally occur when the head abruptly decelerates, which causes the brain to bounce back and forth within the skull (such as in a high-speed car crash or in shaken baby syndrome).
  • Skull fractures are breaks or cracks in one or more of the bones that form the skull. They are a result of blunt force trauma and can cause damage to the membranes, blood vessels, and brain under the fracture. One main benefit of helmets is to prevent skull fractures.
  • Chronic traumatic encephalopathy (CTE) is a progressive neurological disorder associated with symptoms that may include problems with thinking, understanding, and communicating; motor disorders (affecting movement); problems with impulse control and depression; confusion; and irritability. CTE occurs in those with extraordinary exposure to multiple blows to the head and as a delayed consequence after many years. Studies of retired boxers have shown that repeated blows to the head can cause issues including memory problems, tremors, and lack of coordination and dementia. Recent studies have demonstrated rare cases of CTE in other sports with repetitive mild head impacts (e.g., soccer, wrestling, football, and rugby).
  • A single, severe TBI also may lead to a disorder called post-traumatic dementia (PTD), which may be progressive and share some features with CTE. Studies assessing patterns among large populations of people with TBI indicate that moderate or severe TBI in early or mid-life may be associated with increased risk of dementia later in life.

Examples of secondary damage:

  • Hemorrhagic progression of a contusion (HPC) are injuries that occur when an initial contusion from the primary injury continues to bleed in and around the brain and expand over time. This creates a new or larger lesion—an area of tissue that has been damaged through injury or disease. This increased exposure to blood, which is toxic to brain cells, leads to swelling and further brain cell loss.
  • A breakdown in the blood-brain barrier refers to the disruption of the network of cells that controls the movement of cells and molecules between the blood and fluid that surrounds the brain's nerve cells. Once the blood-brain barrier is disrupted, blood, plasma proteins, and other foreign substances leak into the space between neurons in the brain and trigger a chain reaction that causes brain swelling. It also causes multiple biological systems to go into overdrive, including inflammatory responses which can be harmful to the body if they continue for an extended period of time. It also permits the release of neurotransmitters, or chemicals used by brain cells to communicate, which can damage or kill nerve cells when depleted or over-expressed.
  • Increased intracranial pressure is usually caused by brain swelling inside the confined area of the skull as a result of the injury. This pressure can damage brain tissue and can prevent blood flow to the brain and deprive it of the oxygen it needs to function.
  • Other secondary damage can be caused by infections to the brain, low blood pressure or oxygen flow as a result of the injury, hydrocephalus (a buildup of fluid in the brain that can increase pressure on brain tissue), and seizures.

Who is more likely to get a traumatic brain injury (TBI)?

Adults age 65 and older are at greatest risk for being hospitalized and dying from a TBI, most likely from a fall. In every age group, serious TBI rates are higher for men than for women. Men are more likely to be hospitalized and are nearly three times more likely to die from a TBI than women.

The leading causes of TBI include:

  • Falls—According to data from the Centers for Disease Control and Prevention (CDC), falls are the most common cause of TBIs and occur most frequently among the youngest and oldest age groups.
  • Blunt trauma accidents—Accidents that involve being struck by or against an object, particularly sports-related injuries, are a major cause of TBI.
  • Vehicle-related injuries—Pedestrian-involved accidents, as well as accidents involving motor vehicles and bicycles, are the third most common cause of TBI.
  • Assaults/violence Assaults—Abuse-related TBIs are head injuries that result from domestic violence or shaken baby syndrome, and gunshot wounds to the head. TBI-related deaths in children age 4 and younger are most likely the result of assault.
  • Explosions/blasts—TBIs caused by blast trauma from roadside bombs became a common injury to service members in military conflicts. The majority of these TBIs are classified as mild head injuries.

How is a traumatic brain injury (TBI) diagnosed and treated?

Diagnosing TBI

All TBIs require immediate assessment by a professional who has experience evaluating head injuries. A neurological exam will judge motor and sensory skills and test hearing and speech, coordination and balance, mental status, and changes in mood or behavior, among other abilities. Screening tools for coaches and athletic trainers can identify the most concerning concussions for medical evaluation.

Initial assessments may rely on standardized instruments such as the Acute Concussion Evaluation (ACE) form from the Centers for Disease Control and Prevention (CDC) or the Sport Concussion Assessment Tool 2, which provide a systematic way to assess a person who has suffered a mild TBI. Reviewers collect information about the characteristics of the injury, the presence of amnesia (loss of memory) and/or seizures, as well as the presence of physical, cognitive, emotional, and sleep-related symptoms. The ACE is also used to track symptom recovery over time. It also takes into account risk factors (including concussion, headache, and psychiatric history) that can impact how long it takes to recover from a TBI.

Diagnostic imaging. When necessary, medical providers will use brain scans to evaluate the extent of the primary brain injuries and determine if surgery will be needed to help repair any damage to the brain. The need for imaging is based on a physical examination by a doctor and a person's symptoms.

  • Computed tomography (CT) is the most commonly used imaging technology to assess people with suspected moderate to severe TBI. CT creates a two-dimensional image of organs, bones, and tissues and can show a skull fracture or any brain bruising, bleeding, or swelling.
  • Magnetic resonance imaging (MRI) produces detailed images of body tissue. It may be used after the initial assessment and treatment as it is a more sensitive test and picks up subtle changes in the brain that the CT scan might have missed. Significant advances have been made in the last decade to image milder TBI damage. For example, diffusion tensor imaging can image white matter tracts, more sensitive tests like fluid-attenuated inversion recovery can detect small areas of damage, and susceptibility-weighted imaging very sensitively identifies bleeding. Despite these improvements, currently available imaging technologies, blood tests, and other measures remain inadequate for detecting these changes in a way that can help diagnose mild concussive injuries.

Neuropsychological tests to gauge brain functioning are often used along with imaging in people who have suffered mild TBI. Such tests involve performing specific cognitive tasks that help assess memory, concentration, information processing, executive functioning, reaction time, and problem solving.

The Glasgow Coma Scale is the most widely used tool for assessing the level of consciousness after TBI. The standardized 15-point test measures a person's ability to open his or her eyes and respond to spoken questions or physical prompts for movement. 

Many athletic organizations recommend establishing a baseline picture of an athlete's brain function at the beginning of each season, ideally before any head injuries occur. Baseline testing should begin as soon as a child begins a competitive sport. Brain function tests yield information about an individual's memory, attention, and ability to concentrate and solve problems. Brain function tests can be repeated at regular intervals (every one to two years) and also after a suspected concussion. The results may help healthcare providers identify any effects from an injury and allow them to make more informed decisions about whether a person is ready to return to their normal activities.

Treating TBI

Many factors—including the size, severity, and location of the brain injury—influence how a TBI is treated and how quickly a person might recover. One of the critical elements to a person's prognosis is the severity of the injury. Although brain injury often occurs at the moment of head impact, much of the damage related to severe TBI develops from secondary injuries which happen days or weeks after the initial trauma. For this reason, people who receive immediate medical attention at a certified trauma center tend to have the best health outcomes. 

Mild TBI

Some people with mild TBI such as concussion may not require treatment other than rest and over-the-counter pain relievers. Treatment should focus on symptom relief and “brain rest.” Monitoring by a healthcare practitioner is important to note any worsening of symptoms or new ones.

Children and teens who have a sports-related concussion should stop playing immediately and return to play only after being approved by a concussion injury specialist.

Preventing future concussions is critical. While most people recover fully from a first concussion within a few weeks, the rate of recovery from a second or third concussion is generally slower.

Even after symptoms resolve entirely, people should return to their daily activities gradually once they are given permission by a doctor. There is no clear timeline for a safe return to normal activities although there are guidelines such as those from the American Academy of Neurology and the American Medical Society for Sports Medicine to help determine when athletes can return to practice or competition. Further research is needed to better understand the effects of mild TBI on the brain and to determine when it is safe to resume normal activities.

People with a mild TBI should:

  • Make an appointment for a follow-up visit with their healthcare provider to confirm the progress of their recovery
  • Inquire about new or persistent symptoms and how to treat them
  • Pay attention to any new signs or symptoms even if they seem unrelated to the injury (for example, mood swings, unusual feelings of irritability)

These symptoms may be related even if they occurred several weeks after the injury.

Medications to treat some of the symptoms of TBI may include:

  • Over-the-counter or prescribed pain medicines
  • Anticonvulsant drugs to treat seizures
  • Anticoagulants to prevent blood clots
  • Diuretics to help reduce fluid buildup and reduce pressure in the brain
  • Stimulants to increase alertness
  • Antidepressants and anti-anxiety medications to treat depression and feelings of fear and nervousness

Severe TBI

Immediate treatment for someone who has suffered a severe TBI focuses on preventing death; stabilizing the person's spinal cord, heart, lung, and other vital organ functions; ensuring proper oxygen delivery and breathing; controlling blood pressure; and preventing further brain damage. Emergency care staff will monitor the flow of blood to the brain, brain temperature, pressure inside the skull, and the brain's oxygen supply.

Surgery may be needed to for emergency medical care and to treat secondary damage, including:

  • Relieving pressure inside the skull (inserting a special catheter through a hole drilled into the skull to drain fluids)
  • Removing debris or dead brain tissue (especially for penetrating TBI)
  • Removing hematomas
  • Repairing skull fractures

In-hospital strategies for managing people with severe TBI aim to prevent conditions including:

  • Infection, particularly pneumonia
  • Deep vein thrombosis (blood clots that occur deep within a vein; risk increases during long periods of inactivity)

People with TBIs may need nutritional supplements to minimize the effects that vitamin, mineral, and other dietary deficiencies may cause over time. Some individuals may even require tube feeding to maintain the proper balance of nutrients.


After the acute care period of in-hospital treatment, people with severe TBI are often transferred to a rehabilitation center where a multidisciplinary team of health care providers help with recovery.

The rehabilitation team includes neurologists, nurses, psychologists, nutritionists, as well as physical, occupational, vocational, speech, and respiratory therapists.

Therapy is aimed at improving the person's ability to handle activities of daily living and to address cognitive, physical, occupational, and emotional difficulties. Treatment may be needed on a short-term basis or throughout a person's life. Some therapy is provided through outpatient services.

Cognitive rehabilitation therapy (CRT) is a strategy aimed at helping individuals regain their normal brain function through an individualized training program. Using this strategy, people may also learn compensatory strategies for coping with persistent deficiencies involving memory, problem solving, and the thinking skills to get things done. CRT programs tend to be highly individualized and their success varies. A 2011 Institute of Medicine report concluded that cognitive rehabilitation interventions need to be developed and assessed more thoroughly.

Other factors that influence recovery include genes and age.

Genes—Genetics may play a role in how quickly and completely a person recovers from a TBI. For example, researchers have found that apolipoprotein E ε4 (ApoE4) — a genetic variant associated with higher risks for Alzheimer's disease — is associated with worse health outcomes following a TBI. Much work remains to be done to understand how genetic factors, as well as how specific types of head injuries, affect recovery. This research may lead to new treatment strategies and improved outcomes for people with TBI.

Age—Studies suggest that age and the number of head injuries a person has suffered over his or her lifetime are two critical factors that impact recovery. For example, TBI-related brain swelling in children can be very different from the same condition in adults, even when the primary injuries are similar. Brain swelling in newborns, young infants, and teenagers often occurs much more quickly than it does in older individuals. Evidence from very limited CTE studies suggest that younger people (ages 20 to 40) tend to have behavioral and mood changes associated with CTE, while those who are older (ages 50+) have more cognitive difficulties.

Compared with younger adults with the same TBI severity, older adults are likely to have less complete recovery. Older people also have more medical issues and are often taking multiple medications that may complicate treatment (e.g., blood-thinning agents when there is a risk of bleeding into the head). Further research is needed to determine if and how treatment strategies may need to be adjusted based on a person's age.

Preventing TBI

The best treatment for TBI is prevention. Unlike most neurological disorders, head injuries can be prevented. According to the CDC, the following actions can help prevent TBIs:

  • Wear a seatbelt when you drive or ride in a motor vehicle  
  • Wear the correct helmet and make sure it fits properly when riding a bicycle, skateboarding, and playing sports like hockey and football
  • Install window guards and stair safety gates at home for young children
  • Never drive under the influence of drugs or alcohol
  • Improve lighting and remove rugs, clutter, and other trip hazards in the hallway
  • Use nonslip mats and install grab bars next to the toilet and in the tub or shower for older adults
  • Install handrails on stairways
  • Improve balance and strength with a regular physical activity program
  • Ensure children's playgrounds are made of shock-absorbing material, such as hardwood mulch or sand

    What are the latest updates on traumatic brain injuries (TBI)?

    The mission of NINDS is to seek fundamental knowledge about the brain and nervous system and use that knowledge to reduce the burden of neurological disease. NINDS, a component of NIH, supports research across the full range of TBI severity. Here is a list of efforts and developments.

    • Transforming Research and Clinical Knowledge in TBI, or TRACK TBI, is an observational study of adults and children with TBI across the spectrum of injury severity. It is creating a TBI database and provides tools and resources to establish more precise methods to diagnose TBI, improve outcome assessment, and compare the effectiveness and costs of tests, treatments, and services. The data from this study will be available in the Federal Interagency TBI Research database (which enables comparisons across clinical trials and clinical studies.
    • Scientists can now look in real time at how head injury affects thousands of individual cells and genes simultaneously in mice. Using a novel sequencing technique that can quickly analyze the gene activity of a cell, scientists were able to look individual brain cells in the hippocampus, a region of the brain involved in learning and memory, after TBI or in uninjured control animals. Scientists can now pinpoint which genes to treat with new therapies.
    • Researchers are conducting studies to better understand the lasting effects of a single head injury vs. repetitive injuries to the brain, how repetitive TBI might lead to chronic traumatic encephalopathy, and how commonly these changes occur among adults. NINDS researchers are currently working to identify biomarkers (signs that may indicate risk of a disease and aid in diagnosis) for chronic traumatic encephalopathy in order to detect this and similar disorders in living people rather than through brain studies after death. 
    • Researchers are exploring ways to promote the brain's innate ability to adapt and repair itself, known as neuroplasticity.  
    • A developed mouse model of TBI is enabling researchers to look at potential treatments for concussion. Using the model, they found that applying glutathione (an antioxidant that is normally found in our cells) directly on the skull surface after brain injury reduced the amount of brain cell death.  
    • The NINDS-funded Translational Outcomes Project in Neurotrauma (-NT) consortium supports development and creation of better assessment tools for preclinical studies in TBI and spinal cord injury and to support data sharing to improve preclinical studies and clinical trial design.
    •  In February 2018 the U.S. Food and Drug Administration (FDA) authorized marketing the first blood test to help diagnose concussion. NINDS funded the early work on the project and the Department of Defense supported its later development.

    Clinical research

    Despite recent progress in understanding what happens in the brain following TBI, more than 30 large clinical trials have failed to identify specific treatments that make a dependable and measurable difference in people with TBI. A key challenge facing doctors and scientists is the fact that each person with a TBI has a unique set of circumstances based on such multiple variables as the location and severity of the injury, the person's age and overall heath, and the time between the injury and the initiation of treatment. These factors, along with differences in care across treatment centers, highlight the importance of coordinating research efforts so that the results of potential new treatments can be confidently measured.

    NINDS co-leads the Strategies to Innovate EmeRgENcy Care Clinical Trials (SIREN) network, with projects that include TBI trials — one of which is looking at brain tissue oxygen monitoring to improve neurologic outcome in the most severely injured people with TBI.

    Harnessing the efforts of the many physicians and scientists working on developing better treatments for TBI requires everyone to collect the same types of information from people, including details about injuries and treatment results. To lay the groundwork for these studies, NINDS started the Common Data Elements project. This effort brings the research community together to develop data collection standards.

    Interagency and international research collaboration

    • NINDS and the European Commission conduct studies through the International Initiative for TBI research (InTBIR) to collect data and encourage new collaborations to improve diagnosis and evaluate which types of care are associated with the best outcomes in children and adults.
    • NIH and the Department of Defense together lead the Federal Interagency TBI Research (FITBIR) database, which includes both new observational studies and other studies, such as the Child Health After Injury Study.
    • NIH investigators and the FDA are active collaborators in the Department of Defense-led TBI Endpoints Initiative to advance diagnosis and treatment of TBI.
    • NINDS also works with Department of Defense and the Departments of Health and Human Services, Veterans Affairs, and Education to coordinate TBI research for military members. This National Research Action Plan (NRAP) aims to improve prevention, diagnosis, and treatment of TBI and other mental health conditions such as PTSD that affect veterans and their families. The findings resulting from NRAP will be rapidly translated into new effective prevention strategies and clinical innovations, as well as identify biomarkers to detect these disorders early and accurately.

    NIH research projects on TBI and other disorders can be found using NIH RePORTER, a searchable database of current and past research projects supported by NIH and other federal agencies. RePORTER also includes links to publications from these projects and other resources.

    Learn About Clinical Trials
    Clinical trials are studies that allow us to learn more about disorders and improve care. They can help connect patients with new and upcoming treatment options.

    How can I or my loved one help improve care for people with a traumatic brain injury (TBI)?

    Consider participating in a clinical trial so clinicians and scientists can learn more about TBI and related disorders. Clinical research uses human volunteers to help researchers learn more about a disorder and perhaps find better ways to safely detect, treat, or prevent disease.

    All types of volunteers are needed—those who are healthy or may have an illness or disease—of all different ages, sexes, races, and ethnicities to ensure that study results apply to as many people as possible, and that treatments will be safe and effective for everyone who will use them.

    For information about participating in clinical research visit NIH Clinical Research Trials and You. Learn about clinical trials currently looking for people with TBI at

    People with a TBI also can support TBI research by designating the donation of brain tissue before they die. The study of human brain tissue is essential to increasing the understanding of how the nervous system functions.

    The NIH NeuroBioBank is an effort to coordinate the network of brain banks it supports across the country to advance research through the collection and distribution of post-mortem brain tissue. Stakeholder groups include brain and tissue repositories, researchers, NIH program staff, information technology experts, disease advocacy groups, and most importantly individuals seeking information about opportunities to donate. It ensures protection of the privacy and wishes of donors. 

    Where can I find more information about traumatic brain injury (TBI)?

    Information may be available from the following resources:

    Order publications from the NINDS Catalog
    The NINDS Publication Catalog offers printed materials on neurological disorders for patients, health professionals, and the general public. All materials are free of charge, and a downloadable PDF version is also available for most publications.