What Is a Brain Injury?

A brain injury is any change in the way the brain works, or any other sign that the brain has been damaged, caused either by an outside force or by a medical event inside the body.

That one sentence covers an enormous amount of ground. It covers a teenager who took a hit in a hockey game and a grandmother who slipped in her kitchen. It covers a stroke, a cardiac arrest that briefly cut off oxygen, and a car crash on a wet road. What links them is not how they happened. It is that the organ responsible for movement, memory, mood, language and judgment has been affected.

If you are reading this in a hospital corridor, or on your phone at two in the morning after someone used a word you had never heard before, this page is for you. It will not tell you what is happening in your particular situation, because we cannot know that and neither can any website. What it can do is make the next conversation with the care team easier to follow.

The two broad families

Almost everything you will hear falls into one of two groups.

Traumatic brain injury (TBI) is caused by an outside force: a fall, a vehicle crash, a sports collision, a blast, an assault, or being struck by something. It is described as closed when the skull stays intact, and open or penetrating when something breaks through.

Acquired brain injury (ABI) is caused by something that happened inside the body rather than to it: a stroke, a period without enough oxygen (after cardiac arrest, drowning or an overdose), an infection such as meningitis or encephalitis, a tumor, or a burst aneurysm.

One thing that trips people up: the terms are not used consistently. Some clinicians use “acquired brain injury” as the wider umbrella that includes traumatic injuries too. If you hear it both ways in the same week, nothing has gone wrong. It is worth asking which sense someone means.

The types of brain injury, explained

What “mild”, “moderate” and “severe” actually mean

Severity is graded from things measured at or near the time of injury: whether consciousness was lost and for how long, how large the gap in memory around the event is, the score on the Glasgow Coma Scale, and what any scans showed.

Here is the part worth holding on to. “Mild” describes the event, not the experience. A mild traumatic brain injury, which most people know as a concussion, can still leave someone exhausted by lunchtime, tearful without warning, and unable to hold a thread of conversation for months afterward. Plenty of survivors find that word genuinely hurtful, and it is easy to see why. It is a clinical grade for what happened at the moment of injury. It is not a verdict on what you are going through now, and it is not a reason to feel you should be coping better.

Concussion and mild TBI
Moderate and severe brain injury

How common brain injuries are, and who they happen to

Brain injury is far more common than most people assume, and the official numbers are known to be an undercount.

Figures from the US Centers for Disease Control and Prevention. They count hospitalizations and deaths only, so they leave out the very large number of injuries treated in emergency departments, urgent care or a family doctor’s office, and the many that are never treated at all.

The pattern behind the numbers is not the one most people picture. Falls, not car crashes, lead to nearly half of TBI-related hospitalizations, and adults aged 75 and over have the highest rates of both hospitalization and death. Firearm-related suicide is the most common single cause of TBI-related deaths. Motor vehicle crashes, assaults, sports and blast exposure make up much of the rest.

What causes a brain injury
Brain injury by the numbers

What people notice after a brain injury

Because different parts of the brain do different jobs, no two injuries produce quite the same effects. Broadly, the changes people report fall into four areas.

• Body. Headache, nausea, dizziness, tiredness that sleep does not fix, problems with balance or coordination, changes in vision.
• Thinking. Trouble holding attention, gaps in memory, losing the thread mid-sentence, taking longer to process what is being said, difficulty planning or starting a task.
• Mood and behavior. Irritability that arrives out of nowhere, low mood, anxiety, saying things that would once have been left unsaid, crying or laughing at odd moments.
• Senses. Sensitivity to light or noise, ringing in the ears, changes in taste or smell.

Three things are worth knowing about that list. Some of these appear straight away and others surface days or weeks later. Many come and go, so a good Tuesday says little about Wednesday. And frequently the changes are noticed first by the people around the person rather than by the person themselves, which is one of the harder parts of this for families.

This is a description of what people commonly experience. It is not a checklist for working out whether someone has a brain injury, and the presence or absence of any single item on it tells you very little on its own. That assessment belongs to the care team.

Cognitive and behavioral changes

How a brain injury is assessed

In the first hours, the team is answering an urgent and fairly narrow question: is anything happening inside the skull that needs to be acted on right now. The fuller picture of how someone is thinking and functioning comes much later.

• Neurological examination. Reflexes, pupils, movement, speech, orientation. Repeated often, because what matters most is whether it is changing.
• The Glasgow Coma Scale. A score out of 15 covering eye opening, speech and movement. You will hear it quoted a lot, and it is a snapshot rather than a forecast.
• CT scan. Fast, and very good at spotting bleeding, swelling and fractures that may need surgery today.
• MRI. Slower and more detailed. Often used later to see damage a CT cannot pick up.
• Neuropsychological testing. Weeks or months later, mapping memory, attention, language and problem solving to guide rehabilitation.

One honest caveat that is rarely explained: a clear CT scan does not always mean nothing happened. A CT is looking for specific things, and it is excellent at those things. Some injuries simply do not show up on it.

How a brain injury is diagnosed
What a CT scan or MRI shows

What recovery tends to look like

We cannot tell you how a particular injury will turn out, and we would be doing you a disservice if we tried. What we can describe is the shape of it, because that shape surprises people.

Recovery is rarely a line that climbs steadily. Families almost always describe good weeks and bad weeks, real progress followed by a stretch where nothing seems to move, then progress again. A hard week is not evidence that recovery has stopped. Rehabilitation is usually a team effort rather than one treatment: physical therapy, occupational therapy, speech and language therapy, cognitive rehabilitation, and psychological support, in whatever combination fits.

Two things are generally true. Change tends to be fastest early on, which is why the first months feel so intense. And improvement can continue long after that early phase, in ways that are less about the injury healing and more about relearning, adapting and finding what works. Anyone who tells you in the first week exactly how this ends is guessing.

Where to go from here

What you need next depends on where you are standing.

• If someone you love was injured recently, start with the first days and what to expect, then a family’s guide to the ICU.
• If the injury is your own, the guides on managing fatigue and the emotional side of recovery are where most people say it starts to make sense.
• If you have become the person doing the caring, read the first 30 days and, when you can, avoiding burnout.

Common questions