Concussion (Mild Traumatic Brain Injury)

A concussion is a mild traumatic brain injury, caused by a blow, jolt or sudden movement that makes the brain shift inside the skull. It is the most common brain injury there is, and the word “mild” describes how it was graded, not how it feels.

Concussion is the one brain injury most people have heard of, which is part of the problem. Everyone has an opinion about it. You will be told to rest in a dark room, or to walk it off, or that you are fine because the scan was clear. A lot of that advice was standard ten years ago and has since been overturned.

This guide sets out what is currently understood, in plain language, so the conversation with your doctor or your child’s doctor is easier to follow. It cannot tell you whether a particular person has a concussion, or when they should play again. Only the care team can do that.

What actually happens in a concussion

The brain sits in fluid inside the skull, which cushions it well against ordinary movement. A hard enough impact, or a fast enough change of direction, makes it move further than the fluid can absorb. It does not have to be a blow to the head at all. A whiplash injury or a heavy landing can do it, because what matters is the acceleration.

The damage is not a bruise you could point to. It happens at the level of individual nerve cells, which get stretched, and of the chemistry that lets those cells signal to each other, which is briefly thrown out of balance. The brain then uses more energy than usual to do ordinary things while it sorts itself out. That is why concussion feels the way it does: thinking is effortful, noise is too loud, and being tired arrives much earlier in the day than it should.

It also explains why symptoms can appear hours after the event rather than immediately, and why someone can walk off a field looking fine and feel considerably worse that evening.

What is a brain injury
The types of brain injury, explained

Why “mild” is such an unhelpful word

In clinical terms, mild, moderate and severe describe measurements taken at the time of injury: whether consciousness was lost and for how long, the size of the memory gap around the event, the score on the Glasgow Coma Scale, and what any imaging showed. On all of those, a concussion sits at the mild end. That is genuinely good news about the injury.

It says nothing about the next three weeks. A mild traumatic brain injury can leave someone unable to finish a workday, tearful for no reason they can name, unable to tolerate a supermarket, and exhausted by a conversation. People are regularly told they should be over it, sometimes by employers, sometimes by family, sometimes by themselves. If that is where you are, the word is misleading you, and it is misleading the people around you too.

What people notice, and when

Concussion effects cluster into four areas. Most people get some from each, not all from one.

• Body. Headache, nausea, dizziness, balance trouble, blurred vision, sensitivity to light or noise.
• Thinking. Feeling slowed down or foggy, losing the thread, trouble concentrating, gaps in memory around the injury.
• Mood. Irritability, tearfulness, anxiety, feeling unlike yourself in a way that is hard to describe.
• Sleep. Sleeping far more than usual, or not being able to fall asleep at all.

Two things surprise people. Losing consciousness is not required, and most concussions do not involve it, which is why so many go unrecognized. And symptoms often arrive on a delay, which is the reason anyone who has taken a significant knock should be watched for the first 24 to 48 hours rather than assessed once and sent on their way.

This describes what people commonly report. It is not a checklist for deciding whether someone has a concussion, and no single item on it settles the question either way. That assessment belongs to a clinician.

Cognitive and behavioral changes after brain injury

How a concussion is diagnosed, and why the scan is normal

Concussion is diagnosed clinically. That means a person examines you and asks questions, rather than a machine producing a picture with the answer on it. The assessment usually covers memory and orientation, balance, eye movement and coordination, reaction time, and a structured symptom checklist. In sport there are sideline tools designed for exactly this.

Here is the part that causes the most distress, so it is worth stating plainly. A concussion does not show up on a CT scan. CT is looking for bleeding, swelling and fractures, and it is very good at those. It is not looking for the microscopic, chemical injury that a concussion actually is. A scan is ordered to rule out something more dangerous, and when it comes back clear that is exactly the result everyone wanted.

A normal scan is not evidence that symptoms are imagined or exaggerated. If someone tells you your scan was fine so you must be fine, they have misunderstood what the scan was for.

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

What recovery looks like, and what changed

If you were told to lie in a darkened room and avoid all stimulation until you felt better, that advice is out of date. It was standard for years. The evidence now points the other way, and international guidance was revised accordingly in 2023.

The current approach, in outline:

• Relative rest for the first 24 to 48 hours. Take it easy, cut back on screens, but gentle movement such as walking is fine and encouraged.
• Then start moving again. Light aerobic activity introduced within the first two to ten days is associated with better recovery, not worse.
• Use symptoms as a guide, not a stop sign. Guidance accepts a mild increase in symptoms during activity, roughly one to two points on a ten-point scale, provided it settles within about an hour. Complete avoidance of anything that provokes symptoms tends to slow things down.
• Build back gradually, with the steps and the pace agreed with a clinician who knows the case.

On timing: most people recover within four weeks. Between 20 and 30 percent of adults and young people take longer, which is common enough that it should not be treated as a sign something has gone badly wrong.

Going back: school and work first, then sport

Every return-to-play protocol has the same first step, and it is not a sporting one. It is being back to normal daily activities, including a full day of school or work, with a clinician’s go-ahead. Only then does the graded return to sport begin.

The sport progression runs through roughly six stages: light aerobic exercise, then moderate activity, then harder non-contact training, then sport-specific drills, then full-contact practice, then competition. Each stage takes at least 24 hours, and new symptoms mean stepping back rather than pushing through. Contact is not resumed without medical clearance.

For students, returning to learn deserves as much attention as returning to play, and often gets far less. Partial days, reduced screen work, extra time and a quiet space to work are ordinary and reasonable adjustments. Schools deal with this regularly; ask.

When symptoms last longer than expected

When symptoms continue beyond about four weeks, the older literature calls it post-concussion syndrome. Current terminology leans toward persisting symptoms after concussion, partly because “syndrome” implied a single fixed condition, and what people actually experience varies widely: headaches, dizziness, poor concentration, sleep problems, low mood, or some combination.

Two things are worth knowing. This is not rare, so it is not evidence of anything unusual having happened. And it is treatable. Guidance is that anyone still symptomatic past four weeks should be referred to a clinician with specific expertise in concussion, rather than left to wait it out. Targeted rehabilitation, whether vestibular, visual, exercise-based or psychological, is generally more effective than more rest.

Repeat concussions, and what is and is not known about CTE

Sustaining a second concussion before the first has settled is the clearest reason to take the return-to-play steps seriously. Recovery from a second injury tends to take longer. There is also a rare condition, second impact syndrome, in which rapid and dangerous brain swelling follows a second impact in someone still symptomatic. It is very uncommon, and mostly reported in adolescents and young adults, but it is severe enough that the sensible response is simply not to return to contact while symptoms are ongoing.

On chronic traumatic encephalopathy (CTE), honesty serves you better than reassurance or alarm. CTE is a degenerative condition found at autopsy in some people with a history of repeated head impacts, particularly in contact sport and the military. It is associated with memory, mood and behavior changes. What is not established is how much exposure carries how much risk, why some people develop it and others with similar histories do not, or how to diagnose it in a living person. It cannot currently be diagnosed while someone is alive.

Anyone telling you that a couple of concussions will give you CTE is going well beyond the evidence, and so is anyone telling you repeated head impacts carry no risk at all. The reasonable position is to reduce avoidable impacts and to manage each concussion properly, which is what all current guidance recommends.

Where to go from here

• If the concussion is your own, the guides on managing fatigue and going back to work or school are the practical next steps.
• If you are looking after someone who has one, start with the changes you might notice, which is often the hardest part for families.
• If the injury turned out to be more than a concussion, the moderate and severe guide covers what comes next.

Common questions