Do Pupils Dilate With a Concussion?

Most concussions don’t leave the pupils looking dilated or lopsided. A concussion is a temporary disruption in how brain cells work and communicate after a blow or jolt. It usually isn’t visible damage or bleeding, and ordinary scans such as CT are typically normal. What concussion does change is subtler: how the pupils respond to light, rather than how big they look.¹ That’s why a clearly enlarged or uneven pupil after a head knock is treated as a warning of a more serious injury, not a typical concussion sign.²

What a Concussion Actually Does to the Pupils

Normally, pupils shrink when light hits them and widen again in the dark. After a concussion, this reflex tends to start later, move slower, and shrink the pupil less.³ The change affects both eyes the same way, so the pupils still match each other.⁴ Concussion alone shouldn’t leave one pupil bigger than the other.

The differences are tiny: in concussed athletes the pupil shrank to about 2.75 mm versus 3.64 mm in uninjured people, and the delay before it moved was about 0.25 seconds versus 0.21.⁵⁶ That can’t be seen by watching someone’s eyes.

Results don’t all point the same direction, either. Some studies of adults found smaller pupils after concussion.⁷ One study of teenage athletes found larger pupils and faster responses.⁸ So there’s no single pupil size that signals concussion.

The idea that dilated pupils mean concussion is a common myth. It mixes up concussion with severe brain injury, where pupils can blow wide open. Normal-looking pupils don’t rule out a concussion.

Other signs are more telling. Headache, nausea, dizziness, and feeling foggy are common, as are confusion, memory gaps around the hit, and trouble with balance. Sensitivity to light or noise is frequent too. Symptoms can appear hours later, so keep watching the person through the first day.

As for what you can see in the eyes: pupil size varies with room light, so a pupil that looks large in a dim room or small in sunlight is usually normal. Comparing the two pupils side by side in the same light is more useful than judging either one alone. If the person struggles to track a moving finger or reports double vision, mention it to a clinician.

When Big or Uneven Pupils Mean an Emergency

A pupil that stays wide and won’t shrink to light can mean swelling or bleeding is raising pressure inside the skull.⁹ This usually comes with growing drowsiness or confusion.¹⁰ But in one reported case, a man kept talking normally while one pupil was fixed and wide from brain bruising.¹¹ Any new pupil change after a head injury should be checked right away.¹²

The skull is a closed space, so bleeding or swelling inside it has nowhere to go and pushes on brain tissue. One nerve that controls pupil narrowing runs close to areas that get squeezed when pressure rises, which is why a pupil can stop reacting.

Some bleeds start slowly. A person can look okay after the injury and then become increasingly sleepy or confused over hours. A sudden change after an initial stretch of seeming normal should prompt an immediate call for help.

Speed matters. Among hospital patients with fixed, dilated pupils, 42% survived when surgeons removed the bleed or mass, versus 8% without surgery.¹³ Shorter delays before treatment gave better chances of recovery.¹⁴ So call 911 rather than waiting to see if it passes.

Other red flags: a worsening or severe headache that won’t ease, repeated vomiting, seizures, slurred speech, growing sleepiness or being hard to wake, weakness or numbness on one side, or clear fluid from the nose or ears.

While you wait for help, keep the person still, lying down if a neck injury is possible. Don’t give food, drink, or pain pills that cause drowsiness. Note when the pupil change started so you can tell paramedics, and stay with the person, watching their breathing and responsiveness.

About one in five people naturally have slightly unequal pupils. If you know the difference existed before the injury, tell the medics. Still get the person checked, because you can’t be sure at home.

Why a Flashlight Check Can’t Spot a Concussion

Judging pupils with a penlight or flashlight is subjective and not very reliable.¹⁵ A handheld pupil-measuring device, which films the pupil and times its response, measures reliably.¹⁶ Automated measurements also give more repeatable results and less examiner bias.¹⁷

The reaction is over in a fraction of a second, faster than people can reliably judge. A pupil-measuring device shields the eye from room light so every test starts under the same conditions, delivers a standard flash each time, and records video to calculate size, delay, and speed as numbers rather than impressions.

Many things besides injury change pupils: room lighting, strong emotion, medicines, earlier eye injuries, and naturally unequal pupils.¹⁸ Teenage girls tend to have larger pupils than boys, more so after a past concussion.¹⁹ So pupil testing alone can’t confirm or rule out a concussion.²⁰

A home check can catch an obviously big, unequal, or unreactive pupil, which is an emergency sign. It can’t detect the slight slowing concussion causes. A normal check is not clearance to keep playing.

Some cold and allergy medicines can widen pupils, while opioid pain medicines shrink them. Mention any medicines, alcohol, or drugs to whoever examines the person.

How Doctors Use Pupil Tests for Concussion

Pupil-measuring device readings of how fast the pupil starts and shrinks may help assess concussions with more symptoms.²¹ Slower reflex timing showed up even in people with past concussions who no longer had symptoms.²² In military cadets, pupil measurements helped identify injury and predict how severe and how long-lasting symptoms would be.²³

The devices track several features: starting size, how much the pupil shrinks, how quickly it constricts, and how fast it recovers. Each can shift slightly after a concussion.

Phones are being tested too. An iPhone-based test picked up slower pupil responses in concussed athletes.²⁴ Athletes with light sensitivity showed a stronger rebound widening of the pupil.²⁵ A smartphone program comparing an athlete’s own preseason recording to a post-injury one was correct about 91% of the time.²⁶ These tools work best against your own baseline, not as a stand-alone verdict. Baselines matter because pupils vary between people by age and sex.

In a clinic or training room, the person sits in a dim or shielded setting while a device is held close to the eye. A short flash triggers the reflex while the camera records. Each eye takes only a few seconds, and the device returns numbers for the clinician.

The clinician compares those numbers to typical ranges or, ideally, to the person’s own earlier baseline. A result that departs from baseline raises concern but doesn’t alone prove a concussion. It’s read together with symptom checklists, balance tests, thinking tests, and the story of the injury.

Researchers are also combining pupil readings with blood markers of brain injury.²⁷ If you play a sport, ask your doctor or team trainer whether a baseline pupil test is offered.

Pupils and Eyes During Concussion Recovery

After mild brain injury, the eye’s ability to focus up close is also weaker.²⁸ In people with past concussion, pupil size showed they worked harder on easy tasks and hit their limit sooner.²⁹ Reading or screen work may tire them faster.

Focusing up close requires the eyes to turn inward together and the lens to adjust, and both rely on brain pathways that can be disrupted. Reading or phone use can then lead to blurry words or headaches.

Common eye symptoms during recovery include blurry close vision, trouble reading, light sensitivity, headaches with screens, and eye strain or tiredness by afternoon. These usually ease as recovery progresses.

To ease strain, take frequent screen breaks and dim bright displays. Wear sunglasses outdoors if light hurts, and return to reading gradually. See an eye doctor if problems last beyond a few weeks. New pupil inequality later on needs urgent care, and worsening vision is not part of normal recovery.