How to Check Pupils for Concussion: Signs of Head Injury Not to Ignore

A fall off a bike, a hard hit during a pickup game, a bump against a low cabinet door. When something like this happens to someone we care about, most of us do the same thing first. We look them in the eye and check their pupils, as if that one glance could tell us everything we need to know.

It can't, not on its own. Pupil checks matter, and we want to show you how to do them correctly, but a pupil check is one piece of a much larger picture. We'll walk through the safe way to check someone's pupils after a bump to the head, what a size difference actually means, and the complete list of danger signs the CDC says should send you straight to the emergency room, no matter what the pupils look like.

Why Pupils Matter After a Head Injury?

The muscles that open and close our pupils are controlled by a set of nerves that run close to the brain stem. Under normal conditions, both pupils shrink and widen together, matching each other in size and reacting the same way to light.

After a bump, blow, or jolt to the head, pressure or bleeding inside the skull can press on those nerves. When that happens, one pupil may stop responding normally while the other still works the way it should. That is why a new pupil size difference after a head injury is treated so seriously. According to CDC's HEADS UP program, one pupil larger than the other is listed among the specific danger signs that call for immediate emergency care, because it can point to a dangerous collection of blood building up inside the skull.

This is different from a typical concussion, which usually does not involve bleeding or swelling severe enough to affect the pupils. A pupil change points toward something more serious than a routine bump on the head, which is exactly why it gets treated as an emergency signal rather than a wait-and-monitor situation.

How to Check Someone's Pupils Safely

You don't need special training or equipment to check someone's pupils. Follow these simple steps:

1. Check the pupils in normal lighting. Avoid dark rooms, as low light naturally causes pupils to widen and can make them appear uneven. Look at both pupils to see if they are similar in size and shape.

2. Use a small light source. Shine a phone flashlight or penlight briefly into one eye, then the other.

3. Watch how each pupil responds. A normal pupil should constrict quickly when exposed to light and return to its usual size once the light is removed. Compare how both eyes react, not just their size at rest.

4. Keep the examination brief. A few seconds per eye is enough. There's no need to hold the light in place or repeat the test multiple times.

5. Know when to seek emergency care. If one pupil is noticeably larger than the other or one pupil responds much more slowly to light, seek emergency medical attention immediately, especially after a head injury.

Important: Checking someone's pupils does not replace a medical evaluation. If unequal pupils develop after a head injury or are accompanied by symptoms such as worsening headache, repeated vomiting, confusion, seizures, or loss of consciousness, call emergency services right away.

What Unequal Pupils Actually Means

Not all cases of unequal pupils are caused by a head injury. The key is whether the difference is new or has always been present.

If You're Not Sure, Treat It Seriously

Most people don't know what someone else's pupils normally look like. If you notice unequal pupils after a head injury and can't tell whether the difference is new, it's safest to treat it as a potential emergency and seek immediate medical evaluation.

The most important factor isn't simply whether the pupils are unequal; it's when the difference appeared and whether it developed after the head injury.

The Full List of CDC Concussion Danger Signs

Pupils are one entry on a much longer list. According to CDC's HEADS UP program, call 911 or go to the emergency department right away if someone shows any of these signs after a bump, blow, or jolt to the head.

• One pupil larger than the other

• Drowsiness or inability to be awakened

• A headache that gets worse and does not go away

• Weakness, numbness, or decreased coordination

• Repeated vomiting or nausea

• Slurred speech

• Convulsions or seizures

• Difficulty recognizing people or places

• Increasing confusion, restlessness, or agitation

• Unusual behavior

• Loss of consciousness, even briefly

Any one of these on its own is enough to call 911. You do not need to see several signs together before acting, and you do not need to confirm a pupil difference before treating another sign, like slurred speech or repeated vomiting, as serious. Each item on this list exists because it can independently point to a dangerous buildup of blood or swelling inside the skull.

This list is not something to memorize under pressure. It is something to recognize quickly enough to act on, which is exactly the kind of pattern recognition that formal first aid training builds through repetition rather than a single read-through.

General Concussion Signs vs. Danger Signs

Not every symptom after a head injury requires calling 911, but some do. The table below explains the difference.

What Should You Do?

• General concussion signs should be evaluated by a healthcare provider as soon as possible, ideally the same day.

• Danger signs may indicate serious bleeding or swelling inside the skull and require immediate emergency medical care.

• If you're unsure whether a symptom is a danger sign, err on the side of caution and seek emergency medical attention.

What to Do While Waiting for Help

Once you have called 911 or decided to head to the emergency department, a few simple actions help keep the person safe until professional help takes over.

Keep them still and avoid moving their head or neck any more than necessary. A hit hard enough to cause a head injury can also affect the spine, so unnecessary movement is a real risk, not just a precaution. If repositioning becomes necessary and you are trained, our guide to the recovery position in CPR walks through when it is appropriate and how to do it without compromising the airway or the spine.

Keep watching for any of the danger signs above, including ones that were not present a few minutes ago. Symptoms can appear or worsen quickly after a head injury, so this is not a one-time check. Do not leave the person alone, even if they insist they feel fine. A head and neck injury can also change how you respond if the person stops breathing, which is part of why it helps to know when CPR should not be started and how a suspected spinal injury factors into that decision.

Know the Signs, Then Get the Training to Back It Up

Recognizing a pupil change or another danger sign is only half the job. Knowing exactly what to do in the seconds after, how to keep someone still, how to talk to a 911 dispatcher, when to start CPR and when to hold off, is what formal training builds. Our First Aid certification course covers head injury response alongside the full range of everyday emergencies, and it is built to be finished in about an hour, from home, for $19.95.

If you are also responsible for kids, our Adult CPR + First Aid course covers pediatric response too, so you are ready no matter who needs help.

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CPR Certification : Do I have to give CPR or First Aid? CPR Certification : Don’t be Afraid to Step in and Help! CPR Certification: How Long Should I Provide CPR? How Effective is Online CPR Certification Education?