Pupils Different Sizes After Head Injury: When to Go to the ER

When Is a Pupil Difference After a Head Injury an Emergency?

In people with bleeding or a tumor inside the skull, one pupil widening "is a neurological emergency, as it may herald the onset of uncal herniation," which is when rising pressure forces part of the brain out of its normal position (Chaudhry, 2014). A review of unequal pupils, called anisocoria, warns that while it's often benign, it "can also represent the sole sign of a life-threatening disease" (Gross, 2016).

A wide pupil can be part of a third cranial nerve palsy. When the palsy is acute and complete and the pupil is involved, the usual cause is an aneurysm on the posterior communicating artery, and a chronic subdural hematoma (a slow bleed between the brain and skull) can cause it too. Headache is usually present, and the authors write that "urgent diagnosis and treatment are warranted" (Pellegrini, 2022). Double vision from a partly paralyzed third nerve with the pupil involved "may reveal an intracranial aneurysm" (Caignard, 2016). A pupil can also turn small. An emergency medicine review lists a small pupil from carotid artery dissection, a tear in a neck artery, alongside a large pupil from an aneurysm as the serious causes to rule out (Huff, 2016).

The emergency signs come from neuro-ophthalmology and emergency medicine reviews (Chaudhry, 2014; Pellegrini, 2022; Caignard, 2016; Huff, 2016) and from CDC's danger signs after a head injury (CDC, 2025). Only an exam can confirm that a long-standing difference is harmless.

The studies cited here don't set a safe waiting period after a head injury, so the clinic's advice is to get a new difference checked right away instead of waiting to see if it passes. CDC lists one pupil larger than the other among the danger signs after a head injury and says to call 9-1-1 or go to an emergency department right away (CDC, 2025). Emergency teams check the pupils alongside eyelid position, eye movements and alertness, because those signs together separate pressure on the brain from harmless causes (Chaudhry, 2014).

What Controls the Size of Each Pupil?

Each pupil sits between 2 muscles pulling in opposite directions, and each muscle answers to a different branch of the autonomic nervous system. Nerve cells in the ciliary ganglion, a relay station behind the eye, drive the sphincter muscle that narrows the pupil. Nerve cells in the superior cervical ganglion, high in the neck, drive the dilator muscle that widens it (McDougal, 2015).

That split is why the emergency causes look different. If the narrowing pathway fails on one side, that pupil stays wide in bright light. If the widening pathway fails, it stays small, the pattern an emergency medicine review links to carotid dissection (Huff, 2016). Because the 2 eyes are wired the same way, a pupil that behaves differently from its partner can point to a problem on one pathway. It's also why our team treats the pupil as one of the few brain circuits you can watch from the outside, part of how the brain controls the autonomic system.

Healthy Concussion Nerve pressed

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Is It Normal to Have One Pupil Bigger Than the Other?

It's common: in studies of healthy adults, roughly 1 in 7 to 1 in 4 had measurably different pupils under standard lighting, and the share was higher in dim light and darkness. The difference can also come and go. In a study that photographed 128 people over 5 days, 19% had a difference at any single exam, and 41% had one at some point during the 5 days (Lam, 1987).

708 adults, photographs13.7%

128 adults, dim light, one exam19%

126 adults, bright light23.8%

128 adults, any time over 5 days41%

Subgroup tested in darkness43.1%

The numbers swing because pupil size depends on the light and on the threshold each study used. A neurosurgery study of 708 adults found a measurable difference on photographs in 13.7% of people with no eye or brain disease besides refractive error (George, 2019). A study under several lighting levels found it in 23.8% in bright light and in 43.1% of a subgroup tested in darkness (Steck, 2018).

A difference you've always had is a different situation from one that just appeared, and only an exam can sort out which one you have. Researchers describe separating Horner syndrome (a small pupil from a disrupted widening pathway) from harmless anisocoria as "important yet clinically challenging" (Disse, 2024). In an intensive care unit, a nebulized breathing medicine, ipratropium, widened one patient's pupil and mimicked brain herniation; the team told them apart by the normal eyelid, eye movements and alertness (Chaudhry, 2014). That was a hospital patient on a known medicine, so a new difference at home still needs the ER.

What Does a Concussion Do to Your Pupils?

The research on concussion points to a change in how both pupils react to light. In one pupillometry study, people with mild traumatic brain injury had pupil responses that were "significantly delayed, slowed and reduced, but symmetrical in nature, and with a smaller baseline diameter" than people without injury. The authors concluded that measuring the pupil's response "provides several objective biomarkers for the presence of mTBI and photosensitivity" (Ciuffreda, 2017).

The direction of the change isn't settled. A prospective study of 98 teen athletes with sport concussion and 134 controls, tested a median of 12 days after injury, found enhanced reflex metrics rather than reduced ones (Master, 2020). The studies disagree on direction and agree that the reflex changes. A retrospective review of app-based pupil tests reported changes that "persist over the life span," which is a weaker design (Carrick, 2021). The evidence is small, and the studies above don't say how long these changes last for most people.

None of these studies describe concussion as a cause of one pupil becoming larger than the other. In the study that looked at both eyes, the changes were symmetrical (Ciuffreda, 2017). If one pupil is bigger after a hit to the head, treat it as the emergency in the first section, even when the injury seemed mild. When the ER finds nothing and symptoms like light sensitivity, headaches or fatigue linger, that's the pattern of a concussion that doesn't heal.

How Does Our Team Use Pupils to Track Brain Recovery?

Our team uses pupillometry, a measurement of how fast and how far each pupil reacts to light, because the pupil is a light-control pathway the clinic can watch directly. The clinic describes it as a tool for tracking attention, working memory, spatial awareness, cognitive fatigue and light sensitivity. The research on pupils after concussion is still early, with small studies that disagree on direction. For related brain-circuit measures, see eye movement testing and autonomic testing.

In the clinic's experience, the pupil response also shows when a recovering brain is running out of room. Our team uses it to judge fatigue and overload, and to set how much rehab a patient can take in one session, so a session can stop before it sets recovery back. That's the clinic's working method, and the studies above didn't test pupil-guided dosing.

Patients can track a version of this at home. Our team teaches people to notice how their eyes and light tolerance change after demanding tasks like driving, a busy store or a long day of screens. In the clinic's view, rising glare sensitivity or eye strain after a task can be a sign the task cost more than the brain had to spend that day. Watching light tolerance never replaces the emergency list above, and a change in the size of one pupil goes on that list.

Pupils Different Sizes After a Head Injury: FAQ

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