After a concussion, your eyes can show several visible changes, from pupils that don’t match in size to a glassy, unfocused stare. Some of these signs are obvious to anyone looking at you, while others only show up when your eyes try to track movement or focus on a nearby object. Eye changes are one of the most reliable physical clues that the brain has been shaken, which is why medical professionals check the eyes first after a head injury.
The Vacant Stare
One of the most immediately noticeable things about someone with a concussion is a blank, glazed look. This “vacant stare” is recognized as a classic physical sign of concussion. The person looks like they’re staring through you rather than at you, and their eyes appear unfocused or glassy. This happens because the force of the injury disrupts how brain cells communicate. The impact causes a chemical cascade inside the brain: cell membranes get stretched, potassium floods out of neurons, calcium rushes in, and normal nerve signaling is temporarily suppressed. That neurological disruption shows up visually as a person who looks dazed, distant, and not fully present.
Pupil Changes
After a concussion, the pupils often react to light more slowly than normal. In a healthy brain, your pupils shrink quickly and symmetrically when a light is shone into them. After a head injury, that response can be significantly delayed, sluggish, and reduced compared to someone without a concussion. In some cases, one pupil may be noticeably larger than the other, a condition called anisocoria. This uneven pupil size tends to become more apparent in bright light and has been shown to correspond to the severity of the brain injury.
A pupil that is no longer round, or one eye with a dramatically larger pupil that doesn’t respond to light at all, can signal something more serious than a mild concussion. Acute dilation of one pupil after head trauma is treated as a neurological emergency, because it may indicate dangerous pressure building inside the skull.
Eyes That Won’t Track Smoothly
If you ask someone with a concussion to follow your finger with their eyes, you’ll often see problems. Instead of smooth, fluid movement, their eyes may jerk or stutter as they try to follow the target. This is because the brain’s ability to coordinate smooth tracking movements gets disrupted by the injury. When tested formally, researchers find high rates of these tracking abnormalities in concussion patients.
Rapid eye movements between two points are also affected. Up to 30% of concussion patients have trouble quickly shifting their gaze from one target to another. Their eyes take longer to start moving, and they overshoot or undershoot the target. To an observer, the person’s eyes may look like they’re lagging behind, struggling to land on the right spot.
Eyes That Won’t Line Up on Close Objects
Normally, both eyes turn inward together when you focus on something close to your face, like a finger moving toward your nose. Your eyes should maintain that coordinated focus until the object is about 6 centimeters (roughly 2 inches) from your forehead. In concussion patients, this ability breaks down. One or both eyes drift outward too early, sometimes at distances of 5 centimeters or more from the nose, which is the clinical threshold for an abnormal result.
This convergence problem is one of the most common eye-related findings after a concussion. Studies report convergence abnormalities in 14% to 55% of athletes, 48% to 55% of military personnel, and 14% to 49% of outpatient groups after concussion. When it happens, you might notice one eye visibly drifting outward while the other stays focused, and the person will often report seeing double. Up to 40% of patients still have symptoms of convergence dysfunction a month or more after the injury.
Light Sensitivity and Squinting
People with concussions frequently squint, shield their eyes, or avoid well-lit environments. This photophobia happens for several overlapping reasons. The thalamus, a brain structure that filters incoming visual information, can be affected when the arteries supplying it with oxygen are damaged by the injury. When the thalamus isn’t filtering light signals properly, the brain gets overloaded with visual input that it would normally manage without trouble.
The autonomic nervous system, which controls pupil dilation among many other automatic body functions, can also be disrupted. If it’s not regulating the pupils correctly, they may stay dilated wider than they should, letting too much light flood into the eye. The combination of sluggish pupils and a brain that’s already struggling to process visual information makes ordinary room lighting or sunlight feel painfully intense. The vestibular system (your inner-ear balance system) can compound the problem: when it sends signals that conflict with what the eyes are seeing, the brain compensates by amplifying visual input, making light sensitivity even worse.
Blinking Differences
Concussion can change how a person blinks, though this is harder to spot with the naked eye. Research on youth athletes found that concussed players had significantly longer blink durations compared to healthy controls during eye-tracking tasks. Their blinks lasted measurably longer when they were trying to perform rapid gaze shifts and tracking movements. As the athletes recovered, their blink duration decreased by 18% to 24%, suggesting that blink patterns normalize as the brain heals. This is an emerging area of study, but it hints that something as subtle as the way someone blinks can reflect what’s happening inside the brain after a hit.
Blurred and Double Vision
Beyond what an observer can see from the outside, the concussed person’s own visual experience changes dramatically. Blurred vision is one of the most commonly reported symptoms, and it stems from the brain’s impaired ability to adjust focus. Your eyes constantly shift focus as objects move closer or farther away. In studies of young concussion patients, 51% had reduced focusing ability compared to age-matched norms. Among military personnel with mild brain injuries, that number climbed to 65%.
Double vision occurs when the two eyes stop working as a coordinated team. Instead of fusing the images from each eye into one clear picture, the brain receives two slightly offset images it can’t merge. This is directly tied to the convergence and tracking problems described above. For the person experiencing it, objects may appear split or overlapping, particularly when reading, looking at screens, or focusing on anything up close.
How These Signs Are Tested
Clinicians use a structured screening called the Vestibular/Ocular Motor Screening, or VOMS, to check for these eye problems after a suspected concussion. It includes five types of eye tests: smooth tracking of a moving finger, rapid gaze shifts between two points in both horizontal and vertical directions, a convergence test where a target is slowly moved toward the nose, and head-movement tests where the person focuses on a fixed point while turning or nodding their head. After each test, the patient rates their headache, dizziness, nausea, and mental fogginess on a 0-to-10 scale. An increase in any of those symptoms during the tests, or a convergence point of 5 centimeters or more from the nose, is considered an abnormal result pointing toward concussion.
These tests work because eye movements depend on widespread brain networks. Even a mild injury can disrupt the precise coordination needed to track, focus, and align the eyes. That’s what makes the eyes such a useful window into brain function after a hit to the head.