When your pupils are different sizes, the medical term is anisocoria. It’s more common than most people realize: about 14% of healthy people have a slight, natural difference in pupil size that causes no problems at all. But anisocoria can also signal something that needs attention, from a benign nerve issue to a medical emergency, depending on what other symptoms come with it.
Physiologic Anisocoria: The Harmless Kind
The most common reason for unequal pupils is simply how your body is built. A study of 708 healthy subjects found that 13.7% had measurable anisocoria on photography, with no underlying disease. This type is called physiologic anisocoria, and the size difference is usually small, often less than a millimeter. Both pupils still react normally to light and darkness. If you’ve had slightly different pupil sizes for as long as you can remember and no other symptoms, this is the most likely explanation.
What Makes Pupils Change Size
Your pupil size is controlled by two tiny muscles in the iris that work against each other. One muscle constricts the pupil (making it smaller), and the other dilates it (making it larger). These muscles are controlled by different sets of nerves. The constricting muscle gets its signal through a parasympathetic nerve pathway that travels with the third cranial nerve. The dilating muscle gets its signal through a sympathetic nerve pathway that runs from the brain, down through the chest, and back up along the carotid artery to the eye.
When either of these nerve pathways is disrupted, one pupil stops responding the way the other does. That’s what produces a noticeable size difference. Figuring out which pupil is the abnormal one, the bigger or the smaller, is the key to identifying the cause.
When One Pupil Is Too Large
Third Nerve Palsy
The third cranial nerve carries the parasympathetic fibers that tell the pupil to constrict. When something compresses this nerve, those fibers stop working, and the pupil on the affected side dilates without opposition. This is one of the more urgent causes of anisocoria. The most well-known life-threatening cause is a brain aneurysm pressing on the nerve. Only about 50% of patients survive a ruptured aneurysm after six months.
A third nerve palsy often comes with other signs beyond the dilated pupil: a drooping eyelid, double vision, or difficulty moving the eye. If imaging is done, the goal is to rule out a compressive lesion like an aneurysm.
Adie’s Tonic Pupil
This is a much more benign cause of a larger pupil. Adie’s pupil shows up most often in women between the ages of 20 and 40. The affected pupil is noticeably larger than the other, and it loses its normal reaction to light. Both the direct and indirect light reflexes disappear or become very sluggish. The pupil may still constrict slowly when you focus on something close, but it does so in a delayed, “tonic” way. Adie’s pupil is not dangerous, though it can cause some light sensitivity or trouble focusing up close.
When One Pupil Is Too Small
Horner Syndrome
Horner syndrome happens when the sympathetic nerve pathway to the eye is disrupted somewhere along its long route from the brain, through the chest, and up the neck. Because this pathway tells the pupil to dilate, losing the signal results in a pupil that stays smaller than it should. The classic trio of symptoms is a constricted pupil, a slightly drooping upper eyelid, and reduced sweating on that side of the face.
The causes of Horner syndrome vary depending on where the nerve pathway is interrupted. It can result from a stroke, a tumor in the lung or neck, or a tear in the carotid artery (carotid dissection). Because of these possibilities, new-onset Horner syndrome typically warrants investigation even when the symptoms seem mild.
Drugs and Chemicals That Affect Pupil Size
Sometimes anisocoria has a simple external cause. Certain substances can change the size of one pupil if they get into just one eye. Scopolamine patches (used for motion sickness) are a well-documented culprit. If you touch the patch and then rub one eye, the medication can dilate that pupil for hours. Bronchodilator nebulizers used for asthma, like ipratropium, can have the same effect if the mist reaches the eye. Antiperspirant agents containing glycopyrrolate also cause dilation.
On the other side, substances like pilocarpine eye drops and organophosphate insecticides cause the pupil to constrict. If only one eye is exposed, the result is unequal pupils that can look alarming but resolve once the chemical wears off.
Eye Injury and Surgery
Blunt trauma to the eye can directly damage the iris muscles that control pupil size. When the muscle responsible for constriction is torn, the pupil may stay dilated and sometimes becomes irregularly shaped rather than perfectly round. This type of anisocoria can be permanent. Eye surgeries, particularly cataract surgery, can also leave one pupil slightly different from the other if the iris is affected during the procedure.
How Doctors Figure Out the Cause
The first step is determining which pupil is the abnormal one. Doctors do this by examining your pupils in both bright and dim lighting. If the size difference gets bigger in bright light, the larger pupil is the problem because it’s failing to constrict. If the difference gets bigger in dim light, the smaller pupil is the problem because it’s failing to dilate.
Specialized eye drops can help narrow down the diagnosis further. Different concentrations of drops like pilocarpine and cocaine are used to test how each pupil responds. For example, a very dilute concentration of pilocarpine will constrict an Adie’s pupil but won’t affect a normal one. Cocaine drops help confirm Horner syndrome by revealing which pupil can’t dilate properly. These pharmacological tests, combined with a neurological exam, help point to the specific nerve pathway involved.
If a third nerve palsy or Horner syndrome is suspected, brain or neck imaging is usually the next step to look for structural causes like aneurysms, tumors, or artery tears.
Symptoms That Need Emergency Care
Anisocoria by itself, especially when it’s been present for a long time, is often harmless. But certain combinations of symptoms signal a potential emergency. Go to the emergency room if unequal pupils appear suddenly alongside any of these:
- Eye pain or sudden headache
- Blurry or double vision
- Sudden vision loss
- Light sensitivity
- Nausea or vomiting
- Neck pain or stiffness
- Fever
The combination of a newly dilated pupil with a drooping eyelid and headache is particularly concerning, as it can indicate a brain aneurysm compressing the third cranial nerve. This requires immediate imaging. A new small pupil with eyelid droop and neck pain could point to a carotid artery dissection, which also needs urgent evaluation.