Why Is One Pupil Bigger Than the Other?

People often notice unequal pupils in a photo or the mirror and worry it signals a stroke or brain problem. Sometimes a partner or friend points it out. The concern is understandable because pupil size is controlled by nerves connected to the brain, but in many cases the difference is small, old, and harmless.

Pupil size is set by a balance between two systems: one that constricts the pupil (parasympathetic, via the third cranial nerve) and one that dilates it (sympathetic). A difference in size means one pupil is either too large or too small relative to the other, and figuring out which one is abnormal is the key step.

Often the eyes themselves see perfectly well and the difference is simply a normal variation. When a nerve pathway is affected, the pupil abnormality is usually accompanied by other clues, such as a drooping eyelid, difficulty moving the eye, double vision, or changes in sweating on the face, which is why the surrounding signs matter as much as the pupils.

Physiologic anisocoria is the most common cause: a difference of usually less than a millimeter that stays about the same in different lighting and has been present for years, often visible in old photos. It needs no treatment.

When the larger pupil is the problem, causes include a third-nerve palsy (a large, poorly reacting pupil with a drooping lid and an eye that cannot move normally, an emergency because a compressing aneurysm must be excluded), Adie's tonic pupil (a benign condition with a large pupil that reacts slowly), and dilating medications or plant chemicals that reach the eye. When the smaller pupil is the problem, Horner syndrome (a small pupil with a mild lid droop, sometimes reduced facial sweating) reflects interruption of the sympathetic pathway anywhere from the brain and chest to the neck and can be caused by conditions ranging from benign to serious, including, rarely, a carotid dissection, which is urgent when it comes with neck pain. Comparing the pupil difference in light and dark tells the examiner whether the abnormal pupil is the larger one (difference greater in bright light) or the smaller one (difference greater in dim light).

Seek urgent or emergency care if unequal pupils are new and come with any of the following: a drooping eyelid, double vision, or an eye that will not move properly; eye pain or a severe or sudden headache; neck pain; recent head or neck trauma; or neurologic symptoms such as weakness, numbness, or trouble speaking. A newly enlarged, poorly reacting pupil with a droopy lid is especially concerning for an aneurysm-related third-nerve palsy and should be evaluated immediately. A longstanding, stable, small difference with no other symptoms is not an emergency.

The evaluation starts by measuring the pupils in both bright and dim light to determine which pupil is abnormal and how each reacts to light. The doctor examines the eyelids for droop, checks eye movements for a nerve palsy, and looks at the front of the eye and iris for trauma or inflammation. Old photographs help establish whether the difference is longstanding. Specialized pupil eye-drop testing can confirm Horner syndrome or a tonic pupil. When a third-nerve palsy or Horner syndrome is found, urgent imaging of the brain, blood vessels, neck, or chest is arranged, often with neurology, to exclude an aneurysm or dissection.

Physiologic anisocoria requires no treatment; it is simply reassurance once serious causes are excluded. When a medication or chemical caused a dilated pupil, it resolves as the substance wears off. Adie's tonic pupil is generally managed conservatively. When the cause is a nerve problem, treatment is directed at that underlying condition: an aneurysm causing a third-nerve palsy requires urgent neurosurgical or endovascular treatment, and a carotid dissection causing Horner syndrome is managed urgently by the appropriate specialists. In short, harmless causes need no intervention, while dangerous ones are treated at their source, quickly.

• Do not ignore a new size difference that comes with a droopy lid, double vision, eye pain, headache, or neck pain.
• Do not assume unequal pupils are always harmless without checking whether they are new or symptomatic.
• Do not put unprescribed eye drops in to try to even out the pupils.
• Do not delay care for a newly enlarged, poorly reacting pupil with a droopy lid; it may indicate an aneurysm.

If you notice a new difference in your pupil sizes, or an old one that has changed, contact May Eye Care Center so we can determine which pupil is abnormal and whether further workup is needed. If the change comes on suddenly with a drooping eyelid, double vision, eye or neck pain, a severe headache, or any weakness or trouble speaking, seek emergency care immediately.

One pupil being slightly larger is usually harmless physiologic anisocoria, but a new difference, or one paired with a droopy lid, double vision, pain, or headache, can signal a serious nerve problem such as a third-nerve palsy or Horner syndrome and needs prompt evaluation.