The appearance of one pupil being noticeably larger than the other (anisocoria) can range from a harmless biological quirk to a sign of a serious underlying health concern. This difference indicates a disruption in the system that controls light regulation in the eye. Pupils are meant to widen and constrict in synchronization, but anisocoria occurs when one control mechanism is affected. Determining whether this difference is a long-standing, benign variation or a newly developed symptom is the first step toward understanding its significance. Causes are diverse, involving local eye issues, external factors like medication, or complex problems affecting the nervous system.
When the Size Difference is Normal
A subtle difference in pupil size can be a normal, non-pathological finding present in 10 to 20% of healthy individuals. This common variation does not indicate disease or require treatment. The size difference is typically small, often measuring less than one millimeter. This benign condition is generally stable and has likely been present since birth or early childhood. A defining characteristic is that both pupils react equally and briskly to changes in light, maintaining the same small size difference throughout. This normal difference is not accompanied by symptoms such as pain, vision change, or a drooping eyelid.
Eye-Specific Causes and Medications
Unequal pupil sizes can result from issues contained entirely within the eye structure or from external chemical exposure. Trauma to the eye can cause mechanical damage to the iris, the colored muscle that controls pupil size. A previous blunt injury or complicated eye surgery, such as cataract removal, may result in a tear or paralysis of the iris sphincter muscle, leaving the pupil permanently misshapen or dilated.
Inflammation of the iris, known as iritis, can also affect pupil function. The inflammation causes the pupil to become constricted and often painful, typically accompanied by eye pain, redness, and sensitivity to light. Certain medications or chemicals that accidentally contact only one eye are a frequent, temporary cause. Eye drops used for examination can cause one pupil to dilate widely for many hours. Exposure can also occur inadvertently through touching the eye after handling transdermal patches (like those containing scopolamine) or inhaling certain asthma medications. This type of difference is usually temporary and resolves once the chemical wears off.
Underlying Neurological and Systemic Conditions
A sudden difference in pupil size can signal a problem along the nervous system pathways that control pupillary function.
Horner Syndrome
Horner syndrome results from a disruption of the sympathetic nerve supply that travels from the brain to the eye. Since this pathway controls pupil dilation, damage results in a smaller pupil on the affected side. Horner syndrome is classically accompanied by a mildly drooping upper eyelid and a lack of sweating on the corresponding side of the face. The underlying cause of this nerve disruption is varied and can range from a neck injury to a serious condition like a Pancoast tumor in the lung apex or a tear in the carotid artery wall.
Adie’s Tonic Pupil
Adie’s tonic pupil involves a dilated pupil that reacts poorly or very slowly to light. This is typically due to damage to the ciliary ganglion, a bundle of nerve cells behind the eye that manages pupillary constriction. Although the pupil may hardly react to a bright light, it often constricts better when the eye focuses on a near object, a phenomenon called light-near dissociation. Adie’s tonic pupil is generally a benign, isolated event, but in some cases, it can be part of Adie’s syndrome, which also involves a loss of deep tendon reflexes.
Third Cranial Nerve Palsy
The most concerning neurological cause of a dilated, nonreactive pupil is damage to the third cranial nerve, which is responsible for most eye movements and pupil constriction. Compression of this nerve, often from an expanding brain aneurysm, is a medical emergency because the aneurysm is at risk of rupture. This type of nerve damage typically presents with a large pupil that does not constrict in light, paired with significant drooping of the eyelid and difficulty moving the eye in certain directions. Sudden-onset pupil size differences can also be an indicator of a stroke or bleeding within the brain, especially when accompanied by a severe headache or other signs of neurological impairment.
When to Seek Immediate Medical Attention
While many causes of unequal pupils are harmless, the presence of certain accompanying symptoms mandates an immediate medical evaluation. Any sudden, new change in pupil size should be treated with urgency until a serious cause is ruled out. Seek emergency care if the pupil difference is accompanied by a sudden, severe headache, which could signal a serious vascular event like an aneurysm or hemorrhage in the brain. Other urgent warning signs include:
- A newly drooping eyelid.
- Double vision or any loss of visual acuity.
- Neurological symptoms such as confusion, difficulty speaking, or muscle weakness.
- A stiff neck.