The pupils of your eyes usually appear as symmetrical black circles, working in unison to regulate light. However, one pupil may sometimes be noticeably larger or smaller than the other, a condition known as anisocoria. While often a harmless variation, unequal pupil size can signal an underlying health issue. Understanding when this difference is normal and when it requires medical attention is important.
Understanding Anisocoria
The pupil functions like a camera’s aperture, adjusting its size to control the amount of light reaching the retina. In dim conditions, the pupil dilates to allow more light in, while in bright light, it constricts to reduce light entry. This dynamic process, known as the pupillary light reflex, involves both the sympathetic and parasympathetic nervous systems, which control dilation and constriction.
A small difference in pupil size, less than 1 millimeter, is a common and benign phenomenon called physiological anisocoria. This normal variation affects approximately 10% to 20% of the population and does not affect vision. In cases of physiological anisocoria, both pupils react normally to light, maintaining the same difference in size across varying light conditions.
Common Reasons for Unequal Pupils
Beyond physiological variations, several factors can cause unequal pupils. Medications are a common culprit, particularly eye drops for glaucoma or those used to dilate pupils during an eye exam. Certain systemic medications, such as selective serotonin re-uptake inhibitors (SSRIs) or transdermal scopolamine patches, can also cause anisocoria. If medication gets into only one eye, it can cause an uneven effect.
Eye conditions can also cause unequal pupils. Trauma to the eye, including eye surgery complications, can cause mechanical damage to the iris, leading to a distorted or non-reactive pupil. Inflammation of the iris (iritis or uveitis) can cause the affected pupil to constrict. Angle-closure glaucoma may also lead to unequal pupils, though severe eye pain often overshadows this.
Neurological conditions significantly impact pupil size by affecting the nerves that control them. Horner’s syndrome, for instance, disrupts the sympathetic nerve pathway to the eye and face, causing a smaller pupil, a drooping eyelid (ptosis), and decreased sweating on the affected side. Adie’s tonic pupil involves damage to nerves supplying the pupil’s constrictor muscles, resulting in a larger pupil that reacts slowly to light and near focus.
Damage to the third cranial nerve (oculomotor nerve palsy) can result in a dilated pupil, often with a drooping eyelid and misaligned eye movements. More serious neurological concerns like stroke, brain aneurysm, or brain tumor can also cause unequal pupils by compressing or damaging nerve pathways. Cluster headaches, characterized by intense, unilateral head pain, can sometimes cause temporary anisocoria, usually with a dilated pupil that resolves after the headache.
When to Be Concerned and Seek Help
It is important to recognize when unequal pupils warrant medical attention. Sudden onset anisocoria, especially without a clear cause, requires immediate evaluation.
If unequal pupils are accompanied by other symptoms, prompt medical help is advised. These symptoms include severe headache, vision changes (double vision, blurred vision, loss of vision), eye pain or redness, a drooping eyelid (ptosis), dizziness, nausea, weakness, or difficulty speaking.
Any unequal pupil size following a head or eye injury also prompts immediate medical attention. A noticeable change in the difference between pupil sizes or their reaction to light should also be evaluated. A healthcare professional can perform an examination and recommend further tests, such as MRI or CT scans, to determine the underlying cause and guide management.