Unequal Pupils? Learn Anisocoria Causes & When to Worry

Why Are My Pupils Different Sizes? Causes of Anisocoria You Should Know

Have you ever looked in the mirror and noticed one pupil is larger than the other? That difference in pupil size is known as anisocoria — and while it can be completely normal in some people, in others it may be a sign of a more serious underlying condition.

What Is Anisocoria?

Anisocoria is a condition where the pupils of your eyes are unequal in size. Pupils control how much light enters your eyes, so they usually respond together to changes in light. When one pupil is consistently larger or smaller than the other, this is known as anisocoria. The size difference may be subtle (less than 1 mm) or quite noticeable, and it can be constant or temporary, depending on the cause.

Is It Always a Problem?

Not always. In fact, physiological anisocoria — a benign and harmless type — affects up to 20% of the population. In these cases, the pupil size difference is small, doesn’t change much with lighting, and there are no other symptoms.

However, new or sudden anisocoria, especially if accompanied by other signs (like drooping eyelid, blurred vision, or headache), could indicate a serious medical issue.

Common Causes of Anisocoria

Physiological Anisocoria

Normal variation in pupil size

Present from birth or develops without cause

No other symptoms

Pupil size difference usually <1 mm

Horner’s Syndrome

Caused by damage to certain nerve fibres controlling pupil size

Pupil on affected side is smaller and reacts slowly to light

Often includes ptosis (drooping eyelid) and lack of sweating on one side of the face

Can result from stroke, tumour, or trauma

Third Nerve Palsy

The third cranial nerve controls most eye movements and pupil function

Affected pupil is usually larger, and doesn’t react to light

Often comes with eye movement issues, double vision, and ptosis (droopy upper eyelid)

Can be due to aneurysm, brain tumour, or diabetes

Adie’s Pupil (Tonic Pupil)

One pupil is larger and reacts slowly (or not at all) to light but may respond to near focus

More common in young women

Often associated with a viral infection or damage to the ciliary ganglion, which controls pupil movement

Usually benign, though vision can be mildly affected

Eye Trauma or Surgery

Injury to the eye or previous surgeries (e.g., cataract removal) can affect pupil size

May be temporary or permanent, depending on the damage

Medications, Drugs and Chemicals

Certain eye drops (like atropine or pilocarpine), nasal sprays, or systemic medications can affect pupil size

Recreational drugs like cocaine or opioids may also cause pupil dilation or constriction

Some household chemicals, like some fly sprays, can cause temporary dilation of the pupil

When Should You Be Concerned?

Seek immediate medical attention if anisocoria is:

Sudden in onset

Associated with vision changes, pain, or headache

Accompanied by drooping eyelid, double vision, or difficulty moving the eye

Paired with neurological symptoms (e.g., weakness, confusion)

These could signal a neurological emergency, such as a brain aneurysm, stroke, or nerve palsy.

While anisocoria can be a normal variation in many healthy individuals, it’s important not to ignore new or unusual changes in your eyes. If you or someone you know suddenly develops unequal pupils — especially with other symptoms — it’s best to consult an eye health professional promptly.

Regular eye exams are essential for maintaining vision health

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