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Dilated Pupils

Also known as mydriasis, enlarged pupils, anisocoria

Expected for a few hours after dilating drops — worth investigating when one pupil enlarges on its own.

About 1 minute to read

Overview

The pupil is simply a hole, and its size is set by two small muscles in the iris working against each other under nervous system control. Dilation is normal in dim light, with excitement or alarm, and for several hours after the drops used in a dilated eye exam.

What is not normal is one pupil clearly larger than the other when nothing has been put in the eye, or pupils that stay wide and react sluggishly to light. The nerves that control the pupil run a long way through the head, which makes pupil size one of the few eye signs that can point at something neurological.

What are the common symptoms?

  • One pupil visibly larger than the other
  • Pupils that stay large even in bright light
  • Marked glare and light sensitivity
  • Difficulty focusing on near work
  • Sometimes a drooping eyelid or double vision alongside

What causes it, and who is at risk?

Dilating drops are the everyday cause. So are medicines: motion-sickness patches, some inhalers, antihistamines and antidepressants — a patch handled and then rubbed into one eye is a classic one-sided version.

Others include blunt injury to the iris, recreational drugs, Adie tonic pupil, third nerve palsy, migraine, and raised pressure inside the head. A small long-standing difference in pupil size with normal light reactions is usually benign physiological anisocoria.

How is it treated?

Drop-induced dilation needs nothing but time — four to six hours, occasionally into the next day. Sunglasses help, and it is worth arranging not to drive until the glare settles.

Anything else is a diagnostic problem rather than a treatment one: an exam with formal pupil reaction testing, a look at the iris under the slit lamp, and prompt referral for imaging when the pattern suggests a nerve palsy. Treatment then follows the cause, not the pupil.

When to get seen quickly

A newly unequal pupil together with a drooping eyelid, double vision, severe headache, or any head injury, is an emergency. Go to an emergency room rather than waiting for an eye appointment.

This page is general information, not a diagnosis. Only an eye care professional who has examined you can tell you what is actually going on.

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Related conditions

Light Sensitivity Wincing at ordinary light can be a migraine trait, a dry surface, or inflammation inside the eye. Uveitis Inflammation inside the eye — painful, light-sensitive, and needing prompt treatment to protect vision. Ocular Migraine A shimmering arc or zigzag that grows, drifts across your vision for 20 to 30 minutes, and then goes. Glaucoma Progressive damage to the optic nerve that steals peripheral vision silently — which is why screening matters more than symptoms.
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