Mydriasis

Key facts

• Mydriasis is dilation of the pupil beyond its normal size, most often induced deliberately for a clinical eye examination.
• It can also occur as a drug side effect, after head trauma, or with certain neurological conditions.
• Short-acting eye drops such as cyclopentolate or tropicamide dilate the pupil so a clinician can examine the retina, optic nerve, and lens.
• A pupil that suddenly dilates without drops, especially with headache, double vision, or a drooping eyelid, needs prompt medical review.

What causes pupil dilation

Mydriasis means the pupil is wider than normal. In clinical practice it's almost always deliberate: an ophthalmologist or optometrist needs a dilated pupil to examine the retina, optic nerve, and lens clearly. Outside the clinic, pupils can dilate as a side effect of certain drugs, after head trauma, or because of a neurological condition affecting the nerves that control the iris.

How it's managed in eye exams

Clinically induced mydriasis relies on short-acting anticholinergic or adrenergic eye drops from the eye care category. Cyclopentolate acts for several hours and is preferred when the examiner also needs to paralyse the eye's focusing muscle, a process called cycloplegia. Tropicamide has a shorter duration and suits situations where only brief dilation is needed.

What to expect

While the pupils are dilated, bright light feels uncomfortable and near vision blurs temporarily. Both effects fade as the drops wear off, usually within a few hours. Driving or operating machinery while dilated isn't advisable, so plan for someone else to drive if possible.

When to see a doctor

If one pupil suddenly dilates without any drops being used, particularly alongside headache, double vision, or a drooping eyelid, get prompt medical review. This combination can point to a nerve or blood vessel problem rather than a routine drug effect.

This page is educational and does not replace advice from a doctor or pharmacist who knows your health history.