What Is Unilateral Mydriasis?

Common causes

The most likely cause depends on whether the pupil reacts to light and whether other neurologic signs are present. Pharmacologic mydriasis can occur after exposure to anticholinergic medications or certain plants. Trauma and prior eye surgery can damage the pupil sphincter and leave a larger, sometimes irregular pupil. Clinicians also consider:

• Adie's tonic pupil
• Third cranial nerve palsy
• Acute angle-closure glaucoma
• Uveitis with iris changes or synechiae

Symptoms and red flags

Mydriasis may cause glare, light sensitivity, and difficulty focusing at near. Red flags include ptosis, double vision, eye movement limitation, severe headache, or eye pain. A red, painful eye with blurred vision can suggest acute angle closure or severe inflammation. Sudden neurologic symptoms such as weakness, numbness, or confusion require emergency care.

How clinicians evaluate it

Evaluation includes checking pupil size in bright and dim light, testing the light reaction, and looking for ptosis or eye movement abnormalities. A slit-lamp exam assesses the iris sphincter and looks for signs of trauma, inflammation, or surgical changes. Intraocular pressure is measured to rule out angle closure. History of medication exposure and, in selected cases, pharmacologic testing may help differentiate causes.

Treatment and next steps

Treatment targets the cause, so management varies from observation to urgent neurologic or ophthalmic care. Pharmacologic dilation may resolve as the drug effect wears off, while traumatic mydriasis can be temporary or permanent. Third nerve palsy or acute angle-closure glaucoma require urgent evaluation and treatment to protect vision and health. While awaiting care, avoid driving if vision is affected and protect the eye from bright light with sunglasses.