What Is Unilateral Anisocoria?

Common causes

Causes range from harmless to urgent, and history is often the clue. The list below includes common possibilities clinicians consider during evaluation.

• Physiologic anisocoria
• Horner syndrome (smaller pupil that dilates poorly in the dark)
• Third cranial nerve palsy (larger pupil that reacts poorly, often with ptosis and eye movement changes)
• Adie's tonic pupil
• Pharmacologic exposure, trauma, or prior eye surgery

How clinicians evaluate it

Clinicians compare pupil size in bright and dim light to see when the difference is greatest. They check the direct and consensual light responses and look for ptosis or restricted eye movements. A slit-lamp exam assesses iris damage and other eye findings, and intraocular pressure is measured when angle closure is a concern. If a neurologic cause is suspected, urgent imaging may be ordered.

Red flags

Some findings suggest a time-sensitive problem and warrant urgent care. Sudden anisocoria with a severe headache, new double vision, or a drooping eyelid is particularly concerning. A red, painful eye with blurred vision can signal acute angle closure or severe inflammation. New neurologic symptoms such as weakness, numbness, or confusion also require emergency evaluation.

Treatment and next steps

Treatment depends on the cause rather than pupil size alone. Pharmacologic anisocoria may resolve as the medication effect wears off, while traumatic anisocoria may persist. If a third cranial nerve palsy or Horner syndrome is suspected, clinicians prioritize urgent workup to identify the underlying cause. Your care plan may include observation, eye drops, medication changes, or specialist referral based on findings.