Why is one pupil smaller?

The nerve signal that widens the pupil is weaker on that side. The smaller pupil is often most clear in dim light.

Is Horner syndrome an emergency?

It can be. New Horner signs with head or neck pain, trauma, or neurologic symptoms need emergency care.

Can children get Horner syndrome?

Yes. Children may have Horner syndrome from birth injury or other causes. A new child case needs prompt evaluation [1] .

Will the droopy eyelid go away?

Sometimes it improves if the cause improves. Some eyelid changes stay stable and need only observation.

Condition

Horner Syndrome

Also known as Oculosympathetic Palsy, Small Pupil and Droopy Eyelid, Ptosis and Miosis, Anisocoria With Ptosis, Sympathetic Eye Pathway Problem

Updated May 19, 2026·For educational purposes only. Not a substitute for medical advice. See our terms.

Bottom Line

Horner syndrome causes a small pupil and mild droopy eyelid on one side. New Horner signs with head or neck pain can be an emergency.

Horner syndrome happens when the nerve pathway that helps open the eyelid and widen the pupil is interrupted. The classic signs are a smaller pupil, a mild droopy eyelid, and less sweating on that side of the face ¹.

The problem can start in the brain, spinal cord, chest, neck, or eye area. Causes range from birth injury to stroke, tumor, trauma, or carotid artery dissection ².

Horner syndrome itself may not damage vision. The cause can be serious, so new signs need prompt medical evaluation.

How Horner Syndrome Is Diagnosed

Doctors look for a pattern: one smaller pupil, mild droopy eyelid, and sometimes less sweating on the same side ¹.

The pupil difference is often larger in dim light. Eye drops may help confirm the diagnosis in some cases.

Imaging may be needed if the syndrome is new, painful, or linked with neurologic symptoms.

Causes and Treatment

Horner syndrome is a sign, not one disease. The nerve pathway can be affected by stroke, neck injury, chest tumors, surgery, migraine, carotid artery dissection, or birth trauma ².

Treatment depends on the cause. The droopy eyelid and small pupil usually do not need separate treatment unless they bother vision or appearance.

When to Seek Care

Go to the emergency room for new Horner signs with severe headache, neck pain, trauma, weakness, dizziness, or trouble speaking.

Call a pediatric ophthalmologist or pediatrician this week for a new pupil or eyelid difference in a child.

Common Questions About Horner Syndrome

The eyelid droop is usually mild and often does not block vision. The cause of Horner syndrome can still be serious.

Next Steps

1Go to the emergency room for new Horner signs with head or neck pain, trauma, or neurologic symptoms.
2Call a neuro-ophthalmologist for new painless Horner signs in an adult.
3Call a pediatric ophthalmologist or pediatrician this week for a new pupil or eyelid difference in a child.
4Bring old photos to help show when the pupil or eyelid difference began.

Find specialists for Horner Syndrome

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