Slight differences in pupil sizes are found in up to 1 in 5 healthy people. Most often, the diameter difference is less than 0.5 mm, but it can be up to 1 mm.

Babies born with different sized pupils may not have any underlying disorder. If other family members also have similar pupils, then the pupil size difference could be genetic and is nothing to worry about.

Also, for unknown reasons, pupils may temporarily differ in size. If there are no other symptoms and if the pupils return to normal, then it is nothing to worry about.

Unequal pupil sizes of more than 1 mm that develop later in life and do not return to equal size may be a sign of an eye, brain, blood vessel, or nerve disease.

The use of eye drops is a common cause of a harmless change in pupil size. Other medicines that get in the eyes, including medicine from asthma inhalers, can change pupil size.

Other causes of unequal pupil sizes may include:

• Aneurysm in the brain
• Bleeding inside the skull caused by head injury
• Brain tumor or abscess (such as pontine lesions)
• Excess pressure in one eye caused by glaucoma
• Increased intracranial pressure, because of brain swelling, intracranial hemorrhage, acute stroke, or intracranial tumor
• Infection of membranes around the brain (meningitis or encephalitis)
• Migraine headache
• Seizure (pupil size difference may remain long after seizure is over)
• Tumor, mass, or lymph node in the upper chest or lymph node causing pressure on a nerve may cause decreased sweating, a small pupil, or drooping eyelid all on the affected side (Horner syndrome)
• Stroke
• Injury to the carotid or vertebral artery
• Diabetic oculomotor nerve palsy
• Prior eye trauma, including surgery for cataracts

Treatment depends on the cause of the unequal pupil size. You should see a health care provider if you have sudden changes that result in unequal pupil size.

Contact a provider if you have persistent, unexplained, or sudden changes in pupil size. If there is any recent change in pupil size, it may be a sign of a very serious condition.

If you have differing pupil size after an eye or head injury, get medical help immediately.

Always seek immediate medical attention if differing pupil size occurs along with:

• Blurred vision
• Double vision
• Droopy eyelid
• Eye sensitivity to light
• Fever
• Headache
• Loss of vision
• Nausea or vomiting
• Eye pain
• Stiff neck

Your provider will perform a physical exam and ask questions about your symptoms and medical history, including:

• Is this new for you or have your pupils ever been different sizes before? When did it start?
• Do you have other vision problems such as blurred vision, double vision, or light sensitivity?
• Do you have any loss of vision?
• Do you have eye pain?
• Do you have other symptoms such as headache, nausea, vomiting, fever, or stiff neck?

Tests that may be done include:

• Blood tests such as CBC and blood differential
• Cerebrospinal fluid studies (lumbar puncture)
• CT scan of the head
• Electroencephalogram (EEG)
• Head MRI scan
• Tonometry (if glaucoma is suspected)
• X-rays of the neck

Treatment depends on the cause of the problem.

Enlargement of one pupil; Pupils of different size; Eyes/pupils different size

Balcer JL. Pupillary disorders. In: Liu GT, Volpe NJ, Galetta SL, eds. Liu, Volpe, and Galetta's Neuro-Ophthalmology. 3rd ed. Philadelphia, PA: Elsevier; 2019:chap 13.

Nischal KK. Ophthalmology. In: Zitelli BJ, McIntire SC, Nowalk AJ, Garrison J, eds. Zitelli and Davis' Atlas of Pediatric Physical Diagnosis. 8th ed. Philadelphia, PA: Elsevier; 2023:chap 20.

Thurtell MJ, Rucker JC. Pupillary and eyelid abnormalities. In: Jankovic J, Mazziotta JC, Pomeroy SL, Newman NJ, eds. Bradley and Daroff's Neurology in Clinical Practice. 8th ed. Philadelphia, PA: Elsevier; 2022:chap 17.

Updated by: Joseph V. Campellone, MD, Department of Neurology, Cooper Medical School of Rowan University, Camden, NJ. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, MD, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.