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

    Anisocoria is unequal pupil size. The pupil is the black part in the center of the eye. It gets larger in dim light and smaller in bright light.


    Slight differences in pupil sizes are found in up to 1 in 5 healthy people. Usually, the diameter difference is less than 0.5 mm, but it can be up to 1 mm (0.05 inch).

    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 is possibly genetic and 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 eyedrops 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
    • Bleeding inside the skull caused by head injury
    • Brain tumor or abscess
    • Excess pressure in one eye caused by glaucoma
    • Increased intracranial pressure
    • 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)

    Home Care

    Treatment depends on the cause of the unequal pupil size. You should see a doctor if you have sudden changes in pupil size.

    When to Contact a Medical Professional

    You should see a doctor if you have persistent, unexplained, or sudden changes in pupil size. The new development of different sized pupils 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
    • Eye sensitivity to light
    • Fever
    • Headache
    • Loss of vision
    • Nausea or vomiting
    • Eye pain
    • Stiff neck

    What to Expect at Your Office Visit

    Your health care 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?
    • What other symptoms do you have?
      • Is there a headache?
      • Is there nausea?
      • Is there vomiting?
      • Is there blurred vision?
      • Is there double vision?
      • Is there a fever?
      • Is there a stiff neck?
      • Are the eyes light-sensitive (photophobia)?
      • Is there eye pain?
      • Is there loss of vision?

    Tests that may be done include:

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

    Treatment depends on the cause of the problem.


    Baloh RW. Neuro-ophthalmology. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier. 2007: chap 450.

    Griggs RC, Jozefowicz RF, Aminoff MJ. Approach to the patient with neurologic disease. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier. 2007: chap 418.

    Rucker JC. Pupillary and eyelid abnormalities. In: Bradley WG, Daroff RB, Fenichel GM, Jankovic J, eds. Bradley: Neurology in Clinical Practice. 5th ed. Philadelphia, Pa: Butterworth-Heinemann Elsevier; 2008:chap 17.


    • Normal pupil


      • Normal pupil


      Review Date: 4/30/2011

      Reviewed By: Kevin Sheth, MD, Department of Neurology, University of Maryland School of Medicine, Baltimore, MD. Review provided by VeriMed Healthcare Network. Also reviewed by David C. Dugdale, III, MD, Professor of Medicine, Division of General Medicine, Department of Medicine, University of Washington School of Medicine;David Zieve, MD, MHA, Medical Director, A.D.A.M., Inc.

      The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition. A licensed medical professional should be consulted for diagnosis and treatment of any and all medical conditions. Links to other sites are provided for information only -- they do not constitute endorsements of those other sites. © 1997- A.D.A.M., Inc. Any duplication or distribution of the information contained herein is strictly prohibited.

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