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By Medicover Hospitals / 03 Mar 2021
Home | symptoms | anisocoria
  • Anisocoria is a condition in which the pupil of one eye is different in size than the pupil of the other eye. Your pupils are the dark circles in the center of your eyes. They are usually the same size. Anisocoria can be caused by several things. You can be born with this disease or develop it later.
  • Article Context:

    1. What is Anisocoria?
    2. Causes
    3. Diagnosis
    4. Treatment
    5. When to visit a Doctor?
    6. Prevention
    7. FAQ's

    What is Anisocoria?

  • Anisocoria occurs when the pupils in your eye are not the same size. The pupil allows light to enter the eye so that you can see.
  • Anyone can have students of different sizes with no problem. In fact, one in five people have pupils who are normally different heights.
  • Sometimes, however, uneven pupil size can be a symptom of a serious eye problem. People who can get anisocoria include those who have:
    • a nervous system problem
    • a history of eye damage
    • risk of having a stroke
    • viral infection
    • adie's toned student


  • Many cases of mild anisocoria are normal and have no underlying disease or history of trauma. Usually, those cases of anisocoria where one pupil is larger than the other by less than 1.0 mm with no apparent cause are called simple anisocoria, benign anisocoria, or physiological anisocoria.
  • The prevalence of anisocoria did not vary with time of day or day to day; nor was it influenced by the subject's sex, age, or eye color.
  • Significant anisocoria can have many causes, and some can be medical problems. Causes of a large anisocoria include:
    • Eye trauma
    • Certain eye medications: For example, pilocarpine eye drops used to treat glaucoma may make the pupil of the treated eye smaller than the other pupil.
    • Inflammation of the iris: Iritis can cause anisocoria, which is usually accompanied by eye pain.
    • Adie's tonic pupil: This benign condition usually makes one pupil noticeably larger than the other. The affected pupil also does not respond to light. In most cases, the cause of Adie's pupil is unknown. But it can be associated with eye trauma, lack of blood flow, or infection.
    • Neurological disorders: Several conditions that damage nerves in the brain or spinal cord can cause anisocoria. One of the most important is Horner's syndrome.
  • People with nervous system disorders that cause anisocoria often also have a droopy eyelid, double vision, or strabismus. Brain disorders associated with anisocoria include stroke, hemorrhaging, and, less commonly, certain tumors or infections.
  • Diagnosis:

    • The causes of anisocoria range from mild to life-threatening conditions. Clinically, it is important to determine whether anisocoria is more noticeable in dim or bright light to clarify whether the larger or smaller pupil is the abnormal one.
      • Anisocoria that is aggravated in the dark suggests that the small pupil is the abnormal pupil and suggests Horner's syndrome or mechanical anisocoria. In Horner's syndrome, the sympathetic nerve fibers have a defect, so the pupil of the involved eye does not dilate in the dark. If the smaller pupil dilates in response to the instillation of apraclonidine eye drops, this suggests Horner syndrome.
      • Anisocoria which is larger in bright light suggests that the larger pupil is the abnormal pupil. This may suggest a toned Adie pupil, pharmacologic dilation, oculomotor nerve palsy, or damaged iris.
    • A Relative Afferent Pupillary Defect (RAPD) also known as Marcus Gunn's pupil does not cause anisocoria.
    • Some causes of anisocoria are life-threatening, including Horner's syndrome and oculomotor nerve palsy.
    • If the examiner is uncertain whether the abnormal pupil is the constricted or dilated pupil, and if unilateral drooping of the eyelid is present, the abnormally sized pupil can be presumed to be the one on the side of the ptosis. This is because Horner's syndrome and oculomotor nerve damage both cause ptosis.
    • Anisocoria is usually a mild finding, not accompanied by other symptoms. Old photographs of patients' faces often help diagnose and establish the type of anisocoria.
    • It should be considered an emergency if a patient develops acute anisocoria. These cases may be due to brain mass lesions that cause oculomotor nerve palsy. Anisocoria is confusion, decreased mental status, severe headaches, or other neurological symptoms that may signal a neurosurgical emergency. This is because a hemorrhage, tumor, or other intracranial mass may enlarge to a size where the third cranial nerve (CN III) is compressed, resulting in uninhibited dilation of the pupil on the same side as the lesion.


    • Typically, anisocoria does not need to be treated because it does not affect eyesight or eye health.
    • If the anisocoria is related to an eye health problem, this problem will need to be treated.
    • If you have any questions about anisocoria, ask your ophthalmologist. Your ophthalmologist is committed to protecting your eyesight.

    When to visit a Doctor?

  • Contact a physician if you have persistent, unexplained, or sudden changes in pupil size. If there is a recent change in the pupil's size, it may be a sign of a very serious illness.
  • If you have a different-sized pupil after an eye or head injury, see a doctor immediately.
  • Always see a physician immediately if the pupil size differs with:
    • Blurred vision
    • Double vision
    • Eye sensitivity to light
    • Fever
    • Headache
    • Loss of vision
    • Nausea or vomiting
    • Eye pain
    • Torticollis


  • In some cases, you may not be able to predict or prevent anisocoria. However, there are steps you can take to reduce your risk of developing uneven pupils. For example:
    • Report any change in your vision immediately to your doctor.
    • Wear a helmet when engaging in contact sports, cycling, or horseback riding.
    • Wear protective gear when operating heavy machinery.
    • Wear your seat belt when driving.
  • If you notice any differences in the size of your pupils, see a doctor immediately. Your doctor can help you identify and treat the underlying cause of your condition.
  • Frequently Asked Questions:

  • About 20% of the population suffers from anisocoria. The amount of anisocoria can vary from day to day and may even change your mind. Anisocoria that is NOT associated with or due to an underlying medical condition is called physiological anisocoria.
  • Anisocoria signs include:
    • drooping eyelid
    • problems moving your eye
    • eye pain
    • fever
    • headache
    • reduced sweating
  • Physiological anisocoria can occur in 20% of the population. The difference between the sizes of the two pupils is rarely more than 1-2 mm but can vary from time to time.
  • Babies born with different-sized pupils may not have an underlying disorder. If other family members also have similar pupils, then the difference in pupil height could be genetic and there is no cause for concern. Additionally, for unknown reasons, student sizes may temporarily differ.
  • Citations:

  • An approach to anisocoria -
  • Simple Anisocoria -
  • Anisocoria -