• Definition

    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.

    Alternative Names

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


    It is normal for a person's two pupils to be up to 1 mm (.05 inch) different in size.

    Babies born with different sized pupils may not have any underlying disorder. If other family members also 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 a brain, blood vessel, or nerve disease.

    Common Causes

    The use of eyedrops are 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
    • Infection of membranes around the brain caused by 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)