Unequal size of pupils of eyes, normally harmless but sometimes due to life-threatening causes.Encyclopedia Entry for Anisocoria :Anisocoria. 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 (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 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) Diabetic occulomotor nerve palsy Prior eye 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. 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.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 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.Enlargement of one pupil; Pupils of different size; Eyes/pupils different size.Normal pupil Normal pupil.Baloh RW, Jen J. Neuro-ophthalmology.