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.

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.

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)

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

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
• Severe eye pain
• Stiff neck

The medical history will be obtained and a physical examination performed.

Medical history questions documenting differing pupil size in detail may include:

• Is this new for you or have your pupils ever been different sizes before?
• When did it start?
• What other symptoms are also present?
  • 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?

The physical examination will include a neurological examination.

Diagnostic tests that may be performed 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 skull
• X-rays of the neck

Depending on the diagnosis, medication may include drugs to control migraine, corticosteroids (to diminish swelling of the brain tissue), anticonvulsant drugs (to control seizures), pain relievers, antibiotics (for bacterial meningitis), or anticancer drugs.

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.