Anisocoria is a condition characterized unequal pupil sizes. Many healthy people have different-sized pupils. A diameter difference of less than 0.5 millimeters is not uncommon. Pupils fluctuate in size temporarily throughout the day, and there is usually nothing to worry about if they return to normal and additional symptoms aren’t present. However, having a large difference in pupil sizes that does not return to normal may indicate a serious health problem.


There are four main types of anisocoria:

  • Simple (or physiological) anisocoria. This is the most common type of anisocoria. Most cases are benign and do not affect the pupils’ response to light. The cause is unknown.
  • Pathologic anisocoria. This type is caused by an underlying disease or condition, including Horner’s syndrome, Adie’s tonic pupil and third nerve palsy.
  • Mechanical anisocoria. This type develops after trauma to the eye that damages the iris. This could be from optical surgery complications, injury or angle-closure glaucoma.
  • Pharmacologic anisocoria. This type develops as a side effect of medications. Medications that may cause uneven pupils include glaucoma eye drops and certain types of anxiety medications and antidepressants.


  • Difficulty moving eyes
  • Drooping eyelids
  • Eye pain
  • Fever
  • Headache
  • Uneven pupil size


Some cases of anisocoria cannot be prevented. However, you can follow safety steps to prevent head injury, a common cause of anisocoria. These include wearing a helmet and seatbelt when appropriate and removing trip hazards from your home and yard.

Risk Factors

  • Brain aneurysm
  • Brain tumor
  • Glaucoma
  • Infection of the membranes located near the brain
  • Migraine
  • Prior eye surgery
  • Seizure
  • Trauma or injury to the eyes


Comprehensive eye exam. Your eye doctor will first ask you to describe any ongoing symptoms. Be prepared to let your provider know approximately when you began experiencing symptoms, as well as any recent eye pain, loss of vision or other symptoms. Your eye doctor will then examine your pupils to see how they respond to light. The provider will also check your eyes with different instruments and tools to get a closer look to detect any problems.

Further testing. Your primary care physician may need to order additional tests to diagnose the cause of anisocoria. These include blood studies, lumbar puncture, or a CT or MRI scan of the head.


Anisocoria itself does not require treatment. Instead, your provider will treat the underlying condition that is causing your pupils to be uneven.

Follow-up Care

Watch your pupils closely for any changes following treatment. You will want to notify your providers immediately if your pupils change in size again.