Physiologic Anisocoria (unequal pupils)

What is Anisocoria?

Anisocoria is a term which refers to the pupils being different sizes. In many people, the size of the pupils is the same in each eye, and both pupils will become smaller or bigger to let light in at the same time. The presence of anisocoria can be normal (physiologic), or it can be a sign of an underlying medical condition. The condition usually does not affect eyesight or eye health.


What is the pupil?

The colored part of the eye is called the iris. This iris is a circular muscle, similar in shape to a donut. The hole in the middle of the iris, which allows light to enter the eye, is called the pupil. Depending on the lighting conditions, the pupil will change in size to allow more or less light in the eye.


When is anisocoria normal?

Anisocoria that is NOT associated with an underlying medical condition is called physiologic anisocoria. Physiologic 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 may vary from time to time. 


How does the doctor determine whether anisocoria is due to an underlying medical problem or physiologic?

Certain characteristics, such as when the anisocoria was first noted, whether it is more noticeable in bright or dim light, and whether or not there was an event related to the change will help determine the underlying cause. During a complete eye examination by an ophthalmologist, the size of the pupils and how they react to bright and dim light will be checked. Based on the evaluation, the doctor may perform additional tests to make a diagnosis.


How is ansiocoria treated?

Usually anisocoria does not need to be treated since it does not affect eyesight or eye health. If there is an underlying medical condition, that issue will need to be treated.