Anisocoria is a term that refers to an abnormal condition in which the pupils of the two eyes in the same individual are of different sizes. Normally, the size of the pupil is identical in both eyes. Pupillary diameter varies as both irises normally constrict or dilate simultaneously according to the light in the environment.

Anisocoria is a common condition, and it is estimated that one in five individuals have anisocoria to a varying extent. Most of these are what is known as physiologic anisocoria, which is mild (less than one-millimeter difference in size) and considered to be normal.

However, anisocoria can also be indicative of an underlying medical condition in some cases. In fact, severe cases of anisocoria, such as those associated with Horner syndrome, have the potential to be life threatening.

There are many possible causes for anisocoria, including:

• Aneurysm
• Head injury
• Brain tumor
• Glaucoma
• Hemorrhage or stroke
• Infection of the brain membranes or meningitis
• Migraine
• Seizures
• Eye drops
• An adverse effect to certain medications that may enter the eyes such as inhalers