Why Is One Pupil More Dilated Than The Other?

There are a number of reasons why pupils are unequal in size. The medical term for this is called anisocoria.

Sometimes the pupils may differ temporarily in size, but it is nothing to worry about if the pupils go back to normal and you are not experiencing any other symptoms. It is normal for a pupil to have a 1 mm difference in size. Eye drops could also be a harmless cause of a change in the size of a pupil. A baby who is born with unequal sized pupils might not have an underlying disorder. If other members of the family also have similar pupils, then the difference in pupil size could be genetic and normal.

• Symptoms to watch out for

If however, the unequal pupil size develops later on in life and does not return to normal it could be a sign of nerve, blood vessel or brain disease. Always seek the advice of a doctor if there is a sudden, unexplained or persistent change in the size of a pupil. Get immediate medical help if each pupil differs in size after a head or eye injury, or if the difference in the size of each pupil occurs together with double vision, blurred vision, vision loss, severe pain in the eye, fever, headache, vomiting or nausea, stiff neck or when the eyes are extremely sensitive to the light.

• Severe causes of differing pupil sizes

Other severe causes of anisocoria might include aneurysm, excess pressure within one eye because of glaucoma, bleeding inside the head from a head injury, brain abscess or tumor, infections of the membranes of the brain caused from encephalitis or meningitis, migraines, seizure, or a lymph node, mass or tumor within the upper chest that causes nerve pressure resulting in a smaller pupil.