Anisocoria

How it works

The pupil is the black hole in the middle of the iris. Anisocoria means that the two pupils are different sizes⁶. Doctors usually count a difference of 0.4 mm or more¹.

Anisocoria is often harmless⁶. This form is called physiological or simple anisocoria. The difference is seldom more than 0.8 mm¹, and most often less than 0.5 mm⁴. In millimetres, the gap gets a little smaller in bright light. But the ratio of the two pupil areas stays the same, so by eye the difference may not seem to change³.

Doctors use light to find which pupil has a problem. A difference that is larger in bright light points to the large pupil. A difference that is larger in the dark points to the small pupil¹.

Less often, unequal pupils come from a nerve problem or an eye injury¹. Eye drops, or asthma inhaler spray that gets into one eye, can also change one pupil⁴.

How common it is

The best figure comes from a 1987 study. Researchers photographed 128 healthy people in dim light, ten times over five days². At any one session, 19% had unequal pupils. Over the five days, 41% had them at least once, but only 3% had them every time².

Light changes the count. In 104 people, the figure fell from 18% in darkness to 8% in bright light³. A 2019 study of 708 adults in even room light found 13.7%⁵. Doctors who judged by eye found fewer than half of those cases⁵.

Is it a problem?

A small difference that stays about the same in dim and bright light, and that has been there for years, is usually harmless¹. A sudden new difference is not the same thing. It can be a sign of a very serious problem and need an emergency brain scan¹⁴.

Get medical help immediately if unequal pupils come with any of these⁴:

• a droopy eyelid,
• double vision, blurred vision or loss of vision,
• eye pain or sensitivity to light,
• headache, fever or a stiff neck,
• nausea or vomiting.

Also get help immediately after a head or eye injury⁴, or if there is confusion or drowsiness⁶.

If you first notice a difference and have none of these signs, still see a doctor soon. Old photos of your face help doctors see whether it is long-standing¹.

History and culture

David Bowie is often said to have had two eye colours. He had anisocoria instead. A punch in 1962, at age 15, left his left pupil permanently wide, so that eye looked darker⁷. This came from an injury, not from the harmless form⁶⁷.