In the mirror, the pupil of the eye appears as a black circle in the middle of the iris (the colored part of the eye). Uneven pupil size, or anisocoria, may be a normal variation in a person’s eyes or may indicate an underlying problem.
Uneven Pupils: What You Need to Know
Although the pupil looks like a black circle in the center of the colored part of your eye, it is actually a hole that opens and closes to let different amounts of light through to the back of the eye, depending on your environment (e.g., sunny day or dark room).
For new uneven pupil size that is related to new double vision, eyelid droopiness or head, neck or eye pain, it is best to be evaluated in the emergency room.
Symptoms of Uneven Pupils
Uneven pupil size may be noticed by the person or by a health professional during an examination. More often than not, it is pointed out to the person by someone close to them.
An ophthalmologist should be seen to rule out ocular causes of eye pain and pupil asymmetry, especially when vision loss or changes, redness or discharge from the eye(s) is present. This is to rule out eye conditions such as acute angle closure glaucoma or inflammation of the front part of the eye (uveitis or iritis).
What causes uneven pupils?
Slight differences between the two pupils may be present in up to 20 percent of people. This is called “physiologic anisocoria” and is normal. In these cases, there are no other symptoms and both of the person’s pupils react to changes in light.
On the other hand, a person whose pupils are uneven when they were normal before may be experiencing a serious problem such as:
A torn or blocked blood vessel in the neck (usually the result of head or neck trauma), which could cause a mildly droopy eyelid on the side of the smaller pupil.
A brain aneurysm.
A third nerve palsy can result in the inability to move the affected eye normally, in addition to eyelid drooping (which is often significant) on the side of the larger pupil. This may be due to a brain aneurysm, and should be urgently evaluated in the emergency room.
Reaction to certain topical dilating medications (such as a pet’s eye drops, or anti-nausea or motion sickness patches such as scopolamine) that may accidentally get into one eye.
Diagnosis of Uneven Pupils
When a doctor sees a patient for uneven pupil size, the first concern is to determine whether the unevenness is new or long-standing. If the problem is new, the doctor will then focus on which pupil is responding differently. The examination may involve:
Taking a careful history of symptoms, noting when they started and what other problems may be present.
Checking the ability of each pupil to constrict in the presence of bright light and to dilate in the darkness.
Neuroimaging with MRI (occasionally CT) depending on the person’s history and what is found on neuro-ophthalmic and neurologic examinations.
Treatment of Uneven Pupils
Treatment depends on identifying and addressing the underlying problem. For physiologic anisocoria, no treatment is needed.