What Is Anisocoria?

by Lisa Emrich Patient Advocate

Shortly after I began my doctoral studies, I started experiencing problems with my vision which were accompanied by unusual headaches. Having been in a car accident which caused whiplash, I was seeing a chiropractor. However, when my headaches didn’t improve, I visited the campus Eye Clinic (part of the School of Optometry) for evaluation.

For my entire life up to this point, I had only visited one eye clinic for all my care. I had been a patient in the same doctor’s office since I was 4 years old. This was my first time to be examined by anybody I didn’t know. Very interesting to have both students and professionals examine my eyes. I was quite the guinea pig in the campus Eye Clinic.

One thing which concerned the doctors was that my pupils were of different sizes. They wanted to know if my pupils had always been that way. I wasn’t really even aware of the issue, having not noticed the difference in my own reflexion. Since I couldn’t answer the question, they wanted to see photos of me from various years to see if my pupils had always appeared to be different sizes.

Pictures from high school showed that indeed my pupils had NOT always differed in size. This discovery, along with my symptoms and examination, prompted the ophthalmologist to order a brain MRI. I didn’t know enough at the time to ask lots of questions. I do know that the doctors wanted to eliminate the possibility of a brain tumor. No mention of other conditions which might affect the eyes (including MS). My MRI came out clean and the clinic made me a special pair of reading glasses.

Although my brain was clear, my eyes still displayed Anisocoria. My eyes continue to show Anisocoria in addition to the Relative Afferent Pupillary Defect (RAPD), a condition which we discussed last week.

What Is anisocoria?

Anisocoria refers to an inequality of pupil size. Approximately 15-20% of the general population has anisocoria without an underlying medical cause. The amount of different in pupil size can vary from day-to-day and even switch eyes. When anisocoria is not caused by an underlying medical condition, it is called physiologic anisocoria. In cases of physiologic anisocoria, the difference in pupil size is usually mild, not more than one millimeter, and does not differ with the degree of ambient light.

What Are Possible Causes of Anisocoria?

Besides occurring naturally in a low percentage of the popuation, anisocoria can be caused by structural or neurological conditions. Structural causes include ocular infection, trauma, inflammation, or surgery. Neurological causes can involve either the sympathetic nerve input (which dilates the pupil) or the parasympathetic nerve input (which constricts the pupil).

Anisocoria which is greater in dim light or darkness suggests a problem with sympathetic nerve input. The affected pupil will stay constricted rather than dilate to let additional light into the eye. Medical causes include Horner’s syndrome, Argyll Robertson pupil, and cluster headaches.

Anisocoria which is greater in bright light (or less in dim light) suggests a problem with parasympathetic nerve input. The affect pupil will stay dilated rather than constrict to restrict the amount of light which enters the eye. Medical causes include Holmes-Adie pupil, 3rd cranial nerve (oculomotor nerve) palsy, exposure to drugs which dilate the eye (such as cold medications, hallucinogens, and dilating eye drops), or use of anticholinergic agents (which are commonly used to treat asthma, urinary bladder spasms, and motion sickness).

If I Have Anisocoria, Do I Have MS?

Not necessarily (or even likely). There are so many possible causes of anisocoria that MS is not at the top of the differential diagnosis list. However, if multiple sclerosis has caused damage to the 2nd or 3rd cranial nerves, then you may show anisocoria.

Remember that the Relative Afferent Pupillary Defect called Marcus Gunn Pupil is also known as “dynamic anisocoria.” This is caused by damage along the 2nd cranial nerve and is seen more commonly in cases of optic neuritis and MS.

Lisa Emrich
Meet Our Writer
Lisa Emrich

Living with multiple sclerosis and rheumatoid Arthritis, Lisa Emrich is an award-winning, passionate patient advocate, health writer, classical musician, and backroad cyclist. Her stories inspire others to live better and stay active. Lisa is author of the blog Brass and Ivory: Life with MS and RA and founder of the Carnival of MS Bloggers. Lisa frequently works with organizations in support of better policies, patient-centered research, and research funding. Lisa serves on HealthCentral’s Health Advocates Advisory Board, and is a Social Ambassador for the MSHealthCentral Facebook page.