What is anisocoria?
Anisocoria is the medical term for unequal pupil size. Normally our pupils are relatively similar in size. For many of us, our pupil size may even fluctuate between equal and unequal sizes throughout the day.
How do our pupils work?
The pupil is not an actual structure inside the eye - it is just an opening! Inside the eye, the iris (colored portion of the eye) separates the front of the eye (anterior segment) from the back (posterior segment). The iris controls the amount of light that enters the eye because its pigment (color) helps to block the light, and it has muscles that can narrow the pupil opening (pupil constriction, or miosis) or widen it (pupil dilation, or mydriasis).
When light enters the eye, there are special light-sensitive cells in the retina that send a signal to a specific part of the brainstem. This triggers a reflex to both pupils, causing them to constrict at the same time. The more light there is, the more constricted the pupils are. In dim light, the brain sends a signal to the iris muscles to dilate and let more light into the eyes. The nerves that send signals to the dilating muscles are the same nerves controlled by our “fight or flight” system, so when we are excited, aroused, or scared, our pupils will dilate as well.
What causes anisocoria?
The most common cause of unequally-sized pupils is physiologic, meaning that it is simply how our pupils are, and unrelated to any nerve or muscle problem. Physiologic anisocoria, also called simple or essential anisocoria, is found in 15-30% of the normal population. One of the key features of physiologic anisocoria is that the amount of size difference does not change between dim and in bright light.
Many people have fluctuating pupil sizes. Episodes of anisocoria are typically benign and can even occur with migraine headaches, which can be alarming to patients and doctors. Unequal pupils may also be caused by medications, especially certain types of eye drops.
Constant anisocoria can occur if the nerves or muscles that control pupil size are damaged. If the nerves or muscles that control pupil constriction are damaged, the pupil may be abnormally enlarged. Likewise, damage to nerves or muscles that control pupil dilation may cause the pupil to be abnormally small.
Why should I be concerned about anisocoria?
Anisocoria typically does not affect your vision and is not blinding, though some people will notice some blurred vision and/or light sensitivity. However, if your anisocoria is constant, your doctor may be concerned that it represents a more serious neurological condition.
Why should I see a neuro-ophthalmologist?
Neuro-ophthalmologists evaluate anisocoria by getting a detailed history (including looking at old photographs), examining your pupils in different lighting conditions, evaluating your eyelids, and doing a complete eye exam. The neuro-ophthalmologist may use at least 1 eye drop to help diagnose the cause of your anisocoria, and, depending on the cause of the anisocoria, may recommend additional tests, such as MRIs and/or CTs.
Depending on the cause of the anisocoria, you may not need any treatment for your anisocoria. The neuro-ophthalmologist will make sure you do not have anything dangerous causing your anisocoria, and may recommend treatments for the symptoms of anisocoria if you have any. However, because anisocoria does not cause blindness or any permanent damage to your eye, the neuro-ophthalmologist may recommend doing nothing.
- North American Neuro-Ophthalmology Society Patient Brochure (English)
- American Academy of Ophthalmology EyeWiki
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This article was created for educational use only and does not substitute for medical advice.