Anisocoria: Definition, Causes & Symptoms

Instructor: Danielle Haak

Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences

Anisocoria is a condition that causes your pupils to be different sizes at the same time! While this may be normal, it can be a sign of an underlying condition. Read through this lesson to learn the symptoms and causes of anisocoria.

What is Anisocoria?

Anisocoria is a condition that causes the pupils to be two different sizes at a given time (typically, people's pupils are the same size). The pupil is the black part in the middle of the eye. Surprisingly, about 20% of the American population has physiologic anisocoria - you're clearly not looking hard enough if you've never seen it!

Physiologic anisocoria is when the pupils are different sizes but there is no underlying medical condition causing them to be this way. The discrepancy can vary from day to day and may switch eyes. For example, the left pupil may be bigger one day and then smaller the next. Most cases aren't a big deal, though anisocoria can be indicative of a more serious underlying medical problem.

Anisocoria is when the pupils are different sizes.

Causes of Anisocoria

So what causes the pupils to vary in size? This can happen as a result of taking certain medications; however, more serious causes can include brain tumors, meningitis, migraines, seizures, bleeding in the skull, aneurysm, or an abscess in the brain. Sudden head injuries can also cause pupils to vary in size, and this is a sign that you should go see a doctor.

Symptoms of Anisocoria

The good news is anisocoria itself does not usually have symptoms, other than, obviously, the difference in pupil sizes. However, the underlying cause may be accompanied by symptoms. These most often include blurry, double, or loss of vision, fever, headache, nausea, and muscle stiffness (especially in the neck).

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