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The iris (the colorful portion of the eye) controls the size of the pupil, which is always changing. Pupils dilate (expand in size) to take in more light in low-light situations. They also constrict (get smaller) in bright conditions to minimize the amount of light that enters the eye.
On average, a normal pupil in adults is:
When pupils constrict from light, it’s called a direct response.
They also constrict when the eye is focused on a near object, which is called an accommodative response.
If a pupil does not constrict in light or dilate in dark, it’s considered abnormal.
Mydriasis is when your pupils dilate unrelated to light levels. They may dilate in bright light, or stay dilated for a longer time.
When pupils dilate, they expand in size. Therefore, pupils look extra big when they’re dilated. The black pupil takes up more of the iris, so the eye may appear darker with less color.
Pupils dilate for many reasons. Here are a few reasons as to why pupils dilate:
In many cases, mydriasis occurs on its own, with no other causes.
In other cases, mydriasis may be a result of a medication or underlying condition. Especially if your mydriasis is prolonged or unusual.
Potential causes of mydriasis include:
More severe causes of mydriasis include:
BEUM is a temporary condition that causes one pupil to dilate. Other symptoms include:
BEUM itself is not dangerous. However, you should undergo a complete eye examination to make sure your eyes and nervous system are healthy.
Brain injury from physical trauma, stroke, or tumor can increase your intracranial pressure. This often causes one eye to dilate. This is commonly referred to as a "blown pupil."
Cranial nerve neuropathy is the gradual deterioration of nerves that connect to your eye. Your oculomotor nerve controls pupil dilation. So any damage to it can cause mydriasis.
This can occur in one or both eyes. It is often accompanied by impaired vision.
Many drugs can cause dilated pupils. Here are just a few medications and illegal drugs that can cause dilated pupils:
Fortunately, there are some treatment options for larger pupils, including:
Photochromic (transition) lenses automatically darken outside in daylight and lighten up inside in darker conditions. These may help facilitate the dilation process so your pupils adjust easier.
Polarized lenses protect your eyes from intense light conditions. They reduce glares from surfaces like water, snow, asphalt, and sheet metal by filtering out specific wavelengths.
If you have mydriasis caused by brain injury or eye damage, you may require surgery to repair any damage done.
If your dilated pupils are a side effect of substance abuse, you may consider rehabilitation.
Here are answers to some of the most frequently asked questions about dilated pupils.
Pupils are supposed to dilate; it’s a natural pupillary response to low-light situations and to help the eye focus on close-up objects. Pupils may also dilate in response to a variety of stimuli under completely normal circumstances.
However, if the pupils remain dilated or they are unequal in size, seek care from your ophthalmologist. You could have a medical emergency on your hands. Mydriasis and unequal pupil sizes can be signs of serious conditions that affect the brain, such as strokes, tumors, internal bleeding, or head trauma.
Because dilated pupils let light in, if your pupils dilate, you may notice an increased light sensitivity. Dilated pupils also make it harder to read up close.
The pupils shrink in size in brightly lit conditions. They do this to restrict the amount of light that meets the eye. This is a natural protection mechanism.
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