Everything You Need to Know About Pupil Dilation

Pupil dilation occurs when the dark circles in the center of the eyes become larger than usual. The muscles of the iris (the colored part of the eye) control pupil size. The pupils normally dilate in response to certain situations, such as dim lighting, and usually constrict back to their regular size in bright light.

Pupil dilation, also known as a “fixed pupil,” happens when one or both pupils stay enlarged regardless of changes in light. In this case, the pupil may remain dilated even in bright light.

You can visit the Eye Center at Batal Specialized Complex in Saudi Arabia for comprehensive eye examinations performed by highly experienced ophthalmologists who provide top‑quality care and appropriate, effective treatment.

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What Does a Normal Pupil Look Like?

On average, a normal adult pupil is:

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What Does Pupil Dilation Look Like?

When the pupil dilates, it increases in size, so the black center of the eye looks unusually large. The black pupil takes up more of the iris, so the eye may appear darker with less visible color. Sometimes one pupil is more dilated than the other. This common condition is called anisocoria and affects about 20% of the population.

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Normal (Physiologic) Causes of Pupil Dilation

The pupil can dilate for many normal reasons, including:

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However, if the pupils stay dilated or are noticeably different in size, you should see an eye doctor. This may be a medical emergency. Pupil dilation can be a sign of serious conditions affecting the brain, such as:

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Medical Causes of Pupil Dilation

Pupil dilation often happens on its own without any other obvious cause. In other cases, it can result from medications or an underlying medical condition, especially if the dilation is prolonged or unusual. Possible medical causes of pupil dilation include:

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Many prescription and over‑the‑counter medications can cause pupil dilation, including:

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Recreational (illegal) drugs that can cause pupil dilation include:

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Pupil dilation can also be a sign of withdrawal from some of these substances.

BEUM (Benign Episodic Unilateral Mydriasis) is a temporary condition in which one pupil becomes enlarged. The larger pupil usually returns to normal size within a few hours or a few days. Other symptoms may include:

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BEUM itself is not dangerous, but you should still undergo a complete eye examination to be sure the eyes and nervous system are healthy.

A brain injury can increase pressure inside the skull and damage the iris. This often leads to dilation of one pupil, commonly referred to as a “blown pupil.” Common causes include:

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An eye injury that damages the iris muscles or the nerves supplying them can cause pupil dilation. Common examples of eye injuries include:

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In addition to trauma, eye injuries can also result from certain surgical procedures, such as cataract removal.

MCNP (microvascular cranial nerve palsy) occurs when the blood supply to the cranial nerves is reduced or blocked. This can change pupil size and impair vision.

This rare neurological disorder leads to abnormal dilation of one or both pupils.

Cranial neuropathy is the progressive deterioration of the cranial nerves that connect to the eye. The oculomotor nerve (cranial nerve III) controls pupil constriction, so any damage to it can cause pupil dilation. This may affect one or both eyes and is often associated with visual disturbances.

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Treatment of Abnormal Changes in Pupil Size

Treatment for pupil dilation depends on the underlying cause. It is important to see a doctor at the Eye Center in Batal Specialized Complex in Saudi Arabia if you notice abnormal changes in your pupils, as these changes can affect your vision. The doctor may recommend one or more of the following:

Photochromic (transition) lenses automatically darken outdoors in daylight and become clearer indoors or in low‑light conditions. They can help reduce sensitivity to bright outdoor light.

Polarized lenses protect the eyes in intense lighting conditions. They reduce glare from surfaces such as water, snow, asphalt, and metal by filtering specific light waves.

Custom contact lenses can help reduce light sensitivity caused by dilated pupils.

If pupil dilation is caused by trauma, surgery may be needed to repair the damaged structures.

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Pupil dilation occurs when the dark centers of the eyes become larger than normal. The pupils naturally dilate in response to changes in light, but dilation can also result from a medical condition that requires treatment. Schedule a comprehensive eye exam at the Eye Center in Batal Specialized Complex in Saudi Arabia to ensure your eyes are healthy.

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