Dilated Pupils

What are dilated pupils?

The pupils look like round, black dots in the center of the eyes, but they are actually openings that allow light to enter the eyes. The size of the opening is controlled by the iris, the colored portion of the eye surrounding the pupil. The pupils constantly change size in response to light, to the eye’s focusing distance, and to emotions. Normally, the pupils dilate (enlarge) in darker conditions to allow more light to enter the eye. The medical term for dilated pupils is mydriasis.

Medications or other drugs can also cause the pupil to dilate, either intentionally or as a side effect. For example, eye care professionals often use eye drops that dilate the pupils in order to examine structures at the back of eye, such as the retina. Other medications and illicit drugs that can dilate the pupils include cough and cold medications, decongestants, cocaine, and marijuana. In most cases, the dilation caused by these drugs is temporary, and the pupils return to normal as the effect wears off.

Poisoning from biological or chemical substances is another cause of dilated pupils. With drugs or poisoning, both pupils are usually equally dilated.

Certain neurological conditions and diseases of the brain, such as stroke or tumor, can also affect pupil size. These conditions may dilate only one pupil, or they may dilate the pupils to a different degree on each side, resulting in unequal pupil size. Pupils that do not respond to light or other stimuli are called fixed pupils. Often, abnormally dilated pupils are also fixed pupils.

Dilated pupils or pupils that are unequal in size can be a sign of serious conditions affecting the brain, including stroke, bleeding or tumor and often signal the presence of a medical emergency. Head trauma may also produce dilated or unequal pupils that accompany other brain injuries. Seek immediate medical care (call 911) if you have dilated pupils as a result of injury or associated with other symptoms.

If your dilated pupils are persistent or recurrent or cause you concern, seek prompt medical care.