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Mydriasis (Dilated Pupils): What You Need to Know

Dilated Pupils

The pupils naturally dilate or become bigger when you enter a darker environment. But sometimes, pupils dilate even when the light in your environment hasn’t changed.

Light gets into your eyes and the retina through the pupils. The pupils are the openings in the center of the eye that regulates light entry. It may change size according to the amount of light in your surroundings. Naturally, your pupils will become smaller in bright spaces since your eyes need less light. Also, when you enter a darker environment, your pupils naturally widen (or dilate) to allow more light to reach the retina, the delicate sensory tissue at the back of the eyeball.

The pupils regulate the amount of light that gets into the eyes. When a bright light is in the environment, such as during the daytime, the pupil constricts or gets smaller. The pupils get smaller to protect the retina from damage due to excessive light. Yet, there are instances when the pupils dilate, even when no light changes exist.

Mydriasis or dilated pupils are a regular occurrence that should not cause panic. Yet, if it occurs longer than usual, it could indicate an underlying issue. Mydriasis may occur due to injury to the muscles or nerves of the eye and brain injury. The disorder may also arise from increased oxytocin levels, side effects of certain recreational drugs, and other medical conditions.

What Is Mydriasis?

Mydriasis is the term used to describe the dilatation or enlargement of the pupil, which typically happens in reaction to low levels of ambient light. Mydriasis could also refer to a medical condition involving excessive or unusual dilation or widening of the pupils.

Your pupils dilate when the light is dim to allow more light to enter the eye. So, mydriasis as a disorder refers to a condition where the pupils dilate without a change in the levels of light. The abnormality may affect one or both eyes.

The National Institute of Health estimates that one in five infants are born with pupils of slightly different sizes, and their eyes react typically to changes in light. Your pupils naturally dilate depending on the environment. Thus, dilated pupils do not mean you have a severe eye problem. You should see a doctor if the condition doesn’t go away fairly soon. Also, if other symptoms are present such as headaches or confusion, seek urgent medical help.

Causes of Mydriasis

If your pupils are larger than normal, it could indicate an emergency problem. Medical examination can determine the cause of mydriasis and provide next-step treatments.

Some of the common causes of dilated pupils include:

1. Injuries to the Eyes

Eye injuries caused by accidents may result in severe damage to the nerves that control the iris and pupils. Damaged nerves can’t control the pupil hence causing dilated pupils. Mydriasis could lead to injury to the iris sphincter muscle (a muscle that helps with pupil constriction).

Also, cranial nerve damage in one or both eyes may lead to mydriasis and vision impairment.

2. Anticholinergics

Some prescription medications have tiny substances that block the transmission of certain chemicals to the eyes. The pupils become more dilated when these chemicals don’t reach the eyes. Medications that can dilate pupils include antidepressants and antihistamines. Also, medications that treat Parkinson’s disease, nausea, and seizures contain anticholinergic that cause pupil dilation.

3. Brain injury

Brain injury increases intracranial pressure or pressure around the brain. The excess pressure from the injury may damage the oculomotor nerve that controls pupillary constriction. A damaged oculomotor nerve can result in a blown pupil, which usually causes mydriasis in only one eye.

Mydriasis caused by brain injury or headache is an emergency and needs urgent medical attention.

4. Increase in Oxytocin

Oxytocin is a hormone that arouses sexual excitement or feeling. The body releases oxytocin when exercising or during intimate and social interactions. Oxytocin also increases the contraction of a woman’s uterus during labor. The pituitary gland secretes the hormone.

When produced in excess, oxytocin can lead to dilated pupils. The effects of oxytocin on pupil dilation are usually mild but may aggravate during intimate interactions and heightened emotional connections.

5. Plants

Your pupils dilate when you unknowingly inhale particles or touch garden flowers from plants, such as Jimson weed, angel’s trumpet, and belladonna family members.

Signs and Symptoms

Signs and symptoms of mydriasis may differ from one individual to the other. But, a characteristic symptom of mydriasis among most patients is dilated pupils that don’t adjust in response to changes in light.

The more dilated the pupils become, the more sensitive the eyes become to light. This condition can lead to blurry vision and a general feeling of constriction around the forehead and eyes. Other symptoms include headaches, dizziness, eye irritation, trouble sleeping, and mydriasis.

Also, an outward eye movement and a drooping eyelid are signs that a third nerve palsy is the primary cause of your mydriasis.


There’s no specific treatment for mydriasis. Your doctor will prescribe medication depending on the cause of the condition. Most treatment approaches will protect the full functionality of the eyes.

A pupil dilation caused by a reaction to medication is never a serious caused concern. Most doctors recommend waiting for the effects of the drug to lessen and avoiding it in the future.

Mydriasis caused by injuries to the brain and eyes requires more intense treatments. The doctor may prescribe low doses of Pilocarpine, a medication typically used to treat glaucoma. Pilocarpine constricts or narrows the pupils and restores them to their natural position. In severe cases, you may require surgery to repair the damage to the nerves or eye structures.


Mydriasis is a normal response to low levels of ambient light. Several factors may cause dilation of the pupil, including emotional or hormonal changes. Yet, if you experience abnormal or prolonged dilation, you should seek medical attention. Mydriasis arising from brain injury or headache is a medical emergency and requires immediate treatment.

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