Mydriasis is the medical term for an unusual dilation or widening of the pupils. There are various causes of this fixed dilation of the pupils.

Normally, a person’s pupils dilate when the light is dim so that more light can enter the eye. Mydriasis describes a condition where the pupils dilate without a change in the levels of light.

In this article, we look at the causes and possible treatments for mydriasis. We also discuss when a person should see a doctor about this symptom.

Mydriasis. Image credit: Bin im Garten, (2011, March 16).Share on Pinterest
Mydriasis causes unusual dilation of the pupil.
Image credit: Bin im Garten, (2011, March 16).

When someone’s pupils dilate in an unusual way, it is called mydriasis. This may be caused by an injury, psychological factors, or when someone takes certain drugs or medications.

Doctors sometimes refer to more pronounced mydriasis, when the pupils are fixed and dilated, as “blown pupil.” This condition can be a symptom of an injury to the brain from physical trauma or a stroke.

The opposite of mydriasis is called miosis and is when the iris constricts to cause very small or pinpoint pupils.

Mydriasis can affect one pupil at a time or both at once. Mydriasis that affects only one eye is called anisocoria.

An estimated 1 in 5 people are born with pupils of slightly different sizes, and their eyes react typically to changes in light. This condition is called physiologic, simple, or essential anisocoria.

In most cases, having dilated pupils does not mean that an individual has a serious health problem, and the condition will go away on its own fairly rapidly. A person should see a doctor if it does not, or if other symptoms are present such as headaches or confusion.

There are many different potential causes for mydriasis, including:


Several different medications can cause mydriasis in one or both eyes.

Antihistamines, botox, muscle relaxants, and drugs used to treat Parkinson’s disease may all cause the pupils to dilate.

Anticholinergics, which are used to block neurotransmitters in individuals with COPD, urinary incontinence, and other conditions, also have this effect.

Recreational drugs that include cocaine, LSD, ecstasy, and amphetamines can also cause dilated pupils.


Some plants, such as Jimson weed, angel’s trumpet, and members of the belladonna family, can cause the pupils to dilate.

A person might encounter these plants without realizing it through inhaling particles or touching garden flowers.

Benign episodic pupillary mydriasis

A condition called benign episodic pupillary mydriasis happens when an otherwise healthy individual with a history of headaches suddenly develops dilated pupils.

They may also experience discomfort in, around, or behind the eye and have blurry vision.

Episodes typically last for a short time and go away on their own.

Dilated pupil eye exams

Share on Pinterest
Eye drops called mydriatics may be used during eyes exams, causing temporary dilation of the pupils.

Eye specialists called ophthalmologists and optometrists may use special eye drops called mydriatics to dilate the pupils during eye exams.

Around 15 to 30 minutes after this treatment, the pupils will dilate, making it possible for the specialist to get a good look at both the retina and the optic nerve. Being able to see these components of the eye can provide key indicators of eye health and signs of serious problems.

Eye exams may be conducted to look for problems, such as glaucoma, macular degeneration, and other eyesight-threatening conditions. The effects of the eye drops usually wear off in 4 to 6 hours.


Increases in oxytocin levels tend to make the pupils dilate. Oxytocin is a hormone that supports human connection and reproduction. It supports women through labor, promotes bonding with babies, and also affects testosterone levels. Studies have linked oxytocin and pupil dilation to attraction and positive social interaction.

Cranial nerve neuropathy

Damage to the third cranial nerve can cause third nerve palsy, which interferes with the body’s ability to control eye muscles and can result in mydriasis.

Infection, injury, diabetes, and migraines can cause this nerve damage, and people can also be born with it.

Brain injury

Damage to the brain can cause increased pressure within the brain, which can affect eye health and damage nerves. Pressure can come from physical trauma or internal brain damage, such as a stroke.

Eye trauma

When someone has an injury to their eye, it can damage the nerves that control pupil dilation.


When people concentrate, it may be linked to dilation of their pupils.

Researchers are using the practice of pupillometry or measuring the size of the pupils to analyze emotions and intellectual tasks that make the pupils dilate and constrict.

The characteristic symptom of mydriasis is dilated pupils that do not get bigger or smaller in response to changes in light.

When the pupils are dilated, the eyes become more sensitive to light. This can lead to blurry vision, as well as, in some cases, a general feeling of constriction around the forehead and eyes.

Some individuals may also experience headaches, dizziness, eye irritation, and have trouble sleeping along with mydriasis.

Difficulty moving the eye and a drooping eyelid can be signs that an individual’s mydriasis is caused by third nerve palsy.

Share on Pinterest
Wearing sunglasses is recommended for those with mydriasis, as they may be extra sensitive to light.

Many instances of mydriasis, particularly those caused by a reaction to plants or medication, will go away on their own, often within a few hours or days.

Individuals with mydriasis will be extra sensitive to light, as long as their pupils are dilated. It is best for the individual to wear sunglasses when outside and stay away from bright lights. It is also a good idea to limit driving, as much as possible.

If a person develops mydriasis after taking medication, they should avoid taking that drug in the future. They may be able to discuss finding an alternative treatment with the doctor who prescribed the drug.

Individuals with mydriasis caused by an allergic reaction to a plant, such as angel’s trumpet, should take steps to limit their exposure to the plant in the future.

Since dilated pupils can be associated with recreational drug use, people with mydriasis may find themselves fielding embarrassing or awkward questions about whether or not they are “on something.”

Treatment for mydriasis is determined by what caused the condition in the first place. Treatment approaches will try to protect the full functionality of the eyes.

For example, if pupil dilation is caused by a reaction to medication, even though several drugs could quickly get the pupils back to normal size, many providers would simply recommend waiting for the effects of the drug to wear off and avoiding it in the future.

When mydriasis is caused by injuries to the brain and eyes, more intense treatments come into play. In extreme cases, a person may require surgery to repair the damage done to the nerves or eye structures. Individuals may wear an eye patch while their eyes heal.

Low doses of Pilocarpine, a medication typically used to treat glaucoma, can also be used to help constrict, or narrow, the pupils.

If a person notices their pupils are dilated without a clear reason, they should see a doctor or an eye specialist.

If mydriasis develops after an injury to the eyes or head, a person should seek medical treatment urgently.

There are many causes for mydriasis, some of which are temporary and will wear off within hours, such as a response to medications.

In serious cases, a person can have surgery to correct the underlying cause of an enlarged pupil, though this is not always necessary.

If a person notices pupil dilation without a clear cause, they should contact their doctor.