What Is It, Pronunciation, Causes, Glaucoma Risk Factor, and More

Author: Alyssa Haag

Editors: Lisa Miklush, PhD, RN, CNS,Ahaana Singh

Illustrator: Jillian Dunbar

Modified: 28 Jan 2021

What is mydriasis?

Mydriasis occurs when an individual’s pupils (the black opening in the center of the eye that regulates light entry), become dilated, or larger. Although this occurs as a normal response to light, mydriasis may also occur as a result of other, less common, underlying causes.

The pupil functions as the part of the eye that regulates the amount of light that enters the eye at any given moment. When there are low amounts of light in the environment, such as at nighttime, the pupil usually dilates in order to bring in more light for increased vividity. This phenomenon is called mydriasis. Conversely, when there are high amounts of bright light in the environment, such as during daytime, the pupil typically constricts, or gets smaller, in order to protect the back of the eye from excessive light damage. When the pupil constricts, it is called miosis.

How do you pronounce mydriasis?

Mydriasis is pronounced as: mid-dry-uh-sis. 

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What causes mydriasis?

Although mydriasis is a normal occurrence, prolonged or inappropriate mydriasis can be indicative of an underlying cause. Potential causes of unusual mydriasis include injury to the muscles or nerves of the eye, traumatic brain injury, increased levels of oxytocin, and side effects of certain recreational drugs or prescribed medications. Additionally, there are also certain conditions that may result in mydriasis

Brain or eye injury, most often due to trauma, may damage the nerves that control the size of the pupil. Traumatic brain injury, usually caused by a stroke, tumor, or head injury, can increase the intracranial pressure (pressure around the brain), potentially resulting in mydriasis. Traumatic brain injury can also damage the oculomotor nerve which innervates, or sends signals, to constrict the pupil. Damage to the oculomotor nerve can result in a blown pupil and consequent mydriasis. This may affect both or just one eye and is usually considered a medical emergency. Additionally, damage to the pupillary sphincter, a muscle located in the iris (the colored part of the eye that controls the size of the pupil) can affect one’s typical response to light and result in mydriasis.

Another potential cause of mydriasis is increased oxytocin production. Oxytocin is a hormone produced in the body which is normally released during exercise and during physical interactions with others. Recent research has shown that increased oxytocin levels may temporarily cause mild to moderate mydriasis

Conditions that can cause mydriasis include benign episodic unilateral mydriasis and Adie syndrome. Benign episodic unilateral mydriasis causes temporary mydriasis of one eye. The condition is typically accompanied with other signs and symptoms, including headaches, eye pain, light sensitivity, nausea, and blurred vision. Adie Syndrome can cause mydriasis of either one or both eyes, although it most commonly affects just one. It is typically accompanied with pupils that constrict slowly in bright light and diminished deep tendon reflexes, usually of the Achilles tendon, located in the lower legs. Though these conditions are usually not dangerous, an exam to assess an individual’s eye health and nervous system is often necessary to rule out any harmful underlying causes.

Certain recreational drugs can also result in mydriasis. Stimulants, including cocaine, methamphetamines, and ecstasy, as well as certain hallucinogens, including LSD and psilocybin mushrooms, have effects on the brain that can cause dilated pupils as a side effect. 

Finally, a variety of medications, such as mydriatics and anticholinergics, among several others, can also lead to mydriasis.

Which medications cause mydriasis?

The most common medications that can cause mydriasis are anticholinergics and mydriatics. Anticholinergics inhibit specific hormones, or chemical signals, from entering the eye and can result in mydriasis as a side effect. Examples of anticholinergics include atropine, used primarily to increase an individual’s heart rate, and scopolamine, commonly prescribed for motion sickness. Other common side effects of anticholinergics include constipation, decreased sweating, increased heart rate, and decreased saliva production. 

Unlike anticholinergics, mydriatics are specific medications used to intentionally dilate an individual’s pupils. Mydriatics, such as tropicamide, allow healthcare practitioners to take a deeper look at the structures of the internal eye. Effects of mydriatics typically last about four to eight hours and during this time, the individual may be sensitive to bright light. 

Other groups of medications that may cause mydriasis as a side effect include selective serotonin reuptake inhibitors (SSRIs), GABA receptor agonists, and adrenergic agonists.

How does mydriasis cause glaucoma?

Mydriasis is a risk factor for angle closure glaucoma in certain individuals. This is most commonly seen in those with “narrow angles”, which refers to an unusually narrow angle between the outer edge of the iris and the cornea (the clear part of the eye that covers the front of the eye).

Glaucoma is a group of eye disorders associated with the destruction of the optic nerve, a cranial nerve that innervates the eye. While open angle glaucoma is slowly progressive, angle closure glaucoma is characterized by a sudden onset of pain in the eye often accompanied with headache, blurry vision, and halos appearing around lights.

Angle closure glaucoma is caused by a blockage that prevents the aqueous humor (the clear fluid in the space in the front of the eye) from draining. This ultimately results in increased pressure in the eye, also called increased intraocular pressure. With “narrow angles”, mydriasis can cause a closure of the angle and thus may play a role in the development of angle closure glaucoma.

How is mydriasis treated?

Mydriasis is treated by diagnosing and treating the underlying cause. Seeking immediate medical attention is important with unusual or prolonged mydriasis, as prompt intervention may be necessary in more serious causes, such as traumatic brain injury

What are the most important facts to know about mydriasis?

Mydriasis refers to the dilation of the pupil, which normally occurs in response to low amounts of light in the environment. In some cases, prolonged mydriasis occurs when an individual’s pupil remains dilated regardless of the amount of light in the environment. This phenomenon may be caused by injury to the muscles and nerves of the eye, traumatic brain injury, increased levels of oxytocin, benign episodic unilateral mydriasis, or side effects of recreational drugs or prescribed medications. Mydriasis may contribute to the possible development of angle closure glaucoma due to its effect on “narrow angles” of the eye. Treatment for mydriasis depends on the underlying cause.

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Related links

Clinical Reasoning: Toxidromes
Direct cholinomimetics
High Yield: Eye conditions: Refractive errors, lens disorders and glaucoma
Peripheral postsynaptic anti-adrenergics: Alpha blockers

Resources for research and reference

Leknes, S., Wessberg, J., Ellingsen, D. M., Chelnokova, O., Olausson, H., & Laeng, B. (2013). Oxytocin enhances pupil dilation and sensitivity to 'hidden' emotional expressions. Social Cognitive and Affective Neuroscience, 8(7): 741–749. DOI: 10.1093/scan/nss062

Mydriasis: Causes and Treatment for Dilated Pupils. In Medical News Today. Retrieved December 15, 2020, from

Sarao, M. S., Elnahry, A. G., & Sharma, S. Adie Syndrome. In StatPearls [Internet]. Retrieved December 15, 2020, from

Sanders R. D. (2009). Cranial Nerves III, IV, and VI: Oculomotor Function. Psychiatry, 6(11): 34–39.

Skeik, N. & Jabr, F. I. (2011). Migraine with benign episodic unilateral mydriasis. International Journal of General Medicine, 4: 501–503. DOI 10.2147/IJGM.S18613