Evidence Based

Dilated Pupils (Mydriasis)

What Are Dilated Pupils (Mydriasis)?

The iris (the colorful portion of the eye) controls the size of the pupil, which is always changing. Pupils dilate (expand in size) to take in more light in low-light situations and they constrict (get smaller) in bright conditions to minimize the amount of light that enters the eye. On average, a normal pupil in adults is about two to four millimeters in diameter in bright lights and four to eight millimeters in diameter in the dark. Generally, both pupils are about the same size.

When pupils constrict in direct light, it’s called a direct response. They also constrict when the eye is focused on a near object, which is called an accommodative response. If a pupil does not constrict in light or accommodation or dilate in the dark, it’s considered abnormal.

Mydriasis refers to excessive or prolonged pupil dilation that can happen without any change in light. Note that mydriasis is different from anisocoria, a common condition that affects about 20 percent of the population. Anisocoria happens when the pupils both normally react to light but differ in size by amillimeter or less, on average.

What Do Dilated Pupils Look Like?

Mydriasis

When pupils dilate, they expand in size. Therefore, pupils look extra big when they’re dilated. The black pupil takes up more of the iris, so the eye may appear darker with less color.

What Causes Dilated Pupils?

Pupils dilate for many reasons. Here are a few reasons as to why pupils dilate:

  • Pupils dilate in the dark so the eye can let more light in and you can see better.
  • Pupils dilate when the eye focuses on an object that is far away.
  • Your ophthalmologist may use dilating eye drops in an eye exam that purposely cause your pupils to expand.
  • Sexual attraction can cause pupil dilation.
  • Some medicines can cause the pupils to dilate because they affect the eyes’ muscles, preventing them from constricting in increased lighting.
  • Illegal, recreational drug use can cause the pupils to dilate.
  • A head injury or brain injury that puts pressure on the eyes and damages the iris muscles can cause the pupils to dilate.
  • Eye injuries or surgeries, such as cataract removal or corneal transplants, can damage the iris nerves and muscles. This, too, can cause the pupils to dilate.
  • Strokes, tumors, and various eye diseases can cause the pupils to dilate.
  • Horner’s Syndrome, which is caused by damage to the sympathetic nerves that dilate the pupils and raise the eyelids, can affect pupil dilation.
  • Adie's Tonic Pupil (also known as Tonic Pupil) can affect pupil dilation. Tonic pupil is a rare neurological disorder in which one pupil is larger than normal and reacts slowly (if at all) to light.
  • Congenital Aniridia is a condition in which a person is born with a partially or completely absent iris, which leaves them with an extra-large pupil.

Possible Complications of Dilated Pupils

Dilated pupils can cause some unpleasant symptoms:

  • You may feel an increased sensitivity to light.
  • You may experience migraines due to increased light that reaches the eye.
  • You may have blurry vision as the eye struggles to focus with too much light.

Treatment Options for Dilated Pupils

Fortunately, there are some treatment options for larger pupils, including:

  • Photochromic lenses automatically darken outside in daylight and lighten up inside in darker conditions. These may help facilitate the dilation process so your pupils adjust easier.
  • Polarized sunglasses can protect your eyes from intense light conditions. They reduce glares from surfaces like water, snow, asphalt, and sheet metal by filtering out specific wavelengths.
  • Specialty colored contact lenses which improve cosmetic appearance and vision by reducing the amount of light entering the eye.
  • If you have mydriasis caused by a traumatic brain injury or eye damage, your case may require surgery to repair any damage done.
  • If your dilated pupils are a side effect of substance abuse, you may consider rehabilitation.

Dilated Pupils FAQs

Here are answers to some of the most frequently asked questions about dilated pupils.

Is it safe to have dilated pupils?

Pupils are supposed to dilate; it’s a natural pupillary response to low-light situations and to help the eye focus on close-up objects. Pupils may also dilate in response to a variety of stimuli under completely normal circumstances.

However, if the pupils remain dilated or they are unequal in size, seek care from your ophthalmologist. You could have a medical emergency on your hands. Mydriasis and unequal pupil sizes can be signs of serious conditions that affect the brain, such as strokes, tumors, internal bleeding, or head trauma.

How does it feel to have dilated eyes?

Because dilated pupils let light in, if your pupils dilate, you may notice an increased light sensitivity. Dilated pupils also make it harder to read up close.

What does it mean when your pupils are dilated small?

The pupils shrink in size in brightly lit conditions. They do this to restrict the amount of light that meets the eye. This is a natural protection mechanism.

What drugs cause dilated pupils?

Many drugs can cause dilated pupils. Here are just a few medications and illegal drugs that can cause dilated pupils:

  • Atropine for heart problems, stomach issues, and some types of poisoning
  • Anti-seizure medications like topiramate (Topamax) and phenobarbital (Luminal)
  • Botulinum toxin (such as Botox)
  • Antihistamines (such as Benadryl, for example)
  • Tricyclic antidepressants like desipramine (Norpramin) and amitriptyline (Elavil)
  • Decongestants (such as Afrin or Sudafed)
  • Motion sickness and anti-nausea medicines (such as Dramamine)
  • Parkinson's Disease medications (such as Symmetrel) and carbidopa-levodopa (Sinemet)
  • Cocaine
  • Amphetamines
  • LSD
  • MDMA
  • Ecstasy
Author: AnnaMarie Houlis | UPDATED October 5, 2020
Medical reviewer: MELODY HUANG, O.D. 
Resources

Academy, Lakeside. “What Kind of Drugs Cause Dilated Pupils?” What Kind of Drugs Cause Dilated Pupils? | Lakeside Academy, boysteenchallenge.mntc.org/what-kind-of-drugs-cause-dilated-pupils/.

Adie Tonic Pupil, www.aao.org/bcscsnippetdetail.aspx?id=1af235eb-71a5-497f-8fac-308c9ea3a0eb

“Anisocoria and Horner's Syndrome.” Anisocoria and Horner's Syndrome - American Association for Pediatric Ophthalmology and Strabismus, aapos.org/glossary/anisocoria-and-horners-syndrome

“Dilated Pupil.” Eye Care, www.uclahealth.org/eye/dilated-pupil

Lick, David J, et al. “The Pupils Are the Windows to Sexuality: Pupil Dilation as a Visual Cue to Others’ Sexual Interest.” Evolution and Human Behavior, www.sscnet.ucla.edu/comm/kjohnson/Lab/Publications_files/Lick,%20Cortland,%20%26%20Johnson%20%282016%29.pdf

Ou, Yvonne, and University of California. “The Dilated Eye Exam: Why It's So Important.” BrightFocus Foundation, 3 Sept. 2020, www.brightfocus.org/glaucoma/article/dilated-eye-exam-why-its-so-important.

“Polarized Lenses and How They Work.” Dr. Floyd Smith | Optometrist, Westwood, NJ 07675, 31 July 2013, drfloydsmith.com/polarized-lenses-and-how-they-work-3/.

Samant, Monica, et al. “Congenital Aniridia: Etiology, Manifestations and Management.” Expert Review of Ophthalmology, U.S. National Library of Medicine, 2016, www.ncbi.nlm.nih.gov/pmc/articles/PMC6086384/

Spector, Robert H. “The Pupils.” Clinical Methods: The History, Physical, and Laboratory Examinations. 3rd Edition., U.S. National Library of Medicine, 1 Jan. 1990, www.ncbi.nlm.nih.gov/books/NBK381/

“Transition Sunglasses Lenses.” American Academy of Ophthalmology, 29 June 2020, www.aao.org/eye-health/glasses-contacts/pros-cons-of-transitions-lenses

“Why Are My Pupils Dilated? 5 Causes of Dilated Pupils (Mydriasis).” WebMD, WebMD, 6 Aug. 2019, www.webmd.com/eye-health/why-are-pupils-dilated-mydriasis.

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