Why the Eye Doctor Dilates Your Eyes

It's all about how light passes through

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Eye doctors temporarily dilate your eyes with eyedrops to open up the black part of your eye (pupil) so that they can see inside better. This is necessary to assess your vision and prescribe the right glasses or contact lenses.

Most people who don't see well have a refractive error—a defect in their eye that affects how light bends when it passes through it. Shining a light into your eyes while they're dilated can help an eye doctor spot and measure refractive errors, as well as identify other potential problems.

This article reviews the purpose of dilation, what conditions it helps diagnose and treat, when you're more likely to need it done, how long it lasts, and what the side effects can be.

Dilated eye

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Purpose

To understand why it's helpful for a doctor to see inside your eye, it helps to know a bit about how the eye sees:

In a healthy eye, light hits the cornea (the first layer) and is refracted to the lens inside your eye. Just like in a camera, the lens focuses the light onto your retina (tissue at the back of your eye).

Via the optic nerve, the retina sends signals to the brain about the patterns of light it's seen. Your brain then interprets those patterns and tells you what you're looking at.

With a refractive error, light bends at a different angle. A dilated eye exam allows the doctor to measure the degree of light refraction to see if the retina is getting the focused light it needs to allow you to see clearly.

The dilating drops:

  • Make the pupil wider, which gives them a bigger opening to look though
  • Allow more light to pass through and be refracted by the cornea, which makes it easier to see
  • Paralyze muscles that help you focus to keep you from subconsciously focusing your eyes, which can throw off results

The most common vision problems involve refractive errors. They are:

Because dilation allows the doctor to see more parts of your eye, including blood vessels, it also allows them to check for other common eye problems, such as:

  • Glaucoma: A group of diseases that damage the optic nerve and disrupt signals from the eyes to the brain
  • Age-related macular degeneration: A cause of progressive vision loss primarily affecting people over 65
  • Diabetic retinopathy: A complication of poorly treated or untreated type 1 or type 2 diabetes that can cause impaired vision or blindness

In some cases, dilating drops are used as a treatment. These are generally longer-lasting drops that can lower pain and inflammation from eye disease, surgery, or injury. They're also used to treat "lazy eye" (amblyopia) and a progressive form of myopia.

When It's Needed

If you're seeing an eye doctor for the first time, expect to be dilated. However, you probably won't need to be dilated every time you have an eye exam.

Dilation is recommended every few years so your doctor can spot potential problems.

You should get them more regularly, such as every one to two years, if you're:

  • Over age 40 and Black or Hispanic
  • Over age 60 and any other ethnicity
  • Have a family history of glaucoma

To decide how often to dilate you, your doctor will also consider your overall health, eye health history, and whether you've had abnormal findings in earlier dilated exams.

Children​

Children can be hard to examine. They're more likely to focus automatically and less able to control eye movements. They also may not be able to switch focus from close to distant objects when asked.

Because of these potential problems, kids may need to be dilated at every exam so the doctor can properly look at their eyes.

For Diagnosis

Presbyopia is an inability to focus up close. It affects almost everyone older than 40. When it develops before that, it's called premature or pre-presbyopia.

Pre-presbyopia makes you unable to:

  • Quickly change focus from near to far
  • Focus properly at night

The doctor needs to see the inside of the eye to diagnose pre-presbyopia. Expect dilation if you're being checked for this. Dilation at future exams will depend on the findings and other considerations such as age and health.

Dilation can also help a doctor diagnose some serious suspected vision problems, such as:

LASIK Candidates

You need a dilated exam before laser vision correction (LASIK) or other vision correction surgery. The surgeon needs to know exactly how much to change the shape of your cornea to correct the refraction error.

Recap

Dilation is done to assess refractive errors, help diagnose conditions like retinal detachment, and determine candidacy for vision correction surgery. While adults may not need to be dilated at every eye exam, kids may as they are less able to control their eye movements.

Dilation Timeline

It will take between 20 and 30 minutes after the drops are put in for your eyes to be fully dilated. The dilation typically stays in effect for between two and eight hours, though it may last 24 hours or even longer in some people.

The color of your eyes has something to do with this, as pigment binds with the drops and changes how they work.

Brown eyes, which have a lot of pigment, need stronger doses and more time to dilate, but the recovery is on the shorter side. On the other hand, hazel, green, and blue eyes have less pigment. They require lower doses, dilate faster, but recover more slowly.

Children also need stronger drops than adults, so their dilation may last longer as well.

Side Effects

Dilating eye drops may sting for a few seconds. Some of them cause blurriness and light sensitivity for several hours. Occasionally, this will even last overnight.

Eye doctors generally give out disposable sunglasses for when you leave the office dilated. You may want to bring your own so you're sure you have something that fits well.

Some glaucoma patients can experience a jump in their eye pressure when dilated. These patients may receive pressure-lowering drops prior to dilation to temper this effect. In some cases, dilation may not be advised.

Recap

You will be blurry and sensitive to light after dilation. This may last a few hours or, if you have light-colored eyes, as long as a day.

After Dilation

It is not safe to drive after your dilated exam, so be sure to ask if you are set to be dilated and arrange for a ride home if so.

Other activities may also be unsafe or difficult while you are waiting for the drops to wear off. You may want to consider taking some time off or making your eye appointment late in the afternoon so you don't miss a lot of work.

Ask your eye doctor if it's OK for you to put your contact lenses in while your eyes are dilated.

Summary

Dilating eye drops help eye doctors diagnose refractive errors and other vision problems. They widen your pupil, let in more light, and paralyze eye muscles.

You shouldn't need dilation at every exam but certain people, tests, and procedures require it. The temporary light sensitivity and focus issues that result from dilation make driving unsafe and other activities difficult until the drops wear off.

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Article Sources
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  1. New York University, Langone Health. Types of refractive error.

  2. American Association for Pediatric Ophthalmology and Strabismus. Dilating eye drops. Updated April 2020.

  3. University of Utah, U Health. Is it necessary to have your eyes dilated during every eye exam? Updated March 5, 2014.

  4. Anderson HA, Bertrand KC, Manny RE, Hu YS, Fern KD. Comparison of two drug combinations for dilating dark iridesOptom Vis Sci. 2010;87(2):120-124. doi:10.1097/OPX.0b013e3181cc8da3

  5. Whitman J. American Academy of Ophthalmology. Should patients with severe glaucoma allow their eyes to be dilated during exam? April 24, 2019.

Additional Reading
  • National Institutes of Health, National Eye Institute. Refractive errors. Updated August 28, 2020.

  • National Institutes of Health, National Eye Institute. Get a dilated eye exam. Updated May 19, 2021.