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What Does Dilating The Eyes Do?

What Does Dilating The Eyes Do?

There are a few really annoying things that everyone hates during an eye exam. The eye puff test of course tops the list, but dilation of the eyes is another. Afterward you know you can expect 4-6 hours of some blurry vision and bright lights. Yes, it can be an annoying step. But dilating the eyes is an important step to evaluate the overall health of the eye.

Natural Dilation of the eye

The colored part of your eye is called the iris. Like on a camera, this iris serves as an aperture controlling how much light enters the eye. The empty circle in between the iris is called the pupil. This iris will actually respond to natural signals (such as light) by constricting or (when there is less light) dilating.

You can actually test this response at home by grabbing a flashlight or flashlight mode on your phone and shining it in front your friends eye. Note: Don’t use a very powerful flashlight or they won’t be your friend much longer. As the light passes over their eye, you will see their pupil become smaller or constricted. As you bring the light away from their eye, you will see their pupil dilate as their iris dilates. This is called the pupillary light reflex.

Natural dilation of the pupil is important when we are at nighttime. At nighttime, everything is darker because there is less light. In order to see better at night, we need to allow more light into the eye. The eye naturally dilates to help us see at nighttime.

Artificial Dilation of the eye

But savy eye doctors have figured out how to cause the eye to dilate artificially. You see, there are certain receptors on the iris muscles. By triggering those receptors on the iris dilator muscle, you can cause the iris to dilate. The medication in dilation eye drops can trigger those receptors and cause the iris to artificially dilate.

But not just one eye drop was created; multiple different eye drops exists with different roles and different actions. Some are short acting, some are long acting.

  • Some eye drops are quick to take effect in roughly 20-30 minutes. These eye drops last approximately 4-6 hours. There are also drops that take a little bit longer for effect but can last upwards to 36 hours. And lastly, there are eye drops which will keep your eye dilated close to a week!
  • Because of the different durations, the eye drops have different purposes as well. The shorter acting eye drops are used in the office to dilate your eyes for the eye doctor. The longer acting eye drops are used more for therapeutic purposes.

What do you experience after eye dilation?

Well for one, artificial dilation of the eyes causes extra light and glare in our vision. Everything will be much much brighter. During daylight, the eye doesn’t need to be dilated and you will want to have sunglasses afterwards in order to cut down on all this extra light. Often you will be able to pick up a cheap disposable pair of sunglasses on the way out of the office after having your eyes dilated.

But dilation can also cause some blurred vision. In addition to regulating light, there is a reason why the pupil likes to stay a certain size. When the pupil gets larger light starts to pass through the edges of the natural lens in addition to the center of the lens. Our natural lens isn’t perfect. Light that enters the edges of the lens won’t all focus at the same spot as light that enters the dead center of the lens. This effect causes something known as spherical aberration which creates some halos and blurry vision.

For some people, eye dilation will cause even more blurry vision; frequently up close but sometimes even more blurred vision for distance. This is actually unrelated to the size of the pupil. Some eye drops affect more than just the iris. Just underneath the iris is another small muscle in the eye called the ciliary body. This muscle is involved in focusing. When we switch off from looking off into the distance to looking up close, this muscle constricts. Constriction of this muscle causes the natural lens to shift focus up close.

Some dilating drops affect this muscle and prevent it from constricting. If we prevent this muscle from constricting, we prevent the eye from focusing. The most common way people notice this is the fact that the lens can no longer focus up close. After putting your glasses back on, you can’t read up close anymore. But dilation of the eyes sometimes makes people realize that their lens is focusing through extra prescription ALL the time. After being dilated, suddenly they have extra prescription blurring their vision because this lens temporarily isn’t working. These people are farsighted individuals (specifically called latent hyperopia because they don’t realize or benefit from any glasses at the time).

But trust me, dilation still has benefits

Without dilation, eye doctors would be unable to fully examine the back of the eye.

Diagram of the eye

Diagram of the eye, Image by Rhcastilhos. And Jmarchn., CC BY-SA 3.0, via Wikimedia Commons / modified from original

Look at that diagram; the retina in the back of the eye takes up a large real estate! While it is technically possible to have a tiny view to the back of the retina through an undilated pupil, it is very impractical for almost all purposes. It would take a long time to cover every millimeter of retina and near impossible to map and be sure all of it was covered. Dilation is necessary to provide all of those wide views.

But dilation goes beyond just looking at the back of the eye. Dilation can help figure out the true prescription of the eye. As mentioned up above, certain dilating drops will prevent the ciliary body muscle from working and prevent the lens from being able to change the focus of the eye. This is actually really beneficial to prevent this focusing ability of the lens from focusing through extra prescription. This is called a cycloplegic refraction. When given extra nearsighted prescription, this lens often easily focuses through that extra prescription and it can actually "appear" better (extra focusing of the lens can add a small amount of contrast despite not actually improving vision or even worsening vision). To get the best vision possible for glasses or through laser eye surgery, the goal is to get to the true prescription of the eye by eliminating this unnecessary variable.

Note: cycloplegic refractions are especially important with kids. Because young kids can’t do the “better 1 or 2” test, the eye doctor has to measure their prescription through a different technique shining a light into their eye called retinoscopy. If the child was able to change their focus throughout this test, the result would be completely inaccurate.

Other important uses of dilation

If you need eye surgery at some point (typically cataract surgery or retina surgery), you will need your eyes dilated. Dilation is necessary in many eye surgeries to either provide access or to allow for a better view during surgery. In some of these cases, if there isn't enough dilation, surgeons use special devices used to artificially expand the pupil and create a better view. Having a good view is an important component to having a safe surgery.

Outside of the surgery realm, dilation drops are frequently used to treat inflammation inside the eye (called iritis). When you have inflammation inside the eye, the eye becomes very sensitive to light. Whenever light shines in the eye, the iris constricts as normal to make the pupil smaller, but the iris is inflamed from the inflammation. Movement of an inflamed muscle causes discomfort. Long acting dilation drops are used to prevent the iris from moving to make the eye feel more comfortable until the inflammation is treated and goes away.

Alternatives to dilation

Still skeptical of dilation? It is possible to get some of the benefits of dilation without having the dilation drops. A specialized retina camera can be used to capture large areas of the back of the eye without dilation. These cameras have existed in some fashion for quite some time, however until recently they were only capable of limited views to the back of the eye. Advancements in technology have allowed these cameras to approach what we can see on our full exam.

However, nothing can replace the direct exam by a trained eye doctor. At least not yet. This is especially the case when a cycloplegic dilation refraction is necessary to get a perfect eyeglasses prescription. So while dilation is annoying, it has it's benefits to keeping your eyes the healthiest.

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