Definition of Cycloplegic Refraction
The Dilated Pupil
A cycloplegic refraction simply means that the patient’s eyes are dilated prior to performing the test used to determine the patient’s glasses prescription.
The refraction is normally performed before the patient is given dilating drops. However, there are times when it is necessary to dilate before performing a refraction in order to get more accurate results. Reasons to do this can include:
- Prior to LASIK or other refractive surgery
- Patients who are extremely nearsighted
- Prior to cataract surgery in some cases
- Patients under the age of 18
Definition of the Term Cycloplegia
The term cycloplegia is derived from the Greek kyklo meaning circle or ring, and -plegia meaning paralysis. It is the paralysis of both the iris (specifically the sphincter muscle) and the ciliary body of the eye.
Cyclopentolate, also known as Cyclogyl, is the most common dilating drop used to induce cycloplegia prior to refraction, although Mydriacyl (tropicamide) can also be used.
Cyclogyl is used because of its rapid onset (about 30 minutes) and effect on accommodation. Cyclogyl is most commonly used for:
- Dilating patients prior to surgery (LASIK, cataract removal or other ocular surgeries)
- Cycloplegic refractions
- Preventing posterior synechiae in the treatment of iritis or uveitis
- Dilating patients with very dark irises
What is a Refraction?
This is one of the most common questions I’m asked by patients during an exam and most of the time it’s because there is an extra charge for the test. The easy answer is: the refraction is the test used to determine your glasses prescription.
The Normal Undilated Pupil
However, a refraction can tell the technician and the doctor much more than just what your prescription is. It can also show whether or not the vision can be improved simply by changing the prescription.
If it cannot be improved with a prescription change, this tells the tech and the doctor that there is an underlying pathology causing the blurry vision or other symptoms. Conditions that can cause this could include:
- Macular degeneration
- Corneal dystrophy
- Dry eye syndrome
- High blood pressure
Refractions can be difficult to perform. One of the main concerns for the technician performing the test is accommodation, especially in those who are myopic (nearsighted).
The Anatomy of Accommodation
Accommodation is the ability of the eye to focus at different distances. Our ability to see clearly at various distances is the result of a symphony of actions involving several ocular structures including the:
- Ciliary body
- Zonular fibers (part of the cilliary body)
Structures of the Anterior Portion of the Eye
The iris is the pigmented part of the eye that is actually a combination of two muscles: the sphincter and dilator. These muscles are responsible for the constriction and dilation of the pupil, the opening in the center of the iris.
The iris has its roots in the cilliary body which is found directly behind it. Also behind the iris is the lens which is responsible for about 30 percent of our focusing power and can actually be seen through the pupil. It is wrapped in a capsule and suspended by tiny strands called zonular fibers which originate from the cilliary body.
The ciliary body is the muscle that directly controls our ability to accommodate. For example, when we read, the eyes converge together, the pupil constricts and the size and shape of the lens changes due to contraction of the ciliary body. This releases the tension on the zonular fibers and causes the lens to bow forward which increases its thickness and power.
When we are looking at a distance (such as when driving), the opposite occurs. The eyes diverge, the pupil dilates and the ciliary body relaxes. Tension on the zonular fibers increases and the lens becomes thin and flat, decreasing its power.
How Accommodation Affects Refraction
When testing a patient for glasses, this ability to accommodate can lead to an inaccurate prescription, and the patient doesn’t even realize they are accommodating. This is most common in myopic patients because their lens is capable of accommodating at extremely close range.
This means they are capable of accommodating more power at distance. In other words, it’s very easy to give a myopic (nearsighted) patient too much prescription because their eyes can easily adapt to see clearly through the extra power. This is called "over-minused,” and it is the main reason for cycloplegic refractions.
Cycloplegia temporarily neutralizes the patient’s ability to accommodate and allows the technician to get a completely accurate reading of how much prescription the eye actually needs.
This is imperative prior to surgical procedures such as LASIK or cataract surgery. In the case of LASIK, it helps determine the amount of tissue that needs to be ablated (removed). Prior to cataract surgery, cycloplegic refraction will help determine the power of the IOL (intraocular lens).
Cycloplegia is also beneficial when refracting children due to their uncanny ability to accommodate. Often times an underlying refractive error such as hyperopia (farsightedness) will be found during the cycloplegic refraction of a child.
Many children have slight refractive errors that they can easily accommodate, and dilation is the only way to reveal the underlying refractive error.
What to Expect During a Cycloplegic Refraction of the Eye
Cycloplegic refraction is a very simple procedure. In fact, the eye exam is identical to the standard eye exam with one exception: a second refraction will be performed after dilation. The second refraction may be a bit more difficult for you due to glare, so the technician should turn down the lights in the exam room. If they don't, ask them to do so.
Dilation also makes the vision somewhat blurry, so you may not have perfectly clear vision at the end of the second refraction. The eye can’t accommodate at all after cycloplegia which means it can’t focus in anything whether it’s up close or far away (although distance vision is usually considerably better than near vision after the eyes have been dilated). So don’t expect your vision to be crystal clear.
After the Cycloplegic Refraction
After your exam, you will be dilated for at least 24 hours, sometimes longer. Unless you have a pair of reading glasses or are very nearsighted, you won’t to be able to read anything up close. Computer screens will also be a problem. If you can’t miss work, you should schedule your appointment for a Friday afternoon (or the day before your weekend, whatever day that may be).
You will also be light sensitive for about 24 hours. Driving shouldn’t be too much of a problem during the day as long as you have a pair of very dark sunglasses. It’s wise to take a pair of the cheap paper sunglasses at the doctor’s office if they are offered - if they aren’t, ask for them. They won’t make a fashion statement, but they are designed for dilated eyes and are much darker than anything you can buy in a store. Post - surgical sunglasses also work well for this purpose.
Driving at night may not be difficult if you’re in a rural area. However, driving in a brightly lit city or town will be difficult because the vision will be blurred. The dilation will also cause excessive glare from street lights as well as from the headlights of oncoming traffic.
Some people are allergic to Cyclogyl, or the preservative in it (benzalkonium chloride). Itching and/or tearing can indicate an allergy especially when associated with redness that lasts longer than a few hours. This mild allergic reaction can be treated with an over-the-counter antihistamine such as Zaditor. Very rarely will a patient have a severe allergic reaction to Cyclogyl such as anaphylactic shock. If you suspect you’re having an allergic reaction, call your doctor right away.
Questions & Answers
© 2012 Melissa Flagg OSC