Eye Exams for Contact Lenses
So if you are interested in contacts or you already wear them and want to have your contact lens prescription updated make sure you say so when you schedule your appointment for an eye exam. This will ensure your exam includes extra time for your optometrist or ophthalmologist to perform these additional tests needed for a proper contact lens fitting or evaluation.
If you have had your eyes examined, you already may know what to expect from a comprehensive eye exam: Your visual acuity will be tested using an eye chart, your eye doctor will perform a number of tests to determine your eye health, and a refraction will be performed to determine if refractive errors exist and whether prescription eyeglasses are needed to sharpen your vision.
But this is just one component of a contact lens exam.
What To Expect During a Contact Lens Fitting
After standard eye exam testing has been completed, your eye doctor will gather additional information so you can be fitted with contact lenses.
You may be asked general questions about your lifestyle and preferences regarding contact lenses, such as whether you might want to change your eye color with color contact lenses or if you're interested in options such as daily disposables or overnight wear.
Your eye doctor may also discuss the option of rigid gas permeable (RGP or GP) contact lenses, although most people today are fitted with soft lenses.
Your eye doctor also might ask how you want to correct vision problems related to aging. Some time after age 40, you will develop a condition known as presbyopia that decreases your ability to read small print and focus on near objects.
To correct presbyopia, your eye doctor may offer you the choice of multifocal or bifocal contact lenses. Another option is monovision, a special contact lens fitting technique where one eye is corrected for distance vision and the other eye is corrected for near vision.
Contact Lens Measurements
Just as one shoe size doesn't fit all, one contact lens size doesn't fit all.
If the curvature of a contact lens is too flat or too steep for your eye's shape, you could experience discomfort or even eye damage. Other aspects of a contact lens fitting include:
Cornea measurements. An instrument called a keratometer will be used to measure the curvature of your eye's clear front surface (cornea).
The keratometer analyzes light reflections from your cornea and determines the curvature of your eye's surface. These measurements help your eye doctor choose the proper curve and size for your contact lenses.
Because the keratometer measures only a small, limited section of the cornea, additional computerized measurements of your cornea (corneal topography) may be done. Corneal topography provides extremely precise details about surface characteristics of the cornea by analyzing photographic images taken of the way light projected into your eye travels.
With one version of a corneal topographer, you are seated with your forehead resting against a curved brace. Circular patterns of light then are beamed into your eye for analysis. A computer creates and prints out the resulting surface "map" of your eye, with different contours represented by varying colors.
An eye doctor uses a biomicroscope to inspect the surface of an eye. This type of evaluation is needed before you are fitted with contact lenses. (Image: National Eye Institute)
Sometimes, corneal topography measurements are combined with wavefront measurements that can identify higher-order aberrations of the eye. These combined measurements can help your eye doctor determine the type of contact lenses that will give you the sharpest vision possible.
If your eye's surface is found to be somewhat irregular because of astigmatism, you may need a special design of lens known as a toric contact lens that is shaped to offset distortions of your eye to provide sharper vision.
At one time, only rigid contact lenses could correct for astigmatism. But there are now many brands of soft toric lenses. Toric lenses also are available in disposable, multifocal, extended wear, and colored versions.
Pupil and iris measurements. The size of your eye's pupil may also be measured. In a simple approach, a card or ruler showing different pupil sizes is held next to your eye to determine the best match.
Automated instruments that measure pupil size also exist. These instruments are capable of extremely precise measurements, and some simultaneously measure the horizontal and vertical diameter of your pupil.
Similar technologies also may be used to measure the diameter of the colored portion of your eye (iris). Pupil and iris measurements help achieve a proper sizing and orientation of your contact lens.
Your eye doctor may hold a pupil gauge next to your eye to determine the approximate size of your eye's pupil as part of your contact lens eye examination. The lettering at the end of the rule shown here is used to measure near vision in general eye examinations. (Image: Richmond Products)
Tear film evaluation. Contact lens fittings may also include a tear film evaluation.
Your body's ability to produce tears may be evaluated through use of small strip of paper inserted underneath your lower eyelid. You close your eyes for about five minutes, and then the paper is removed. The length of the paper moistened by your tears is measured to assess your tear production and determine if you have dry eyes.
Another method of detecting dry eye involves adding fluorescein dye to the tear layer on your eye via eye drops or a moistened paper strip containing the dye, and then evaluating how long it takes for your tears to evaporate.
If you don't produce enough tears and you have severe dry eye, contact lenses may not be right for you. In some cases, certain contact lenses for dry eyes such as those made of silicone hydrogel material might work better for you if you have mild dryness-related discomfort when wearing conventional contact lenses.
Evaluation of your eye's surface and contact lens fit. The health of your cornea will be evaluated using a biomicroscope also called a slit lamp). This lighted instrument provides a highly magnified view of the cornea and other tissues to enable your eye doctor to evaluate the health of the front of your eyes and detect any changes caused by contact lens wear.
The biomicroscope also is used to evaluate the fit of a trial contact lens, because it enables your doctor to observe the alignment and movement of the lens as it rests on the surface of your eye.
When trial lenses are used, you typically will need to wear them a few minutes so that initial tearing of the eye stops and the lenses stabilize. Your eye doctor can then make a proper evaluation of how the lenses fit without the presence of excess moisture caused by tearing.
In follow-up visits, your eye doctor may stain your eye with fluorescein to check for defects and make sure your contact lenses are not damaging your eye's surface. You usually will need to remove your contact lenses before this test is performed.
After finding contact lenses that fit properly, are comfortable for you, and provide good vision, your eye doctor can write your contact lens prescription. This prescription will designate contact lens power, a shape matching the curvature of your eye (base curve), and diameter.
It typically takes about two office visits to complete the contact lens fitting. After that, your eyes will need to be examined once annually so that your eye doctor can monitor the health of your eyes. Some patients may need contact lens progress evaluations prior to the annual visit.
Keep in mind that if you wear contact lenses, your annual eye exams typically will cost more than a routine exam for someone who doesn't wear contacts, due to the additional contact lens-related tests that are included.
About the Author: Gary Heiting, OD, is senior editor of AllAboutVision.com. Dr. Heiting has more than 25 years of experience as an eye care provider, health educator and consultant to the eyewear industry. His special interests include contact lenses, nutrition and preventive vision care. Connect with Dr. Heiting via Google+.
Original version of this article was by Michelle Stephenson.
[Page updated June 2011]
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