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What is a contact lens eye exam and fitting?

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Eye exams for contact lenses include special tests and measurements that are not included in routine eye exams for eyeglasses.

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 eye doctor to perform everything required for a proper contact lens fitting or prescription update.

What to expect during a contact lens exam

During your eye exam for contact lenses, your visual acuity will be tested using an eye chart, and a number of tests will be performed to determine your eye health and whether prescription eyewear is required to correct refractive errors.

Your eye doctor will gather additional information so you can be fitted with contact lenses, or the fit of your current contact lenses can be evaluated.

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, which often provide sharper vision than soft lenses.

Your eye doctor also might ask how you want to correct vision problems related to aging. After age 40, it’s normal to experience 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, which is a special contact lens fitting technique where one eye is corrected for distance vision and the other eye is corrected for near vision.

Measurements taken during a contact lens exam

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 damage to your eye.

The following tests are performed during an eye exam for contact lenses to make sure you see clearly, comfortably and safely with contacts.

Cornea measurements

An instrument called a keratometer is used to measure the curvature of your eye's clear front surface (cornea).

A 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 may be performed using an automated instrument called a corneal topographer.

Corneal topography provides extremely precise details about surface characteristics of the entire cornea. It does this by measuring how your eye reflects light.

Sometimes, corneal topography measurements are combined with wavefront measurements that provide even more specific information about the shape of your cornea by identifying tiny imperfections called higher-order aberrations. 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 or be fitted with GP contact lenses.

Pupil and iris measurements

The size of your eye's pupil also may be measured during your eye exam for contact lenses.

To obtain this measurement, your eye doctor may simply hold a special ruler up to your eyes and compare the size of your pupils to illustrations of different pupil sizes on the ruler.

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 techniques might be used to measure the diameter of the colored portion of your eye (iris).

Pupil and iris measurements help your ECP choose contact lenses that are of a proper size to fit well and look best on your eyes — especially if you are interested in color contact lenses.

Tear film evaluation

Contact lens fittings may also include a tear film evaluation.

Your eye doctor will use one or more techniques to make sure your eyes produce enough tears for comfortable and safe contact lens wear, and that you don’t have unusually dry eyes.

If you have a significant dry eye condition, you might have to avoid or discontinue contact lens wear. But in cases of contact lens discomfort due to mild dryness, special contact lenses for dry eyes may enable you to wear contacts safely and comfortably.

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 instrument provides a highly magnified view of the cornea and other structures in the front of the eye so your eye doctor can make sure you are a good candidate for contact lens wear or (if you already wear contacts) that contact lens wear isn’t harming your eyes.

The slit lamp 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 watery eyes.

In follow-up visits, your eye doctor might apply a special dye to the surface of your eyes to make sure your contact lenses aren’t irritating your corneas or causing dry eyes.

After your contact lens fitting, you will be asked to return for one or more follow-up visits so you eye doctor can confirm that the lenses are continuing to fit well and that you are seeing clearly, comfortably and safely with them. Once this is confirmed (and only after it is confirmed), your eye doctor will then finalize and sign your contact lens prescription.

Your prescription for contact lenses will include a number of designations that are not found on an eyeglass prescription, including:

  • The brand of the contact lenses

  • The curvature of the optical zone of the lenses (called the base curve, or BC)

  • The diameter of the lenses

  • An expiration date for purchasing replacement lenses

Your eye doctor might also specify wearing instructions on your contact lens prescription, such as, “Not to be worn overnight; replace every two weeks.”

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