Contact Lenses: The Importance of Accurate Measuring and Fitting
Having contact lenses fitted is far more complicated than getting eyeglasses. As the lenses sit directly on the cornea, it is essential that they are the correct measurements for your eyes. If you are thinking of switching to contact lenses, would like to have your contact lens prescription updated, or are considering color contacts for cosmetic reasons, you should mention this when you are scheduling an appointment with your eye care specialist. This will ensure that you are reserved a suitably lengthy time slot, giving the doctor the extra time needed to perform special tests and discuss your options with you. This will ensure not only clear vision but also comfortably fitting lenses.
Choosing the right contact lenses
As with a regular eye exam, your visual acuity will be checked, one eye at a time, likely using an eye chart. Tests will also be performed to check for other vision issues and any eye health problems. After this, if the doctor has decided that it is safe for you to wear contacts, he or she will need additional information to help you choose the right lenses. Your lifestyle, preferences for replacement and wear schedules, and other factors will be discussed. Many people choose soft lenses, but if you have an eye condition which would benefit from rigid gas permeable lenses then this option will also be considered. Your age is important too, as around the age of 40 presbyopia develops, causing near vision to worsen as the natural lenses in your eyes lose flexibility. This may affect whether you are will need multifocal or monovision lenses. All these consideration require careful thought, as your quality of life will likely be affected by the choices you make.
One-size-fits-all may apply to many things, but a contact lens isn't one of them. Much like feet need to be measured to ensure proper fitting shoes, your eyes must be measured in order to get the right lenses. If the curvature of the lens is too flat or too steep it will be uncomfortable and possibly even harmful to your eye.
A good eye doctor will measure your eye’s surface and curvature using a device called a keratometer. You may be asked to rest your chin on a support while the instrument takes photographs of your eye. The keratometer analyzes light reflections from the cornea and, based on that, calculates the curvature of your eye and the size of contact lens you will need. For those with hard-to-fit corneas, further computerized measurements may be taken using corneal topography to get the more precise shape of the corneal surface. Wavefront measurements may also be used to detect and correct certain vision problems, like higher order aberrations.
In the case of astigmatism, where the eye surface is irregular and the ‘bulge’ causes poor focusing, a toric contact lens may be required. This type of lens is specially shaped to counter the distortions of the eye and, as you can probably imagine, require more detailed measurements than a standard lens. Toric lenses are available in rigid gas permeable and soft lens options and can be colored, disposable, multifocal, and/or extended wear lenses.
Additional measurements and testing
The size of your pupil must be measured for a good fit. This may be taken using a ruler or an automated instrument. The diameter of the iris of your eye is also required for proper sizing.
Tear film evaluation may also be taken. A small strip of paper is inserted under the lower eyelid; you close your eyes for several minutes and the paper is then removed. This can assess your tear production and will determine if you have dry eyes. Other methods include using a fluorescein dye in eye drops to see how long tears take to evaporate. For those with dry eyes, contact lenses may not be the best option, although the newer silicone hydrogel lenses do work better for dry eyes than their predecessors.
Contact lens fitting
Another instrument that may be used is a biomicroscope, which provides a highly magnified view of the cornea, allowing the doctor to observe the fit of a trial contact lens. It can also be used to assess the health of the cornea and any changes following contact lens wear.
If trial lenses are used, you will have to wear them for at least 15 minutes in order for them to settle and the natural tearing to cease. The eye doctor will then examine the eye and evaluate the fit. There will likely be follow-up visits scheduled even if there are no apparent problems. During these later exams, the doctor may stain the eye to make sure that no damage is occurring from the lenses. (Expect to remove your contacts for this process.) Once the lenses are fitted and comfortable your eye doctor will write the final prescription for you. It will show the corrective lens power required, as well as the base curvature and the diameter needed.
Most fittings take at least two visits. Once a final prescription has been determined, your eyes should be examined annually to monitor general vision and eye health and to see if you require any lens changes.
Now that you understand the complicated measuring and fitting process required for contact lenses, you can better appreciate why these appointments take more time and are more expensive than eyeglass fittings.
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