I often get this question from patients: “I can see fine. My contacts are fine. Why do I have to have a contact lens exam?” This is not an unusual conversation in my office. Now is a great opportunity to explain the different tests done in a Contact lens exam.
One important thing we need to understand is that contact lenses are FDA approved medical devices. They are actually small, very thin, pieces of plastic that sit directly on the cornea. The cornea of the eye is a very small, yet complicated structure that needs to be examined regularly when a patient wears contacts. Misuse of contacts and/or poor fitting contacts can cause serious harm to the health of the cornea and ultimately your vision.
My vision is fine. Why do I need another exam?
There are several tests your optometrist will do to determine that contacts are safe, healthy on your eyes and that the prescription is correct. We measure the curvature of the surface of the cornea – using a Keratometer or a Topographer – to determine the right size of contact to put on. Taking this measurement every year will help ensure that the contacts are not causing any subtle damage to the surface of the cornea. There is also a highly specialized microscope that some doctors are beginning to use that is able to count the number of corneal cells to make sure that they are staying stable while using contact lenses. These tests are early detectors of corneal changes due to the use of contacts that may not initially affect the vision or comfort of your lenses.
The contact lens exam also includes a detailed tear analysis to determine if you have dry eyes or allergy eyes. These common conditions can affect the comfort of your lenses and your vision. There are many different types of materials that contacts are made of and some are not compatible with certain eye surface conditions. For example, someone might need a contact with more water in it, others might need one that transmits more oxygen to the cornea. All of these factor are determined in the contact lens exam.
Next, the contact lens prescription needs to be determined. Due to the fact that that contact lens sits on the eye, not in front of the eye like glasses do, there is actually a different determination of the contact lens prescription that needs to be done in some cases. So, no … the contact lens prescription is not the same as the glasses prescription.
Once all of the parameters of the contact lens is chosen – size, material and optical prescription – the lens needs to be evaluated on the eye. We look at how the lens moves on the eye and how it sits to determine if it is going to be a healthy lens for you. Sometimes, differences in eyelids, blinking and tears can affect how a lens sits on the eye and adjustments might need to be made.
Finally, when your optometrist is happy with the fit and the vision through the contacts, then your contact lens examination is complete. Depending on the difficulty of your prescription, any eye surface disease, such as dry eyes or allergies, and if you are an experienced wearer or not, this process can take from one visit to several. Work with your optometrist during the process so that your can safely wear contact lenses for many years to come.