PLEASE READ THIS: The information contained herein is not intended
as a substitute for medical advice and care from qualified, licensed
health care providers. The reader should regularly consult a physician
in matters relating to his or her health and partic ularly with
respect to any symptoms that may require diagnosis or medical
attention. The information is presented here to educate and inform
and to guide you to an understanding of cause, prevention as well
as treatment.
Contact lenses are prescription medical devices designed to provide corrective optics for the human vision system. Regular check-ups are vital to assure that the lenses do that, and nothing more. The information presented below applies to both rigid and soft contact lenses.
In order to better understand and appreciate the information
presented here, I'd recommend obtaining a labeled
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print and have accessible the Glossary
of Terms (click here to download) found here in the EyeCare
Connection.
The order of the tests a nd the techniques may vary according to the practice of the individual practitioner. The contact lens examination, also called check-up or follow-up, consists of tests which help to evaluate a number of factors, including: visual acuity, lens optical po wer, physical fit of the lens on the cornea, condition of the lens material following a period of use, the effect of the lens on the eye tissues, including the conjunctiva and cornea, the effect of wearing lenses on the structure and curvature of the c ornea, the effect of lens care chemicals on your eye, and other long-term eye health issues. Below is a discussion of these topics.
Tests are performed to determine if the contact lenses provide an optical correction consistent with the best quality vision. The over-refraction is the term used for this test. Your vision is tested with the contact lenses on your eyes. Additional spectacle lenses are placed before your eyes to determine if more or less powe r is needed to allow for maximum vision. This test is performed with either hand-held lenses or with the phoropter, the instrument eye doctors use to test vision. Often the doctor will measure the optical refractive error of your eye as if for prescri bing eyeglasses and convert and compare this number with the refractive power of your contact lenses. The end result is a determination of the optical characteristics of your lenses needed for you to experience the best quality vision.
Using the slit-lamp/biomicroscope, the contact lens is observed on your eye. As you blink, the lens moves over the cornea, floating on a very thin layer of tears. It also comes in contact with the conjunctiva on the inside of y our upper lids, and in the case of soft lenses, the conjunctiva of the lower lid and overlying sclera. A soft lens should center evenly over the cornea and completely cover it, extending somewhat over the cornea and onto the sclera (white of your eye). A rigid lens is evaluated for its relative position on the cornea and its interaction with the eyelids. Many doctors use a dye called sodium fluorescein which is added to your tears to help see them as they flow under the lens and to assess whether or not the lens is pressing onto the cornea. This is especially important for rigid lenses.
The slit-lamp/biomicroscope is also used to view the lens for edge defects, scratches, tears or fractures in the lens and protein or other materials adherent to the lens surface. A microscopic defect in the lens may result in discomfort out of proportion to the damage observed. Defects in the lens can also result in damage to eye tissue, as discussed below.
The contact lens is removed from the eye for evaluation of the ocular surface. A poorly fitting or damaged lens, or one that does not allow adequate oxygen to reach the cornea, can seriously effect your eye health. Som e people are concerned about eye infections occurring during contact lens wear. Infections are caused by bacteria, viruses and, rarely, fungus and protozoa. When a lens irritates or otherwise damages the cornea, it creates a wound, much like a cut on y our skin, that opens the tissue for invasion by a pathogen---resulting in an infection. Recent studies indicate that insufficient oxygen and reduced cornea cell turnover rate can predispose a contact lens wearer to infection.
Looked at in a differ ent way, this also means that a quality contact lens properly cared for and worn responsibly is very unlikely to result in a problem from infection.
The slit-lamp/biomicroscope is used to view the cornea under magnification. The fluorescein dye he lps to outline areas of tissue damage. The doctor uses the instrument to look for corneal scars (a sign of previous damage or infection), active infection, swelling of the tissue (edema, often resulting from insufficient oxygen reaching the eye), neovas cular growth (new blood vessels growing into the normally transparent cornea, also a result if insufficient oxygen), and other signs of compromised eye health.
He/she may 'evert' the upper eyelid (flip the eyelid inside out) to expose the inside surface and look for signs of tissue reaction to chemicals, allergy, or irritation from the edge of the lens. Contact lens care solutions often contain chemicals (disinfecting agents and preservatives) and detergents (for cleaning). Occasionally , people develop chemical sensitivity or allergic reactions to these chemicals. This insult to the eye tissues can result in damage to the eye. Symptoms include: itching, burning, tearing, often noticed some time after exposure to the offending chemical , often worsening during the day.
While the lens is still removed, the cornea is observed with the keratometer and the curvature of the front of your eye is measured. The keratometer also helps evaluate the corne al surface for irregularities. Poor fitting lenses or lack of oxygen can cause changes in the shape of the cornea, resulting in a change in the fit of the lens.
There are a number of remedies to be considered if problems are noticed:
Contact lenses have a limited useful life. Disposable lenses generally last 4 to 20 days, frequent replacement lenses hold up for up to 4 months, sometimes longer. Standard soft lenses are good for 6-12 months. One material, GMMA, has been know to maintain its shape and optical characteristics for as long a two years. Rigid gas permeable lenses of low to mid range gas permeability last up to two years and those of higher 'dk' values may begin to decompose after less than a year. Of course lens care and handling play a very important role in how long your lenses can continue to safely provide good vision and eye health.
Your prescription can change! It is always a good idea to update your lenses as needed.
Replace damaged lenses. Refit with a lens of different size, curvature, oxygen transmissi bility as necessary to provide good eye health. Sometimes it is necessary to temporarily discontinue lens wear as the eye heals.
Not everyone can wear lenses overnight w ithout some negative effect on eye health.
Use hypoallergenic care systems. In situations of severe allergic conjunctivitis, topical medication may be used to help the tissue return to good health .
The contact lens examination is designed to verify the status of your contact lens, with regard to both eye health and best vision correction. The recommended intervals for contact lens check-ups is at least annually, or as directed by your doctor, and twice a year if you sleep in your lenses. And remember: Contact lenses do not cause infections. People do!
