Vision Correction: Part 3

by Food and Drug Administration (FDA)

(Page 3 of 4)

Contact lens quality continues to improve. Soft contacts contain from 25 percent to 79 percent water, are easy to adjust to, and are more comfortable than rigid gas permeable (RGP) lenses, thanks to their ability to conform to the eye and absorb water. Soft lenses aren't likely to pop out or capture foreign material such as dust underneath, as hard lenses are. Extra-thin soft lenses are available for very sensitive eyes.

While the ability to hold water increases oxygen permeability of soft lenses, it also makes them more fragile. And soft lenses are more likely to absorb chemicals and residues on the wearer's hands.

RGP lenses are more durable and resistant to deposit buildup, and they generally give clearer, crisper vision. They tend to be less expensive over the life of the lens, but the initial cost often is higher. RGP contacts last several years, while soft contacts, depending on the type, are meant to be replaced after periods ranging from a day to about a year. In addition, RGP lenses can be marked to show which lens is for which eye, and they're less likely to tear or rip, making them easier to handle. However, it often takes several weeks to get used to wearing rigid lenses, compared with several days for soft.

Many changes are occurring in the world of disposable (defined by the FDA as used once and discarded) and frequent- or planned-replacement contacts. The latest innovations include daily disposables, bifocals and toric contacts for astigmatism.

"It's healthier to replace lenses more often," says James Saviola, O.D., chief of the vitreoretinal and extraocular devices branch in the FDA. "And if you reuse your lenses, you need to do something more than store them in saline solution." The FDA approved in 2000 the first "no-rub" cleaning solution for contact lenses. The solution adds a safeguard for people who do not rub their lenses — but should — when cleaning. The no-rub directions for this first solution initially applied to lenses replaced within a month or less. Now, it has been expanded to include lenses that are replaced after a month or more. Other products also are available that have no-rub directions for lenses replaced within a month. But Saviola reminds people that in some cases, rubbing is still necessary to keep their lenses clean.

A new generation of lens materials is being studied. Lenses made of these materials provide a greater amount of oxygen permeability, says Saviola. Two types have received FDA approval, one for seven days of continuous wear, the other for 30 days. Others, such as the 30-day continuous wear contact, now are being considered.

The most serious safety concerns with any contact lens deal with overnight use, or extended-wear. Rigid or soft, wearing these types of contacts overnight increases the risk of corneal ulcers — infection-caused eruptions on the cornea that can lead to blindness. Symptoms include vision changes, eye redness, eye discomfort, and excessive tearing. Extended-wear rigid lenses also can cause unexpected, sometimes undesirable reshaping of the cornea. Saviola advises that keeping lenses clean, replacing them often, and wearing them as prescribed by your eye-care specialist increases the safety of wearing contacts.

People should not wear contact lenses longer than the time prescribed by their eye-care practitioner. But whatever he or she prescribes, be sure to ask for written instructions and follow them carefully. Patient package inserts usually accompany contact lenses, and Saviola emphasizes that people who are not offered this information by their doctors should ask for it.

For those who haven't been able to wear contacts, implantable lenses may be an option in the future.

Orthokeratology

Orthokeratology, or Ortho-K, is a procedure that uses RGP contact lenses to change the curvature of the cornea to improve its ability to refract light and successfully focus on objects. Unlike regular RGPs, Ortho-K RGPs have a design that can reshape the curvature of the cornea. This method, however, does not produce a permanent result.

With conventional Ortho-K, the lenses are worn about eight hours a day. After the cornea has achieved the best shape for optimal vision, the lenses are worn less frequently — perhaps for a few hours every two or three days. If someone starts and then discontinues Ortho-K, says Saviola, the corneas will eventually return to their natural state. People choose Ortho-K over refractive surgery because Ortho-K's effects are not permanent.

One disadvantage of Ortho-K is that clear vision may fluctuate during the day. Also, Ortho-K may take many months to change a person's vision. A more advanced technique known as "accelerated Ortho-K" takes less time, and may be recommended to achieve a rapid effect.

Since 1998, Saviola says the FDA has cleared a number of daily wear Ortho-K lenses, but overnight Ortho-K lenses have not been approved.

The best candidates for prescription Ortho-K are people of any age who have low amounts of nearsightedness or astigmatism. The goal is to bring the person's vision to at least 20/40. But for some, Ortho-K will provide 20/20 vision.

Corneal Ring Segments

In 1999, the FDA approved a non-laser surgical procedure for correcting small amounts of nearsightedness. Corneal ring segments are tiny, clear crescent-shaped pieces of plastic polymer that are implanted in the cornea. The ring segments reshape the cornea so that it becomes flatter, allowing it to focus light rays onto the retina and producing sharp vision. The procedure takes about 15 minutes and is done on an outpatient basis. Before surgery, anesthetizing drops are placed in the eyes.

Corneal rings are still being studied to treat mild hyperopia and astigmatism, although these uses have not been approved by the FDA. Several other intraocular and corneal implants, from several companies, also are in various stages of clinical study.

About the Author

www.fda.gov
FDA is A United States government body that oversees medical devices, including contact lenses, intraocular lenses, excimer lasers and eyedrops. In the US, these products must be approved by the FDA before they can be marketed.