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A Focus on Vision

By Carol Rados

Eye problems, in general, tend to get overlooked in a crowd of broader health issues such as heart disease and cancer. For this reason, the vision health care community has been working hard in recent years to emphasize the importance of proper eye care.

The focus, primarily, has been on increasing the number of people who receive regular vision checks, and addressing diseases, injuries and, according to the National Eye Institute (NEI), the most frequent eye problems in the United States--defects or refractive errors--most often responsible for impairing vision. As a result, vision goals have been added recently to a set of national health objectives, called Healthy People 2010, which are aimed at preventing disease and promoting health.

"These objectives are important because they give vision a prominent place on the public health agenda," says Rosemary Janiszewski, the Healthy People 2010 coordinator for the NEI. "It is an acknowledgment from our country's leading health officials that vision plays a significant role in the nation's overall health."

How We See

The "Snellen Eye Chart," a series of letters arranged in lines, is the standard for measuring how well each eye sees. People view the chart at a distance of 20 feet. One eye is covered while the other is tested.

Having 20/20 vision means seeing at 20 feet what a person with normal vision sees at 20 feet. Someone able to read additional lines smaller than the line representing normal vision has 20/15, or even 20/10, vision. A person who has worse-than-normal vision and can only read letters larger than the 20/20 line has 20/40 vision, or higher. As a result, a person who has 20/40 vision can see at 20 feet what the person with normal vision sees at 40 feet. And so on.

The eye does not actually "see" objects. Instead, it sees the light that objects reflect. To see clearly, light striking the eye must be bent or "refracted" through the cornea--the clear window at the front of the eye that provides most of the focusing power. Light travels through the lens, where it is fine-tuned to focus properly on the nerve layer that lines the back of the eye, the retina, and is then sent to the brain through the optic nerve. The retina acts like the film in a camera, and clear vision is achieved only if light from an object is precisely focused on it. If not, the image you see is blurred. This problem is called a refractive error.

Refractive Errors

Refractive errors usually occur in otherwise healthy eyes, and are caused mostly by an imperfectly shaped eyeball, cornea, or lens, according to the NEI. Nearsightedness (myopia) and farsightedness (hyperopia) are the most common refractive errors. People with myopia see near objects clearly, while distant ones are blurred. People with hyperopia experience just the opposite--they see distant objects clearly, while near ones are blurred. Uneven focus or distorted vision (astigmatism) and aging eye that can't focus close up (presbyopia) are other common refractive errors.

The magnitude of refractive error is measured in units called diopters. Each diopter of refractive error affects a person's ability to read smaller lines of an eye chart.

Why refractive errors develop is not known. The NEI says that most infants have some degree of hyperopia, but that vision becomes more normal with age, usually leveling off by age 6. However, some children remain farsighted, or become so later in life. While some children may be nearsighted early in life, most myopia occurs later during adolescence. Refractive error can continue to change over a person's lifetime. According to the NEI, 60 percent of Americans have refractive errors that need correcting for sharper vision.

Glasses, contact lenses, and various eye surgeries and procedures are aimed at reducing refractive errors by focusing light rays properly on the retina. The past 20 years have seen many innovations in vision correction methods, including implantable intraocular lenses and different types of lasers used to reshape parts of the eye, which are regulated as medical devices by the Food and Drug Administration.

The FDA says that it's important to learn as much as possible about the differences between the available corrective lenses, new and older surgeries, and any other vision correction procedures. It's also important to know what factors make some a good candidate for certain procedures but a poor candidate for others.

Malvina B. Eydelman, M.D., director of the FDA's Division of Ophthalmic and Ear, Nose and Throat Devices, adds that it's important to weigh the benefits and risks of each vision correction option, and to have realistic expectations.

Corrective Eyewear

The NEI estimates that more than 150 million Americans spend over $15 billion each year on corrective eyewear to compensate for refractive errors. Discussing the latest alternatives to corrective eyewear with an eye care practitioner will help ensure that any risks are minimized.

All contact lenses are regulated by the FDA as medical devices. By law, people need a prescription to buy them, even for "plano" lenses, which are worn solely to change the appearance of the eye.

In addition, because people have many choices in how, where, and from whom to buy contact lenses, the Federal Trade Commission (FTC) enforces the Contact Lens and Eyeglass Rules, which help increase the ability to shop around. In this way, the FTC works to prevent fraudulent, deceptive, and unfair business practices regarding contact lenses.

Contact lens quality continues to improve. Advances in materials have made several types of precision contact lenses available for more people. While different types of plastics offer options for replacement and wear schedules, contact lenses are divided into two main groups: soft and rigid gas-permeable (RGP), also called hard contact lenses. From there, the lenses are broken down based on what they're made of, how often they need replacing, and whether they can be worn overnight.

RGP lenses give clearer, crisper vision for some people, according to the NEI. They tend to be less expensive over the life of the lens, but the initial cost often is higher. RGPs last for several years, while soft contacts, depending on the type, are meant to be replaced after short periods. 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. It may take several weeks, however, to get accustomed to wearing rigid lenses, compared with several days for soft lenses.

Daily-wear soft contacts contain from 25 percent to 79 percent water, are easy to adjust to, and are initially more comfortable than RGPs, due to their ability to conform to the eye and absorb water. Soft lenses aren't as likely to pop out or capture foreign material, such as dust, as hard lenses. There are a variety of soft lens materials available for some people with very sensitive eyes.

The development of hyper-oxygen-transmissible lens materials, for both rigid and soft lenses, has created a new generation of extended-wear contacts that are intended to decrease the incidence of, and the risks for, lens-related eye infections. Silicone hydrogel contact lenses, which, according to the NEI, allow physiological levels of oxygen to reach the ocular surface, have improved the safety of extended- or continuous-wear contacts. Extended-wear lenses are available for overnight, and extended-wear disposables are soft lenses worn from one to six days and then discarded.

In October 2002, the FDA approved a new type of soft contact lens, safe enough to wear continuously for up to 30 nights. These lenses allow six times more oxygen to reach the eye than previously approved lenses. All extended-wear contact lenses, however, carry a greater risk of serious eye infections than lenses that are removed before the wearer retires for the day.

The replacement schedule of contact lenses refers to the length they can safely be worn. RGPs generally are replaced every couple of years because they are made of a durable material, although a prescription change would mean new lenses. Soft contacts come in a wider variety of replacement schedules.

Some special features of many contact lenses, both soft and hard, include bifocals, colored contacts, plano lenses, torics for astigmatism, and UV-blocking contacts.

The rule of thumb for contact lens wearers, says James Saviola, O.D., chief of the FDA's Vitreoretinal and Extraocular Devices Branch, "is to practice good hygiene and follow manufacturers' instructions for proper use, cleaning, and storage of the lenses." Report any signs of infection to your doctor, he adds. People should not wear contact lenses longer than the time prescribed by their eye care practitioner. But whatever is prescribed, Saviola says, be sure to ask for written instructions and follow them carefully. Patient package inserts usually accompany contact lenses, and people who are not offered this information by their doctors should ask for it.

The most serious safety concerns with any contact lens deal with overnight use, or extended wear. Rigid or soft, wearing these types of contact lenses overnight increases the risk of corneal ulcers--infection of the cornea that can lead to blindness. Symptoms include vision changes, eye redness, eye discomfort, and excessive tearing. Saviola advises that keeping lenses clean, replacing them often, and wearing them as prescribed by your doctor minimize the risks of wearing contacts.

Orthokeratology (Ortho-K) is a nonsurgical 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.

The Ortho-K system was initially approved for daily wear. But in 2002, the FDA approved the lenses for overnight use. A person takes them out in the morning to enjoy the day free of contacts. This method, however, does not produce a permanent result, and Saviola says that a doctor must be certified to fit Ortho-K lenses.

Plano Lenses--Wearer Beware

Also known as zero-powered, decorative, or noncorrective lenses, plano lenses at one time were considered cosmetic devices. Their purpose is to temporarily change, for example, a brown-eyed person's eye color to blue, or to make a person's eyes look "weird" by portraying Halloween themes or the logos of a favorite sport team. But because these lenses carry the same infection risks to the eye as corrective contact lenses, in 2005, they became medical devices by law.

"FDA strongly believes that eye care providers are needed to fit decorative lenses," Saviola says, because of concerns about the potential for eye problems, such as pink eye (conjunctivitis) and corneal ulcers. He says that the agency also informed health care professionals of the risk of blindness and other eye injuries "if non-corrective, decorative, or cosmetic lenses are distributed without an eye care professional's involvement."

The FDA further advises people to never buy such decorative lenses at any store that doesn't ask for a valid prescription from an eye care professional. "The FDA has never cleared an over-the-counter novelty lens," says Saviola. Such sales are illegal in the United States, and for good reason: wearing contact lenses that don't fit properly is dangerous and can cause serious vision problems, abrasions, and infections.

Maria Higgins, O.D., F.A.A.O., an optometrist who practices in Pittsburgh, was part of the National Contact Lens Enforcement Petition in 2003 that strongly encouraged the FDA to amend the medical device laws to include regulation of all contact lenses.

"I have had numerous experiences where a patient who was new to my office had purchased lenses at an establishment that was less than optimal," she says. Two girls, in particular, came in with flaring, red eyes, Higgins recalls. They were diagnosed with corneal ulcers as the result of overwearing colored, nonprescription contact lenses purchased from a Dollar Store. Both women had worn two-week, disposable lenses for over four months.

"I am not against patients being able to purchase lenses in places other than my office," Higgins says, "but I want my patients to be safe." Fortunately, she adds, since the new law requiring all contact lenses be dispensed by prescription only, "I've found that patients do realize the importance of being fitted by a professional." Plano lenses are as safe as any other contact lenses, Higgins adds, as long as people follow the same rules for corrective contact lenses.

Corrective Surgeries

Refractive surgery includes several surgical procedures designed to help reduce the need for glasses or contact lenses. These procedures correct refractive errors by changing the focus of the eye. Common procedures such as photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK) do this by reshaping the curve of the cornea to move the point at which light is focused onto the retina.

Various procedures with different capabilities are available. There are now four categories of refractive surgery procedures: excimer laser, implant, thermal, and other refractive procedures.

In PRK, an excimer laser capable of removing precise amounts of tissue with micron accuracy is used to reshape the central cornea--to flatten it to correct myopia, or to steepen it to correct hyperopia. PRK can also be used to correct astigmatism. The layer of cells covering the cornea, the epithelium, is removed, and the laser sculpts the cornea to correct the refractive error. A bandage contact lens is placed over the eye after the procedure to speed the epithelial healing process.

PRK gained popularity in the mid-1990s, but also was met with limitations. It worked best in patients with low-to-moderate myopia, because with higher levels, there was a risk of corneal haze. The procedure also was associated with some physical discomfort after surgery, since the cornea needed several days to heal. In some cases, it could take several months to reach the best level of vision.

By far the most popular vision correction procedure has been LASIK. Surgeons use a surgical knife, called a microkeratome, to create a hinged flap on the surface, fold it over to sculpt the underlying cornea into a new shape, and fold it back onto the cornea.

To encourage her daughter to consider LASIK, Becky Ricketts, 51, of Mt. Airy, Md., underwent the procedure for severe astigmatism in both of her eyes two years ago.

"I decided to be the guinea pig," she says. "My daughter's eyes were so bad, I just believed she would be better off having LASIK, based on results of the people I knew who'd had it done."

Ricketts's eyesight, though not as poor as her daughter's, was such that she wore glasses every day for most of her life, but not so bad that she was legally required to wear them to drive. "I've always passed my driving tests without glasses," she says. She does admit, however, that she squinted in front of the computer screen, and claims that without glasses, "everything had a fuzzy look." She was not able to wear contact lenses because the astigmatisms were so severe that "if I blinked, the contacts moved and I couldn't see." In fact, any movement of the head, Ricketts says, caused her contact lenses to move.

The advantages of LASIK include a quicker visual rehabilitation, reduced pain and discomfort, and the surgeon's ability to treat higher levels of refractive error without the limitations associated with PRK.

Three years after LASIK, Ricketts says, "My vision couldn't be better. I'm happy I had the surgery," she says, "but I didn't spend my life wanting to have it done." Although she still wears glasses to correct presbyopia, Ricketts is currently considering a relatively new procedure that would reduce her need for reading glasses.

Doctors say that one of the keys to a successful LASIK procedure is the measurement that an ophthalmologist takes to determine refractive error. Small imperfections in the eye may cause some light to travel through the eye at different angles, making light strike the retina in different places. Collectively, these imperfections are called optical aberrations.

Traditional laser technology allows for correction of the refractive errors myopia, hyperopia, and astigmatism, also known as "lower order" aberrations. A new excimer laser procedure, called wavefront-guided LASIK, treats lower order and "higher order" aberrations, which are subtle focusing imperfections in an eye's optical system that can result in less-than-optimal clarity.

Wavefront, or custom LASIK, uses a measuring device to create a "map" of how a person's eye focuses light to precisely assess the unique irregularities and variations of the eye. These variations, experts claim, can be as unique as a person's fingerprints.

The FDA approved the excimer laser for use in wavefront-guided LASIK in 2003. Ricketts's 28-year-old daughter, Lindsey Hocker, of Frederick, Md., underwent the relatively new custom corneal surgery less than one year after it first became available.

"Regular LASIK came highly recommended to me by several people, and seeing the success that Mom had with LASIK convinced me to do it," Hocker says. "But because of the problems I had, I decided to go with my doctor's recommendation for the custom cornea."

The wavefront map is very detailed: Instead of simply creating a general description of the eye's focusing power, for example, nearsightedness, farsightedness, or astigmatism, it records every subtle distortion in the pathway of light moving through the eye.

"Immediately after the surgery," Hocker says, "I could see the clock on the wall for the first time since the fourth grade." The only side effect she has experienced in two years was dry eyes after surgery.

Although it's natural for people to want to hear the success stories of others who have undergone a type of surgery, the FDA recommends that people avoid being influenced by others encouraging them to have such procedures. Not everyone is a candidate for every procedure.

Laser Epithelial Keratomileusis, or LASEK, is a variation of LASIK, and corrects myopia, hyperopia, and astigmatism. The epithelium, or outer surface of the cornea, is loosened with alcohol, not with the microkeratome used in LASIK. It is then peeled back to expose the cornea. The same excimer laser used in LASIK is applied to the cornea, but only to the surface. The epithelium is placed back into position, and a bandage contact lens is placed on the eye to promote healing. Like LASIK, the recovery time is rapid. Discomfort is somewhat increased, compared with LASIK.

LASEK is similar to PRK. The difference is that with LASEK, the epithelium is replaced after surgery. In PRK, the epithelium is discarded. Both PRK and LASEK are similar to LASIK in that they use the excimer laser to shape the cornea.

While the FDA regulates excimer lasers, the agency doesn't have the authority to regulate a doctor's practice of medicine or the off-label use of medical products. Therefore, the FDA does not tell doctors what to do when running their businesses or what they can or cannot tell their patients. Consequently, people considering laser surgery should ask questions and fully understand any procedure they might be considering.

The idea of a person walking into a doctor's office and an hour later walking out with perfect vision is a very attractive one, but the reality is that these are surgical procedures with potential complications, and perfect results are not guaranteed, experts say. Everette Beers, Ph.D., chief of the FDA's Diagnostic and Surgical Devices Branch, reminds people that refractive surgeries are elective procedures, some of which can't be undone.

"People need to remember that you can change glasses or contacts, but not implants or surgery," he says. Be sure to consult with a refractive surgeon to determine your eligibility for surgery. Beers also warns that surgical procedures are not without some risk, and that "the long-term effects of many procedures are still unknown."

According to the American Academy of Ophthalmology (AAO), more than 90 percent of people who have refractive surgery for myopia and astigmatism end up with 20/40 vision or better without glasses, a correction sufficient enough to allow them to drive legally without glasses. Sixty percent to 70 percent of patients achieve 20/20 vision or better.

Implant Procedures

Corrective artificial lens implants give people who don't want to bother with eyeglasses or manual insertion of contact lenses another option to consider.

Intrastromal corneal ring segments are semicircular pieces of plastic that are implanted within the cornea to treat mild forms of myopia. They also are sometimes used for other conditions affecting the cornea. The inserts are designed to change the shape of the cornea by adjusting the focusing power of the eyes so that light is focused onto the retina. A small incision is made near the upper edge of the cornea, in which the ring segments are inserted. The incision is closed with two small sutures that are usually removed two to four weeks after surgery.

While tissue removed during laser eye surgeries cannot be replaced, the intrastromal corneal ring segments are removable.

Phakic Intraocular Lenses (phakic IOLs) are new devices made of plastic or silicone, approved by the FDA for correcting nearsightedness. These thin lenses are implanted into the eye to help reduce the need for glasses or contact lenses. A small incision is made in the front of the eye, in which the phakic lens is inserted. Phakic refers to the lens being implanted into the eye without removing the eye's natural lens. Since phakic IOLs involve entering the eye, unlike LASIK and PRK, the risk of complications is higher.

Phakic lenses are intended to be permanent. If a cataract develops, however, the natural and phakic lenses would be removed and replaced with artificial lenses, says Kesia Alexander, Ph.D., chief of the FDA's Intraocular and Corneal Implants Branch. But, she adds, "there's no guarantee that the eye will return to its previous level of vision." Alexander also says that while phakic lenses are a good alternative for people who are very myopic and can't be corrected with LASIK, "there's no guarantee that you won't always be able to go without glasses."

Thermal Procedures

Conductive keratoplasty (CK) uses radio frequency energy, instead of a laser, to bend the cornea. Also known as "blended vision," CK corrects for hyperopia. By overcorrecting the cornea, CK causes the eye to become nearsighted. "CK achieves its correction of presbyopia," says Beers, "by inducing monovision with one nearsighted eye."

CK does not involve making an incision, but instead, a tiny probe releases controlled amounts of very low heat from radio frequency energy, causing the outside area of the cornea to tighten like a belt, making the central cornea steeper. CK causes little or no discomfort or irritation, and vision improvement is almost instantaneous. Unlike other types of refractive surgery, such as LASIK, however, correction from CK may be temporary and re-treatment may be necessary.

Other Refractive Surgery Procedures

Accommodative and multifocal IOLs are used to treat nearsightedness, farsightedness, and the inability to focus up close because of age. These artificial lenses are surgically implanted in the eye. Unlike the phakic IOLs, which are implanted in front of the eye's natural lens, accommodative and multifocal IOLs actually replace the eye's natural lens once a cataract has developed. These lenses enable the eye to regain its focusing and refractive ability.

Monovision is a corrective technique used to treat people with presbyopia. The intent is for the person to use one eye for distance viewing and one eye for near viewing. Having each eye configured for different focusing distances can reduce or eliminate the need for eyeglasses or contact lenses.

The practice was first applied to contact lenses, and more recently to LASIK and other surgeries. In refractive surgery, the technique treats one eye to focus at close proximity, while the other eye is left untreated or, if needed, treated to be able to focus at a distance. This method may be difficult to adjust to at first but, according to the International Society of Refractive Surgery, about six to eight weeks after the monovision procedure, most people's brains are able to adjust to the different focusing ability of the eyes.

The FDA recommends that anyone considering monovision try the contact lens procedure first, as a trial run, before having the surgery, which is permanent. Also, it's important to check state drivers' license requirements with monovision.

Eyeglasses--The Reliable Standby

In some cases, modern technology can provide the best vision correction option. In those cases in which it can't, eyeglasses may be the way to go. Glasses correct refractive errors by adding or subtracting focusing power to the cornea and lens. The power needed to focus images directly on the retina is measured in diopters. This measurement is also your eyeglass prescription.

Like contact lenses, glasses come in all shapes and sizes, offering an array of choices for both function and fashion. Eyeglass frames, for example, are more durable and tout materials such as titanium and new "memory metals." Manufacturers are making lenses that are thinner, stronger, and lighter. And lens options include antireflective coating, light-changing tints, line-free (progressive) bifocal, and polycarbonate--the most impact-resistant lens material available.

Regular eye exams are important because they can detect early signs of disease and refractive error long before either leads to vision impairment. Doctors recommend that everyone have an eye exam shortly after birth, and at least every few years until age 40. After that, the eyes should be routinely checked every two or three years. People with diseases such as diabetes and hypertension should have their eyes checked more frequently.

To read the rest of this great article from the Food and Drug Administration, please click here: http://www.fda.gov/fdac/features/2006/406_vision.html