Refractive errors can be corrected with glass or plastic lenses mounted in a frame (eyeglasses) or with a small lens made of plastic floating or resting on the cornea (contact lens). Good vision correction is possible with either eyeglasses or contact lenses. For most people, the choice is a matter of appearance, convenience, cost, risk, and comfort.
Eyeglasses
Eyeglass lenses can be made of either plastic or glass.
Plastic lenses for eyeglasses are lighter but are more likely to scratch. These lenses are more commonly used than glass because they are thinner and can also be coated with a substance that helps them resist scratches.
Glass lenses for eyeglasses are more durable and less likely to scratch than plastic but are more likely to break, potentially causing injury.
Both glass and plastic lenses can be tinted or treated with a chemical that darkens them automatically when exposed to light. Lenses can also be coated to reduce the amount of potentially damaging ultraviolet light that reaches the eye.
Newer plastic lenses (made from polycarbonate or a urethane-based monomer) completely block ultraviolet light, are highly resistant to breaking, and also can be coated with a scratch-resistant substance.
Bifocals are eyeglasses that contain 2 lenses—an upper lens that corrects the view of distant objects and a lower lens that corrects the view of nearby objects, such as reading material. However, people also need to focus at middle distances, such as when viewing a computer screen. Trifocals are eyeglasses that meet this need because they contain a third lens for middle distance. Continuously variable lenses (progressive lenses or no-line bifocal lenses) also permit focusing at middle distances and have a cosmetic advantage in that there is no line or sharp division between the regions of the eyeglass lens.
Contact Lenses
Many people think contact lenses allow them to be more active or make them feel more attractive than when wearing eyeglasses. Some people think that vision is more natural with contact lenses. However, contact lenses require more care than eyeglasses and, rarely, they can damage the eye. Some people, particularly people with a tremor or people with arthritis, may have trouble handling contact lenses and placing them in their eyes.
Contact lenses provide better peripheral vision (the outer limits of the field of vision) than do eyeglasses.
Contact lenses can be made that correct for different visual problems (such as distance and close-up vision) in different parts of the lens. These are so-called bifocal or multifocal contact lenses.
Contact lenses can correct
Aniseikonia (a difference in image size)
Aphakia (absence of a lens in the eye—can be present at birth [congenital] or can occur when no lens is implanted to replace the one removed during cataract surgery)
Keratoconus (a cone-shaped cornea)
Either soft or rigid (hard) contact lenses are used to correct nearsightedness and farsightedness. Soft toric lenses (which have different curvatures molded onto the front lens surface) or rigid contact lenses can correct astigmatism, but require expert fitting.
Presbyopia can also be corrected with contact lenses. In one approach, called monovision, one eye is corrected for reading and the other is corrected for distance vision. However, some people have difficulty adjusting to monovision. Another approach is to wear a bifocal or multifocal contact lens in each eye.
Neither rigid nor soft contact lenses offer the eyes the protection against a blunt or sharp injury that eyeglasses do.
Rigid contact lenses
Rigid contact lenses, which are usually gas-permeable, are thin disks made of hard plastic. They are smaller than soft contact lenses and cover only part of the cornea. Oxygen, which the cornea needs to function properly, did not pass easily through the plastic of the older style hard contact lenses. Modern gas-permeable contact lenses (GPCLs), which are made of plastics such as newer fluorosilicone acrylate compounds, permit more oxygen to reach the cornea. Rigid contact lenses can be used to correct nearsightedness, farsightedness, and irregularities in the cornea (astigmatism and keratoconus).
GPCLs can be designed to fit the eye exactly. They usually require some time for the eye to adapt to their presence and need to be worn for about 4 to 7 days before they feel comfortable for a prolonged period. The contact lenses are worn for a gradually increasing number of hours each day. Although rigid contact lenses may be uncomfortable at first, they should not be painful. Pain usually indicates an improper fit. People who wear GPCLs may have temporary (less than 2 hours) blurred vision when they put eyeglasses on after removing the contact lenses. Vision with rigid contact lenses is usually sharper than vision with soft contact lenses, particularly in people with astigmatism.
Scleral contact lenses
Scleral contact lenses are made of rigid gas-permeable materials, and they are larger than corneal contact lenses. They are called scleral contact lenses because the peripheral portion of the lens rests on the sclera (the white of the eye). These lenses are more comfortable because they allow a thick tear film to bathe the surface of the eye. As a result, even people who have severe disease of the eye's surface can usually wear them with ease.
Because each pair of scleral contact lenses is custom-made, they are generally easier to adapt to than traditional lenses of smaller diameter.
Scleral contact lenses can correct high degrees of irregular astigmatism (for example, advanced keratoconus).
Soft contact lenses
Soft hydrophilic (water-absorbing) contact lenses are made of flexible plastics and are 30 to 79% water. They are larger than rigid contact lenses and cover the entire cornea. Soft contact lenses can be used to correct nearsightedness, farsightedness, astigmatism, and presbyopia. Not all soft contact lenses allow oxygen to reach the cornea easily.
Because they are larger, soft contact lenses are less likely than rigid contact lenses to fall out or to allow dust and other particles to get trapped underneath. In addition, soft contact lenses are usually comfortable from the first wearing. Soft contact lenses require scrupulous care to prevent problems, because the risk of infection is higher with soft contact lenses than with rigid contact lenses. When dry, soft contact lenses are brittle and break easily.
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Care and complications of contact lenses
Contact lenses should be removed and cleaned every day at bedtime using a contact lens disinfecting solution. Tap water should never be used, particularly to clean soft or gas-permeable contact lenses. Hands should be washed with soap and water before handling lenses. Each lens is cleaned separately. A lens is placed in the palm of one hand, and several drops of fresh solution are dripped onto it. The fingertip of the index finger of the other hand is used to clean the lens, using a gentle back-and-forth motion on both surfaces of the lens. Another several drops of solution are then used to rinse the lens. After cleaning, all contact lenses should be disinfected, by storing them in fresh disinfectant solution in a contact lens case overnight.
After the lenses are inserted the next day, the disinfectant solution should be removed from the lens case. The lens case is then rinsed with fresh solution and placed in a cabinet or drawer and allowed to dry by leaving it open to the air. The case should not be left open on the countertop around the sink in the bathroom because tiny droplets that disperse after brushing teeth or flushing the toilet may contain microorganisms and can settle in the case and start an infection. The lens case should be replaced every 1 to 2 months.
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Wearing contact lenses longer than recommended or longer than the eye can tolerate can cause eye redness, watering, and light sensitivity (called contact lens overwear syndrome or contact lens overuse syndrome). Once the lenses are removed, these symptoms tend to resolve over a day or so, but symptoms that continue longer could be a sign of a more serious infection.
Some contact lenses require weekly treatment with an enzyme cleaner. Daily disposable contact lenses are discarded after a day of use. Some regular or disposable soft contact lenses are designed so that they may be kept in the eye during sleep for a number of days (extended wear). Most can be kept in place for up to 7 days, but some contact lenses are available that can be kept in place for up to 30 days. However, the risk of infection is much higher when contact lenses are worn overnight. Therefore, it is best to remove contact lenses before going to sleep.
Wearing contact lenses poses a risk of serious, vision-threatening, painful complications, including the formation of ulcers on the cornea. Corneal ulcers can be caused by bacteria, viruses, fungi, or amebae and can cause loss of vision. The risks can be greatly reduced by strictly following the eye doctor's lens care instructions.
The risk of serious infections increases when people swim or shower while wearing their contact lenses and clean their lenses with homemade saline solution, saliva, tap water, or distilled water. Sleeping while wearing any type of contact lens also greatly increases the risk of serious infections. The risk of infection increases for every night a person sleeps in soft contact lenses. The best way to reduce the risk of infection is to not sleep in contact lenses, disinfect correctly, and not expose the contact lens to tap water. If a person has intense eye pain, excessive watering of the eye, pain with exposure to light, vision changes, or eye redness, the contact lenses should be removed immediately. If the symptoms do not resolve quickly, the person should contact an eye doctor.
