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Refractive errors can be corrected with glass or plastic lenses mounted in a frame (eyeglasses) or with a small lens made of plastic floating on the cornea (contact lens). Good vision correction is possible with both eyeglasses and contact lenses. For most people, the choice is a matter of appearance, convenience, cost, risk, and comfort.
Plastic lenses for eyeglasses are lighter but are more likely to scratch. Glass lenses are more durable but are more likely to break, potentially causing injury. Plastic lenses are more commonly used because they are thinner and can also be coated with a substance that helps them resist scratches. 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.
Bifocals contain two 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 meet this need by adding a lens for middle distance. Continuously variable lenses (progressive add 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.
Many people think contact lenses are more attractive than eyeglasses, and some 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 older people and people with arthritis, may have trouble handling contact lenses and placing them in their eyes.
Contact lenses may provide better visual acuity and peripheral vision than do eyeglasses. Contact lenses can correct nearsightedness, farsightedness, astigmatism, anisometropia, aphakia after removal of a cataract, and keratoconus (a cone-shaped cornea—see Keratoconus). Either soft or rigid (hard) contact lenses are used to correct nearsightedness and farsightedness. Soft toric lenses or rigid contact lenses can often 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.
Neither rigid nor soft contact lenses offer the eyes the protection against a blunt or sharp injury that eyeglasses do.
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, does not pass easily through the plastic of the older style hard contact lenses. Gas-permeable contact lenses (GPCLs), which are made of plastics such as newer silicone 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 usually 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.
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, and sometimes astigmatism. 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.
Contact lenses should be removed and cleaned every day using a contact lens disinfecting solution. Tap water should never be used, particularly to clean soft or gas-permeable contact lenses. After cleaning, all contact lenses should be stored in a disinfectant solution in a contact lens case overnight. After the lenses are inserted the next day, 100% of the disinfectant solution should be removed from the lens case. The lens case is then allowed to dry by leaving it open to the air. The lens case should be replaced every 1 to 2 months. 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 newer contact lenses are available that can be kept in place for up to 30 days. However, the risk of infection is higher with contact lenses that 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. Ulcers can be caused by bacteria, viruses, fungi, or amoebas and can cause loss of vision (see see Corneal Ulcer). The risks can be greatly reduced by following the eye doctor's instructions and using common sense.
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 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. 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.
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