Nearsightedness and farsightedness
Nearsightedness (myopia) and farsightedness (hyperopia) are vision problems.People who are nearsighted see objects clearly when they are close to the eye, while distant objects appear blurred or fuzzy. Symptoms of hyperopia can vary from no visual problems at all, to clear distance vision combined with blurry near vision, to blurry distance and near vision.
To understand nearsightedness and farsightedness, it helps to know somethingabout the main parts of the eye's focusing system: the cornea, lens, and retina. The cornea is a tough, transparent, dome-shaped tissue that covers the front of the eye, lying in front of the iris (the colored part of the eye). Thelens is a transparent structure located behind the iris. The retina is a thin membrane that lines the rear of the eyeball. Light-sensitive cells in the retina convert incoming light rays into electrical signals that are sent to the brain, which then interprets the images. In people with normal vision, parallel light rays enter the eye and are bent by the cornea and lens to focus precisely on the retina, producing a crisp, clear image. In a nearsighted person, the cornea and lens bend the light rays too much for the length of the eye, and the rays converge in front of the retina. The resulting image that goesto the brain appears fuzzy. In farsightedness, the point of focus falls behind the retina.
Typical symptoms of nearsightedness are blurred distance vision, eye discomfort, squinting, and eye strain. People usually are diagnosed with this problemduring the first several years of elementary school when a teacher notices achild having difficulty seeing the blackboard, reading, or concentrating.
Newborn babies usually are slightly farsighted, but their vision tends to improve over time. However, some people have hyperopia for life. If the condition is not too severe, the lens may be able to adjust enough to bring the imageback onto the retina. When this happens, a person can see clearly at a distance, but the constant refocusing may cause headaches or eyestrain. Nearby objects may appear blurry because the eyes have to shift from focusing on something at a distance to focusing on something near. If the lens cannot adjust enough to bring images of distant objects onto the retina, those objects may appear blurry. This explains why symptoms of farsightedness are so variable.
The usual treatment for nearsightedness and farsightedness is corrective lenses, which may be worn in eyeglasses or as contact lenses. For people who findglasses and contact lenses inconvenient or uncomfortable, and who meet certain selection criteria, corrective eye surgery may be an option. Several typesof surgery, including radial keratotomy (RK), photorefractive keratectomy (PRK), and laser-assisted in-situ keratomileusis (LASIK) are used to correct these vision problems.
Presbyopia is another type of vision problem that occurs as a part of normalaging and has symptoms similar to those of farsightedness. As people age, thelens becomes less flexible, and the muscles that adjust its curvature becomeless powerful. As a result, it loses its ability to focus the rays of lightcoming from nearby objects. People with presbyopia have trouble reading smallprint and doing close work. The problem may be worse early in the morning, in dim light, or when the person is tired.
Symptoms of presbyopia usually begin to appear between age 40-45 and continueto develop until the condition stabilizes 10-20 years later. Presbyopia cannot be cured, but the problem can be corrected with eyeglasses or contact lenses.