Ametropia, which is often caused by a congenital malformation, refers to vision disorders characterized by the eyes inability to correctly focus the images of objects on the retina. Its forms include myopia (nearsightedness), hyperopia (farsightedness), and astigmatism. Strictly speaking, presbyopia is not a form of ametropia, but rather a natural phenomenon associated with aging. Most cases of ametropia can be corrected, either by wearing glasses or contact lenses, or by surgery.
Myopia, or nearsightedness, is caused by an overly long eyeball, an overly curved cornea, or an anomaly in the lens. It causes blurred vision of faraway objects, but does not affect near vision. Myopia usually appears in early adolescence and stabilizes at adulthood. Very strong nearsightedness increases the risk of developing certain eye diseases, like cataracts and retinal detachment. As a result, myopic people should go for regular, routine checkups.
Presbyopia, or presbytia, is the lens's declining power of accommodation, associated with aging. It usually appears around the age of 40, as difficulty seeing clearly at short distances (e.g. for reading), while far vision usually remains intact. Presbyopia may be accompanied by headache and visual fatigue.
An astigmatism is the result of an irregularity in the curve of the cornea. It leads to deformed images and defective vision at all distances. Often hereditary, it can also be caused by injury or by surgical operation for cataracts or keratoconus (a genetic disease that entails the gradual deformation of the cornea).
Hyperopia, or farsightedness, is caused by insufficient length of the eyeball, an overly flat cornea, or an anomaly in the lens. Images focus behind the retina, creating defective vision at short distances. The eyes lens naturally corrects moderate hyperopia, although it may still cause headache. In children, farsightedness can cause strabismus and may lead to a decrease in visual acuity.
Glasses are lenses attached to a frame, designed to correct a vision defect or to protect the eyes. The lenses may be concave for myopia, convex to treat hyperopia and presbyopia, or aspherical (nonspherical) with a curve adjusted to improve the focusing of rays in the eye and to correct astigmatism. Some lenses may have multiple curves (bifocal, trifocal, and progressive lenses). Often prescribed for presbyopic patients, these lenses are composed of different areas with different optical features. One area, often towards the bottom of the lens, is designed to improve near vision, while another, towards the top, is assigned to far vision. Wearing tinted glasses is recommended for diseases like keratitis and albinism, but also for everyday life, when in the presence of bright light.
Contact lenses, or corneal lenses, are optical prostheses placed directly on the cornea to improve vision. They may be hard or soft and can easily correct myopia and hyperopia, but are less helpful for astigmatism and presbyopia. Compared with glasses, contact lenses provide the advantage of covering the entire field of vision and of being less visible. On the other hand, contact lenses require adaptation and may result in intolerance (dry eye, conjunctivitis, keratitis, etc.). Strict, regular care must be followed for contact lenses, to prevent the risk of infection.
Myopy, mild hyperopia, and astigmatisms can be treated by corneal surgery, which involves reshaping the curve of the cornea. Corneal surgery uses a number of techniques, including photorefractive keratectomy (PRK) and LASIK. Photorefractive keratectomy is the microscopic abrasion of the cornea using a laser beam. LASIK, which also uses laser beams, abrades the cornea deeper inside. This surgery first requires the removal of the superficial layer of the cornea, which is put back in place at the end of the operation. In this way, LASIK can correct stronger forms of ametropia than photorefractive keratectomy. These simple, relatively nontraumatic corneal operations are now commonplace in industrialized countries.