Myopia, Hyperopia, Astigmatism & Presbyopia
The major role of the lens of the eye is to refract and focus light from both distant and near objects. The lens achieves this by changing its shape. To see at a distance, muscle fibers attached to the lens tighten to flatten the shape of the lens and focus light coming from a distance onto the retina. For close vision, these muscle fibers relax, and the lens reverts to its naturally rounder shape, which focuses light from close objects onto the retina. This process is known as accommodation.
Refractive errors, the most common kind of vision impairment, are caused by a deviation in the way light focuses on the retina, and they affect approximately one third of people 40 years and older in the United States. The four types of refractive errors are nearsightedness (myopia), farsightedness (hyperopia), astigmatism, and presbyopia.
In nearsighted people, either the eyeball is too long or the cornea has too much curvature. Light focuses on a point in front of the retina; as a result, near objects can be seen clearly, but distant ones do not come into proper focus.
In farsighted people, the eyeball is too short or the cornea is too flat, and light focuses on a point behind the retina. Distant objects can be seen clearly, but close ones do not come into proper focus.
Astigmatism occurs when the cornea or lens is slightly irregular in shape. This causes light to focus at different points in the eye. The result is blurred or distorted vision. Astigmatism is common in people with nearsightedness.
Presbyopia occurs when the lens begins to lose its ability to accommodate. With age, the lens gradually becomes thicker and more rigid and is less able to change its shape to bring close objects into focus. Presbyopia affects most adults over age 35.
All four of these conditions can be corrected with eyeglasses or contact lenses. The primary treatment for presbyopia is reading glasses. Someone who is also nearsighted or has astigmatism needs prescription bifocals, trifocals, or contact lenses.
A popular alternative to glasses and contacts, especially for nearsightedness, is laser vision correction (IntraLase, laser-assisted in situ keratomileusis [LASIK], laser-assisted subepithelial keratomileusis [LASEK], or photorefractive keratotomy [PRK]). Laser vision correction has been shown to be safe and effective at middle age and beyond. Additional options like intraocular lenses also are available, and new ones are being investigated. Some surgical treatments are available for presbyopia, but researchers are investigating new ones.