Common Refractive Problems

  Refraction refers to the bending of light rays as they pass through one medium to another, such as from air to water. As the light rays pass through the tissues of the cornea and the lens, they are bent (refracted) in a manner that brings them into focus upon the retina.
  Three components determine an eye's refractive characteristics: the shape of the cornea, the power of the lens, and the length of the eyeball. When these components are in correct proportion to one another, light is focused directly on the retina resulting in clear vision (normal). However, in many eyes, these components are not in the correct proportion to one another, resulting in the refractive problems of myopia (nearsightedness), hyperopia (farsightedness), and astigmatism.
    Another common problem, presbyopia, begins to affect persons over 40 years of age. Presbyopia, or "aging vision," occurs as the natural lens of the eye becomes less pliable, which make focusing on near objects increasingly difficult.


  Myopia, also known as nearsightedness, is the most common of all refractive errors. Over 25% of the population has some degree of nearsightedness. Nearsighted people can see various ranges of near objects clearly but distant objects are out of focus. The most common cause of myopia is from an elongated eyeball. This extra length causes light from distant objects to converge to a focal point before reaching the retina. Beyond the focal point, the light rays diverge. Since the retina only captures the quality of the image that reaches it, the brain receives an image that is out of focus.


  Hyperopia, also known as farsightedness, is a refractive problem caused by an eyeball that is too short or a cornea that is too flat. When the eyeball is too short, the reduced length does not give the cornea and lens sufficient space to bring the light rays to a focal point upon the retina. When the cornea is too flat, it does not bend the light rays from near objects sufficiently to bring them in focus upon the retina.


  Astigmatism is an overall inability of the eye to focus clearly at any distance because of uneven curvatures of the cornea. Instead of having uniform curvatures in all meridians, astigmatic corneas have more curvature in one meridian than the others. For example, the cornea may be more curved horizontally than vertically. Corneas with pronounced astigmatism are shaped more like a football than a well-rounded basketball.
   Every cornea has some degree of astigmatism. Fortunately, for many individuals, the distortion due to uneven curvature is so slight that it has little impact upon vision. However, as the degree of astigmatism increases, the level of distortion increases proportionately. Astigmatism often occurs in conjunction with nearsightedness and farsightedness.


  In young individuals, the natural lens of the eye is soft and pliable. This innate flexibility permits the natural lens to change its shape, allowing it to focus on objects near the eye. As the years pass, the lens loses its flexibility and can no longer vary the focus of the eye. This condition usually becomes noticeable sometime between 40 and 50 years of age. People with normal vision up to that time find it increasingly difficult to focus on near objects, like words on a page or a computer screen, and need to wear glasses for reading and other close up activities.


*Eye Disorders PDFs:

Fuchs' Dystrophy
Recurrent Erosions
Diabetes and the Eye
Macular Degeneration
Dry Eye Syndrome



Eye Disorders Links:

Eye Care Handbook
Galaxy Eye Diseases
Merck Manual Eye Diseases
National Eye Institute
St. Lukes Eye Center
Washington Eye MD


General Medical Links:

Health Central


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Copyright © 2009 Robert C. Arffa, M.D., 1370 Washington Pike, Bridgeville, PA 15017