Refractive Disorders

In a perfectly normal eye, the cornea and lens contribute to sharp focusing of incoming light rays from distant objects directly on the retina.

The cornea acts as a natural lens and its curvature determines its power. Cornea is responsible for 80% and the lens is responsible for about 20% of the eye’s refractive power. Nerve fibers in the retina transmit this image to the brain, where it is interpreted as vision. This perfect eye system sees 20/20.  If the light rays do not come to a pinpoint focus on the retina, the patient has a refractive error. There are four types of refractive disorders. These are:

Myopia (also known as near-sightedness)

If a person can see objects nearby with no problem, but reading road signs or making out the writing on a bill board is more difficult, he may be near-sighted. Myopia occurs when light rays are focused in front of the retina, causing distant images to appear blurry. Far-objects appear blurry because the light rays are spread apart instead of focused when they strike the retina. Myopia is the result of an eyeball which is too long, or a cornea which is too curved.

Myopia is not a disease, nor does it mean that you have "bad eyes." It simply refers to a variation in the shape of your eyeball. Myopia usually starts in childhood and gets progressively worse through adolescence. It usually stops changing by the late teens, but it can sometimes continue to get worse into the mid-twenties. Corrective concave lenses (“negative” lenses that are thicker at the edge than in the center) or contact lenses are prescribed to focus light more precisely on the retina, where a clear image is formed.

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Hyperopia or Hypermetropia (also known as far-sightedness)

Hyperopia, or far-sightedness, is the opposite of myopia. If a person can see objects at a distance clearly but has trouble focusing well on objects close up, he may be farsighted. It occurs when light rays are focused behind the retina, causing near images to appear blurry. Near-objects appear blurry because the light rays are spread apart instead of focused when they strike the retina. Very far-sighted people have difficulty seeing clearly the distant objects as well. Hyperopia is the result of an eyeball which is too short, or a cornea which is less curved. This creates blurred vision that is more noticeable for near objects than distant ones.

Hyperopia is not a disease, nor does it mean that you have “bad eyes.” It simply means that you have a variation in the shape of your eyeball. Hyperopia may be present in childhood but does not usually become apparent until people are in their late 20’s or 30’s when they can no longer see up close. When these patients get into their 40’s or 50’s, they begin to have difficulty seeing distant objects as well. Corrective convex lenses (“positive” lenses that are thicker in the center than at the edge) or contact lenses are prescribed to focus light more precisely on the retina, where a clear image is formed.

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Astigmatism

Astigmatism is an overall inability of the eye to focus clearly at any distance. It occurs when cornea is shaped more like a football (with two different curvatures) than a well-rounded baseball. Images appear blurred or ghost-like because light rays are refracted unequally. In extreme cases, images both near and far, appear blurred. Many people who have myopia also have some degree of astigmatism. This creates eye strain. Many people live with minor astigmatisms that do not require any type of treatment. Others with more severe astigmatism can have it corrected by wearing glasses or contact lenses.

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Presbyopia

Presbyopia is the inability to focus the eyes at near. 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 myopia as well as hyperopia have presbyopia after age 40.

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Information courtesy of Neo Vision Eye Center