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The eye’s surface is convex, and light rays that hit it bend toward its center. In an eye that has a normally curved cornea and the correct shape, an image focuses exactly on the retina.
When the eyeball is too long, light rays focus in front of, rather than on, the retina. Under these circumstances, near objects are perceived clearly but distant objects are not.
When the eyeball is too short, light rays entering the eye focus behind the retina. Distant objects are seen clearly but near objects are not.
Vision becomes distorted when the surface of the cornea has an uneven curvature; sometimes, it is the eye’s lens that is irregularly shaped. This type of irregularity causes light to focus on more than one spot in the back of the eye, causing blurred vision.
Over time, the eye’s lens gradually loses its elasticity and its ability to change shape to see close objects. Bifocals or reading glasses are the traditional prescription for remedying this presbyopic loss of accommodation, but recent technology makes it possible to exchange the inflexible lens for one designed to compensate for changes in the eye and improve functional vision at all distances.
Like the lens of a camera, the eye’s lens focuses to keep the images of both close and distant objects clear. Over time, the lens becomes less transparent; studies suggest accumulated exposure to ultraviolet light causes the natural lens to cloud. Most often, this clouding takes place slowly as proteins within the lens degenerate.
The eye has about one million tiny nerve fibers that carry visual information from the back of the eye to the brain. Glaucoma destroys these nerve fibers. It was once thought that the destruction of these fibers was due to high pressure within the eye, but we now know that even patients with normal eye pressure can have glaucoma and experience loss of this important nerve function.
Age-related macular degeneration (AMD) affects the area of the retina called the macula. This small area is responsible for producing sharp, central vision required for "straight ahead" activities such as driving, reading, recognizing faces, and performing close-up work.
While scientists are uncertain of its cause, AMD destroys cells in the area of the macula in two different forms, "dry" and "wet." Dry AMD can advance so slowly that people hardly notice it, or it can rapidly progress to the "wet" type with vision loss in one or both eyes.
Diabetes affects the blood vessels throughout the body, particularly in the kidney and in the eye. Diabetic retinopathy is the name we give to diabetes’ adverse affects on the blood vessels in the eye. In the United States, diabetic retinopathy is the leading cause of blindness among adults. Risk of developing diabetic retinopathy increases over time. An adult who has had diabetes for 15 years or longer stands an 80 percent chance of experiencing damage to retinal blood vessels.