Refractive Eye Problems
Astigmatism
In order for the eye to work properly, light coming into the eye must be properly focused on the retina (or the back of the eye). When the image is not focused, there is an irregularity in the eye. This irregularity can be the overall shape of the eye or the curvature of the cornea (the clear outer covering of the eye), or both. The cornea should be curved equally in all directions. Astigmatism occurs when the cornea is curved more in one direction than another.
Astigmatism is quite common and, in the vast majority of cases, it is due simply to variations between people. Just as different people have different shaped feet or hands, people also have different shaped corneas. Rarely astigmatism is caused by lid swellings such as chalazia, and corneal scars, or by keratoconus (a rare condition in which the cornea becomes misshapen and pointed rather than smooth and rounded).
Astigmatism may cause blurred vision, eye strain or even headaches. It can also cause images to appear doubled, particularly at night. Small amounts of astigmatism can be ignored. But if any of its symptoms are present, astigmatism can be corrected by glasses or contact lenses. In most patients hard contact lenses do a better job of correcting for astigmatism than soft contact lenses.
Hyperopia (Farsightedness)
The cornea and the lens work together to focus images from the visual world on the back of the eye (the retina). If an image is out of focus, it is typically because the overall shape of the eye is incorrect or the cornea does not have the proper curvature. Farsightedness or hyperopia occurs when the eye is too small or the cornea is too flat. When this happens, visual images are focused behind the retina.
A person with hyperopia is able to see objects at a distance, but has trouble with objects up close, like books or newspapers. Many people are not diagnosed with hyperopia without a complete eye exam. School screenings typically do not discover this condition because they test only for distance vision.
Treatment includes contact lenses or glasses which correct for near vision. Corrective lenses should be worn for near tasks, such as reading, but do not need to be used for distance vision tasks, such as driving.
Myopia (Nearsightedness)
The cornea and lens of the eye work together to properly focus visual images on the retina. If an image is out of focus, it is because the overall shape of the eye is incorrect or because the cornea does not have the proper curvature. When the eye is too big or the cornea is too steep, visual images are focused in front of the retina. This condition is called nearsightedness or myopia.
Myopia normally starts to appear between the ages of eight and twelve years old, and almost always before the age of twenty. Once myopia starts, as the body grows, the myopia often increases. It typically stabilizes in adulthood. Changes in glasses or contact lens prescriptions are necessary during growth periods.
Someone with myopia has an inability to see objects at the distance, such as street signs, chalk boards and television. Many times, myopia is diagnosed during school screenings.
The treatment for nearsightedness includes lenses which allow visual images to be focused on the retina. These lenses can be in the form of contact lenses or glasses. Once the eye has stabilized and myopia is no longer progressing, laser vision correction is an option for many.
Presbyopia
During the early and middle years of life, the lens of the eye provides for the capability to focus both near and distant images. To accomplish this feat, the lens changes shape, getting thicker for near objects and thinner for distant objects.
Presbyopia occurs when the lens of the eye is no longer able to change shape. This typically takes place around age forty. Some persons may be older, closer to fifty, and some younger, less than thirty-five, when the lens loses its flexibility. For people who have presbyopia, vision is blurred when looking at near objects, such as during reading. Also, it may become difficult adjusting focus when switching from near to distance vision.
The amount of power that is needed in glasses to correct for presbyopia is dependent on the strength of the glasses needed for distance vision. For persons who are nearsighted, removal of the glasses may make it easier to read up close. For those not nearsighted, glasses or bifocals are needed to see well up close. A complete eye examination will determine the strength of lenses needed to see well at all distances.
