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Refractive Disorders

Emmetropia An emmetropic eye is one that has no refractive error. It will focus on the retina the image of an object viewed from a distance of 20 feet or more (optical infinity).

An ametropic eye is one that has a refractive error, a mismatch between its optical power and axial length. To make up for refractive error, glasses or contact lenses are placed in front of the eye. There are three kinds of refractive errors: myopia, hyperopia, and astigmatism.

Myopia The eye's excessive refractive power causes light rays to focus in front of the retina. Myopia is corrected with a spherical concave lens that moves the focus back to the retina.

Hyperopia The eye's insufficient refractive power causes light rays to focus behind the retina. Hyperopia is corrected with a spherical convex lens that moves the focus forward onto the retina.

Astigmatism The eye's abnormal corneal curvature prevents a point focus on the retina. That is not all optical planes are in focus. The patient sees a blurred image. Astigmatism cannot be corrected with a simple spherical lens, as myopia and hyperopia are. It must be corrected with a cylinder, which has more focusing power in one axis than another.

Presbyopia Describes the loss of the eye's accommodative ability with aging.

Before age 40, the lens is flexible enough so that contraction of the ciliary muscle allows it to assume a convex front surface and focus objects viewed at reading distance.

After age 40, the lens proteins gradually degenerate, the lens stiffens, and will not round up to allow focusing of near objects on the retina. In other words, accommodation is progressively lost, and by age 60 it is gone altogether. This age-related loss of accommodation, or presbyopia, is a form of induced hyperopia for viewing near objects.

To correct presbyopia, one places a convex lens in front of the eye. Because it is used only for seeing near objects, it is prescribed either as a half-glass or as the bottom part of a bifocal if the patient needs a correction for distance viewing.

What to do?
Patients who do not complain of blurred vision usually do not need refractive care. But some people who report discomfort around their eyes or a ghost image when viewing may have refractive errors.

Myopia usually sets in during the first or second decades and progresses until the third decade. Hyperopia sets in at any time and tends to progress very little. Presbyopia starts in mid-life (40's) and progresses until age 60. Astigmatism usually accompanies myopia or hyperopia.

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