FIGURE. Errors of refraction compared with the normal eye. Corrective lenses are shown for nearsightedness and farsightedness. (Adapted from Scanlon, VC, and Sanders, T: Essentials of Anatomy and Physiology, ed 5. FA Davis, Philadelphia, 2007, p 208, with permission.)
Refractive errors are defects in visual acuity resulting from the inability of the eye to effectively focus light on the surface of the retina. Four common refractive errors follow.
- Hyperopia. This condition occurs when light entering the eye comes to a focus behind the retina so that vision is better for distant objects. For this reason, the condition is commonly called farsightedness, which causes difficulties in seeing objects that are close. Hyperopia often results when the globe of the eye is abnormally short in length from front to back.
- Presbyopia. This refractive error is a form of farsightedness that causes the eye to lose its ability to focus. Unlike hyperopia, however, presbyopia results from a loss of elasticity in the crystalline lens of the eye. When the eye focuses on a distant object, muscles encircling the lens contract, stretch, or flatten it. When the eye focuses on a nearby object, the muscles relax, allowing the lens to resume a more spherical shape. In presbyopia, however, the lens remains in a comparatively flattened position after the muscles have relaxed. This condition is a consequence of advancing age.
- Myopia. This condition occurs when light entering the eye comes to a focus in front of the retina so that vision is better for nearby objects. Consequently, the condition is commonly called nearsightedness. Myopia is a condition in which close objects are seen clearly, but objects farther away appear blurred. Myopia often results when the eyeball is abnormally long from front to back.
- Astigmatism. This refractive error occurs when light entering the eye is focused unevenly or diffusely across the retina so that some of the visual field appears properly focused while some does not. The condition is caused by variations in the curvature over certain portions of the lens or cornea of the eye, creating out-of-focus vision.
Except for presbyopia, which is a consequence of advancing age, it is not known what causes some individuals to develop visual defects, whereas others do not. Some types of refractive errors, however, show a strong familial pattern, suggesting a genetic predisposition to acquiring them. For example, when both parents have astigmatism, their offspring will also have astigmatism.
Signs and Symptoms
In addition to the characteristic visual deficits described, general symptoms of refractive errors may include squinting, headaches, and frequent rubbing of the eyes.
The diagnosis of refractive errors usually involves testing for visual acuity using the Snellen eye chart, ophthalmoscopic examination of the interior of the eye, and tests to detect eye muscle function.
Treatment of refractive errors involves the prescription and fitting of corrective lenses in the form of either eyeglasses or contact lenses. An alternative to corrective lenses is laser eye surgery, which permanently changes the shape of the cornea. Laser-assisted in situ keratomileusis (LASIK) can be performed on individuals with varying degrees of nearsightedness, farsightedness, and astigmatism. In the surgery, a microsurgical knife cuts a hinge flap in the cornea that is then lifted out of the way. A laser reshapes underlying corneal tissue. The flap is then replaced and quickly adheres to the eyeball. Other surgical alternatives include the following:
- Photorefractive keratectomy (PRK) that does not create a corneal flap but uses laser to treat myopia, hyperopia, and astigmatism. PRK involves the use of a cool ultraviolet light-beam laser to ablate or remove very tiny bits of tissue from the surface of the cornea in order to reshape it. When the cornea is reshaped, it works better to focus light into the eye and onto the retina, providing clearer vision than before. Both nearsighted and farsighted people can benefit from PRK.
- Astigmatic keratectomy is an incision to reduce certain degrees of astigmatism by smoothing an irregular cornea into a more normal shape.
- Intrastromal corneal rings use clear, thin polymer inlays to correct low myopia only. Two ultrathin arcs are surgically implanted in the peripheral area of the cornea. The arcs flatten the cornea to the degree required to correct the myopic condition. The procedure does not cut or remove any tissue, although the segments can later be removed or replaced to correct for possible sight changes as the eye ages.
The American Academy of Ophthalmology identifies a number of studies regarding the effectiveness of complementary therapy for several eye diseases and disorders. See http://one.aao.org/CE/ PracticeGuidelines/Therapy.aspx for details.
If multifocal lenses are prescribed, clients may need continued encouragement to get used to them. If contacts, either hard or soft, are prescribed, clients need to be fully informed of their care, use, and safety. In the case of laser surgery, clients must understand good postoperative follow-up and the importance of keeping the eye moist.
With corrective lenses and/or surgery, the prognosis is good for persons with refractive errors.
There are no specific preventive measures for any of these refractive errors.