• Refractive errors occur when the focal point of light- waves entering the eye fails to align properly on the RETINA (ASTIGMATISM, nearsightedness, farsightedness).
• Functional limitations result when corrected vision remains insufficient to allow participation in activities or occupations that require sight.
• Legal blindness exists when corrective measures can- not restore VISUAL ACUITY to 20/200 or VISUAL FIELD to greater than 20 degrees.
Health and Disorders of the Eyes
More than 150 million Americans have a VISION IMPAIRMENT that requires CORRECTIVE LENSES (eye- glasses or contact lenses)—30 percent of men and 40 percent of women. About 12 million Americans have uncorrectable vision impairments that result in functional limitations; 10 percent of them meet the criteria for legal blindness. Among those who have uncorrectable vision impairments, 50 percent are age 65 or older. Though the eyes arise directly from the evolving brain very early in fetal development, their formation becomes complete during the final 12 weeks of PREGNANCY. Infants born before 32 weeks of gestation are at risk for RETINOPATHY of prematurity, a leading cause among children of vision impairments ranging from STRA- BISMUS (inability to focus both eyes on the same object) to legal blindness.
Traditions in Medical History
As refractive errors are very common, practitioners throughout history have tried various and sometimes hazardous methods for improving or restoring vision. The earliest documentation of corrective lenses for this purpose dates to 16th China. European traders who traveled to China noted the elderly holding quartz crystal lenses to see objects close to them. Eyeglasses set in frames and worn on the face began to appear in Europe in the 17th and 18th centuries. By the late 19th century inventors were experimenting with glass lenses placed directly on the eye. These attempts produced large, heavy, and ultimately unfeasible lenses that covered the entire surface of the eye. The contact lens finally became a reality in the 1950s with the advent of high-tech plastics that were lightweight, optically clear, and inert (did not react with body fluids). Subsequent advances over the next 30 years brought about lenses made of surgical plastics that allow oxygen to reach the cornea, much improving comfort and safety. By the 1990s, daily wear disposable contact lenses became the standard of contact lens correction.
CATARACT, the clouding and discoloration of the eye’s lens that develops with aging, has for centuries been the leading cause of blindness in adults. It also is one of the earliest documented vision problems for which practitioners used surgical treatments to remedy, perhaps because the cause of the problem, the cloudiness, was so apparent. CATARACT EXTRACTION AND LENS REPLACE- MENT has become so commonplace in contemporary ophthalmology that the procedure is no less an expectation for restoring vision than are eye- glasses for correcting refractive errors. In about 20 minutes, the ophthalmologist removes the clouded lens and replaces it with a synthetic one. Ancient physicians, lacking the benefits of the anesthetics that make the surgery painless for today’s patients, became skilled at “couching” a cataract in only seconds. The procedure required the doctor to distract the patient long enough to puncture the cornea and push the lens out of the line of vision. The lens remained within the eye as though resting, hence the term “couching.” The result was less than perfect because the person lost the refractive ability of the lens, but the procedure restored enough vision to allow one to function in daily life. In the 1950s ophthalmologists began removing the cataract from the eye, but not until the 1970s did technology and technique converge in procedures that incorporated a replacement lens.
Breakthrough Research and Treatment Advances The evolution of knowledge and advances in laser technology are converging to present treatment options that were science fiction a decade ago. New procedures are greatly expanding the potential for permanent correction of disorders and defects of the eye, including refractive disorders, that reduces and may eventually even eliminate the need for corrective lenses. Refined laser techniques such as LASIK allow ophthalmologists to reshape the cornea in precise, microscopic increments. Implantable rings inserted around the edge of the cornea can help flatten and reshape it to alter its refractive ability. Permanent contact lenses attached over the lens can have similar effect. Implantable replacement lenses are expanding beyond their initial application in cataract extraction and replacement to offer nearly ideal vision for people with severe astigmatism or myopia (nearsightedness).
Cataract extraction and lens replacement now routinely restores sight for more than 90 percent of people who otherwise would lose vision to cataracts. Other surgical procedures offer hope for altering the course of glaucoma. New treatments may stem the loss of vision due to AGE-RELATED MACULAR DEGENERATION (ARMD). These conditions are the leading causes of vision impairments that lead to functional limitations or legal blindness among adults. And research continues to explore a “bionic” PROSTHETIC EYE that can convert light- waves to nerve impulses and transmit them to the brain. Such a prosthesis would function similarly to the COCHLEAR IMPLANT used to restore some types of neurosensory HEARING LOSS. Because many of the conditions that result in vision impairment are not preventable, technological innovations such as these appear to be the future of ophthalmologic treatment.