Myopia is a refractive error (inability to focus clearly) in which people can see near objects clearly but not distant ones, which usually look blurry. It is not an eye disease in the strict sense.
Myopia or nearsightedness is a common refractive error in which a person cannot see distant objects clearly. In most cases the problem is caused by an abnormally long eyeball, a steeply curved cornea, an overly thick lens, or a combination of all three factors. The length of the eyeball or curvature
of the cornea or lens causes images to be focused in front of the retina (the light-sensitive tissue at the back of the eye) rather than on it.
People with myopia not only need prescription lenses for nearsightedness when they are young but may also require bifocals as they age because the eye gradually loses its ability to accommodate (change focus) as a person gets older.
Myopia is a common refractive error worldwide, although the rates vary from country to country and from age group to age group. According to a British medical journal, the prevalence of myopia (including mild cases that do not require the person to wear glasses) has been reported as high as 70–90 percent in East Asia and Japan, 30–40 percent in Europe and the United States, and 10–20 percent in Africa. It is estimated that 44 percent of Japanese and Taiwanese adults have myopia severe enough to require corrective lenses, whereas in India, the rate is 7 percent of the general adult population. A study of British university students showed that 50 percent were myopic, whereas the rate of myopia in Greek university students is 37 percent.
In the United States, the rate of myopia in the general population severe enough to require corrective lenses is thought to be between 20 and 25 percent. Although a few children are born with myopia, the condition is most likely to appear between ages five and twenty. About 25 percent of Americans in this age group are nearsighted. About 26 percent of people in the United States between the ages of twelve and fifty-four have myopia severe enough to require correction.
Ethnicity makes a difference in rates of nearsightedness in the United States. About 78 percent of Asian Americans have myopia, followed by Hispanics (13 percent), African Americans (7 percent), and Caucasians (5 percent). As far as is known, myopia is equally common in men and women.
Myopia is thought to result from a combination of genetic factors and close visual work over an extended period of time in childhood. Myopia is known to run in families. In addition, the different rates of myopia among different races and ethnic groups points to some kind of genetic cause. More recently, the PAX6 gene on chromosome 11 has been identified as a gene that appears to affect the length of the human eyeball.
Seven Centuries of Eyeglasses
Many people think of eyeglasses (or spectacles) as a modern invention when in fact they date back to the late thirteenth century. Salvino d'Armate (1258– 1312), an Italian living in Florence, is credited with inventing the first spectacles around 1284. These early glasses did not have temple bars like present-day models but perched on the bridge of the wearer's nose. The first portrait of a person wearing eyeglasses—a clergyman reading in his library—was painted in France in 1352. The earliest eyeglasses were intended to correct farsightedness. It was not until the fifteenth century that eyeglasses were created for people with myopia.
Eyeglass development increased rapidly after 1604, when Johannes Kepler (1571–1630), a German mathematician and astronomer, published the first explanation of how properly shaped glass lenses can correct farsightedness and nearsightedness. American statesman Benjamin Franklin (1706–1790), who suffered from both farsightedness and nearsightedness in midlife, invented bifocals in 1784 when he grew tired of having to switch between two pairs of eyeglasses. British astronomer George Airy (1801–1892) created the first lenses to correct astigmatism in 1825. The first contact lenses were developed in Germany in 1892, although these early models were large by modern standards and could not be worn comfortably for more than several hours at a time.
Persons with myopia in the twenty-first century have a wide variety of eyeglasses, contact lenses, and surgical treatments available for vision correction. More flexible frames, designer styling, and the use of lighter shatterproof plastic lenses instead of glass make eyeglasses increasingly attractive and comfortable to wear.
Some doctors refer to the development of myopia in the elementary grades as school myopia, as the close work involved in learning to read
and write appears to trigger nearsightedness in about 10 percent of children in the United States.
The symptoms of myopia often emerge during a child's first years in school. Parents may notice that the child holds a book very close while reading or leans close to the desk surface while writing. He or she may squint a lot or sit very close to the television or blackboard. Other symptoms include headaches and failure to notice distant objects.
Myopia and other refractive errors are evaluated by a series of vision tests. After the examiner takes a history of the patient's symptoms (including a family history of eye problems), the patient is usually asked to read the letters on an eye chart known as a Snellen chart. Each eye is tested separately. The examiner may also shine lights into the eyes or administer eye drops that allow him or her to see all the structures inside the eye clearly. This part of the examination allows the doctor to evaluate the severity of the patient's nearsightedness.
To measure the strength of the lens needed to correct the patient's myopia, the examiner uses a device called a photopter (or refractor). The photopter is placed in front of the patient's eyes and the examiner moves various lenses in and out of the device while the patient rereads the letters on the Snellen chart. The photopter can also be used to measure the correction needed for a bifocal lens.
Very mild myopia may not need corrective treatment. A person should see an eye doctor, however, if he or she is developing headaches or eye strain, or if blurry vision is interfering with daily activities.
People whose nearsightedness is severe enough to require correction have several options:
- Eyeglasses. These can be used to correct nearsightedness caused by uneven curvature of the lens or cornea as well as the length of the eyeball. Eyeglasses are prescribed by an optometrist or ophthalmologist but made and fitted by an optician.
- Hard contact lenses. These usually provide more effective correction of nearsightedness than soft contact lenses.
- Orthokeratology (Ortho-K). This is a procedure in which the person wears hard contact lenses for several hours overnight in order to gradually correct the curvature of the cornea. The lenses are removed during the day. Ortho-K is also referred to as corneal molding. It does not permanently improve vision. If the patient stops wearing the retainer lenses, his or her vision may return to its original condition. Ortho-K, however, is ineffective in correcting myopia caused by an abnormally long eyeball.
- Laser surgery. If a person's nearsightedness is related to the shape of the cornea, an ophthalmologist can use lasers to reshape the cornea either by making a flap in the surface of the cornea and reshaping the tissue of the cornea under the flap, or by completely removing the upper layer of tissue in the cornea before reshaping the lower layers of tissue.
- Lens implantation. Lens implantation is a controversial treatment for moderate or severe myopia. The ophthalmologist surgically inserts a clear corrective lens inside the eye in front of the natural lens. The procedure was not performed very frequently in the early 2000s, however, because it has a high risk of complications.
There are drawbacks to surgical correction of refractive errors. These include the risks of infection, development of haze in the cornea, or dry eyes. In some cases the surgeon may need to perform a second operation if the first one either overcorrected or undercorrected the shape of the patient's cornea.
It is important for a patient with myopia to discuss all the treatment options with the optometrist or ophthalmologist, as no two people have exactly the same degree of visual blurring or the same lifestyle. In addition, patients with diabetes require very careful evaluation before any type of eye surgery because diabetes weakens the retina of the eye and increases the risk of glaucoma.
The prognosis of myopia depends partly on its severity. People with any degree of myopia can have their vision corrected satisfactorily by eyeglasses, contact lenses, or surgery. People with severe myopia (about 30 percent of nearsighted patients), however, have an increased risk of retinal disorders and glaucoma after age forty. They should therefore schedule regular eye examinations to reduce the risk of these complications.
Myopia is still largely considered a matter of heredity and cannot be prevented by any method known. There have been various attempts to slow
the progression of nearsightedness in schoolchildren by eye exercises or such alternative therapies as biofeedback, but none have proved to be successful. People can, however, live comfortably with nearsightedness by visual screening in childhood, regular eye checkups at all ages, and wearing corrective lenses when needed.
Nearsightedness is such a common refractive error in the general population that it is not likely to disappear in the near future. It is possible that the present variety of treatment options will be improved or expanded by further research.
SEE ALSO Astigmatism; Glaucoma; Hyperopia
WORDS TO KNOW
Accommodation : The medical term for the eye's ability to change its focus automatically for viewing objects at different distances.
Astigmatism : A refractive error caused by irregularities in the shape of the cornea or the lens of the eye.
Cornea : The transparent front part of the eye where light enters the eye.
Ophthalmologist : A doctor who specializes in diagnosing and treating eye disorders and can perform eye surgery.
Optician : An eye care professional who fills prescriptions for eyeglasses and corrective lenses.
Optometrist : An eye care professional who diagnoses refractive errors and other eye problems and prescribes corrective lenses.
Photopter : A device positioned in front of a patient's eyes during an eye examination that allows the examiner to place various lenses in front of the eyes to determine the strength of corrective lenses required.
Refractive error : A general term for vision problems caused by the eye's inability to focus light correctly.
Retina : The light-sensitive layer of tissue at the back of the eyeball.
Snellen chart : A series of letters arranged in lines on a chart to be viewed from a distance of 20 feet (6 meters). It is used to measure how well someone can see.
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American Optometric Association (AOA). Myopia (Nearsightedness). Available online at http://www.aoa.org/x4688.xml?Play=true (accessed May 31, 2008). The page includes six animations about myopia that have a total playing time of about two and a half minutes.
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Montreal Vision Clinic. EyeMotion: Myopia and Hyperopia. Available online at http://www.eyemotion.com/eyemotion/library/_montrealeyefr.php?src=8 (accessed May 31, 2008). This is a one-minute animation of the refractive errors involved in nearsightedness and farsightedness.
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