Myopia | Definition | Description | Causes and symptoms | Treatment

Definition

Myopia is the medical term for nearsightedness. People with myopia see objects more clearly when they are close to the eye, while distant objects appear blurred or fuzzy. Reading and close-up work may be clear, but distance vision is less sharply defined.

Description

To understand myopia it is necessary to have a basic knowledge of the main parts of the eye's focusing system: the cornea, the lens, and the retina. The cornea is a tough, transparent, dome-shaped tissue that covers the front of the eye (not to be confused with the white, opaque sclera). The cornea lies in front of the iris (the colored part of the eye). The lens is a transparent, double-convex structure located behind the iris. The retina is a thin membrane that lines the rear of the eyeball. Light-sensitive retinal cells convert incoming light rays into electrical signals that are sent along the optic nerve to the brain, which then interprets the images.

In people with normal vision, parallel light rays enter the eye and are bent by the cornea and lens (a process called refraction) to focus precisely on the retina, providing a crisp, clear image. In a myopic eye, the focusing power of the cornea (the major refracting structure of the eye) and the lens is too great with respect to the length of the eyeball. Light rays are bent too much, and they converge in front of the retina. This inaccuracy is called a refractive error. In other words, an overfocused fuzzy image is sent to the brain.

There are many varieties of myopia. Some common types include:

·         physiologic

·         pathologic

·         acquired

By far the most common form, physiologic myopia develops in children sometime between the ages of five and 10 and gradually progresses until the eye is fully grown. Physiologic myopia may include refractive myopia (the cornea and lens-bending properties are too strong) and axial myopia (the eyeball is too long). Pathologic myopia is a far less common abnormality. This condition begins as physiologic myopia, but rather than stabilizing, the eye continues to enlarge at an abnormal rate (progressive myopia). This more advanced type of myopia may lead to degenerative changes in the eye (degenerative myopia). Acquired myopia occurs after infancy. This condition may be seen in association with uncontrolled diabetes and certain types of cataracts. Antihypertensive drugs and other medications can also affect the refractive power of the lens.

Eyecare professionals have debated the role of genetics in the development of myopia for many years. Most believe that a tendency toward myopia may be inherited, but the actual disorder results from a combination of environmental and genetic factors. Environmental factors include close work, work with computer monitors or other instruments that emit some light (electron microscopes, photographic equipment, lasers, etc.), emotional stress, and eye strain.

A variety of genetic patterns for inheriting myopia have been suggested, ranging from a recessive pattern with complete penetrance in people who are homozygotic for myopia to an autosomal dominant pattern; an autosomal recessive pattern; and various mixtures of these patterns. One explanation for this lack of agreement is that the genetic profile of high myopia (defined as a refractive error greater than −6 diopters) may differ from that of low myopia. Some researchers think that high myopia is determined to a greater extent by genetic factors than low myopia.

Another explanation for disagreement regarding the role of heredity in myopia is the sensitivity of the human eye to very small changes in its anatomical structure. Since even small deviations from normal structure cause significant refractive errors, it may be difficult to single out any specific genetic or environmental factor as their cause.

Genetic markers and gene mapping

Since 1992, genetic markers that may be associated with genes for myopia have been located on human chromosomes 1, 2, 12, and 18.There is some genetic information on the short arm of chromosome 2 in highly myopic people. Genetic information for low myopia appears to be located on the short arm of chromosome 1, but it is not known whether this information governs the structure of the eye itself or vulnerability to environmental factors.

In 1998, a team of American researchers presented evidence that a gene for familial high myopiawith an autosomal dominant transmission pattern could be mapped to human chromosome 18 in eight North American families. The same group also found a second locus for this form of myopia on human chromosome 12 in a large German/Italian family. In 1999, a group of French researchers found no linkage between chromosome 18 among32 French families with familial high myopia. These findings have been taken to indicate that more than one gene is involved in the transmission of the disorder.

Family studies

It has been known for some years that a family history of myopia is one of the most important risk factors for developing the condition. Only 6-15% of children with myopia come from families in which neither parent is myopic. In families with one myopic parent, 23-40% of the children develop myopia. If both parents are myopic, the rate rises to 33-60% for their children. One American study found that children with two myopic parents are 6.42 times as likely to develop myopia themselves as children with only one or no myopic parents. As of 2001, the precise interplay of genetic and environmental factors in these family patterns, however, is not yet known.

One multigenerational study of Chinese families indicated that persons in the third generation had a higher risk of developing myopia even if their parents were not myopic. The researchers concluded that, at least in China, the genetic factors in myopia have remained constant over the past three generations while the environmental factors have intensified. The increase in the percentage of people with myopia over the last 50 years in the United States has led American researchers to the same conclusion.

Myopia is the most common eye disorder in humans around the world. It affects between 25 and 35% of the adult population in the United States and the developed countries, but is thought to affect as much as 40% of the population in some parts of Asia. Some researchers have found slightly higher rates of myopia in women than in men.

There is considerable variation in the age distribution of myopia in the United States. The prevalence of myopia rises among children and adolescents in school until it reaches the 25-35% level in the young adult population. It declines slightly in the over45 age group. Approximately 20% of 65yearolds have myopia. The figure drops to 14%for Americans over 70.

Other factors that affect the demographic distribution of myopia are income level and education. The prevalence of myopia is higher among people with above-average incomes and educational attainments. Myopia is also more prevalent among people whose work requires a great deal of close focusing, including work with computers.

Causes and symptoms

Myopia is said to be caused by an elongation of the eyeball or a cornea that is steeply curved. This means that the oblong (as opposed to normal spherical) shape of the myopic eye causes the cornea and lens to focus at a point in front of the retina. A more precise explanation is that there is an inadequate correlation between the focusing power of the cornea and lens and the length of the eye.

People are generally born with a small amount of hyperopia (farsightedness), but as the eye grows this decreases and myopia does not become evident until later. This change is one reason why some researchers think that myopia is an acquired rather than an inherited trait.

The symptoms of myopia are blurred distance vision, eye discomfort, squinting, and eye strain. Headaches may accompany eyestrain.

Diagnosis

The diagnosis of myopia is typically made during the first several years of elementary school when a teacher notices a child having difficulty seeing the chalkboard, reading, or concentrating. The teacher or school nurse often recommends an eye examination by an ophthalmologist or optometrist. An ophthalmologist is an MD or DO (Doctor of Osteopathy) who is a medical doctor trained in the diagnosis and treatment of eye problems. Ophthalmologists also perform eye surgery. An optometrist (OD) diagnoses, manages, and treats eye and visual disorders. In all states, optometrists are licensed to prescribe diagnostic and therapeutic drugs.

A person's distance vision is tested by reading letters or numbers on a chart posted a set distance away (usually 20 ft [6 m]). The doctor asks the person to view images through a variety of lenses to obtain the best correction. The doctor also examines the inside of the eye and the retina. An instrument called a slit lamp is used to examine the cornea and lens. The eyeglass prescription is written in terms of diopters (D), which measure the degree of refractive error. Mild to moderate myopia usually falls between −1.00D and −6.00D. Normal vision is commonly referred to as 20/20 to describe the eye's focusing ability at a distance of 20 ft (6 m) from an object. For example, 20/50 means that a myopic person must stand 20 ft (6 m) away from an eye chart to see what a normal person can see at 50 ft (15.2 m). The larger the bottom number, the greater the myopia.

Treatment

People with myopia have three main options for treatment: eyeglasses, contact lenses, and, for those who meet certain criteria, refractive eye surgery.

Eyeglasses

Eyeglasses are the most common method used to correct myopia. Concave glass or plastic lenses are placed in frames in front of the eyes. The lenses are ground to the thickness and curvature specified in the eyeglass prescription. The lenses cause the light rays to diverge so that they focus further back, directly on the retina, producing clear distance vision.

Contact lenses

Contact lenses are a second option for treatment. Contact lenses are extremely thin, round discs of plastic that are worn on the eye in front of the cornea. Although there may be some initial discomfort, most people quickly grow accustomed to contact lenses. Hard contact lenses, made from a material called PMMA, are virtually obsolete. Rigid gas permeable lenses (RGP) are made of plastic that holds its shape but allows the passage of oxygen into the eye. Some believe that RGP lenses may halt or slow the progression of myopia because they maintain a constant, gentle pressure that flattens the cornea. As of 2001, the National Eye Institute was conducting an ongoing study of RGP lenses called the Contact Lens and Myopia Progression (CLAMP) Study, with results to be released in 2003.

A procedure called orthokeratology acts on this principle of corneal molding. However, when contact lenses are discontinued for a period of time, the cornea will generally go back to its original shape. Rigid gas permeable lenses offer crisp, clear sight. Soft contact lenses are made of flexible plastic and can be up to 80% water. Soft lenses offer increased comfort and have the advantage of extended wear. Some can be worn continuously for up to one week. While oxygen passes freely through soft lenses, bacterial contamination and other problems can occur, requiring replacement of lenses on a regular basis. It is very important to follow the cleaning and disinfecting regimens prescribed because protein and lipid buildup can occur on the lenses, causing discomfort or increasing the risk of infection. Contact lenses offer several benefits over glasses, including: better vision, less distortion, clear peripheral vision, and cosmetic appeal. In addition, contacts don't steam up from perspiration or changes in temperature.

Refractive eye surgery

For people who find glasses and contact lenses inconvenient or uncomfortable, and who meet selection criteria regarding age, degree of myopia, general health, etc., refractive eye surgery is a third treatment alternative. As of 2001, four types of corrective surgeries are available:

·         radial keratotomy (RK)

·         photorefractive keratectomy (PRK)

·         corneal rings

·         laser-assisted in-situ keratomileusis (LASIK), which is still under clinical evaluation by the Food and Drug Administration (FDA)

Refractive eye surgery improves myopic vision by permanently changing the shape of the cornea so that light rays focus properly on the retina. These procedures are performed on an outpatient basis and generally take 10 to 30 minutes.

RADIAL KERATOTOMY. Radial keratotomy (RK), the first of these procedures made available, has a high associated risk of an unfavorable outcome. It was first developed in Japan and the Soviet Union, and introduced into the United States in 1978. The surgeon uses a delicate diamond-tipped blade, a microscope, and microscopic instruments to make several spoke-like "radial" incisions in the non-viewing (peripheral) portion of the cornea. As the incisions heal, the slits alter the curve of the cornea, making it more flat, which may improve the focus of images onto the retina. With the advent of laser surgeries, this procedure has become almost obsolete.

PHOTOREFRACTIVE KERATECTOMY. Photorefractive keratectomy (PRK)involves the use of a computer to measure the shape of the cornea. Using these measurements, the surgeon applies a computer-controlled laser to make modifications to the cornea. The PRK procedure flattens the cornea by vaporizing small amounts of tissue from the cornea'ssurface. As of early 2001, only two excimer lasers are approved by the FDA for PRK, although other lasers have been used. It is important to make sure the laser being used is FDA approved. Photorefractive keratectomy can be used to treat mild to moderate forms of myopia. The cost is approximately $2,000 per eye.

LASER-ASSISTED IN-SITU KERATOMILEUSIS. Laser-assisted in-situ keratomileusis (LASIK) is the newest of these procedures. It is recommended for moderate to severe cases of myopia. A variation on the PRK method, LASIK uses lasers and a cutting tool called a microkeratome to cut a circular flap on the cornea. The flap is flipped back to expose the inner layers of the cornea. The cornea is treated with a laser to change its shape and focusing properties, then the flap is replaced.

Risks

All of these surgical procedures carry risks, the most serious being corneal scarring, corneal rupture, infection, cataracts, and loss of vision. In addition, a study published in March 2001 warned that mountain climbers who have had LASIK surgery should be aware of possible changes in their vision at high altitudes. The lack of oxygen at high altitudes causes temporary changes in the thickness of the cornea.

Since refractive eye surgery doesn't guarantee 20/20 vision, it is important to have realistic expectations before choosing this treatment. In a 10-year study conducted by the National Eye Institute between 1983 and 1993, over 50% of people with radial keratotomy gained 20/20vision, and 85% passed a driving test (requiring 20/40 vision) after surgery, without glasses or contact lenses. Even if a person gains near perfect vision, however, there are potentially irritating side effects, such as postoperative pain, poor night vision, variation in visual acuity, light sensitivity and glare, and optical distortion. Refractive eye surgeries are considered elective procedures and are rarely covered by insurance plans.

Myopia treatments under research include corneal implants and permanent surgically placed contact lenses.

Alternative treatments

Some eye care professionals recommend treatments to help improve circulation, reduce eye strain, and relax the eye muscles. It is possible that by combining exercises with changes in behavior, the progression of myopia may be slowed or prevented. Alternative treatments include: visual therapy (also referred to as vision training or eye exercises), discontinuing close work, reducing eye strain (taking a rest break during periods of prolonged near vision tasks), and wearing bifocals to decrease the need to accommodate when doing close-up work.

Prognosis

Glasses and contact lenses can (but not always) correct a person'svision to 20/20. Refractive surgery can make permanent improvements for the right candidates.

While the genetic factors that influence the transmission and severity of myopia cannot be changed, some environmental factors can be modified. They include reducing close work, reading and working in good light, taking frequent breaks when working at a computer or microscope for long periods of time, maintaining good nutrition, and practicing visual therapy (when recommended).

Health care team roles

Ophthalmologists and optometrists diagnose myopia. Both may prescribe corrective lenses (glasses or contact lenses). Ophthalmologists perform surgery to correct myopia. Various individuals can fill prescriptions for corrective lenses. This is governed by individual state laws.

Prevention

Eye strain can be prevented by using sufficient light for reading and close work, and by wearing corrective lenses as prescribed. Those with corrective lenses should have regular eye examinations to see if their prescription has changed or if any other problems have developed. This is particularly important for people with high (degenerative) myopia who are at a greater risk of developing retinal detachment, retinal degeneration, glaucoma, or other problems.

KEY TERMS

Accommodation— The ability of the lens to change its focus from distant to near objects. It is achieved through the action of the ciliary muscles that change the shape of the lens.

Cornea— The outer, transparent tissue that covers the front of the eye. The cornea is part of the eye's focusing system.

Diopter (D)— A unit of measure for describing refractive power.

Laser-assisted in-situ keratomileusis (LASIK)— A procedure that uses a cutting tool and a laser to modify the cornea and correct moderate to high levels of myopia.

Lens— The transparent, elastic, curved structure behind the iris (colored part of the eye) that helps focus light on the retina.

Ophthalmologist— A medical doctor (MD or DO) who specializes in the diagnosis and medical and surgical treatment of eye diseases and disorders.

Optic nerve— A bundle of nerve fibers that carries visual messages in the form of electrical signals to the brain.

Optometrist— Doctors of optometry are primary health care professionals who examine, diagnose, treat, and manage diseases and disorders of the visual system, the eye, and associated structures, as well as diagnose related systemic conditions. They prescribe glasses, contact lenses, low vision rehabilitation, vision therapy and medications, as well as perform certain surgical procedures.

Orthokeratology— A method of reshaping the cornea using a contact lens. It is not considered a permanent method to reduce myopia.

Peripheral vision— The ability to see objects and movement to the side, outside of the direct line of vision.

Photorefractive keratectomy (PRK)— A procedure that uses an excimer laser to make modifications to the cornea and permanently correct myopia. As of 2001, two lasers have been approved by the FDA for this purpose.

Radial keratotomy (RK)— A surgical procedure involving the use of adiamond-tipped blade to make several spoke-like slits in the peripheral (non-viewing) portion of the cornea to improve the focus of the eye and correct myopia by flattening the cornea.

Refraction— The bending of light rays as they pass from one medium through another. Used to describe the action of the cornea and lens on light rays as they enter they eye. Also used to describe the determination and measurement of the eye's focusing system by an optometrist or ophthalmologist.

Refractive eye surgery— A general term for surgical procedures that can improve or correct refractive errors by permanently changing the shape of the cornea.

Retina— The light-sensitive membrane that lines the back of the eye. The retinal cells process and send visual signals to the brain through the optic nerve.

Visual acuity— The ability to distinguish details and shapes of objects.

Resources

BOOKS

Grosvenor, Theodore P., David A. Goss, and Henry W. Hoffstetter. Clinical Management of Myopia. Woburn, MA: Butterworth-Heinemann Medical, 1998.

Machat, Jeffrey J., Stephen G. Slade, and Louis E. Probst. The Art of Lasik. 2nd ed. Thorofare, NJ: Slack Inc.,1999.

Olitsky, Scott E., and Leonard B. Nelson. "Disorders of Vision." In Nelson Textbook of Pediatrics, 16th ed. Edited by Richard E. Behrman, et al. Philadelphia: Saunders, 2000.

Ong, Editha, and Kenneth J. Ciuffreda. Accommodation, Nearwork, and Myopia. Santa Ana, CA: Optometric Extension Program Foundation, 1998.

Rosenfield, Mark, and Bernard Gilmartin. Myopia and Nearwork. Woburn, MA: Butterworth-Heinemann Medical, 1998.

PERIODICALS

Chan, C. K., F. C. Lawrence. "Macular Hole After Laser in Situ Keratomileusis and Photorefractive Keratectomy." American Journal of Ophthalmology 131, no. 5 (2001): 666-667.

Marr, J. E., et al. "Associations of High Myopia inChildhood." Eye 15 (2001): 70-74.

Miller, A. E., et al. "Patient Satisfaction After LASIK for Myopia." Contact Lens Association of Ophthalmologists Journal 27, no. 2 (2001): 84-88.

Romano, P. E. "Much Can be Done for Your Child'sMyopia." Optometry and Visual Sciences 78, no. 4(2001): 186-187.

Singh, D. "Is Refractive Surgery Justified?" Journal of the Indian Medical Association 98, no. 12 (2001): 748-767.

Wu, H. M., et al. "Does Education Explain Ethnic Differences in Myopia Prevalence? A Population-based Study of Young Adult Males in Singapore." Optometry and Visual Sciences 78, no. 4 (2001): 234-239.

ORGANIZATIONS

American Academy of Ophthalmology. PO Box 7424, San Francisco, CA 94120. (415) 561-8500.〈http://www.eyenet.org〉.

American Optometric Association. 243 North Lindbergh Blvd., St. Louis, MO 63141. (314) 991-4100.〈http://www.aoanet.org〉.

Optometric Extension Program Foundation, Inc. 1921 E. Carnegie Ave., Ste. 3-L, Santa Ana, CA 92705-5510.(949) 250-8070. 〈http://www.healthy.net/oep〉.

OTHER

American Optometric Association. 〈http://www.aoanet.org/cvc-myopia.html〉.

Harvard Medical School. 〈http://www.med.harvard.edu/publications/On_The_Brain/Volume4/Number3/Myopia.html〉.

Internet Ophthalmology. 〈http://www.ophthal.org〉.

Massachusetts Eye and Ear Infirmary. 〈http://www.meei.harvard.edu/shared/ophtho/ophtho.html〉.

Mayo Clinic. 〈http://www.mayo.edu/ophtharst〉.

Rush University School of Medicine.〈http://www.rush.edu/worldbook/articles/013000a/013000207.html〉.

Stanford University School of Medicine. 〈http://www.med.stanford.edu/school/eye〉.