Myopia

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 blurry.

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 the 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 types 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 5-10 years 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.

Genetic profile

Eyecare professionals have debated the role of genetics in the development of myopia for many years. Some 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. One explanation for 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 by genetic factors to a greater extent 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.

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 myopia with 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 and 32 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.

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. The precise interplay of genetic and environmental factors in these family patterns, however, is not yet known.

One multigenerational study of Chinese subjects indicated that subjects 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.

The debate continued with more recent reports. In the summer of 2004, one report said that scientists were close to identifying the myopia gene, located on chromosome 11. Another report reviewed several studies and claimed that lifestyle was to blame for myopia. For instance, a study found that 80% of 14- to 18-year old boys studying in schools in Israel that emphasize reading religious texts have myopia, while the rates for boys in state school was just 30%. It is likely that genes and environment play a role.

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.

The age distribution of myopia in the United States varies considerably. Five-year-olds have the lowest rate of myopia (less than 5%) of any age group. The prevalence of myopia rises among children and adolescents in school until it reaches the 25%-35% mark in the young adult population. It declines slightly in the over-45 age group; about 20% of 65-year-olds 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. 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.

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—M.D. or D.O. (Doctor of Osteopathy)—is a medical doctor trained in the diagnosis and treatment of eye problems. Ophthalmologists also perform eye surgery. An optometrist (O.D.) diagnoses and manages and/or treats eye and visual disorders. In many states, optometrists are licensed to use diagnostic and therapeutic drugs.

A patient's distance vision is tested by reading letters or numbers on a chart posted a set distance away (usually 20 ft). The doctor asks the patient 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 from an object. For example, 20/50 means that a myopic person must stand 20 ft away from an eye chart to see what a normal person can see at 50 ft. 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 some 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 is conducting an ongoing study of RGP lenses called the Contact Lens and Myopia Progression (CLAMP) Study, with results to be published 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 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 do not 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. There are three types of corrective surgeries available as of 2001: 1) radial keratotomy (RK), 2) photorefractive keratectomy (PRK), and 3) laser-assisted in-situ keratomileusis (LASIK). 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 10-30 minutes. LASIK is approved by the FDA, though certain lasers are in various stages of approval at a given time. Patients should check with the FDA and ask for patient references when choosing a LASIK provider.

RADIAL KERATOTOMY. Radial keratotomy (RK), the first of these procedures made available, has a high associated risk. 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.

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's surface. 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 the 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 does not 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, more than 50% of people with radial keratotomy gained 20/20 vision, and 85% passed a driving test (requiring 20/40 vision) after surgery, without glasses or contact lenses. Even if the patient 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 not always 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 the patient's vision to 20/20. Refractive surgery can make permanent improvements for the right candidates.

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 transparent structure of the eye over the lens that is continous with the sclera in forming the outermost, protective, layer of the eye.

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

Laser-assisted in-situ keratomileusis (LASIK)— Aprocedure 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 physician specializing in the medical and surgical treatment of eye disorders.

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

Optometrist— A medical professional who examines and tests the eyes for disease and treats visual disorders by prescribing corrective lenses and/or vision therapy. In many states, optometrists are licensed to use diagnostic and therapeutic drugs to treat certain ocular diseases.

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 that are not located directly in front of the eye. Peripheral vision allows people to see objects located on the side or edge of their field of vision.

Photorefractive keratectomy (PRK)— A procedure that uses an excimer laser to make modifications to the cornea and permanently correct myopia.

Radial keratotomy (RK)— A surgical procedure involving the use of a diamond-tipped blade to make several spoke-like slits in the peripheral (nonviewing) 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 layer of tissue in the back of the eye that receives and transmits visual signals to the brain through the optic nerve.

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

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).

Eye strain can be prevented by using sufficient light for reading and close work, and by wearing corrective lenses as prescribed. Everyone 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.

Resources

PERIODICALS

"Blame lifestyle for myopia, not genes." Biotech Week July 28, 2004: 65.

Naiglin. L., et al. "Familial high myopia: evidence of an autosomal dominant mode of inheritance and genetic heterogeneity." Annals of Genetics 42, no. 3 (1999): 140-146.

Pacell, R, et al. "Role of genetic factors in the etiology of juvenile-onset myopia based on a longitudinal study of refractive error." Optometry and Visual Science 76 (June 1999): 381-386.

Saw, SM, et al. "Myopia: gene-environment interaction." Annals of the Academy of Medicine of Singapore 29 (May 2000): 290-297.

"Scientists close to identifying myopia gene." Chemistry and Industry August 16, 2004: 7.

Wu, MM, and MH Edwards. "The effect of having myopic parents: an analysis of myopia in three generations." Optometry and Visual Science 76 (June 1999): 341-342.

ORGANIZATIONS

American Academy of Ophthalmology. PO Box 7424, San Francisco, CA 94120-7424. (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〉.

International Myopia Prevention Association. RD No. 5, Box 171, Ligonier, PA 15658. (412) 238-2101.

Myopia International Research Foundation. 1265 Broadway, Room 608, New York, NY 10001. (212) 684-2777.

National Eye Institute. Bldg. 31 Rm 6A32, 31 Center Dr., MSC 2510, Bethesda, MD 20892-2510. (301) 496-5248. 2020@nei.nih.gov. 〈http://www.nei.nih.gov〉.

Myopia

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 myopia with 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〉.

Myopia

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 blurry.

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 the 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 types 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 years 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.

Genetic profile

Eye care professionals have debated the role of genetics in the development of myopia for many years. Some 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 by genetic factors to a greater extent 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 myopia with 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 and 32 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 six times as likely to develop myopia themselves as children with only one or no myopic parents. The precise interplay of genetic and environmental factors in these family patterns, however, is not yet known.

One multigenerational study of Chinese patients indicated that third generation family members 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.

Demographics

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.

The age distribution of myopia in the United States varies considerably. Five-year-olds have the lowest rate of myopia (less than 5%) of any age group. The prevalence of myopia rises among children and adolescents in school until it reaches the 25%-35% mark in the young adult population. It declines slightly in the over-45 age group; about 20% of 65-year-olds 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.

Signs and symptoms

Myopia is said to be caused by an elongation of the eyeball. 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.

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—M.D. or D.O. (Doctor of Osteopathy)—is a medical doctor trained in the diagnosis and treatment of eye problems. Ophthalmologists also perform eye surgery. An optometrist (O.D.) diagnoses, manages and/or treats eye and visual disorders. In many states, optometrists are licensed to use diagnostic and therapeutic drugs.

A patient's distance vision is tested by reading letters or numbers on a chart posted a set distance away (usually 20 ft). The doctor asks the patient 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 from an object. For example, 20/50 means that a myopic person must stand 20 ft away from an eye chart to see what a normal person can see at 50 ft. The larger the bottom number, the greater the myopia.

Treatment and management

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 some 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 is conducting an ongoing study of RGP lenses called the Contact Lens and Myopia Progression (CLAMP) Study, with results to be published 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.

Soft contact lenses are made of flexible plastic and can be up to 80% water. Soft lenses offer increased comfort and 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 will not fog 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. There are three types of corrective surgeries available as of 2001: 1) radial keratotomy (RK), 2) photorefractive keratectomy (PRK), and 3) 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 10-30 minutes.

RADIAL KERATOTOMY Radial keratotomy (RK), the first of these procedures made available, has a high associated risk. It was first developed in Japan and the Soviet Union, and was 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.

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's surface. 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 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 the 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 warns 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 does not 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 20/20 vision, and 85% passed a driving test (requiring 20/40 vision) after surgery, without glasses or contact lenses. Even if the patient 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 the patient's vision 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).

Eye strain can be prevented by using sufficient light for reading and close work, and by wearing corrective lenses as prescribed. Everyone 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.

Resources

BOOKS

Birnbaum, Martin H. Optometric Management of Nearpoint Vision Disorders. Boston: Butterworth-Heinemann, 1993.

Curtin, Brian J. The Myopias: Basic Science and Clinical Management. Philadelphia: Harper & Row, 1985.

Rosanes-Berrett, Marilyn B. Do You Really Need Eyeglasses? Barrytown, NY: Station Hill Press, 1990.

Zinn, Walter J., and Herbert Solomon. Complete Guide to Eyecare, Eyeglasses, and Contact Lenses. Hollywood, FL: Lifetime Books, 1996.

PERIODICALS

Edwards, M. H. "Effect of parental myopia on the development of myopia in Hong Kong Chinese." Ophthalmic Physiologic Optometry 18 (November 1998): 477-483.

Naiglin, L., et al. "Familial high myopia: evidence of an autosomal dominant mode of inheritance and genetic heterogeneity." Annals of Genetics 42, no. 3 (1999): 140-146.

"Nine Ways to Look Better: If You Want to Improve Your Vision–Or Just Protect What You Have–Try These Eye Opening Moves." Men's Health 13 (January-February 1998): 50.

Pacella, R., et al. "Role of genetic factors in the etiology of juvenile-onset myopia based on a longitudinal study of refractive error." Optometry and Visual Science 76 (June 1999): 381-386.

Saw, S. M., et al. "Myopia: Gene-environment interaction." Annals of the Academy of Medicine of Singapore 29 (May 2000): 290-297.

Wu, M. M., and M. H. Edwards. "The effect of having myopic parents: An analysis of myopia in three generations." Optometry and Visual Science 76 (June 1999): 341-342.

Young, T. L., et al. "Evidence that a locus for familial high myopia maps to chromosome 18p." American Journal of Human Genetics 63 (July 1998): 109-119.

Young, T. L., et al. "A second locus for familial high myopia maps to chromosome 12q." American Journal of Human Genetics 63 (November 1998): 1419-1424.

ORGANIZATIONS

American Academy of Ophthalmology. PO Box 7424, San Francisco, CA 94120-7424. (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>.

International Myopia Prevention Association. RD No. 5, Box 171, Ligonier, PA 15658. (412) 238-2101.

Myopia International Research Foundation. 1265 Broadway, Room 608, New York, NY 10001. (212) 684-2777.

National Eye Institute. Bldg. 31 Rm 6A32, 31 Center Dr., MSC 2510, Bethesda, MD 20892-2510. (301) 496-5248. 2020@nei.nih.gov. <http://www.nei.nih.gov>.

Rebecca J. Frey, PhD

Risa Palley Flynn

Myopia

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 blurry.

Description

Myopia affects about 30% of the population in the United States. To understand myopia it is necessary to have a basic knowledge of the main components involved in the eye's focusing system: the cornea, lens, and 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 the 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 results in what is called a refractive error. In other words, an overly focused, fuzzy image is sent to the brain.

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

• physiologic
• pathologic
• acquired

By far the most common, physiologic myopia develops sometime between the ages of five to 10 years and gradually progresses until the eye is fully grown. This may include refractive myopia (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, or 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.

Causes & symptoms

Myopia is said to be caused by an elongation of the eyeball. 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.

Myopia is considered to be primarily a hereditary disorder, meaning that it runs in families. 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. Because of this, it is sometimes argued that myopia is not inherited, but acquired. Some eyecare professionals 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, stress , and eye strain.

The symptoms of myopia are blurred distance vision, eye discomfort, squinting, and eye strain.

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 exam by an ophthalmologist or optometrist. An ophthalmologist—M.D. or D.O. (Doctor of Osteopathy)—is a medical doctor trained in the diagnosis and treatment of eye problems. Ophthalmologists also perform eye surgery. An optometrist (O.D.) diagnoses and manages and/or treats eye and visual disorders. In many states, optometrists are licensed to use diagnostic and therapeutic drugs.

A patient's distance vision is tested by reading letters or numbers on a chart posted a set distance away (usually 20 ft, or 6 m). The doctor has the patient 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 20 ft away from an object. For example, 20/50 means that a myopic person must be 20 ft away from an eye chart to see what a normal person can see at 50 ft (15 m). The larger the bottom number, the greater the myopia.

Treatment

Nutritional therapy

The following nutritional supplements may help improve vision:

• Vitamin A: essential vitamin for healthy eyes.
• Bioflavonoids. These plant chemicals can help myopic people see better, especially at night
• Zinc: may improve night vision
• Ginkgo extract: increases blood supply to the eye. It may help prevent deterioration in vision.

Eye exercises

Some eye care professionals recommend exercises to help improve circulation, reduce eye strain, and relax the eye muscles. The Bates method is a common set of exercises. 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.

Acupuncture

Acupuncture , by acting on eye muscles, causes changes in the shape of the eyeball and thus, may be able to correct nearsightedness. Approximately 10 sessions followed by daily eye exercises are needed to see significant and prolonged results.

Allopathic 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 diverge

the light rays so they focus further back, directly upon 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. 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 changes in temperature or perspiration.

Refractive eye surgery

Recommended for people who find glasses and contact lenses inconvenient and uncomfortable, refractive eye surgery improves myopic vision by permanently changing the shape of the cornea so light rays focus properly on the retina. These procedures are performed on an outpatient basis and generally take 10-30 minutes. There are three types of corrective surgeries available as of 1998: (1) radial keratotomy, (2) photorefractive keratectomy, and (3) laser-assisted in-situ keratomileusis (LASIK). Each of these surgery techniques changes rapidly in price and effectiveness. Patients should investigate the procedures and ask many questions of their doctors or others who have had the procedures before having them done.

RADIAL KERATOTOMY. Radial keratotomy (RK), the first of these procedures made available, is considered the riskiest. The surgeon uses a delicate diamond-tipped blade, a microscope, and microscopic instruments to make several spoke-like, "radial" incisions in the nonviewing (peripheral) portion of the cornea. The slits surgically alter the curve of the cornea, making it flatter, which may improve the focus of images onto the retina.

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's surface. 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. Laserassisted 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 form 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 the shape and focusing properties, then the flap is replaced.

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

Expected results

Glasses and contact lenses can (but not always) bring vision to 20/20. Refractive surgery can make permanent improvements for the right myopic candidate. Ophthalmologists continue to improve upon and develop new techniques to correct myopia. Alternative treatments have not been widely studied.

Prevention

Myopia is generally considered a hereditary condition, which means that it runs in families. From this perspective there is nothing that can be done to prevent this disorder. However, because the percentage of people with myopia in the United States has steadily increased over the last 50 years, some believe that the condition results from a combination of genetic and environmental factors. If this is true, then it may be possible to prevent or control myopia by reducing close work, reading and working in good light, maintaining good nutrition , and practicing visual therapy (when recommended). In fact, a 2002 study showed that children's diets high in starches may be adding to the high prevalence of myopia. Diets high in refined starches from breads and cereals increase insulin levels, which in turn affect development of the eyeball. Increasing protein consumption has been shown to slow the progression of myopia in children.

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

Resources

BOOKS

Birnbaum, Martin H. Optometric Management of Nearpoint Vision Disorders. Boston: Butterworth-Heinemann, 1993.

Curtin, Brian J. The Myopias: Basic Science and Clinical Management. Philadelphia: Harper & Row, 1985.

Rosanes-Berrett, Marilyn B. Do You Really Need Eyeglasses? Barrytown, NY: Station Hill Press, 1990.

The Burton Goldberg Group. "Vision Disorders." Alternative Medicine: The Definitive Guide. Tiburon, CA: Future Medicine Publishing, Inc., 1999.

"Vision Disorders." Reader's Digest Guide to Medical Cures & Treatments. Canada: The Reader's Digest Association, Inc., 1996.

Zinn, Walter J., and Herbert Solomon. Complete Guide to Eye-care, Eyeglasses, and Contact Lenses. Hollywood, FL: Lifetime Books, 1996.

PERIODICALS

Carey, Benedict. "Goodbye Glasses: New Surgery Can Deliver Sharp Vision to the Nearsighted-Without a Single Cut of the Scalpel (Photorefractive Keratotomy)." Health 10 (September 1996): 46.

"Catching Your Eye (Photorefractive Keratotomy Evaluation.)" People's Medical Society Newsletter 15 (August 1996): 6.

Fox, Douglas." Blinded by Bread: Are Diets High in Starch Making Kids Short-Sighted?." New Scientist (April 6, 2002):9–11.

"Insight on Eyesight: Seven Vision Myths: Blind Spots About Vision Can Cause Needless Worry, Wasted Effort, and Unnecessary Treatment." Consumer Reports on Health 9 (April 1997): 42.

"9 Ways to Look Better: If You Want to Improve Your Vision—Or Just Protect What You Have—Try These Eye Opening Moves." Men's Health 13 (Jan.-Feb. 1998): 50.

Schwartz, Leslie. "Visionquest (Use of Lasers in Treatment of Nearsightedness or Myopia)." Shape 16 (March 1997): 28.

ORGANIZATIONS

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

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

Myopia International Research Foundation. 1265 Broadway, Room 608, New York, NY 10001. (212) 684-2777.

National Eye Institute. NIH Bldg. 31, 9000 Rockville Pike, Bethesda, MD 20892. (301) 496-5248. http://www.nei.nih.gov.

Mai Tran

Teresa G. Odle

Myopia

Definition

Myopia, or nearsightedness, is a condition in which objects in the distance are blurred either because the eye is too long or too strong. It is the result of both environmental and genetic factors.

Description

The degree to which one is myopic depends on the powers of the cornea and the lens of the eye and the length of the eyeball. In a normal eye the incoming visual images meet on the retina in the back of the eye. If these visual images converge in front of the retina instead of on the retina, then one is myopic.

There are several types of myopia, of which simple myopia is the most common. Individuals with simple myopia have eyes that are either too long or too powerful. Congenital myopia develops in infants. Individuals with high myopia, greater than six diopters, can develop pathological changes in the retina, called degenerative myopia. Nocturnal myopia, another type of myopia sometimes referred to as "night blindness," is blurred vision only in darkness. Myopia can also be induced by co-existing medical problems and drug exposure.

A child's refractive status or power of the eye when he or she begins school is a good indicator of whether the child will become nearsighted. Most children are hyperopic, or far-sighted, at birth and experience a decrease in far-sightedness throughout early childhood. Myopia is less likely to develop by age 13 if a child still has at least 0.75 diopters of hyperopia at age eight. But if a child has become at least 0.25 diopters myopic at this age then there is a 60 percent chance that the child will require spectacle correction for nearsightedness by age 13.

Myopia does decreases in later life. This appears not to be related to a decrease in close work as is often suggested but rather due to some factor intrinsically related to ageing. It has been hypothesized the power of the lens of the eye changes in later life.

High myopia has been associated with various syndromes: Ehlers-Danlos, Marfan, Down, and Stickler syndromes. Myopia is often observed in retinopathy of prematurity (ROP). ROP is seen in 68 percent of infants with low birth weights and over 80 percent of infants born with ROP will be myopic. The myopia associated with ROP increases through age five, after which it stabilizes.

Transmission

Although no gene for myopia has been isolated, heredity is believed to play a role in myopia. If both parents are myopic, then the odds that the child will be myopic are as high as 60 percent. This drops to at most 40 percent when only one parent is nearsighted, and for 15 percent of myopic children, neither parent has myopia. High myopia is especially likely to have a genetic component.

Demographics

Overall, 25 percent of those living in the United States are myopic. Myopia is slightly more prevalent among females than males, and among those with advanced academic training. Less than 5 percent of five year olds are myopic, but this percentage increases to 25 percent by late teens and to approximately 35 percent for young adults and to 42 percent of those middle-aged. These percentages decrease to 20 percent by age 65 and to less than 13 percent by age 80.

Myopia is more prevalent in Asian countries; as much as 70 percent of the Chinese population is nearsighted.

Causes and symptoms

Congenital myopia develops because of an obstruction along the visual pathway such as cataract. The eye becomes elongated in response to blur these causes, creating a myopic eye.

A first sign that a child might have myopia is difficulty in seeing things in the distance, such as the chalkboard. The child may not see things in the distance as well as a classmate or sibling.

For the 2 percent of the population who are extremely nearsighted, an inherently weak sclera, whose fibers are not held together tightly, causes the eye to stretch. This stretching can continue into adulthood, increasing myopia.

Other causes of increasing nearsightedness include difficulty with converging, the process through which the eyes move inwardly together when reading, and esophoria, the condition in which the eyes are more comfortable positioned close to the nose. Doing a lot of close work, such as playing video games and using the computer for extended periods, may increase myopia for these children.

Causes of induced myopia include cataracts and elevations of blood sugar in diagnosed or undiagnosed diabetics. Some drugs, such as corticosteroids, antihistamines , and some antibiotics , including sulfonamides , can induce myopia.

Another cause of increasing myopia is the over wearing of contact lenses . Swelling of the cornea can occur if the eye does not have sufficient oxygen causing a transient increase in myopia. Silicone contact lenses allow a marked increase in oxygen to reach the eye decreasing the probability of myopic increases.

For the child with diabetes, fluctuations in blood sugars can cause swelling of the cornea of the eye, leading to transient increases in myopia, which stabilize once the diabetes is controlled. But the child may independently become more myopic later in life.

In the early 2000s, it has been suggested that insulin resistance, which accompanies type 2 diabetes and prediabetes, may increase myopia in children and adolescents. The level of insulin-like growth factor binding protein 3 (IGFBP-3), a hormone that works with insulin to lower blood glucose levels, is low in individuals who are insulin resistant. This decreased level, in turn, decreases the sensitivity of ocular tissue to another compound called retinoic acid, which prevents increases in the length of the eye. Thus, if insulin levels are higher than normal, the risk of myopia may be increased.

Asthenopia or a feeling of eye strain is not common in myopia. If a child complains of eye strain, then usually there is another cause of the eye strain, including an astigmatism, a condition in which the eye is football shaped; anisometropia, a condition in which the eyes are of different powers; or difficulty with focusing.

When to call the doctor

There are many reasons why a child cannot see well in the distance. Myopia is the most common cause of distance blur, and since much of what a child learns comes from vision and visual cues, correction of myopia is important. Also, problems with vision may be a sign of a more serious ocular problem, such as cataracts, or of a medical problem, such as diabetes.

Diagnosis

Myopia is diagnosed by determining a child's unaided vision and is confirmed objectively by the eye care practitioner with various techniques, including retinoscopy and refraction.

The type and extent of myopia is determined by additional testing. These tests include an evaluation of the child's binocular vision, his eye movements, his ability to converge and focus on objects close-up, and his ocular health. Dilation of the eye allows the doctor to check for complications of ROP, diabetes, or degenerative myopia. Since children are capable of over focusing, dilation can help the eye care practitioner determine a child's true prescription because the drugs used to dilate also impair this tendency to over focus. Over focusing can cause a child to appear to be myopic when he or she is actually not.

Treatment

Myopia is most commonly treated with spectacles or glasses. Myopia in preschool children does not need to be corrected with glasses, unless either anisometropia, a condition in which there is a difference of more than 1.00 diopters between the two eyes, or amblyopia , a condition in which a child cannot be corrected to 20/20 with spectacles, is present. As the child enters school, distance vision becomes critical for learning, and children with prescriptions of at least 1.00 diopter of myopia or who have 20/40 vision or worse should be given glasses. Once a child is diagnosed with myopia, he or she should be examined every six months to a year, and each eye should be corrected to 20/20 at each visit. The glasses are then usually worn full time, except for children with difficulty with convergence (esophoria), who may remove their glasses for close work.

Some adolescents may want to wear contact lenses. Wearing contact lenses can improve appearance. Peripheral vision is improved with contact lenses, especially for those with high degrees of myopia or who have anisometropia.

Rigid gas permeable (RGPs) contact lenses are used to correct myopia. The rate at which myopia increases may be slowed by RGP lenses. RGPs are also employed in orthokeratology, a technique in which RGP lenses of gradually decreasing flatness are worn for specified amounts of time. These lenses, called ortho-K lenses, flatten the cornea, changing the power of the cornea over time and decreasing myopia. This effect of ortho-K lenses is not permanent and an ortho-K lens must be worn periodically or the original myopia reappears. For some individuals—for example, those with keratoconnus, a disease of the cornea—RGPs may offer the only way to correct vision.

For some children the development of myopia may be slowed with reading glasses or bifocals. If bifocals are prescribed, then either progressive or no-lines, or a lined bifocal may be given. If a lined bifocal is prescribed, then the line is always placed higher for the child than for the adult. This is done to encourage use of the power of the bifocal.

Refractive surgery is also used to correct myopia, but only on fully grown individuals. A child's eyes change and the safety of these procedures have not been established in the growing eye.

The most common surgical procedure performed to correct myopia is laser in situ keratomileusis (LASIK). Other techniques to correct myopia include photorefractive keratectomy (PRK), radial keratomy (RK), laser epithelial keratomileusis (LASEK), intraocular lens implants and intrastromal corneal rings. Inflammation of the eye, increased dryness of the eye, and cataracts are some of the risks associated with refractive surgery.

Alternative treatment

Cycloplegic drugs, such as atropine, may decrease myopia, but they may hinder the child's ability to see up close. As of 2004 pirenzepine, which has shown to decrease the rate of myopia in children without sacrificing the ability of the child to do close work, is in clinical trial.

Prognosis

Most infants are born far-sighted and eventually reach emmetropia or normal vision, by age six. Over one third of children go on to become myopic as adults.

Patients with high myopia, greater than 6.00 diopters, have an increased risk of developing a retinal tear, hole, or detachment; a posterior staphyloma; a posterior vitreous detachment; or glaucoma. Rarely are these pathological changes of myopia seen in children or adolescents. Retinal detachments and tears are possible, however, in highly myopic children or adolescents who play contact sports . If a retinal problem is diagnosed or suspected, referral to a retinal specialist is necessary.

Prevention

For individuals who have difficulty with convergence or focusing or who are esophoric, close work may increase myopia. Children diagnosed with these problems would benefit from frequent breaks while doing close work. Increases in myopia for these children may be slowed with bifocals and/or removal of glasses for reading and homework.

Nutritional concerns

Since elevated levels of insulin may be associated with increased myopia; a diet low in those foods that increase insulin secretion, such as refined carbohydrates, may help decrease myopia.

Parental concerns

Children rely on their vision in their learning processes; if they have difficulty seeing this handicap affects academic performance. Thus, any vision problem should be corrected promptly. Once myopia is diagnosed, it typically increases through childhood and vision correction is needed for classroom work and for sports. If a child is not corrected fully and continues to have blurred vision, the eye may elongate in response to blur, perpetuating the myopia.

Myopia cannot be diagnosed by school screenings or by simply reading eye chart at the pediatrician's office. A comprehensive eye exam as given by an ophthalmologist or an optometrist is needed, if myopia is suspected. Most cases of myopia result from changes within the eye, but the condition can be a manifestation of other more serious problems, such as cataract or diabetes.

KEY TERMS

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

Anisometropia —An eye condition in which there is an inequality of vision between the two eyes. There may be unequal amounts of nearsightedness, farsightedness, or astigmatism, so that one eye will be in focus while the other will not.

Astigmatism —An eye condition in which the cornea doesn't focus light properly on the retina, resulting in a blurred image.

Cataract —A condition in which the lens of the eye turns cloudy and interferes with vision.

Convergence —The natural movement of the eyes inward to view objects close-up.

Cornea —The clear, dome-shaped outer covering of the eye that lies in front of the iris and pupil. The cornea lets light into the eye.

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

Emmetropia —Normal vision.

Retina —The inner, light-sensitive layer of the eye containing rods and cones. The retina transforms the image it receives into electrical signals that are sent to the brain via the optic nerve.

Sclera —The tough, fibrous, white outer protective covering of the eyeball.

See also Diabetes mellitus; Ehlers-Danlos syndrome; Marfan's syndrome.

Resources

BOOKS

Cordain, Loren. The Paleo Diet. Hoboken, NJ: John Wiley & Sons, 2002.

Murillo-Lopez, Fernando H. "Myopia." In Current Ocular Therapy. Philadelphia: Saunders, 2000.

Prett, Ronald C. "Pathologic Myopia." In Principles and Practice of Ophthalmology. Philadelphia: Saunders, 2000.

Steidl, Scott M., and Ronald C. Pruett. "Myopia and Systemic Disorders." In Principles and Practice of Ophthalmology. Philadelphia: Saunders, 2000.

PERIODICALS

Byrne, Jennifer. "Pirenzepine Showing Promise in Clinical Trials for Topical Myopia Treatment." Primary Care Optometry News 8, no. 10 (October 2003): 10–1.

Cordain, Loren, et al. "An Evolutionary Analysis of the Aetiology and Pathogenesis of Juvenile-Onset Myopia." Acta Ophthalmologica Scandinavica 80, no. 2 (April 2002): 125–35.

Karpecki, Paul M. "What's New in Refractive Surgery." Review of Optometry (May 15, 2001): 71–4.

Karpecki, Paul M., and Marc Bloomenstein. "Phakic IOLs: The LASIK Alternative." Review of Optometry (February 15, 2003): 91–2.

Mutti, Donald O., and Karla Zadnik. "Age-Related Decreases in the Prevalence of Myopia: Longitudinal Change of Cohort Effect?" Investigative Ophthalmology & Visual Science 41, no. 8 (July 2000): 2103–07.

WEB SITES

"Myopia (Nearsightedness)." All About Vision. Available online at <www.allaboutvision.com/conditions/myopia.htm> (accessed November 30, 2004).

Martha Reilly, OD