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 5-10 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 (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). 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. 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 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. 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. 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 risking 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 changes in temperature or perspiration.

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. Developed during the last two decades, there are three types of corrective surgeries available as of 1998: 1) radial keratotomy, 2) photorefractive keratectomy, and 3) laser-assisted in-situ keratomileusis, 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 light rays focus properly on the retina. These procedures are performed on an out-patient basis and generally take 10-30 minutes.

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 non-viewing (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 (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 1998, only two eximer 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 (LASIK) is the newest of these procedures and, as of early 1998, is still under clinical investigation. 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.

All of these surgical procedures carry risks, the most serious being corneal scarring, corneal rupture, infection, cataracts, and loss of vision. 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, 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 you gain 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 treatment

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) bring vision to 20/20. Refractive surgery can make permanent improvements for the right myopic candidate.

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).

Try to prevent eye strain 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.

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 which 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. As of early 1998, the eximer laser is not approved by the FDA for this use.

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 (M.D or D.O.) 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

A doctor of optometry (O.D.) is trained and licensed to examine and test the eyes for disease and to treat visual disorders by prescribing corrective lenses and/or vision therapy. In many states, they are licensed to use diagnostic and therapeutic drugs, and if so, they can treat certain ocular diseases.

Orthokeratology

A method of reshaping the cornea using a contact lenses. 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 a laser to make modifications to the cornea and permanently correct myopia. As of early 1998, only two lasers have been approved by the FDA for this purpose.

Radial keratotomy (RK)

A surgical procedure involving the use of a diamond-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.

Further Reading

For Your Information

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

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

Gale Encyclopedia of Medicine. Gale Research, 1999.