Causes and Symptoms (Magill’s Medical Guide, Sixth Edition)
The problem of astigmatism is caused by a difference in the focal length of the eye for two perpendicular directions, which can occur if the eyeball becomes slightly deformed, like a grape being squeezed between two fingers. The curvature of the cornea would be flattened in one plane but remain more rounded in the other one. A deformed eye lens can also cause astigmatism.
During an eye examination, the optometrist tests for astigmatism by showing the patient a diagram of straight lines radiating outward from the center of the picture. A person with normal eyes will see all the lines in focus, but someone with astigmatism will see only one line sharply focused while the other ones are fuzzy. For example, if the horizontal line is perceived to be in focus, then the vertical line will be blurred while the lines in between will be partially out of focus. Each eye must be tested individually because the amount of astigmatism can differ.
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Treatment and Therapy (Magill’s Medical Guide, Sixth Edition)
To correct for astigmatism, the optometrist can add a cylindrical correction to the eyeglass prescription, which changes the focal length of the eye in only one plane. A cylindrical lens can be pictured to be thick in the middle and thin at the edges, like a slice cut off from the outside edge of a cylindrical object.
A typical prescription for a person who is farsighted and also has astigmatism might be “+2.0D + 0.5 cyl axis 90.” The “+2.0D” is the strength of a typical converging lens for a farsighted person, expressed in diopters. Diopters are equal to the inverse of the focal length, so 2.0 diopters equals a focal length of 0.5 meter, or 50 centimeters. The correction for astigmatism here specifies a cylindrical lens of +0.5 diopters situated at an angle of 90 degrees to the horizontal axis.
Astigmatism can be corrected with hard contact lenses because the lens makes contact with the cornea over a layer of tears. The tears fill the space between the lens and the misshapen eyeball, providing the extra focusing that is required.
Laser surgery has become a highly successful procedure to correct for vision problems, including astigmatism. The surgeon first cuts through a paper-thin layer of the outer eye surface (called the cornea) and lifts it like a flap. A computer-controlled laser beam is then used to remove layers of the inner cornea, reshaping it to restore horizontal and vertical symmetry...
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For Further Information: (Magill’s Medical Guide, Sixth Edition)
American Medical Association. American Medical Association Family Medical Guide. 4th rev. ed. Hoboken, N.J.: John Wiley & Sons, 2004.
Cameron, John R., James G. Skofronick, and Roderick M. Grant. Medical Physics: Physics of the Body. Madison, Wis.: Medical Physics, 1992.
Slade, Stephen G., Richard Baker, and Dorothy Brockman. The Complete Book of Laser Eye Surgery. Naperville, Ill.: Sourcebooks, 2002.
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Astigmatism (Encyclopedia of Medicine)
Astigmatism is the result of an inability of the cornea to properly focus an image onto the retina. The result is a blurred image.
The cornea is the outermost part of the eye. It is a transparent layer that covers the colored part of the eye (iris), pupil, and lens. The cornea bends light and helps to focus it onto the retina where specialized cells (photo receptors) detect light and transmit nerve impulses via the optic nerve to the brain where the image is formed. The cornea is dome shaped. Any incorrect shaping of the cornea results in an incorrect focusing of the light that passes through that part of the cornea. The bending of light is called refraction and focusing problems with the cornea are called diseases of refraction or refractive disorders. Astigmatism is an image distortion that results from an improperly shaped cornea. Usually the cornea is spherically shaped, like a baseball. However, in astigmatism the cornea is elliptically shaped, more like a football. There is a long meridian and a short meridian. These two meridians generally have a constant curvature and are generally perpendicular to each other (regular astigmatism). Irregular astigmatism may have more than two meridians of focus and they may not be 90° apart. A point of light, therefore, going through an astigmatic cornea will have two...
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Astigmatism (Encyclopedia of Alternative Medicine)
Astigmatism is a problem of visual distortion caused by a misshapen cornea. The cornea acts as a focusing lens for the eye. If the cornea does not have the proper shape, the eye is unable to properly focus an image. Most people have a certain degree of astigmatism. Corrective measures are necessary only in cases where the distortion is severe.
Light rays entering a normal eye come to a point of focus on the retina through a transparent, dome-shaped layer called the cornea. In astigmatism there is unequal curvature of the cornea, and the light rays come to focus at more than one point on the retina. This causes the person to see a blurred or doubled image. Astigmatism is usually present at birth and may increase during childhood as the eye tissue develops. Usually the degree of astigmatism remains fairly constant throughout adulthood.
Causes & symptoms
It is unknown why some people develop a misshapen cornea. It is possible that astigmatism is an inherited trait. Factors such as stress, continual reading in dim lighting, or excessive close-up work may also contribute to the development of astigmatism. It is sometimes caused by pressure from chalazion, a condition that causes the eyelid to swell; from scars on the cornea; or from keratoconus, a...
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Astigmatism (Encyclopedia of Nursing & Allied Health)
Astigmatism results from an inability of the cornea to properly focus an image as a single point onto the retina, instead focusing the image on two different locations. The result is a blurred image.
The cornea is a transparent layer that bends light and helps to focus it onto the retina, where light is detected. Any incorrect shaping of the cornea results in an incorrect focusing of the light that passes through it. Usually the cornea is spherically shaped, like a baseball. However, in astigmatism the cornea is elliptically shaped with a long meridian and a short meridian. These two meridians generally have a constant curvature and are generally perpendicular to each other (regular astigmatism). Irregular astigmatism may have more than two meridians of focus and they may not be 90° apart. A point of light, therefore, will have two points of focus instead of one. This causes blurry vision.
Some astigmatism is caused by problems in the eye's lens. Minor variations in the curvature of the lens can produce minor degrees of astigmatism (lenticular astigmatism). Infants generally have the least amount of astigmatism. Astigmatism may increase during childhood as the eye develops.
Causes and symptoms
The main symptom of astigmatism is blurred vision. Patients may also experience headaches and eyestrain. Astigmatism is suspected when the child can see some part of a pattern or picture more clearly than others. For example, lines going across may seem clearer than lines going up and down.
Regular astigmatism can be caused by the weight of the upper eyelid upon the eyeball, creating distortion; surgical incisions in the cornea; trauma or scarring to the cornea; tumors of the eyelid; or a developmental anomaly. Irregular astigmatism can be caused by scarring or keratoconus, a condition in which the cornea thins and becomes cone shaped. Although the etiology of keratoconus is unknown, it may be hereditary and may be worsened by chronic eye rubbing. Diabetes can also play a role in astigmatism. High blood sugar levels can cause shape changes in the intraocular lens. This usually occurs slowly and is often noticed only when the diabetic starts treatment. The return to a more normal blood sugar allows the lens to return to normal; this is sometimes manifested as farsightedness. Diabetics should wait until their blood sugar is under control for at least one month before being refracted for eyeglasses.
Patients seek treatment because of blurred vision. Ophthalmologists and optometrists use a variety of tests to detect astigmatism. The patient may be asked to describe the astigmatic dial, a series of lines that radiate outward from a center. People with astigmatism will see some of the lines more clearly than others. Another diagnostic instrument is the keratometer, which measures the curvature of the central cornea. A computerized corneal topographer can measure a larger area encompassing the center and mid-periphery of the cornea. A keratoscope projects a series of concentric light rings onto the cornea. Misshapen locations of the cornea are revealed by noting areas of the light pattern on the cornea that do not appear concentric. It is also important to perform a refraction to determine if the lens is also contributing to the astigmatism. A refraction is when different lenses are placed in front of the eyes of the patient, who is asked which one permits better vision.
Astigmatism is treated with cylindrical lenses. These can be eyeglasses or contact lenses. Lenses are shaped
to counteract the abnormalities of the cornea that are causing the difficulty.
Generally, if visual acuity is good and the patient experiences no overt symptoms, treatment is not considered necessary. Larger amounts of astigmatism, or astigmatism treated for the first time, may not be totally corrected. The cylindrical correction in the eyeglasses may be initially difficult. It may take a week or so to adapt to the corrective lenses. Patients experiencing ongoing problems should be referred back to their eye doctor, who might want to recheck the prescription.
Contact lenses used to correct astigmatism are called toric lenses. When a person blinks, a contact lens rotates. In toric lenses, it is important for the lens to return to the same position each time. Lenses have a prism to weight the lenses, thin zones, cut-off areas (truncations), or other methods of stabilization.
In 1997, the Food and Drug Administration (FDA) approved laser treatment of astigmatism. Patients considering this should verify the surgeon's experience with the procedure and should discuss the possible risks or side effects. The most common laser surgery technique is known as LASIK (laser in situ keratomileusis). Another widely used technique is PRK (photorefractive keratectomy).
In the case of keratoconus, a corneal transplant can be performed if hard contact lenses do not provide adequate vision. Approximately 100% of keratoconus patients will require corneal transplantation. Early in 2001, the FDA approved an initial feasibility study of a keratoconus treatment involving prescription inserts instead of transplantation. The researchers hoped to also obtain approval to utilize the inserts to treat complications arising in some LASIK patients.
Astigmatism may be present at birth. It may also be acquired if something is distorting the cornea. It may worsen with time. Vision can generally be corrected with eyeglasses, contact lenses, or refractive surgery. The major risks of surgery (aside from surgical risks such as infection, night-vision problems, and reduced contrast sensitivity) are over-and under-correction of the astigmatism. Some lasers are able to treat over-correction, and under-correction can be solved by repeating the operation.
Health care team roles
Diagnosis is made by an ophthalmologist or optometrist; nurses, because of their roles in vision screening programs in schools, day care institutions, and other settings, are often front-line workers in initial detection of astigmatism.
At present, there are no known ways to prevent astigmatism, although early detection can prevent the condition from worsening.
Meridian section of a sphere, for example, longitude or latitude on the globe. Or, on a clock, a section going through 12:00-6:00 or 3:00-9:00, etc.
Ophthalmologist medical doctor who specializes in eye surgery.
Optometrist primary eye care doctor who examines, diagnoses, treats, and manages diseases and disorders of the visual system, the eye, and associated structures as well as diagnoses related systemic conditions. An optometrist prescribes glasses, contact lenses, low vision rehabilitation, vision therapy, and medications, as well as performs certain surgical procedures.
Albert, D.M., and F.A. Jakobiec. Principles and Practice of Ophthalmology. New York: W.B. Saunders Company, 1994.
Berkow, Robert, ed. Merck Manual of Medical Information. Whitehouse Station, NJ: Merck Research Laboratories, 1997.
Newell, F.W. Ophthalmology, Principles and Concepts, 8th ed. St. Louis, MO: Mosby, 1996.
David L. Helwig