Astigmatism

When the cornea or lens of an eye is irregularly shaped, vision may be out of focus at any distance.

Image courtesy of Eyemaginations

Astigmatism occurs due to the irregular shape of the cornea or the lens inside the eye. The cornea and lens are primarily responsible for properly focusing light entering your eyes allowing you to see things clearly.

The curvature of the cornea and lens causes light entering the eye to be bent in order to focus it precisely on the retina at the back of the eye. In astigmatism, the surface of the cornea or lens has a somewhat different curvature in one direction than another. In the case of the cornea, instead of having a round shape like a basketball, the surface of the cornea is more like a football. As a result, the eye is unable to focus light rays to a single point causing vision to be out of focus at any distance.

Sometimes astigmatism may develop following an eye injury or eye surgery. There is also a relatively rare condition called keratoconus where the cornea becomes progressively thinner and cone shaped. This creates a significant amount of astigmatism resulting in poor vision and may progress to a point where a corneal transplant is necessary.

A phoropter and a retinoscope are instruments commonly used by optometrists to measure refraction.
Astigmatism can be diagnosed through a comprehensive eye examination. Testing for astigmatism measures how the eyes focus light and determines the power of any optical lenses needed to compensate for reduced vision. This examination may include:

Visual acuity – As part of the testing, you’ll be asked to read letters on a distance chart. This test measures visual acuity, which is written as a fraction such as 20/40. The top number is the standard distance at which testing is done, twenty feet. The bottom number is the smallest letter size you were able to read. A person with 20/40 visual acuity would have to get within 20 feet of a letter that should be seen at forty feet in order to see it clearly. Normal distance visual acuity is 20/20.
Refraction – Using an instrument called a phoropter, your optometrist places a series of lenses in front of your eyes and measures how they focus light using a hand held lighted instrument called a retinoscope. Or the doctor may choose to use an automated instrument that automatically evaluates the focusing power of the eye.
Keratometry – A keratometer is the primary instrument used to measure the curvature of the cornea. By focusing a circle of light on the cornea and measuring its reflection, it is possible to determine the exact curvature of the cornea’s surface. This measurement is particularly critical in determining the proper fit for contact lenses.
This testing may be done without the use of eye drops to determine how the eyes respond under normal seeing conditions. In some cases, such as for patients who can’t respond verbally, or when some of the eyes focusing power may be hidden, eye drops may be used. They temporarily keep the eyes from changing focus while testing is done.

Using the information obtained from these tests, along with the results of other tests of eye focusing and eye teaming, your optometrist can determine if you have astigmatism. He or she will also determine the power of any lens correction needed to provide clear vision. Once testing is complete, your optometrist can discuss options for treatment.

Persons with astigmatism have several options available to regain clear vision. They include:

• eyeglasses
• contact lenses
• orthokeratology
• laser and other refractive surgery procedures

Eyeglasses are the primary choice of correction for persons with astigmatism. They will contain a special cylindrical lens prescription to compensate for the astigmatism. This provides for additional lens power in only specific meridians of the lens. An example of a prescription for astigmatism for one eye would be -1.00 -1.25 X 90. The middle number (-1.25) is the lens power for correction of the astigmatism. The “X 90” designates the placement (axis) of the lens power. The first number (-1.00) indicates that this prescription also includes a correction for nearsightedness in addition to astigmatism.

Generally, a single vision lens is prescribed to provide clear vision at all distances. However, for patients over about age 40, a bifocal or progressive addition lens may be needed. These provide different lens powers to see clearly in the distance and to focus effectively for near vision work.

A wide variety of lens types and frame designs are now available for patients of all ages. Eyeglasses are no longer just a medical device that provides needed vision correction. Eyeglass frames are available in a many shapes, sizes, colors and materials that not only allow for correction of vision, but also enhance appearance.

For some individuals, contact lenses can offer better vision than eyeglasses. They may provide clearer vision and a wider field of view. However, since contact lenses are worn directly on the eyes, they require regular cleaning and care to safeguard eye health.

Soft contact lenses mold to the shape of the eye, therefore they are not as effective as rigid contact lenses in correcting for astigmatism due to irregular curvature of the cornea. However, special toric soft contact lenses are available to provide a correction for many types of astigmatism. Since rigid gas permeable contact lenses do not mold to the cornea, they offer an effective way to compensate for its irregular shape and improve vision for persons with astigmatism and other refractive errors.

Orthokeratology (ortho-k) involves the fitting of a series of rigid contact lenses to reshape the cornea, the front outer cover of the eye. The contact lenses are worn daily for limited periods, such as overnight, and then removed. Persons with moderate amounts of astigmatism may be able to temporarily obtain clear vision for most of their daily activities. Orthokeratology does not permanently improve vision and if you stop wearing the retainer lenses, your vision can return to its original condition.

Astigmatism can also be corrected by reshaping the cornea using a highly focused laser beam of light. Two commonly used procedures are photorefractive keratectomy (PRK) and laser in situ keratomileusis (LASIK).

PRK removes tissue from the superficial and inner layers of the cornea. LASIK does not remove tissue from the surface of the cornea, but only from its inner layer. To do this a section of outer corneal surface is cut and folded back to expose the inner tissue. Then a laser is used to remove the precise amount of tissue needed and the flap of outer tissue is placed back in position to heal. By altering the shape of the corneas both procedures refocus light onto the retina.

People who have higher amounts of astigmatism or whose corneas are too thin to allow the use of laser procedures now have another option. They may be able to have their astigmatism corrected using small lenses surgically implanted in their eyes. They look like contact lenses, only they provide the needed optical correction directly inside the eye.

Individuals with astigmatism have a wide range of options to correct their vision problems. In consultation with your optometrist, you can select the treatment that best meets your visual and lifestyle needs.