Astigmatism is detected during a routine eye exam with the same instruments and techniques used for the detection of near-sightedness and farsightedness.
Your Optometrist can estimate the amount of astigmatism you have by shining a light into your eye while manually introducing a series of lenses between the light and your eye. This test is called retinoscopy.
Though many Optometrists continue to perform retinoscopy, this manual procedure has been replaced or supplemented in many eye care practices with automated instruments that provide a faster preliminary test for astigmatism and other refractive errors.
Whether your eye exam includes retinoscopy, an automated refraction, or both, your optometrist or ophthalmologist will perform another test called a manual refraction to refine the results of these preliminary astigmatism tests.
In a manual refraction (also called a manifest refraction or subjective refraction), your eye doctor places an instrument called a phoropter in front of your eyes. The phoropter contains many lenses that can be introduced in front of your eyes one at a time so you can compare them.
As you look through the phoropter at an eye chart at the end of the exam room, your eye doctor will show you different lenses and ask you questions along the lines of, "Which of these two lenses makes the letters on the chart look clearer, lens A or lens B?" Your answers to these questions help determine your eyeglasses prescription.
Astigmatism, like near-sightedness and farsightedness, usually can be corrected with eyeglasses, contact lenses or refractive surgery.
The axis of astigmatism in eyeglass and contact lens prescriptions describes the location of the flatter principal meridian of the eye using the above 180-degree rotary scale.
In addition to the spherical lens power used to correct near-sightedness or farsightedness, astigmatism requires an additional "cylinder" lens power to correct the difference between the powers of the two principal meridians of the eye.
So an eyeglasses prescription for the correction of myopic astigmatism, for example, could look like this: -2.50 -1.00 x 90.
• The first number (-2.50) is the sphere power (in diopters) for the correction of myopia in the flatter (less near-sighted) principal meridian of the eye.
• The second number (-1.00) is the cylinder power for the additional myopia correction required for the more curved principal
meridian. In this case, the total correction required for this meridian is -3.50 D (-2.50 + -1.00 = -3.50 D).
• The third number (90) is called the axis of astigmatism. This is the location (in degrees) of the flatter principal meridian, on a
180-degree rotary scale where 90 degrees designates the vertical meridian of the eye, and 180 degrees designates the horizontal
If you wear soft toric contact lenses for astigmatism correction, your contact lens prescription will likewise include a sphere power, cylinder power and axis designation.
Gas permeable contact lenses are also an option. Because these lenses are rigid and optically replace the cornea as the refracting surface of the eye, a cylinder power and axis may or may not be needed, depending on the type and severity of astigmatism correction required.
Refractive surgery such as LASIK also can correct most types of astigmatism. Discuss with your eye doctor which procedure is best for you.
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