Astigmatism (uh-STIG-muh-tiz-um) is a refractive error that prevents sufferers from seeing objects clearly from a distance or up close. Astigmatism may occur in varying degrees in each eye and can accompany myopia or hyperopia. Mild astigmatism is usually not noticeable, or causes only slight blurriness, while severe astigmatism causes objects to appear blurry at any distance. Approximately 80 percent of Americans have some degree of astigmatism, but many cases do not require correction.
A normal cornea is shaped like a perfect sphere. The eye’s natural lens is also curved in equal degree in all directions. The corneas or lenses of people with astigmatism do not have equal curves. One side may be steeper than the other, making the cornea look more like a football than a basketball. Because of this, light entering the eye is not focused correctly on the retina, resulting in a blurred image.
Signs and symptoms of astigmatism may include
When to see a doctor
If your quality of vision detracts from your enjoyment of activities or interferes with your ability to perform everyday tasks, see an eye doctor. An eye doctor can determine whether you have astigmatism, and if so, to what degree. He or she can then advise you of your options to correct your vision.
If you're a healthy adult older than 40, have your eyes examined about every two to four years until age 55. After age 55, have them checked every one to three years for signs of eye disease or problems, and then every one to two years after age 65. If you have eye problems, such as astigmatism, you may need to have your eyes checked more frequently. If you're at risk of certain eye diseases, such as glaucoma, or you have diabetes, check with your doctor to see how often you need to have your eyes examined.
Tests and diagnosis
To diagnose astigmatism, your eye doctor may:
Measure reflected light. By measuring light reflected from the surface of your cornea, a device known as a keratometer quantifies the amount and orientation of corneal astigmatism.
Measure the curvature of your cornea. Using light to project rings on to your cornea, a device called a keratoscope measures the amount of curvature to your cornea's surface and can confirm the presence of astigmatism. Observation through the keratoscope of the reflection of light from your cornea and inspection of the shape and spacing of the rings provide information about the degree of astigmatism.
To measure the change in corneal surface curvature, a process called corneal topography is used. Corneal topography uses a videokeratoscope, which is a keratoscope fitted with a video camera.
Levels of Astigmatism
Astigmatism is measured in units of diopters. In a prescription, plus and minus signs in the ‘cylinder’ box indicate the astigmatism prescription, which is then followed by numbers indicating the location (axis) of astigmatism. Here is a rough breakdown of the different degrees of astigmatism:
0.25 to 0.75 diopters = mild astigmatism
1.00 to 2.50 diopters = moderate astigmatism
2.75 to 4.75 diopters = severe astigmatism
5.00 diopters or higher = extreme astigmatism
To prescribe corrective wear for astigmatism, measurements are taken from a vertical and horizontal, or oblique approach, forming an axis. This is done because light enters the eye from different directions. Both the vertical and horizontal measurements will be different with astigmatism.
Let’s use the letter T as an example. You have a vertical and horizontal part to the T. Light from each part of the T enters the eye at the same time. The vertical and horizontal parts of the T each have different prescriptions for the eye with astigmatism. Let’s say the vertical part of the T has a prescription of -3.00 while the horizontal part has a prescription of -5.00. Because the two prescriptions are 2.00 diopters or more apart, the person is considered to have severe astigmatism. In general, higher levels of astigmatism show a greater disparity between two prescriptions, and with milder astigmatism, the values are much closer to each other.
The following are a few other abbreviations you may encounter on your eyeglass prescription:
- SVD - Single Vision Distance, or glasses for distance only
- SVN - Single Vision Near, or glasses for reading only
- Sphere - Spherical power has the same power in all meridians
- Cylinder - A cylinder power corrects astigmatism and represents the difference in the greatest power of the eye and weakest power of the eye, usually separated by 90 degrees.
- Axis - indicates the angle (in degrees) between the two meridians of an astigmatic eye
- PD - (pupillary distance, or distance between the centers of the two pupils between the eyes) This measurement is essential to designing glasses that comfortable to wear and optically perfect.
- Prism - Prism is not commonly prescribed. It is often prescribed to displace the image in a certain direction for patients with crossed-eye (strabismus) or other eye muscle or focusing disorders.
Patients seek treatment because of blurred vision. A variety of tests can be used to detect astigmatism during the eye exam. 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.
Place yourself at approximately 1m from the screen
Cover one eye with your hand, without pressing on the lid, and take the test.
Cover the other eye and begin the test again. If some of the lines appear grayer and some blacker, you probably have an astigmatism - consult your eye care specialist.
One diagnostic instrument used is the keratometer. This measures the curvature of the central cornea. It measures the amount and direction of the curvature. A corneal topographer can measure a larger area of the cornea. It can measure the central area and mid-periphery of the cornea. A keratoscope projects a series of concentric light rings onto the cornea. Misshapen areas of the cornea are revealed by noting areas of the light pattern that do not appear concentric on the cornea. eResearch by Navid Ajamin -- summer 2013
Because these instruments are measuring the cornea, it is also important to have a refraction in case the lens is also contributing to the astigmatism. The refraction measures the optics or visual status of the eye and the result is the eyeglass prescription. The refraction is when the patient is looking at an eye chart and the doctor is putting different lenses in front of the patient's eyes and asks which one looks better.
Astigmatism can be treated by the use of cylindrical lenses. They can be in eyeglasses or contact lenses. The unit of measure describing the power of the lens system or lens is called the diopter (D). The lenses are shaped to counteract the shape of the sections of cornea that are causing the difficulty.
Because the correction is in one direction, it is written in terms of the axis the correction is in. On a prescription, for example, it may say −1.00 × 180°. Cylinders correct astigmatism, minus spheres correct myopia, and plus spheres correct hyperopia.
There is some debate as to whether people with very small amounts of astigmatism should be treated. Generally, if visual acuity is good and the patient experiences no overt symptoms, treatment is not necessary. When treating larger amounts of astigmatism, or astigmatism for the first time, the doctor may not totally correct the astigmatism. The cylindrical correction in the eyeglasses may make the floor appear to tilt, thus making it difficult for the patient at first.
Generally, the doctor will place lenses in a trial frame to allow the patient to try the prescription at the exam. It may take a week or so to get used to the glasses, however, if the patient is having a problem they should contact their doctor, who might want to recheck the prescription.
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Reference: 1.eyehealthweb.com 2.mayoclinic.com 3.vision.about.com 4.encyclopedia.com