Here you will learn about the different types of astigmatism and how to correct them.
What is Astigmatism?
Astigmatism 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.
Levels of Astigmatism
This section is designed to help you better understand your astigmatism prescription. 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.
Types of Astigmatism
Now that you understand the different degrees of astigmatism, it is important to understand the different types as well. There are several types of astigmatism, including:
- Myopic astigmatism: One or both principal meridians of the eye are nearsighted
- Hyperopic astigmatism: One or both principal meridians of the eye are farsighted
- Mixed astigmatism: One principal meridian is nearsighted while the other is farsighted
Astigmatism can also be classified as either regular or irregular. Regular astigmatisms are more common, and are what give the cornea its football shape. The principal meridians are 90 degrees apart and perpendicular to each other. Irregular astigmatism can appear after certain types of eye surgery, or it can be caused by an eye injury or by the condition known as keratoconus. In cases of irregular astigmatism, the principal meridians are not perpendicular to each other.
In mild cases of astigmatism, symptoms are hardly noticeable. In fact, treatment may not be needed. In more severe cases, astigmatism makes it difficult to see fine details, either up close or far away. Astigmatism generally does not cause symptoms such as eye pain, watery eyes, or eye redness. People with severe astigmatism may suffer from headaches, eye fatigue, and fluctuating vision, especially while reading a book, staring at a computer screen, or looking off into the distance.
What Causes Astigmatism?
The cause of astigmatism is unknown, but many experts believe it is present at birth, which suggests it could be genetic. Astigmatism occurs when the shape of the cornea or lens is irregular. The cornea and lens are the parts of the eye responsible for focusing light, allowing you to see clearly. When the curvature is abnormal, light entering the eye is bent irregularly. The eye is unable to focus light rays to a single point, causing blurry vision.
In some cases, astigmatism develops after an injury to the eye or after eye surgery. A rare condition called keratoconus may also cause astigmatism. In keratoconus the cornea progressively thins out and becomes more cone-shaped, which results in astigmatism. Although most cases of visual distortion from keratoconus can be corrected with contact lenses, the condition is known to progress to stages that require surgery, including collagen cross linking and even corneal transplants.
Astigmatism can be easily diagnosed after a standard eye exam with a refraction test. To measure the curvature of the cornea, a keratometer may be used in a keratometry exam. In cases where fine detail of the shape of the cornea needs to be determined, a more sophisticated test called a corneal topography may be performed. However, additional testing is almost never required. If the patient is unable to respond normally during the refraction test—as may be the case with young children or persons with disabilities—their refraction can be measured by a test called a retinoscopy, which uses reflected light. During a retinoscopy the ophthalmologist uses an instrument called a retinoscope. The retinoscope focuses light into the eye. The ophthalmologist looks at the light reflex in the pupil while placing different lenses in front of the eye.
People with astigmatism have varying degrees of blurred vision. Treatment for astigmatism includes eyeglasses, special contacts, and certain refractive surgeries.
In most cases astigmatism is best corrected by eyeglasses. Contact lenses, more specifically toric contact lenses, can be specially designed for people with astigmatism. Minor degrees of astigmatism can be corrected with soft toric lenses. High degrees of astigmatism are better corrected with eyeglasses or RGP toric contact lenses. Toric contact lenses are more expensive than normal contacts because of the extra correction provided in them. Surgical treatments for astigmatism include LASIK Eye Surgery (Laser in-situ keratomileusis), PRK (photorefractive keratectomy), and astigmatic keratotomy (AK). LASIK reshapes the cornea by removing eye tissue. In astigmatic keratotomy, an older treatment, an eye surgeon makes incisions in the periphery of the cornea to change its shape. Orthokeratology, or Ortho-K, uses RGP contact lenses to gradually reshape the cornea. The reshaping of the cornea is not permanent, however, and the special contact lenses will still be worn a few hours a day to keep the new shape. Only mild astigmatism is treated with this method, and most ophthalmologists feel it can be potentially harmful to the eye and do not advocate this treatment.
It is important to know that most types of astigmatism are related to the curvature of the cornea. Because of that, only surgery can effectively correct the condition. Eyeglasses and contact lenses are good for correcting vision while they are being worn, but they do not cure the condition. Astigmatism is constantly being researched, and recent advances in materials and technology are expected to help people with astigmatism to see clearly again.
Talking to Your Eye Doctor
People with astigmatism have a wide range of options to correct their vision problems. Here are some questions to ask your eye doctor about astigmatism:
- What prescription do I have?
- Based on the degree of my astigmatism, what are my treatment options?
- Is there an adjustment period for this treatment option?
- If the initial treatment is not effective, what are my next options?
- How long will it be before I can see clearly?
- How long will it take for my symptoms to go away?
- How often should I have my prescription checked?
- Are there any new treatment options you recommend?
- What new symptoms should I watch for after treatment begins? If new symptoms develop, how quickly should I come in to see you?
- J. DiGirolamo, MD “The Big Book of Family Eye Care” (Basic Health Publications, Inc. 2011) 52-55; 62; 77-78; 200
- J. Anshel, MD “Smart Medicine for Your Eyes” (SquareOne Publishers, 2011) 161-163
- J. Weizer, MD; J. Stein, MD, MS “Reader’s Digest Guide to Eye Care” (Quantum Publishing, Ltd. 2009) 30