What Is Astigmatism?
Astigmatism is a common vision condition that can cause blurry vision far away and up close. Astigmatism is not an eye disease, but a type of refractive error.
A refractive error is when your eyes cannot focus light correctly, causing blurry vision. Typically, your eyes focus light directly onto your retina, which is the tissue lining the back of your eyes. If you have a refractive error, light focuses in different areas instead:
- Myopia (nearsightedness) occurs when your eye focuses light in front of the retina, causing blurry vision far away.
- Hyperopia (farsightedness) occurs when your eye focuses light behind the retina, causing blurry vision at near.
- Astigmatism occurs when your eye focuses light at multiple points. Some points are located in front of your retina, and some are behind the retina.
You can also have a combination of refractive errors, such as myopia with astigmatism or hyperopia with astigmatism.
We can categorize astigmatism into two types:
- Regular astigmatism is when the two curvatures (called meridians) of astigmatism are perpendicular to each other (90 degrees apart). Most people with astigmatism have regular astigmatism.
- Irregular astigmatism is when the two meridians of astigmatism are not perpendicular. Irregular astigmatism is not common.
Causes of Astigmatism
Astigmatism is the result of an irregular curvature of your eyes. This irregularity can occur in a couple of ways:
- Your cornea, which is the clear covering over the surface of your eye, has an irregular shape. Instead of a symmetrically round shape (like a basketball), your cornea is shaped like an egg. Corneal astigmatism is the most common form of astigmatism.
- Your natural lens, which sits behind the colored iris, changes with age. If the curvature of the lens changes, you can develop astigmatism. This is called lenticular astigmatism.
The causes of regular astigmatism are complex, but current research shows that your genes influence the development of astigmatism. Irregular astigmatism results from an eye injury, eye surgery, or corneal diseases such as keratoconus.
Keratoconus is an eye disease that causes irregular and often high amounts of astigmatism. This disease also causes your cornea to develop a cone-like shape, which is how keratoconus gets its name.
This condition usually starts after puberty and stabilizes around 40 years of age. Studies show that genetics and external factors (like excessive eye rubbing and eye allergies) both play roles in keratoconus. There are several treatments for keratoconus. They range from keratoconus contact lenses to surgical options such as corneal implants, collagen cross-linking, and corneal transplants.
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Approximately one out of three people have astigmatism.
If you have astigmatism, you may notice symptoms such as blurry vision far away and close up, fluctuating vision, eye strain, and headaches. You may also notice difficulty seeing at night, including glare and starburst patterns around lights. Your symptoms will depend on the severity of your eyeglass prescription.
When you look at your eyeglass prescription, you may see numbers written under “cylinder” and “axis.” The cylinder refers to how strong your astigmatism correction is. The axis refers to the location of the astigmatism on your cornea. You may also see the word diopters (D), which are the units your eye doctor uses to measure your prescription.
We can categorize your astigmatism by strength:
- Mild astigmatism is 1.00 D or less.
- Moderate astigmatism ranges from 1.00 to 2.00 D.
- Severe astigmatism ranges from 2.00 to 4.00 D.
- Extreme astigmatism is anything above 4.00 D.
Patients with mild astigmatism may not notice symptoms and have decent vision without any correction. Patients with moderate astigmatism and higher usually notice problems with their vision and require eyeglasses or contacts. Most people use their corrective lenses full-time since they help with both distance and near vision.
How to Correct Astigmatism
For this article, we will be focusing on treatments for regular astigmatism, not irregular astigmatism. Besides glasses or contact lenses, there are a variety of ways to treat astigmatism:
Orthokeratology lenses are rigid gas permeable contact lenses you use while sleeping. These lenses temporarily reshape your corneas overnight. You remove the contacts after waking up and can see fairly well for the day without any glasses. Your corneas return to their original shape after a day or two, so you will need to use the lenses each night. Although orthokeratology mainly corrects myopia, this treatment can also correct moderate amounts of astigmatism.
Refractive Lens Exchange
A refractive lens exchange removes the natural lens in your eye, which is the same procedure as cataract surgery. The surgeon inserts a toric intraocular lens, which is a particular implant that corrects astigmatism. This option is better for older patients who have higher astigmatism corrections and may develop cataracts soon.
Phakic Intraocular Lens Implant
This procedure is typically reserved for patients with high myopia, but can also correct prescriptions with myopia and astigmatism. In contrast with a refractive lens exchange, a phakic intraocular lens procedure keeps your natural lens. Instead, the surgeon inserts the implant in front of your natural lens, which is called a posterior chamber phakic lens. Or, the surgeon may choose an anterior chamber phakic lens, which is placed in front of the iris.
Laser Refractive Surgery
PRK and LASIK are laser eye surgeries that can treat astigmatism. Laser refractive surgery can correct up to about 6.00 D of astigmatism. If you also have myopia or hyperopia, laser eye surgery may only be able to correct a lower amount of astigmatism. If you have higher astigmatism and thin corneas, you may not be eligible for laser refractive surgery. You can consult your eye surgeon to see if you are a candidate for LASIK or PRK.
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