Like nearsightedness and farsightedness, astigmatism is a common refractive error that affects how light reflects on the retina, the light sensitive tissue at the back of your eye. Astigmatism occurs when the cornea (the clear front cover of the eye) is asymmetrically shaped, or due to the curvature of the lens inside the eye. When the lens and cornea are perfectly spherical shapes, light that enters the eye focuses at a single point. An eye with astigmatism has imperfections in the corneal or lens curvature, causing light to scatter and focus at multiple points. This causes blurred vision and is termed corneal or lenticular astigmatism, depending on which structure has the asymmetric shape.
Astigmatism can range in severity from very mild to severe. In fact, most people are likely born with some degree of astigmatism, but for more than one-third of the population, astigmatism can significantly interfere with vision —meaning it measures about one diopter or more. Astigmatism may also be present in addition to myopia or hyperopia, compounding the visual defect. It can also lead to headaches.
What Causes Astigmatism?
No one knows for sure why some people have astigmatism and others do not, but there is a strong hereditary link. It is also usually present at birth, though it can change with time.
Astigmatism seems to affect some ethnicities more than others. For instance, Asians and Hispanics tend to have a higher prevalence of astigmatism than Caucasians and African-Americans.
Sometimes, astigmatism is induced by an ocular disease, injury or a surgical procedure. For example, astigmatism can occur in people who have keratoconus, a condition that causes corneal thinning, or as an unwanted side effect of cataract or refractive surgery.
How Is Astigmatism Diagnosed?
Symptoms of undiagnosed astigmatism include headaches, eyestrain, and blurry vision. However, if you visit your eye doctor regularly, it is unlikely that your astigmatism would be overlooked. Detecting astigmatism is a routine part of most eye exams and it is very easy for an eye doctor to detect.
There are several different tools that your doctor can use to identify and measure astigmatism. For instance, a doctor can estimate how much astigmatism you have during retinoscopy (an objective measurement of refractive error by shining light at the back of the eye) as well as during manual refraction with a phoropter (an instrument used to check the power of the eyes). Your corneal curvature can also be measured using a keratometer (an instrument used to measure the front curvature of the eye).
Sometimes, astigmatism is detected even before you meet the doctor in the exam room. During pretesting, astigmatism may be detected by the doctor’s technician using an autorefraction machine. Other technology that may be used to detect astigmatism includes advanced diagnostic machines, called corneal topographers and wavefront aberrometers, which can precisely map how much astigmatism you have and where on your eye it is located.
Surgical Treatments for Astigmatism
As long as you don’t have too much astigmatism, it may be possible to correct it with refractive surgery. In procedures such as PRK or LASIK, a laser changes the shape of the cornea by removing a small amount of eye tissue, essentially correcting the irregular shape.
If you are already starting to develop age-related eye disease, cataract surgery may also present a unique opportunity to surgically correct astigmatism.
Scleral Lenses for Astigmatism
In some cases, astigmatism can be corrected with a soft toric contact lens. Toric lenses utilize different lens designs to stabilize the lens including thin-thick zones, lens truncation (the bottom of the lens is cut off a little) and prism ballasting (the lens is a little thicker or heavier in different areas).
Unfortunately, these lenses can move around and, when they do, the prescription changes and vision can get temporarily blurry.
In an effort to provide more stable vision, many doctors prefer to fit patients with astigmatism in rigid, also known as gas permeable (GP), lens materials. When GP lenses are placed on an eye with astigmatism, the space between the back of the GP lens and the surface of the cornea is filled up by tears, thereby correcting the imperfections caused by an otherwise irregular shape.
There are many different GP lens designs and many of them are a great choice for people with various degrees of astigmatism. However, some people are unable to tolerate GP lenses and find them uncomfortable. In those cases of astigmatism, a scleral lens can provide improved all day comfort.
Scleral lenses feature a large-diameter that allows the lens to vault (not touch) the cornea and rest on the white part of the eye, known as the sclera. These lenses are very versatile and patients with all types of astigmatism can wear them, including those with against-the-rule astigmatism. People with against-the-rule astigmatism sometimes struggle with traditional GP lens designs because, like soft toric lenses, they can slide on the eye.
If you have astigmatism, visit your eye doctor to determine the most appropriate management strategy for you.