Definition and causes of astigmatism
Astigmatism is not a disease; it is simply a type of refractive error or vision condition, which causes blurry, fuzzy or distorted vision.
Astigmatism is a condition caused by subtle flaws in the way your eye bends light (a process called refraction) that passes to the retina, the light-sensitive area at the back of the eyeball. The optic nerve transmits nerve impulses from the retina to the brain, which translates them into eyesight.
If your vision works properly, your cornea and the lens behind it refract light into the retina. Alas, imperfections in the curvature of the cornea or the lens (or both) can prevent images from properly focusing on the retina. The brain perceives these ocular foul-ups as unclear or distorted images.
Astigmatism is one of the three most common kinds of refractive error. You’ve most likely heard of the other two: myopia (nearsightedness) and hyperopia (farsightedness). There are several types of astigmatism, including astigmatism presenting with myopic or hyperopic refractive errors.
What is astigmatism?
Your cornea is a transparent dome covering the front surface of the eye. It’s curved like the lens on a telescope or camera. A perfectly curved cornea and internal (intraocular) lens focus light signals on the retina to produce crisp, clear vision in the brain.
Unfortunately, corneas and lenses may have imperfect curves that interfere with refraction, generating the fuzzy or distorted vision that we call astigmatism. Defects in other structures within the eye may also contribute to astigmatism.
How do doctors diagnose astigmatism?
During a vision test, your eye doctor will scan your eyeball to measure its entire surface in three dimensions. Sections of the eye called meridians help your eye doctor identify high and low spots on the cornea that generate astigmatism. A measurement called a diopter estimates how much refraction happens in each meridian.
The meridians of the eye look like the face of a traditional clock. One meridian, for example, is the line connecting the 6 (bottom) to the 12 (top). Another meridian is the line from 9 (left) to 3 (right).
Eye doctors assess the differences between the meridians of the eye to determine the proper prescription to produce clear vision. Tests such as a retinoscopy involve shining a light in your eye while you look through different lenses until your doctor finds one that best clarifies your eyesight.
SEE RELATED: What to expect in an astigmatism test
Astigmatism in children
Babies are often born with astigmatism, though it generally clears up within the first year. Few children show signs of astigmatism by the time they’re 5 to 9 years old.
However, some children do not outgrow their astigmatism, and uncorrected astigmatism can negatively impact learning and development.
Thus, it is important to schedule regular eye exams for children in order to detect astigmatism and/or other uncorrected refractive errors.
What are the most common types of astigmatism?
Astigmatism has three primary classifications, defined by the principal meridians (the steepest and flattest meridians of the eye).
Myopic astigmatism: Principal meridians of the eye are nearsighted (myopia + astigmatism).
Hyperopic astigmatism: Principal meridians are farsighted (hyperopia + astigmatism).
Mixed astigmatism: Principal meridians are both near and farsighted (astigmatism is the primary refractive error).
You may also see astigmatism described as:
Corneal: Defects in the cornea.
Lenticular: Defects in the lens of the eye.
Regular: Principal meridians are perpendicular (90 degrees apart).
Irregular: Principal meridians are not perpendicular (sometimes caused by injuries and scarring of the cornea). Keratoconus, a long-term thinning of the cornea, and some forms of eye surgery also can cause irregular astigmatism.
SEE RELATED: Eye Doctor Q and A about Astigmatism
How is astigmatism corrected?
Eyeglasses. Prescriptive lenses correct refractive errors in the cornea and lens of the eye. A regular astigmatism should be correctable to 20/20, according to the American Academy of Ophthalmology, though an astigmatism may be too severe to correct vision to normal. An irregular astigmatism often cannot be corrected to 20/20.
Contact lenses. Many people choose to correct their astigmatism with contact lenses, which come in three primary varieties:
Toric. These soft contact lenses can have different levels of refraction that align with individual meridians of the eye, enabling precise vision corrections. Because of their complex designs, toric lenses often are more expensive than other types.
Gas permeable. These rigid lenses have a uniform shape that essentially replaces the refraction effect of the cornea. These often provide sharper vision than their softer toric counterparts. Note that some gas-permeable lenses use toric designs. Gas-permeable lenses may be less comfortable than toric lenses initially but provide superb vision correction and comfort with time.
Hybrid. These lenses are rigid in the center for higher visual clarity but soft at the edges for more comfort. These often provide the best of both worlds for correcting astigmatism with contact lenses. Hybrid and gas permeable lenses require more time and expertise to get a precise fitting, so factor that into your buying decisions.
Surgery. Operations to reshape the cornea and remove cataracts can correct astigmatism. For instance, LASIK surgery uses high-precision lasers to make the cornea more symmetrical and can correct moderate amounts of astigmatism permanently.
Moreover, an eye surgeon can implant a toric intraocular lens (IOL) in the eye to correct astigmatism and remove a cataract at the same time.
Note that a toric IOL may be considerably more expensive if your health insurance does not cover the procedure. Surgery is permanent, so you have to weigh the risks and make sure your doctor is certain that you’re a good candidate for it.
Everybody’s vision is different, which means every form of astigmatism has a unique impact. This means you have to pay extra attention to the fit and function of your eyeglasses or contact lenses. And don’t dive into surgery until you’ve researched the topic thoroughly and discussed with your eye doctor.
Your doctor will review all available treatment options with the goal of developing clear, comfortable and efficient vision.
Page updated August 3, 2020