To understand astigmatism, imagine the normal eye as being evenly rounded, like a basketball. Astigmatism causes the eye to be egg- or oval-shaped, similar to an American football. Astigmatism can be classified into two types:
- horizontal astigmatism (when the eye is wider than it is tall), and
- vertical astigmatism (when the eye is taller than it is wide)
With either type of astigmatism, near and far vision is blurry because of the eye’s irregular shape.
Cause
Astigmatism is caused by a cornea or lens that is not normal in shape. Astigmatism is extremely common. Doctors are baffled as to why the shape of the cornea or lens differs from person to person. They are aware, however, that the risk of developing astigmatism is inherited (passed down from parents).
People can also get astigmatism from:
- eye disease
- eye injury
- or after surgery
It is a myth that reading in low light or sitting very close to the television causes astigmatism or makes it worse.
How Astigmatism Affect Vision
The cornea and lens at the front of the eye are normally round in shape. This helps to sharply focus light rays onto the retina, allowing you to see clearly.
Light rays do not refract (or bend) properly as they enter the front of the eye in people who have astigmatism. Light rays either fall short of or behind the retina, causing blurry vision at close and far distances.
Astigmatism can coexist with other refractive errors, such as:
- Myopia (nearsightedness) or
- Hyperopia (farsightedness)
Diagnosis
During a comprehensive eye exam, ophthalmologists can diagnose astigmatism. Testing may include:
Eye Chart: You will be tested on your ability to read letters on an eye chart. This test measures your visual acuity, or the sharpness of your vision at various distances.
Phoropter: This is where you look through a large binocular-like device and tell your doctor which letter you can see better. Based on your answers, your doctor writes a prescription to provide you with the clearest vision possible.
Autorefractor: This device also helps measure your astigmatism or other refractive error. It works by shining a light into the eye and measuring how it changes as it bounces off the back of the eye.
A keratometer measures the curvature of your cornea. Your eye doctor may also perform a corneal topography scan to detect imperfections and distortions on the corneal surface.
How Is Astigmatism Treated?
Contact lenses or glasses. They function by refocusing light on the retina at the back of your eye, allowing you to see more clearly. Astigmatism contacts are classified into two types: toric soft lenses and RGP contacts (rigid gas permeable). RPG lenses may be the best option for people with severe astigmatism.
Refractive surgery is a type of eye surgery. A laser reshapes the cornea during LASIK and other procedures to change how light passes through it.