astigmatism

Corneal topography of someone with regular astigmatism.

The cooler tones indicate the flatter parts of the cornea while the warmer tones indicate the steeper parts.

As you can see, this cornea is more curved along the vertical plane. 

Uh-stig-muh-tizum. It sounds like a terrible disease, but it's not. It is a type of refractive error- it affects how light bends, or refracts, when it enters your eye. Nearsightedness (myopia) and farsightedness (hyperopia) are also refractive errors. Astigmatism is an irregular curvature of the front part of the eye (the cornea and/or the lens). Most people have at least a small amount of astigmatism, but not everyone's vision is affected by it. To explain astigmatism in more detail, let's take a quick step back and review refractive errors.

Image: NEI

A quick review of refractive errors: 
We refer to the curvature of the cornea and lens as either spherical or astigmatic. When you have a spherical cornea or lens, it has the same roundness all over, like a basketball. Because of this uniform curvature, light entering the eye focuses at one point.

• If that one point is in front of the back of the eye (the retina), you are nearsighted. You need minus power in your glasses/contacts to make the light focus ON the retina so you can see clearly. 
• If that one point is behind the retina, you are farsighted. You need plus power in your glasses/contacts to make the light focus ON your retina so you can see clearly. 

With an astigmatic cornea or lens, the curvature is NOT the same all the way around (more like a football than a basketball), and light entering the eye focuses at two different points. This distorts and blurs things up close and far away, and it's especially noticeable when looking at street signs or electronic displays. As someone who has a fair amount of astigmatism, reading signs at the airport is a nightmare without my astigmatism correction!

There are different types of astigmatism. If the irregular curvature is found in the clear part of the front of your eye (the cornea), it's called corneal astigmatism. If it is found in the part of the eye that sits behind the iris (the lens), it's called lenticular astigmatism. You can have either or both. Corneal astigmatism is more common.

Astigmatism can also be classified as either regular or irregular. Be warned: this part will bore you.

• Regular astigmatism occurs when the primary meridians/curvatures are 90 degrees apart, producing a bow-tie or figure-eight pattern on corneal topography (see below). This is much more common than irregular astigmatism. Regular astigmatism can be further classified based on which plane is steepest: 
• With-the-rule astigmatism is when the vertical plane is the steepest. This would be similar to a football laying on its side.
• Against-the-rule astigmatism is when the horizontal plane is the steepest. This would be similar to an upright football on a kick-off tee.
• Oblique astigmatism is when the principal planes are not at or close to (within 30 degrees) the vertical or horizontal. 

Types of regular astigmatism, as seen on corneal topography
Image: Optometric Management

• Irregular astigmatism is present when the curvature is not regular, and is generally a result of surgery, scarring, or disease. An example would be keratoconus. 

How do I know if I have astigmatism?
Astigmatism is diagnosed by your optometrist or ophthalmologist during a comprehensive eye exam. There are three instruments that may be used to detect astigmatism:

• Keratometer- this instrument measures the average curvature of the central part of the cornea. In most offices, an automated keratometer is combined with an autorefractor (AKA the thing that has a picture of a hot air balloon or barn in it). 
• Corneal topographer- this instrument maps the front surface of the eye, indicating where the cornea is steepest and flattest. The image at the top of this post is a corneal topogram.
• Phoropter- AKA the "better one or two" instrument. By using the dials below, your eye doctor can figure out how much astigmatic correction you need and where it needs to be.

How do you treat astigmatism?

• Glasses- Spectacle lenses can correct astigmatism. Unlike a spherical prescription, an astigmatic prescription has 3 numbers:

• The first number is the spherical component- the amount of nearsightedness (-) or farsightedness (+).
• The second number is the cylinder- the amount of the astigmatism. This can be written in either plus or minus form.
• The third number is the axis- a number between 1 and 180 that indicates the position of the astigmatism.
• Contact lenses- Contact lenses that correct astigmatism may be referred to as toric lenses. Soft contact lenses have come a long way in recent years, and are now available in a wide range of astigmatic prescriptions. However, they are not available in every axis and every cylinder, so they are not for everyone. With larger amounts of astigmatism, gas permeable contacts may give sharper, more consistent vision. Gas permeable contacts are an especially great option in patients with irregular astigmatism.
• Surgery- Refractive surgeries, such as LASIK, are also an option.

CliffsNotes: Astigmatism isn't a disease; it's a refractive error. It means the cornea and/or lens is not completely round, so light doesn't focus at one point. That distorts and blurs things, but it can be corrected with glasses, contact lenses, or refractive surgery. 

Additional recommended resources:

• AOA: Astigmatism
• All About Vision: Astigmatism
• NEI: Astigmatism
• AAO: Astigmatism