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

Keratoconus

A topographical map of the front surface of the eye in a patient with keratoconus.

The red indicates the steepest part of the cornea (the cone).

What is keratoconus?
Keratoconus is a degenerative disease in which the cornea (the clear tissue of the front of the eye) progressively thins and bulges. This area of bulging resembles a cone. The National Keratoconus Foundation (NKCF) has a good introductory video here.

What are the symptoms?
The cornea is normally smooth and dome-shaped, and this allows light to focus clearly on the retina. With keratoconus, the irregularity of the cornea prevents light from focusing clearly on the retina, causing blurred and distorted vision. Glare and light-sensitivity may occur as well. Symptoms usually present in the late-teens/early-twenties. The disease generally affects both eyes, though one eye can be more severely affected than the other.

How is it treated?
Early in the disease process, eyeglasses or soft contact lenses may do the job. But as the cone progresses, the cornea becomes more irregular, and vision cannot be adequately corrected with glasses or soft contacts. Thus, you need something that will help mask the irregularity of the front surface of the eye. That's where rigid gas permeable (RGP or GP) contact lenses come in.

Image from NKCF

The type of contact lens that is best depends on the severity of the disease and the location of the cone. Some of the options available include:

• Custom soft contact lenses- custom-made soft lenses specifically designed for keratoconus.
• Corneal GP lenses- small rigid lenses that sit on the clear part of the eye (cornea).
• Scleral GP lenses- large-diameter rigid lenses that sit on the white part of the eye (sclera). 
• Hybrid lenses- lenses with a GP center and a soft periphery, or skirt.
• "Piggybacking"- when a GP lens is placed on top of a soft lens for increased comfort.

Fitting a keratoconic patient in any of these contact lenses is typically considered a specialty fitting, so you can expect to spend a little more time and money on the process and the lenses compared to a standard soft contact lens fitting. Be sure to discuss the options with your optometrist to figure out what is best for your individual case. If he/she is not able to perform the specialty fitting, he/she can certainly refer you to an optometrist in your area that can.

What are the surgical treatment options?
For various reasons, some cases of keratoconus cannot be treated with the above methods. In such cases, surgery may be indicated.

• Intacs® are small, semi-circular inserts that are implanted into the middle layer of the cornea to flatten the cone-like area. Intacs are FDA approved for the treatment of keratoconus.

Intacs®

• Collagen cross-linking (CXL) is a treatment procedure that involves administering riboflavin eyedrops and then exposing the cornea to UV-A light. This promotes cross-linking of the collagen fibers of the cornea, which stiffens the cornea and prevents further bulging. The outer layer of the cornea is removed in epithelium-off CXL, and left intact in epithelium-on CXL. CXL gained FDA approval in the US in April 2016, and we are starting to see some insurance coverage for the procedure.

• Conductive keratoplasty (CK) involves using heat in the form of radiofrequency energy to change the curvature of the cornea. Using a thin probe, radiofrequency energy is applied to small areas of the cornea in a circular pattern, creating constriction of the corneal tissue and reducing astigmatism. This procedure is being done by some doctors (off-label) in conjunction with CXL and/or Intacs.

• Corneal transplants may be necessary in 10-20% of keratoconus cases. In a full-thickness corneal transplant, or penetrating keratoplasty (PK), the diseased cornea is removed and replaced with a healthy donor cornea.  Deep anterior lamellar keratoplasty (DALK) is a partial-thickness corneal transplant that leaves the bottom layer of the patient's cornea intact, only transplanting a portion of the donor cornea. This potentially allows for a faster healing time and less risk for graft rejection (1).

Photo of a patient post-PK

CliffsNotes: Keratoconus is an eye disease that causes irregularity in the front surface of the eye. The best treatment is determined on a case-by-case basis, so talk to your eye doctor about what your options are.

Additional Recommended Resources:

• National Keratoconus Foundation
• AOA: Keratoconus
• All About Vision: Keratoconus