Palmetto Cataract and Eye Specialists

Astigmatism: Quick Facts

Astigmatism

Introduction

Astigmatism occurs in about 1 in 3 adults. It is a very common refractive disorder of the eye. A refractive disorder occurs when the eye cannot clearly focus images. The three most common types of refractive disorders are: Astigmatism can occur with or without near-sightedness and far-sightedness.

Causes

Astigmatism can occur for two reasons. It can occur because the cornea, the clear front layer of the eyeball, has an irregular shape or curvature. This is called corneal astigmatism. Astigmatism can also occur because of irregularities in the natural lens inside the eye, otherwise known as lenticular astigmatism.





If your eye is shaped like a basketball, it is perfectly round and you have no astigmatism.*
*Assuming you have no astigmatism inside your lens.

If your eye is shaped like a football, where one side of the football is more curved (in this case, from top to bottom) than the other side (in our example, from side to side), your eye contains astigmatism.

Light rays passing through an eye with astigmatism cannot focus at a single point. This may result in blurred vision.

Types

Astigmatism can be regular or irregular. Regular astigmatism means that the steeper side is perpendicular to (90 degrees away from) the flatter side. Again, this is similar to the shape of a football. If you were to grip a football with your fingers, the steeper side of the football runs parallel to your fingers. The flatter side, running from one tip to the other tip, is 90 degrees away. Irregular astigmatism occurs when the steeper side is not perpendicular to the flatter side. Irregular astigmatism can also occur if the degree of steepness changes along the length of the curve. Conditions affecting the cornea, or the clear covering of the eye, can lead to irregular astigmatism. Keratoconus and pellucid marginal degeneration are two corneal diseases which may lead to irregular astigmatism. It is especially important that these patients never undergo refractive surgery such as LASIK because they have a higher risk of corneal instability.

Symptoms

If you have mild astigmatism, you may not have any symptoms (doctors use the term "asymptomatic" to describe when the condition is not causing any symptoms or complaints to the patient). With higher amounts of astigmatism, you may have blurry vision, eye strain, eye fatigue, or headaches. You may also see halos around lights, ghosting, and double vision. Keep in mind that other conditions such as near-sightedness, far-sightedness, dry eyes, and cataracts can cause similar symptoms.

Diagnosis

Most people will complain of blurred vision, headaches, or eye strain before they are diagnosed with astigmatism. Children may fail vision screening exams at school because of astigmatism.

Your eye care professional will conduct multiple tests to determine whether you have astigmatism. The exam begins with a vision test.

If you have normal vision, your vision will be recorded as 20/20. If you have abnormal vision, the second number (or the denominator) will increase. For instance, if your vision is 20/60, you can only see at 20 feet what a normal eye should be able to see from 60 feet away.

If your vision is not 20/20, your doctor may perform a refraction. A machine holding lens (called a phoropter) is held in front of your face. Your doctor will offer you choices between two different lens to determine whether you have a refractive disorder such as astigmatism. If you hear the doctor asking you "if one or two is better or sharper?", you are undergoing a refraction. A refraction is not only used to determine a need for glasses, but it is also crucial to determine the best corrected vision. This is helpful to determine whether a cataract may be ready for surgical removal.

Additional instruments can be used to determine whether your astigmatism is due to the cornea or due to the lens.

Prescription Basics

One way to tell if you have astigmatism is to look at your glasses prescription. Your glasses prescription should be written in this format:

Prescription: Sphere Cylinder Axis

Prescription #1: -1.00 +2.00 x 180 or

Prescription #2: -1.00 -2.00 x 180


The first number is called the sphere. If it is a negative number, you are near-sighted. If it is a positive number, you are far-sighted. The larger the number, the greater the degree of your near-sightedness or far-sightedness. If you only have one number in your prescription (ie, -1.00), do you NOT have any astigmatism. Some doctors will denote this as -1.00 sphere (or abbreviated, -1.00 sph).

The second number is the degree (or cylinder) of your astigmatism. In the above example, prescription #1 has +2.00 cylinders of astigmatism, prescription #2 has -2.00 cylinders of astigmatism.

The third number is the direction (or axis) of your astigmatism.

In general, ophthalmologists will write your prescription in positive cylinder format while optometrists will use negative cylinder format.

Treatment

You should see an eye care professional if you have blurred vision, headaches, or eye strain in order to determine whether you have astigmatism (or another eye condition).

If you have regular astigmatism, it can be corrected with eyeglasses, contact lens, Toric intraocular lens implants (after cataract surgery), or refractive surgery.

The word Toric may apply to a contact lens or an intraocular lens - both are used to correct astigmatism.

If you have irregular astigmatism, eyeglasses and soft contact lens may not provide clear vision. You may need to wear hard contact lens, also called rigid gas permeable (RGP) lens, to fully correct the irregular astigmatism. Irregular astigmatism can also be corrected with refractive surgery (LASIK is one type of refractive surgery).

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