The Simple Explanation of What Is Astigmatism
Astigmatism always conjures intense anxiety in people. Many people recoil at just the mention of the word. But does astigmatism deserve the bad rap it gets? Maybe, maybe not.
Astigmatism is a type of prescription error in your eyes which causes you to have blurry vision. Most frequently, astigmatism is caused by the cornea being shaped more like a football than a basketball.
But astigmatism doesn't have to provoke fear. All it takes is understanding what it is.
So what is it?
In normal eyes, the light that enters focuses on a perfect point on the retina. That is normal eyes without any astigmatism and without even any nearsightedness or farsightedness.
In astigmatism, light can’t focus properly. And it all boils down to a cornea which isn’t a perfect sphere.
Everyone's favorite way of describing astigmatism is that the eye is shaped like a football rather than a basketball. And that is pretty much true! A football is different from a basketball in that one direction of the football is steeper than the other direction. A cornea with astigmatism has this same property. One direction of the cornea is steeper and the other flatter.
In someone without astigmatism, the cornea is shaped like a basketball. Because of this, light that enters the eye focuses at the same point no matter the direction the light comes from.
But with a cornea shaped like a football, this is different. Light from one direction may hit the retina perfectly while the light from another direction completely misses focusing on the retina. In effect, the overall image is distorted and blurred since only part of it hits the retina. In severe cases, astigmatism will even cause double vision (see also Will Astigmatism Cause Double Vision?).
Blur from astigmatism, image by English: The original uploader was Tallfred at English Wikipedia. Português: O uploader original foi Tallfred em en.wikipedia, BSD, via Wikimedia Commons / cropped from original
What can be done about it
Fortunately, normal astigmatism can be corrected quite easily.
A standard pair of glasses corrects three different things or combinations of those things.
- Nearsightedness (also known as myopia)
- Farsightedness (also known as hyperopia)
The first two (nearsightedness and farsightedness) cause you to have blurry vision either in the distance or up close. These are called spherical errors. If you have zero astigmatism (your cornea is shaped like a basketball or sphere) and you can see up close but not in the distance - you are nearsighted. You need a symmetric or spherical lens to correct your vision.
Astigmatism, however, needs a lens shaped like a cylinder instead. The problem with astigmatism is that there is a mismatch between the steep and flat parts of the cornea. Notice that a cylinder has a flat portion and a steep portion. By matching up the flat portion of the cylinder to the steep part of the cornea, you can counteract that mismatch to make the cornea all equal again. This corrects the blurry vision from astigmatism.
Cylinder lens; image by !Original: WingkLEEVector: Chrischi, CC0, via Wikimedia Commons
Measurements of astigmatism
To find out the right cylinder to use in your glasses, the astigmatism must be measured appropriately. There are two things we want to know. How strong is the astigmatism? and What direction do we need to put the cylinder to cancel out the astigmatism?
How strong is the astigmatism?
This is simply the difference between the steepest part of the cornea and the flattest part of the cornea.
Note: lenses and the focusing of light is measured in a unit called the diopter. This mathematical measurement allows us to calculate exactly where light is going to focus after passing through a lens (which allows us to make glasses to correct vision).
In very small amounts of astigmatism, there may only be a 0.25 diopter difference between the steepest part of the cornea and the flattest part. This is a pretty insignificant amount; you more than likely won’t even notice this amount.
In general, astigmatism starts to become significant when there is more than a 1.00 difference between the steep and flat parts of the cornea. Above this level, it becomes more important to correct the astigmatism with glasses, contact lenses or surgery to allow for good vision.
What direction do we need to put the cylinder to cancel out the astigmatism?
Normal astigmatism has a direction. One direction will be flat and one direction will be steep. (remember, this is the definition of astigmatism). These directions are also called the axis of the astigmatism.
Now is time to dig out your protractor. The axis of the astigmatism is given in degrees. Time to nerd out a little:
Zero degrees is set as horizontal. That would make 180 degrees also horizontal. 90 degrees and 270 degrees would be up and down respectively.
But normal astigmatism only has one flat direction and one steep direction. One flat side is symmetric with the opposite flat side and the same with one steep side (once again, similar to a football or cylinder). Because of this, astigmatism really only needs to be measured between 0 and 180 degrees (an astigmatism axis of 90 degrees is exactly the same as an axis of 270; usual astigmatism is symmetric).
The actual measurement
In general, astigmatism in your vision is measured by the “better 1 or better 2” test (also known as a refraction). This is done at a phoropter. Within the phoropter are different cylinder lenses that correct all the powers of astigmatism (up to 6.00 diopters of astigmatism; which is very very high). These lenses can also be rotated to match up with every direction of your astigmatism.
The very basics of the test involve switching between different powers of cylinder lenses and directions until one is found that corrects your vision the best. Once we find the lens that corrects all your astigmatism, we now know the power and the direction of your astigmatism.
When you visit your eye doctor, your astigmatism will be measured as part of standard care. You don’t need to request to have your astigmatism measured.
Putting it all together
Now that we know the strength and the direction of the astigmatism, we can use that information to create a glasses prescription.
A standard glasses prescription has three numbers:
- Number #1 - The spherical prescription error (remember, this is how nearsighted or farsighted you are). If you are nearsighted, your prescription will be a negative number. If you are farsighted your prescription will be positive number
- Number #2 - The power of your astigmatism
- Number #3 - The axis of your astigmatism
It’s worth noting that the astigmatism number can be either positive or negative. Whether this number is positive or negative determines what the axis is telling us. If your astigmatism power is negative, the axis is telling us where the flat part of your cornea is. If your astigmatism power is positive, the axis is telling you the steepest part of the cornea. Confusing? Yeah. But in general, just ignore whether it’s positive or negative. The magnitude of the number is more important for you to understand how strong your astigmatism is.
Note: Generally when you visit an optometrist, you will have a negative number and if you visit an ophthalmologist you will have a positive number. The reasons behind this difference are based upon on differences between correcting astigmatism with glasses vs surgery.
With those three numbers, glasses can be created which equalize the astigmatism in your vision. Providing the clear vision you are looking for.
Astigmatism sounds complex, but it’s a relatively straightforward thing. The takeaway is that astigmatism will cause your vision to be blurry. There are different powers and directions of astigmatism and glasses are used to cancel out the astigmatism in your vision to provide you with clear vision.
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