Astigmatism remains one of the most misunderstood eye conditions. Untreated, astigmatism can have a variety of effects on your vision. This includes double vision.
In large amounts, astigmatism can cause double vision or a shadowing effect to your vision. But with smaller amounts of astigmatism, this double vision effect may be too small to notice.
Astigmatism is only one of a variety of different causes of double vision. You should most definitely seek medical care if you do develop any new onset double vision. But let's learn about what astigmatism is and how it causes double vision.
What exactly is astigmatism
If we look at a perfect lens (such as those used for photography), it’s completely symmetrical. A lens accepts light from any direction. All this light needs to focus at the same point. By being symmetrical, light that comes from one side will be focused equally to light coming from the other side.
Photography lens; image by AlbertGy, CC BY 4.0, via Wikimedia Commons
With astigmatism, the lens isn't completely symmetrical. One direction of the lens (such as up and down) is steeper than the other direction. This causes the lens to take on more of a football shape.
The cornea is the front clear part of our eye. Our corneas work just like a camera lens. It takes light in front of us and focuses it into the eye and onto our retina. A perfect cornea allows us to see everything without glasses.
But not all corneas are perfect. When one direction of the cornea is steeper than the other direction, the cornea has astigmatism.
How astigmatism affects our vision
The main symptom people experience with astigmatism is blurred vision. Because the cornea isn't symmetrical, light that enters the steep part of the cornea focuses at a different point on our retina than light that enters the flat part. Because of this mismatch, you have blurred vision. (When like focuses at a single point, everything is crisp and sharp).
But when this mismatch becomes large, it causes double vision as well. Instead of seeing one sign, you start to see two.
Double Vision; image by pixel parker on Unsplash / modified from original
Types of Double Vision
There are actually different types of double vision (also known as diplopia).
- Double vision can happen when the eyes aren't focusing on the same target. Misalignment of the eyes can cause this problem. This causes the brain to receive two completely separate images from the two eyes. If you cover or close one of the eyes, this double vision goes away. This type is called binocular diplopia. This type of double vision causes you to see two distinct images (one from each eye).
- A separate type occurs within the individual eyes. If you close the other eye, you still have the double vision. This type of double vision is a focusing issue. Something is going on in the eye (such as astigmatism) which is causing light to hit the retina at two different points. In addition to astigmatism, cataracts are another common cause of this type of double vision - called monocular diplopia. This type of double vision causes you to see more of a shadowing effect rather than two distinct images.
Astigmatism can cause the monocular diplopia type. Thus, the double vision you can have from astigmatism causes objects to take on a shadow rather than causing you to see two distinct images.
The power of the astigmatism also matters. Astigmatism is measured in something called a diopter. Most people have low amounts of astigmatism, typically less than 1.0 diopters (note: you can check how much astigmatism you have by looking at the middle of the three numbers on your glasses prescription. That number gives the power of your astigmatism). But astigmatism can go to very high amounts of over 6.0 diopters; but that is much much less common.
If you have a low amount of astigmatism, the shadowing effect may be subtle enough where you don't notice it. As you get to larger and larger amounts, such as astigmatism greater than 2.0 or 3.0, you start to notice the double vision effect a whole lot more. You have a greater mismatch between light that enters the steep part of the cornea vs light that enters the flat part.
Correcting astigmatism
If you are experiencing double vision (and in general also if you are experiencing blurry vision), you should visit an eye doctor. Through testing, an eye doctor can determine whether your double vision is being caused by astigmatism and provide a prescription to correct it. Yes, astigmatism is correctable!
Glasses (or contact lenses) can be prescribed to correct astigmatism. For the flatter parts of the cornea, the glasses provide a steeper correction. For the steeper parts of the cornea, the glasses provide a flatter correction. All of this balances the astigmatism out to provide complete symmetry of light entering the eye; just like a camera lens. All this symmetry allows light to focus at a single point on our retina to provide a crystal clear image.
Summary
Astigmatism can cause double vision. This type of double vision causes a shadowing effect in vision. But not everyone may notice this double vision. It may be too subtle in lower amounts of astigmatism for one to notice. As you get to the larger prescriptions, the double vision becomes more pronounced. But fortunately, astigmatism can be corrected with glasses or contact lenses to provide great vision!
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