Lots of things change when we get older. And as much as we would hope, the eyes aren’t immune to changes with age. In particular, one of the thing that changes is the prescription and astigmatism of our eye.
Astigmatism will change over time. Gradually astigmatism can shift directions on our cornea. This can initially cause our astigmatism to decrease before causing our astigmatism to steadily increase and get worse with age.
These changes can cause someone who has never had astigmatism before to develop it over time. To learn why this is the case, first let’s learn about what actually causes astigmatism.
What Causes Astigmatism
Much of the astigmatism in our vision comes from our cornea. In the most simple explanation, the cornea is shaped more like a football and less like a basketball. Like a football, the cornea is steep in one direction and flat in another direction. These different curves prevent all the light that enters your eye from focusing at a single point. Thus, prescription glasses that correct astigmatism account for these different curves in order to allow light to focus at the same spot.
This is a football; image by Dave Adamson on Unsplash
Astigmatism has two measurements: power and direction. Power is how strong our astigmatism is, ie the difference between the steep and flat part of the cornea and how much prescription we need to correct it. The direction of the astigmatism measures where the steep part of the cornea lies on a 360 degree axis.
Advanced learning: because a normal cornea is symmetric, the measurement axis only needs to be between 0 and 180 degrees. And to be even more confusing, the direction of the astigmatism can also indicate the flat axis instead if used with a negative power. Those come into play when talking about prescribing glasses for astigmatism and are irrelevant to today’s discussion.
When we further classify astigmatism, we can separate it into two different categories based upon the direction of the astigmatism (these categories may seem unimportant right now but bear with me).
- "With the rule" astigmatism - this is astigmatism that runs up and down on your cornea. The steepest parts of the cornea are at the top and the bottom and the flattest part of the cornea is on the sides.
- "Against the rule" astigmatism - this is astigmatism that runs from side to side on your cornea. The steepest parts of the cornea are on the sides and the flattest parts are on the top and the bottom.
Ok. Let’s see why those two categories are important.
Corneal topography map of "with the rule" astigmatism
Astigmatism WILL Change Over Time
There are a few different factors which can influence how much astigmatism you have. The biggest factor however is the position of the eyelids on our cornea.
Those that have eyelids that sit tighter on the cornea tend to have larger amounts of astigmatism. Because the eyelids sit on the top and the bottom of the cornea, it tends to cause those parts of the cornea to be more steep. This leads to "with the rule" astigmatism.
In general, when you are young and your eyelids are tighter (and if you have astigmatism), you have "with the rule" astigmatism.
But over time, our eyes change. One thing that changes is the tension that the eyelid exerts onto our eye. There is less tension on cornea from the eyelid over time. As this occurs, the cornea begins to flatten where the eyelids sit. Eventually this flattens enough where the top and the bottom of the cornea are flatter than the sides of the cornea. This causes a change in the astigmatism to "against the rule" astigmatism.
If you notice, "with the rule" astigmatism and "against the rule" astigmatism are opposites of each other. If you reduce "with the rule" astigmatism too much (by flattening the steep part of the cornea), you create "against the rule" astigmatism. This is what happens over time.
Gradually over time, our astigmatism changes from "with the rule" astigmatism to "against the rule" astigmatism.
Initially astigmatism may decrease
Let’s say you start off with astigmatism. And your astigmatism is "with the rule". Much of your astigmatism is being formed by your eyelids on your cornea. If you think about it, less tension from the eyelids will cause your astigmatism to go down. And this is true.
Less tension from the eyelids makes the top and the bottom of the cornea less steep and the power of the astigmatism goes down. This will cause you to have less astigmatism in your vision. This transition tends to happen in our 40s.
Later astigmatism will increase
Eventually the top and the bottom of the cornea will be equally flat to the sides of the cornea. All the "with the rule" astigmatism is gone. But the cornea still continues to change because of the effect of the eyelids.
This change continues to flatten the top and the bottom of the cornea. And suddenly the sides of the cornea are steeper than the top and the bottom. You now have "against the rule" astigmatism. Your astigmatism has switched directions.
As this process continues to happen, this "against the rule" astigmatism builds over time and astigmatism increases over time. This tends to occur in the 50s and up.
How Much Astigmatism Are We Talking About
Is this something to be worried about? Fortunately these changes to astigmatism aren’t massive amounts. This is a very gradual process.
Over the course of a ten year period, one can experience approximately 0.30 diopters of change. For reference, the smallest unit that we can measure astigmatism in glasses is 0.25 diopters. And in general, astigmatism doesn’t become significant to vision until it reaches approximately 0.75 diopters (see also Is Astigmatism Common?). Ie, we are talking small changes to the cornea over time.
For some people with large amounts of astigmatism, they may not notice much change at all as these numbers may be comparatively too small.
For people with no astigmatism, they may suddenly find out that they have developed astigmatism over time with age.
When does this matter than?
For many people these changes may not matter much to them in the scheme of things. But ultimately these changes in astigmatism become important when we talk about correcting vision with cataract surgery.
Many people desire to get out of glasses with cataract surgery. To achieve that goal, we also need to get rid of astigmatism. Uncorrected astigmatism will cause vision to be blurry.
During cataract surgery, special lenses or techniques can be used to correct astigmatism. (see also This Is The Best Cataract Surgery For Astigmatism)
When your eyes are evaluated for cataract surgery, your surgeon may see astigmatism on your cornea that you never knew existed (since it may not have been there in the past) and discuss different options to correct that astigmatism. Your surgeon will also take into account the gradual shift in astigmatism when planning the appropriate treatment. Thus, allowing you to have the best result and best vision after cataract surgery.
Summary
As we get older, the astigmatism on our cornea changes. This causes it to change directions and eventually causes us to develop more astigmatism as we get older. This is important when considering cataract surgery to correct your vision and get out of glasses.
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