So you have astigmatism. But where did it come from? Perhaps your father also has a large amount of astigmatism. So is your astigmatism just a result of unfortunate genes?
There is no strong evidence that your astigmatism is genetic or inherited from your parents. There may be some genetic component but more than likely your astigmatism came from other things such as the position of your eyelids.
Of course we want to know as much as possible about astigmatism and whether your kids are going to grow up having astigmatism. Let’s see what has been discovered.
First off, what is astigmatism?
If you need glasses or contact lenses, you have blurred vision from something called refractive error. Essentially light that enters your eye isn’t focused on the retina on the back of your eye. Like an out of focus camera, everything becomes blurry.
There are three primary refractive errors one can have:
- Nearsightedness: up close vision is sharp
- Farsightedness: distance vision is sharper than up close (but both may be blurry)
- And Astigmatism: both distance vision and up close vision are blurred
Like nearsightedness and farsightedness, astigmatism prevents light from focusing properly onto your retina.
But the confusing thing about astigmatism is that it causes light to focus in different locations. With astigmatism, your cornea is shaped more like a football than a basketball. A football has a direction which is steep and a direction which is less steep. If we picture a cornea shaped like this, light that enters the steep part of the cornea will be focused in a different location than light that enters the flatter part of the cornea. This is where astigmatism causes blurred vision.
How is astigmatism measured
Astigmatism can be mild or it can be severe. If there is a very small difference between the flat part of the cornea and the steep part of the cornea, there is less blurry vision from the astigmatism. The astigmatism is mild.
If there is a very large difference between the flat and steep parts, the astigmatism is severe. The light from each direction doesn’t focus anywhere close to where it needs to be.
Ok, now that we got the basics out of the way, how genetic is astigmatism?
The genetics
It can be tricky to determine the exact genetics of something. There is a lot about us which comes from both genetics and from the environment around us. Within all of our cells is DNA. This DNA comes from our parents. And for the most part, we have two copies of all of our DNA (with the exception being in males with the Y chromosome). These DNA copies come from our parents, one from each.
Sometimes a trait or feature is entirely 100% genetic. Perhaps a single gene is able to cause a certain feature (called autosomal dominant). Neurofibromatosis type I is an example of an autosomal dominant condition.
Perhaps you need two copies of the gene to cause that feature (called autosomal recessive). Cystic Fibrosis is a condition inherited in an autosomal recessive manner.
The way these traits are discovered are by documenting detailed family trees over generations. By figuring out which family members and which offspring possess the trait, one can determine how it is inherited.
But this only works if the trait is entirely 100% genetic.
There are many examples in which the environment influences the genetics or even in which other genes influence other genes. It starts to get complex determining whether something is genetic and how genetic it is.
This is another way that can help: take a look at twins.
There are two types of twins.
- The first type of twins come from the exact same egg. These identical twins have identical DNA.
- The second type of twins are born at the same time but come from two different eggs. These fraternal twins are genetically distinct from each other (at least as genetically distinct that two siblings will be).
By comparing these two groups of twins, you can get an idea on how much a trait is based upon genetics or how much it is based on environment.
A trait with a very high genetic component will be exactly the same in the identical twins but can have differences in the fraternal twins. Note: because the fraternal twins may both inherit the DNA in question, these studies require large groups of twins to assess any relationships.
A trait with less genetics will show some differences between both categories of twins.
So is astigmatism genetic?
Researchers have done both types of studies looking at astigmatism. They have compared the measurements of astigmatism in families between children and their parents. They have looked at differences between twins. But the results have come back mixed.
There is no strong genetic relationship for astigmatism. For the other refractive errors, nearsightedness and farsightedness, there is a strong relationship, but not for astigmatism.
Within families, some have found a small relationship between the astigmatism from the parents and their children but not all researchers have found the same result. Amongst twins, most researchers have found no differences between the identical twins and the fraternal twins.
There is no clear inheritable pattern for astigmatism. While there may likely be a small genetic basis for astigmatism, genetics don’t explain the whole picture.
If not genetics, then what?
Well, the easy answer is the environment. But that’s pretty vague.
There are a few other things that have been identified as influencing the development of astigmatism. Perhaps the biggest cause may be how our eyelids are positioned against our eye.
If our eyelids are tight against our eye, they will push in on the cornea. BUT, the eyelids are only at the top and bottom of our corneas. Thus the eyelids push and steepen the top and bottom of the cornea. The sides of the cornea remain flat. This creates astigmatism.
While not exactly purely “the environment" (since genetics such as ethnicity can have some influence on eyelid position), the position of the eyelids can help explain some development of astigmatism.
Summary
The causes of astigmatism are incompletely understood. However, when compared to other refractive errors such as nearsightedness and farsightedness, genetics for astigmatism appear to play a much small role. Just because your mother has astigmatism doesn’t necessarily mean that you are going to develop it as well.
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