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Is Astigmatism Common?

Is Astigmatism Common?

Odds are that you have astigmatism. Surprised? Astigmatism is much more common than you may think. But you may not even realize you have astigmatism because astigmatism can exist in small insignificant amounts.

Between 40-60% of the population can have some amount of astigmatism. However, the vast majority only have small amounts. Only about 5-20% of the population has any real significant amount of astigmatism. Astigmatism, however, can change with age and depending on ethnicity.

But what counts as a significant amount of astigmatism? What even exactly is astigmatism? Let's delve into the numbers!

What is astigmatism?

There are two main prescription errors which can cause you to have blurry vision.

  • One of them is being near-sighted or far-sighted. If you are near-sighted, you see fine up close, you just can't see off in the distance. If you are far-sighted, you are the opposite. These are called spherical errors.
  • Astigmatism is the other error. Unlike near-sighted or far-sighted, astigmatism causes blurry vision everywhere.

Most astigmatism in our vision comes from our cornea. Instead of being shaped like a sphere or basketball, the cornea is shaped more like a football. This causes one direction or axis of the cornea to be steep and one direction or axis to be flat.

Astigmatism causes blurry vision. Light that enters the flat part of the cornea doesn't focus on the retina at the same point as the light that enters the steep part of the cornea. This creates a blur.

Blur from astigmatism

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

Fortunately, normal astigmatism can be corrected easily with glasses. While a spherical lens is used to correct spherical errors, a cylindrical or football-shaped lens is used to correct astigmatism. This lens is flat in one direction and steep in the other (to match the astigmatism). by lining up this lens opposite to the astigmatism, this lens cancels out the astigmatism and brings things into focus.

Different powers of astigmatism

Not all astigmatism is equal. Even a slight football shape to the cornea technically causes astigmatism. But this astigmatism may blur vision very little. This would be a very low power of astigmatism.

The power of astigmatism is determined by the difference between the steep direction of the cornea and the flat direction. The greater the difference, the higher the power and stronger the astigmatism.

A low power of astigmatism is generally thought of as anything less than 1.00. Anything above this value starts to cause more issues with vision.

A glasses prescription will typically have three numbers. The last two numbers of the prescription represent the astigmatism (with the first number describing the spherical error). The middle number is the power of the astigmatism, the last number is the axis or direction of the astigmatism on a 180º scale.

How often does astigmatism occur

Because astigmatism can exists in very small amounts, astigmatism ends up being very common. Large population studies have demonstrated that 63% of young adults have some amount of astigmatism!

However this “some amount” includes the smallest amount of astigmatism that we can measure: 0.25. Of this 63%, the vast majority have very small amounts of astigmatism. If we increase the smallest included amount of astigmatism to 0.50 (still very small), only 46% of the population will have astigmatism.

This percentage rapidly falls off as we approach the higher amounts of astigmatism. Less than 20% have a significant amount (more than 1.00) and only 5% of the population will have astigmatism greater than 1.50. Most people don't need to worry about large amounts of astigmatism. The vast majority of people who have astigmatism have low and mostly insignificant levels.

Percentage of Population with Astigmatism

Differences over age

These large population studies, however, mostly involved adults. But astigmatism changes throughout our life.

In our first few months after birth, we actually will have a very high amount of astigmatism. Gradually over the course of our first few years, this astigmatism largely disappears. And by the time that we hit pre-school age, we hit “normal” population levels of astigmatism. A large study on 6 year olds showed that about 5% had a significant amount of astigmatism.

However, children with high amounts of astigmatism are at risk for losing vision. Up until the age of 12, the vision is constantly developing. These ages are called the critical period for vision. It is essential during these years to have good vision so that good vision develops.

High astigmatism will cause blurry and poor vision. If this blurry vision is not corrected with glasses, good vision will not develop. This is called amblyopia. From birth through childhood, it is important to continually screen children for astigmatism to allow for the healthiest development of the eyes.

But after our vision develops through childhood, astigmatism remains relatively stable until our late-adult years. Once we pass through our 40s, as our eyes and cornea change, our astigmatism can continue to change. Our astigmatism can get smaller, disappear, change direction and then grow larger again. Even if you never had astigmatism before, you can potentially develop a little as your cornea changes with age.

Differences by ethnicity

Not all ethnicities have the same levels of astigmatism. The general population numbers above don't necessarily apply to everyone. There are a few ethnicities which are notable for having higher levels of astigmatism.

In general, East Asians, Hispanics and Native Americans are more likely to have astigmatism. The percentage of those with significant astigmatism in these populations can be almost 2 or 3 times greater than the general population!

Astigmatism doesn't exist in isolation

People with astigmatism are more likely to be near-sighted than the rest of the population. As you increase the amount of astigmatism, this increases the chance that you are also nearsighted.

This association has been studied in children. In children with astigmatism, the more astigmatism they had, the greater amount of near-sightedness that developed over time.

The relationship between astigmatism isn't fully understood. But it is hypothesized that blurry vision from astigmatism in a developing eye serves as a trigger for more near-sightedness to develop.

Summary

Astigmatism is actually quite common. But not all the astigmatism is the same. You may have very low levels of astigmatism and not even be aware of it. Actual significant astigmatism occurs at a much lower level. So while you may have astigmatism and you may even be unaware of it, chances are you don't need to worry much about it.

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