Astigmatism is one of the most common eye conditions: an imperfection in the curve of the cornea (the clear front window of the eye) or of the lens inside it. The NHS describes the eye as "shaped more like a rugby ball than a football", so light is focused at more than one place instead of at a single sharp point on the retina. It is not an eye disease, and in most people it is harmless.
How common is it? A 2018 global meta-analysis in the Journal of Current Ophthalmology found astigmatism in about 40% of adults and 15% of children, making it the most common refractive error worldwide. It usually occurs together with short-sightedness or long-sightedness, according to the NHS.
What astigmatism vision looks like. The US National Eye Institute (NEI) lists the typical symptoms as:
- blurry vision
- needing to squint to see clearly
- headaches
- eye strain
- trouble seeing at night
The American Academy of Ophthalmology (AAO) adds "areas of distorted vision" and eye discomfort. Many people describe letters with shadows or edges that smear in one direction. With mild astigmatism you may notice nothing at all, which is why it is often found only at a routine eye test.
Regular and irregular astigmatism. Your surgeon will tell you which type you have, because it decides your treatment:
| Type | What it means | How it is usually treated |
|---|
| Regular astigmatism | The two main curves of the cornea sit at right angles, like a rugby ball. Most astigmatism is this type. | Glasses, toric contact lenses, laser eye surgery, toric ICL or toric lens |
|---|
| Irregular astigmatism | The curves are uneven or not at right angles, often because of keratoconus, scarring or previous surgery | Rigid contact lenses, corneal cross-linking for keratoconus; standard laser is usually not suitable |
|---|
EyeWiki (the AAO's clinical encyclopaedia) explains that regular astigmatism is further described by its axis: "with-the-rule" (common in children), "against-the-rule" (the most common type in adults over 40) and "oblique". You will see the axis on your glasses prescription as a number between 0 and 180, next to the "cylinder" (CYL) value that shows how much astigmatism you have.
What causes it? Nobody knows exactly why corneas differ in shape. The NEI says some people are born with astigmatism and others develop it in childhood or young adulthood; an eye injury, eye disease or surgery can also cause it. The AAO calls it a myth that reading in low light or sitting close to the television causes astigmatism or makes it worse. Astigmatism can change gradually over the years: it may worsen, improve or stay the same.
When it needs checking soon. The NHS advises an eye test if you have symptoms or have not had one for two years, partly because astigmatism can be linked to keratoconus, a thinning and bulging of the cornea. Untreated astigmatism in children can lead to amblyopia (lazy eye), according to the AAO. Astigmatism that keeps increasing year after year, especially in teenagers and young adults, should be checked with a corneal scan.