What is astigmatism? Symptoms, causes and correction
By See Better · Updated · 5 min read
The short answer
Astigmatism is a common focusing problem in which the front of the eye is curved more in one direction than the other, like a rugby ball instead of a soccer ball. Light focuses at two points instead of one, so vision is blurred or slightly stretched at all distances. It is corrected with glasses or contact lenses.
Key points
- Astigmatism is a difference in the curve of the eye. It is not a disease.
- It causes blur at all distances, eye strain, headaches and streaky lights at night.
- Most people have at least a little, often together with short- or long-sightedness.
- On a prescription it appears as the CYL and AXIS values.
Astigmatism in simple terms
The clear front window of your eye is called the cornea. In an eye without astigmatism, the cornea is curved evenly in every direction, like the surface of a soccer ball. Light passing through it comes to a single sharp point on the retina.
In an eye with astigmatism, the cornea is steeper in one direction and flatter in the other, more like the side of a rugby ball. Light is bent differently depending on where it enters, and it comes to a focus at two points instead of one. The result is a picture that is slightly smeared or stretched. Less often, the cause is the shape of the lens inside the eye and not the cornea.
Astigmatism is a refractive error, in the same family as short-sightedness and long-sightedness. It is not an eye disease, and it does not mean your eyes are unhealthy.
Symptoms of astigmatism
A small amount of astigmatism may cause no symptoms at all. Larger amounts can cause:
- blurry or slightly distorted vision at all distances
- letters that look shadowed or doubled
- confusing similar shapes, such as 8 and 3, or H and N
- eye strain and tired eyes, especially after reading or screen work
- headaches
- squinting
- glare, halos or streaks around lights at night
That last symptom is often the one people notice first. Headlights and street lights seem to flare or smear into lines. See trouble seeing when driving at night.
What causes it
Most astigmatism is simply the shape you were born with, and it often runs in families. It can also develop or change after an eye injury, eye surgery or with certain eye conditions.
Some things do not cause astigmatism, although people often blame them: reading in poor light, sitting too close to the TV, using screens, or wearing the wrong glasses.
How common is it?
Very. Most people have at least a small amount, and it usually comes together with short-sightedness or long-sightedness. Children can have it too. Because they do not know their vision is blurred, significant astigmatism in a child is usually found at an eye test, not through complaints.
Astigmatism on your prescription
Two numbers on a glasses prescription describe astigmatism:
- CYL (cylinder) is the amount, in dioptres. Up to about 0.75 is mild, 1.00 to 2.00 is moderate, and more than that is high.
- AXIS is a number from 0 to 180 that gives the direction of the astigmatism. It is an angle, not a strength.
If the CYL box on your prescription is empty, you have no astigmatism that needs correcting. How to read your glasses prescription covers the other values.
How astigmatism is corrected
- Glasses. The lens is made with extra power in one direction to balance the uneven curve of the eye. This is the simplest and most common solution.
- Toric contact lenses. These are soft contact lenses designed to sit at a set angle on the eye. They need to be fitted by an optometrist.
- Laser eye surgery. It reshapes the cornea and is an option for some adults with a stable prescription.
A first pair of glasses for astigmatism, or a change in the astigmatism correction, can make floors and walls look slightly tilted for a few days. This passes as your brain adjusts. Getting used to new glasses explains what to expect.
Does astigmatism get worse?
It usually changes slowly, if at all, and small shifts over the years are normal. One exception is worth knowing about. Astigmatism that increases quickly in a teenager or young adult, particularly with frequent changes of glasses, can be a sign of keratoconus, a condition in which the cornea thins and bulges. It is treatable and best caught early, so prescription changes like this should be followed up. Rubbing your eyes hard and often is linked to keratoconus and is a habit worth breaking.
Frequently asked questions
Is astigmatism serious?
No. In almost all cases it is a harmless variation in the shape of the eye that is fully corrected with glasses or contact lenses. Significant astigmatism in a young child should be corrected so that vision develops normally.
Can astigmatism go away by itself?
Not usually in adults. It tends to stay fairly stable. In babies and young children it often reduces during the first years of life as the eye grows.
Why do lights look streaky at night with astigmatism?
At night your pupils widen and let in light through more of the cornea, which shows up its uneven curve. Points of light are stretched into streaks or starbursts. The right glasses, ideally with an anti-reflective coating, reduce this.
Can you have astigmatism in only one eye?
Yes. The amount and direction of astigmatism are often different in each eye, and it is possible to have it in one eye only.
This article is general information and is not a diagnosis or a substitute for an eye examination. If you are worried about your eyes or your sight, see an optometrist or a doctor. Sudden loss of vision, eye injuries and severe eye pain are emergencies.
