Astigmatism is when the shape of the eye isn't completely round. The eye has a curved rugby ball or egg shape, which changes how light rays travel into the eye. This is a type of 'refractive error'.
So, what does astigmatism mean for your eyesight?
Astigmatism causes things at any distance to look blurry or distorted. This can be corrected with prescription glasses or contact lenses.
Astigmatism test and diagnosis
Routine optician appointments include an astigmatism test. Your optician will check your sight and eye health by doing a few different tests. These will include:
- A visual acuity test – you'll read an eye chart without lenses if you don't have a prescription already or wearing your current prescription.
- A refraction test – this can be automated or done manually. You may look at an eye chart while wearing different lenses and say which makes the chart look clearer.
- A test to measure the curve of your astigmatism.
- Tests to check the health of your eye, such as using an ophthalmoscope (specialist torch) and slit lamp to look closely at the front and back of the eye.
Your prescription includes three numbers:
- Spherical – indicating whether you're short or long-sighted and to what degree. A positive number indicates long-sightedness, a negative number indicates near-sightedness. The bigger the number, the stronger the prescription.
- Cylinder – this measures the degree of any astigmatism in dioptres. A prescription of 1.5 or more generally means you need glasses or contact lenses to correct your vision.
- Axis – this describes the location of any astigmatism on the cornea.
Types of astigmatism
Astigmatism is what's known as a type of refractive error. It affects how the eye reacts to light. Astigmatism happens when the cornea or lens curves more one way than another, causing blurred vision, eye strain and headaches if untreated. This can be corrected by lenses, with different strengths needed depending on the curve of the eye.
Both regular and irregular astigmatism can be myopic, hyperopic or mixed, as follows:
Astigmatism is very common, and although it can't be cured, there are straightforward treatments that can correct your sight.
The main treatment options for astigmatism are:
- Corrective lenses – glasses or toric contact lenses can help correct your vision problems caused by astigmatism.
- Refractive surgery – this type of laser surgery can adjust the curvature of the cornea to help you see better. This is usually either LASIK or PRK eye surgery.
If you have astigmatism that hasn't been diagnosed and corrected, you might experience symptoms. The symptoms of astigmatism can include:
- Blurred vision
- Eye strain and irritated eyes
- Trouble with night vision
See an optician if you experience these astigmatism symptoms, especially if they're bothering you as you go about daily life. Having astigmatism doesn't mean something is wrong with your eye health; it's a refractive error that can easily be corrected. Find out more about living with astigmatism.
Astigmatism in children
Children often can't report problems with their sight, so it's important they have regular eye tests, free for children on the NHS. This helps ensure any issues with your child's sight are caught early, preventing avoidable problems that can develop without treatment.
What causes astigmatism?
Astigmatism happens in children and adults. The underlying causes of astigmatism aren't known for sure, but in most people the cornea is not perfectly round. Astigmatism may occur if there is:
- A family history of astigmatism
- A history of eye surgery, for example, for cataracts
- Thinning or scarring of the cornea
- Severe near-sightedness or long-sightedness
Though not caused by ageing, growing older is associated with worsening astigmatism, as changes occur in the cornea.
When to get medical advice
Children and adults should have regular eye tests. If you feel your vision has changed, or if you have any concerns, see your optician. A routine eye exam with an optician can spot astigmatism. All children should have a vision screening test around four to five years old, usually at school.