What is astigmatism?
In order to see perfectly clearly, any object we look at needs to be uniformly in focus at the back of the eye. Astigmatism usually occurs when either the cornea or crystalline lens of the eye (or in some cases both) are not the ideal spherical ‘football’ shape, but more like the pointed end of a rugby ball. This means the eye focuses light at two separate points on the retina, which is the light-sensitive layer at the back of the eye, this can cause blur and distortion in the vision.
Most glasses wearers will have some form of mild astigmatism. In cases of more significant astigmatism, when not corrected by glasses or contact lenses, symptoms could include:
- Having to squint to see clearly at all distances
- Eye strain or tiredness when focusing on prolonged tasks
What causes astigmatism?
Astigmatism is usually something you’re born with and is also more common in premature or low birth weight babies. Astigmatism can sometimes develop later on in life and may occur following trauma or surgery to the eye.
It’s not clear why it happens if you are born with it, but it’s most likely linked to your genetics and tends to be hereditary. Astigmatism is commonly found in conjunction with long or short-sighted prescriptions, mild levels of astigmatism are very common as very few eyes are perfectly spherical.
More significant astigmatism is often seen if you suffer from an eye condition known as keratoconus which causes your cornea to get progressively thinner and more irregular in shape. This then leads to a more irregular focus with your eyes and results in astigmatism that can develop rapidly as the eye shape changes as the condition progresses.
Lots of children are born with astigmatism, but as they grow and their eyes change shape, it can improve with age. If astigmatism is in one eye only it may cause a lazy eye (amblyopia), a condition that often occurs in children as a result of one eye not seeing as clearly as the other during the development of the child’s vision. It’s important to get your child’s eyes tested to ensure that they can see okay, especially prior to starting school.
Astigmatism could also develop after:
- An eye injury
- Eye surgery
- An eye disease
During your eye test your optometrist will speak to you about any concerns you have about your eyesight and will review the outcome of your pre-test exams.
One of these will be the results of the autorefractor test which measures the shape and focus of your eyes while you focus on a point of light. In looking for refractive errors, the autorefractor notes whether the reflected light coming from the eye is focused differently in one direction to the other, which could indicate astigmatism.
Your optometrist will perform both subjective and objective tests to check your vision and eye health. One of the objective tests used to help detect astigmatism will be a small, handheld instrument called a retinoscope which is used to shine a beam of light into your eye. The optometrist uses the instrument to test how the light beam moves once it enters the eye and is reflected back towards them. The type of reaction shows if astigmatism is present and in what quantity.
Another test used by the optometrist to check for astigmatism will be the Jackson cross cylinder test, which relies on your answers. During this subjective test, the optometrist will show you two choices of lens on a number of occasions and ask which one makes some dots or a letter look the clearest of the two. Both will appear slightly blurry, but this is a way of fine-tuning the power needed to correct the astigmatism.
What is the difference between myopia and astigmatism?
The main difference between myopia (short-sightedness) and astigmatism is that they are two different refractive errors. Astigmatism causes blurred vision at any distance, whereas myopia causes far away objects to appear out of focus.
In most cases, astigmatism is easily corrected with either glasses, specialised contact lenses (called toric lenses) or, in severe cases, surgery.
Your optometrist will be able to detect astigmatism during a routine eye test and they’ll recommend the best option for you.
Glasses can be used to treat regular astigmatism – that’s when the cornea is more curved in one direction than the direction at right angles to it, so the lens can be adjusted to focus light correctly on to the retina in each direction.
If your astigmatism is irregular, which is more common after an eye injury or in cases of keratoconus, for example, then the cornea will be uneven and curved in numerous ways. In this case contact lenses may be the best solution to even out the irregularities in the cornea.
Toric soft contact lenses are made with additional power in one direction specific to the prescription in order to correct regular astigmatism, so they can be worn like standard lenses. We have several different types of toric contact lenses including Acuvue Oasys for astigmatism.
Rigid gas permeable lenses can correct both regular and irregular corneal astigmatism. For higher levels of regular corneal astigmatism and for irregular corneal astigmatism they can give a more stable vision due to them being more rigid.
There’s more information about astigmatism and contact lenses in our detailed article.
Also known as refractive surgery, laser eye surgery corrects focusing problems by reshaping the cornea (the transparent ‘window’ at the front of the eye) with a laser. Although we don’t offer this procedure, we’ve put together this guide for those considering treatment.