Astigmatism

If you have blurred vision or your vision is quite sharp but you experience headaches, squinting or other discomfort, you may have astigmatism.

Astigmatism is a condition that prevents your eye from forming a clear image on the retina. It’s usually due to an irregularly-shaped cornea (the transparent outer layer that covers the front of the eye). Sometimes astigmatism is due to scarring of the cornea or to abnormalities in the shape of the crystalline lens (the fine-focussing lens inside the eye). The condition can usually be corrected using spectacle lenses or contact lenses. Most people have astigmatism to some extent but in many cases it’s so slight that it doesn’t need correcting.

It’s worth being aware that astigmatism is a hereditary condition. In many cases children don’t realise that their vision is poor, as they have nothing really to compare it with. Astigmatism in children is often not identified until it begins to affect their schoolwork. This underlines the importance of regular eye checks performed by a professional.

The Effects of Astigmatism

In the normal eye, the cornea and the lens each have a convex, perfectly spherical surface rather like a slice taken from a football (the lens is actually bi-convex, having both sides spherical). This optical system ensures that all of the light that enters the eye is focussed sharply on to the retina.

In the astigmatic eye, the shape of the cornea (or sometimes the lens) is more oval, much like the shape of a rugby ball. In this case the cornea has two different curvatures whose ‘meridians’ or ‘axes’ are at right angles to each other. Only the light that is focussed by one of these curvatures forms a sharp image on the retina. The remainder focuses either behind or in front of the retina, according to the degree of long- or short-sightedness. The result is blurred vision.

How Astigmatism is Treated

Astigmatism is treated using corrective spectacles, contact lenses or laser technology.

Most spectacle wearers will have seen their prescription written down. It looks something like this:

Sphere
+2.50

Cyl
+1.00

Axis
135

The number shown under ‘sphere’ is the strength of the lens that is needed to correct the eye for long- or short-sightedness. The power of a lens is measured in dioptres and is either positive (+) to correct long sight or negative (-) to correct short sight. The ‘cyl’ (if present) is the additional power needed only in the meridian that is out of focus, the ‘axis’, to correct the astigmatism. In this case the axis is at 135° to the horizontal. ‘Cyl’ is short for cylinder, as the shape of this correction on its own is cylindrical. Cyl is also measured in dioptres. The axis is always between 0° and 180°. Lenses used to correct astigmatism are known as ‘toric’ lenses.

Technology has long existed to produce spectacle lenses that correct astigmatism. Today, toric contact lenses can also do the job, sometimes better than spectacles. Contact lenses may be the ‘soft’ type or rigid gas permeable (RGP). RGP lenses are able to hold the shape of the cornea and generally provide a crisper image, although they are sometimes harder to adapt to. As the name implies, RGP contact lenses allow oxygen to permeate through to the cornea, which helps to keep it healthy. RGP lenses are usually used for correcting high astigmatism or astigmatism that is irregular, such as that caused by scarring from surgery or injury to the cornea.

Laser treatment permanently changes the shape of the cornea to virtually eliminate refractive errors. In rare cases, astigmatism is also treated by implanting a corrective lens within the eye.

If you think you might have astigmatism and you have some of the symptoms, arrange an eye test. Astigmatism isn’t a disease and the diagnosis and treatment is completely painless! The optician will also check the general health of your eyes – that's never a bad thing when you consider how you would manage without your eyesight.