Astigmatism is a focusing disorder of the eye that distorts vision. The higher the level of astigmatism, the poorer the person's quality of vision will be if it is not corrected.
Astigmatism is very common eye condition. A study of more than 11,000 spectacle wearers conducted recently in the UK, showed that almost 50 per cent had some degree of astigmatism in one or both eyes1. Astigmatism often accompanies either myopia (short sight) or hyperopia (long sight) and affects people of all ages.
Astigmatism usually results from the cornea (the transparent front part of the eye) not being totally spherical in shape but having one axis steeper than the other (like a rugby ball rather than a football). It can also be due to irregularity or tilting of the natural lens inside the eye.
Although it was once considered more difficult to correct astigmatism with contact lenses than with glasses, this is no longer the case. Many contact lenses are now available to correct astigmatism (also known as 'toric' lenses) and they come in a wide range of lens types, materials, fittings, powers and designs. Bifocal and multifocal lenses that correct presbyopia (loss of near vision focusing power with age) and astigmatism are also available.
Soft contact lenses wrap to take up the shape of the front of the eye like a glove and are, therefore, unable to correct astigmatism in their basic spherical form. However, they can be made with one axis able to correct more refractive error than the other (toric design). This type of lens requires more care in fitting than a standard soft contact lens, as the angle and power of the astigmatic correction needs to be matched to that of the eye.
These lenses are usually worn during the day only and replaced daily (daily disposable), two-weekly or monthly depending on the lens and your eye. Custom-made and specialty lenses for higher order and irregular astigmatism are also available and are generally designed to be replaced at longer intervals. Toric lenses (lenses for astigmatism) come in silicone hydrogel materials as well as traditional soft lens materials.
Rigid gas-permeable (RGP) contact lenses do not conform to the shape of the eye due to their rigid nature and are, therefore, able to correct low to medium levels of astigmatism in their basic form. For higher levels of astigmatism, they can be made with one axis able to correct more refractive error than the other (toric design).
Again, this type of lens needs more care in fitting than basic gas-permeable contact lenses as the angle and power of the astigmatic correction needs to be matched to that of the eye. These lenses are usually worn during the day only and generally last for six to 12 months.
To learn more about astigmatism and your suitability for contact lenses, visit your eyecare practitioner who will be able to advise you on the best lens for your needs.
1. Young G, Sulley A, Hunt C. (2011) Prevalence of astigmatism in relation to soft contact lens fitting. Visioncare Research Ltd, Farnham, UK.
Illustrations courtesy of CooperVision.