Astigmatism causes the focusing error because the light that enters the eye in one direction is focused differently to light that enters the eye in another. This results in a blurry image being formed at the retina (the light-sensitive) tissue at the back of the eye). Astigmatism usually leads to difficulty in seeing fine detail, as well as eye strain and headaches if the degree of astigmatism is high. Astigmatism can be corrected with glasses or contact lenses, although higher levels of astigmatism are better corrected with rigid and scleral lenses (see below).
There are 2 types of astigmatism: regular and irregular. A good way to think of the surface shape of the cornea is to think of everyday objects: a basketball (no astigmatism), a football (regular astigmatism) and an egg (irregular astigmatism). Astigmatism can also be classified as mixed, hyperopic or myopic. This is mainly academic, so we won’t get into it.
1. Regular astigmatism: caused by a cornea which has only 2 curvatures. Most people who don't have keratoconus have this type of astigmatism.
2. Irregular astigmatism: caused by a cornea which has more than 2 curvatures at varying angles. This is most commonly seen in keratoconus but can also be seen in people who have had unsuccessful laser eye surgery and eye injuries.
Keratoconus is a progressive, non-inflammatory thinning and protrusion of the cornea. This creates corneal distortion, and always results in irregular astigmatism. So all people who have keratoconus have astigmatism, but not all people who have astigmatism have keratoconus. Keratoconus usually starts in youth or adolescence, although the age of onset can be anywhere between 7 and 50. It is usually in both eyes, but almost always worse in one eye than the other. It is estimated that 1 in 2000 people have keratoconus in Australia, although this number is probably higher because not all Optometrists have a corneal topographer to diagnose the milder forms of the condition.
Eye rubbing behaviour is frequently associated with keratoconus, so it comes as no surprise that people who have keratoconus also tend to have eye allergies. The distortion produced by keratoconus is not fully correctable with glasses. Rigid contact lenses provide the clearest vision for people with keratoconus. An exciting new solution is emerging which promises much better comfort than standard rigid contact lenses and Kerasoft lens
