Astigmatism

WHAT IS ASTIGMATISM?

An eye with astigmatism has different powers in different meridians of the eye. This causes light entering the eye to focus in different places, rather than at one single point.The astigmatic cornea has an uneven curvature, so that there is no point of

focus of the light rays on the retina. A cylindrical lens placed in front of the eye with its axis corresponding to the abnormal plane on the cornea will focus the light rays.

CAUSES OF ASTIGMATISM

The surfaces of the cornea and crystalline lens are the major refracting (focusing)

surfaces of the eye. Normal eyes (those without refractive error) as well as hyperopic and myopic eyes all have spherical refracting surfaces. A spherical surface is like that of around ball, and has the same curvature in all meridians (directions) on its surface.

In astigmatism, the refracting surfaces of the eye do not have the same curvature in all meridians (directions).The refracting surfaces of an astigmatic eye are more like thesurfaces of a rugby ball or an egg. A surface that is like that of a rugby ball or egg – not equally round or curved in all directions – is called a toric surface. A toric surface has two meridians: one meridian is steeper (morecurved), the other meridian is flatter (less curved).

CAUSES OF ASTIGMATISM

The most common cause of astigmatism is a toric cornea. This is because the cornea is the eye’s main refracting (light bending) surface.Because the two meridians of an astigmatic eye have different powers, the spectacle lens needed to correct astigmatism must also have different powers in different meridians. Each meridian can be corrected by a cylindrical lens. Usually two cylindrical lenses are needed (one for each meridian), and these cylindrical lenses are joined together to make a single sphero-cylindrical lens.

Sometimes it is easier to think of a toric eyeball as simply being like a soccer football that has been squashed a little bit. A person with astigmatism usually has a corneal surface that is toric, but this will not be noticeable just by looking at them. Special instruments can measure a toric corneal surface, but if you just look at an astigmatic eye it will look perfectly round like a normal eye.This is because the amount of corneal toricity only needs to be very small in order to create significant amounts of astigmatism.

Figure 2 This cornea is not spherical so it does not focus light at one single point causing astigmatism in this eye. A surface which makes two focal points like this is called a toric surface.