Astigmatism is one of the most common refractive complaints.
In normal circumstances, the curvature of the cornea is equal along all meridians like a football (or pool ball). In the case of astigmatism, one of the meridians is more curved than the perpendicular meridian, so the surface is more oval-shaped like a rugby ball (or an egg). This difference in curvature between the two meridians means that the image seen on the retina is out of focus. In fact, two focus planes are created corresponding to the two principle meridians – the ones with maximum and minimum curvature.
- Myopic astigmatism:if the astigmatism seen is myopic, the focus point is found in front of the retina.
- Hypermetropic astigmatism:if the astigmatism seen is hypermetropic, the focus point is found behind the retina.
- Mixed astigmatism:this occurs when one of the planes is found in front of the retina and the other is behind.
Usually astigmastism is produced due to the differences in curvature of the rear face of the cornea, i.e. because it is oval-shaped. On rare occasions however, it may be due to the toricity of the lens. This is known as internal or lenticular astigmatism.