What is astigmatism?
Astigmatism is a defect in the curvature of the cornea (the dome-like transparent structure which covers the iris and the pupil) or in the shape of the eye lens.
Normally, the cornea and the lens are regular and are curved in the same shape throughout. This helps to focus light clearly onto the retina at the back of the eye. Nevertheless, if the cornea or the lens are not smooth or do not have a regular curve, the rays of light do not refract correctly, which causes a refraction problem.
In a normal eye, the cornea and the lens focus the light onto the retina.
In an astigmatic eye, the images get focused either in front of or behind the retina, which means that the images appear blurred.
Astigmatism of the cornea and the lens
Corneal astigmatism is when the cornea has an irregular shape. This is the more frequent type.
Nevertheless, there is also astigmatism of the lens in which case it is the lens that is deformed. Both cause near and far objects to appear blurred and distorted.
Regular and irregular astigmatism
- Regular astigmatism. The principal meridians are perpendicular to each other and form a 90º angle. Most astigmatisms are regular and are of the cornea.
- Irregular astigmatism. The principal meridians are not perpendicular. It may be the consequence of an injury or surgery that has caused the scarring of the cornea. In addition, it may be caused by a keratoconus, an eye problem which causes the thinning and deformity of the cornea.
Simple and compound astigmatisms
- Simple myopic astigmatism. One of the two principal meridians of the eye focuses light rays in front of the retina. The other focuses correctly onto the retina.
- Simple hypermetropic astigmatism. One of the two principal meridians focuses rays of light behind the retina. The other focuses correctly onto the retina.
- Compound myopic astigmatism. The two main meridians of the eye focus light rays in front of the retina.
- Compound hypermetropic astigmatism. The two principle meridians focus light rays behind the retina.
- Mixed astigmatism. One principle meridian focuses the light in front of the retina and the other behind.
Some people are born with this defect. In fact, the majority have a certain degree of astigmatism that may be combined with other refraction defects, such as myopia or hypermetropia.
Adults with a high degree of astigmatism may realize that something is wrong with their vision due to blurred or distorted vision, visual fatigue, headaches, the need to squint to focus better or other visual problems which, although not necessarily indicating the presence of this refraction defect, do indicate the need to visit the ophthalmologist for a check-up.
In the case of children, they may not realize that they are suffering from this refractive defect, and may not complain of blurred or distorted vision. Nevertheless, if it goes uncorrected, it may affect the child’s performance at school. This is why it is essential that they see the ophthalmologist.
Correction of astigmatism
Normally, low to medium prescriptions can be corrected with glasses or contact lenses (soft or hard). In cases of higher prescriptions, glasses or hard contact lenses are normally more suitable, but this will depend on each specific case.
On the other hand, for some with this refractive defect, various surgical techniques may be an option for correcting their vision.