Astigmatism is a type of refractive error wherein the refraction varies in the different meridian. Consequently, the rays of light entering in the eye cannot converge to a point focus.
Varieties of ASTIGMATISM
A.Regular Astigmatism: The astigmatism is regular when the refractive power changes uniformly from one meridian to another
Depending upon the position of the two focal lines in relation to retina, the regular astigmatism is further classified into three types:
- Simple astigmatism, wherein the rays are focused on the retina in one meridian and either in front (simple myopic astigmatism) or behind (simple hypermetropic astigmatism) the retina in the other meridian.
- Compound astigmatism. In this type the rays of light in both the meridian are focused either in front or behind the retina and the condition is labelled as compound myopic astigmatism or compound hypermetropic astigmatism, respectively.
3. Mixed astigmatism refers to a condition wherein the light rays in one meridian are focused in front and in other meridian behind the retina. Thus in one meridian eye is myopic and in another hypermetropic. Such patients have comparatively less symptoms as ‘circle of least diffusion’ is formed on the retina
Treatment
1. Optical treatment of regular astigmatism comprises the prescribing appropriate cylindrical lens, discovered after accurate refraction.
i. Spectacles with full correction of cylindrical power and appropriate axis should be used for distance and near vision.
ii. Contact lenses. Rigid contact lenses may correct upto 2-3 of regular astigmatism, while soft contact lenses can correct only little astigmatism. For higher degrees of astigmatism toric contact lenses are needed. In order to maintain the correct axis of toric lenses, ballasting or truncation is required.
2. Surgical correction of astigmatism is quite effective.
B. Irregular Astigmatism : It is characterized by an irregular change of refractive power in different meridia. irregular astigmatism is found in patients with extensive corneal scars or keratoconus.
Treatment 1. Optical treatment of irregular astigmatism consists of contact lens which replaces the anterior surface of the cornea for refraction.
