Astigmatism is a type of refractive error wherein the refraction varies in the different meridia. Consequently, the rays of light entering in the eye cannot converge to a point focus but form focal lines. Broadly, there are two types of astigmatism: regular and irregular.
REGULAR ASTIGMATISM
The astigmatism is regular when the refractive power changes uniformly from one meridian to another (i.e., there are two principal meridia).
Etiology
1. Corneal astigmatism is the result of
abnormalities of curvature of cornea. It constitutes the most common cause of astigmatism.
2. Lenticular astigmatism is rare. It may be:
i. Curvatural due to abnormalities of curvature of lens as seen in lenticonus.
ii. Positional due to tilting or oblique placement of lens as seen in subluxation.
iii. Index astigmatism may occur rarely due to variable refractve index of lens in different meridia.
3. Retinal astigmatism due to oblique placement of macula may also be seen occasionally.
Types of regular astigmatism
Depending upon the axis and the angle between the two principal meridia, regular astigmatism can be classified into the following types :
1. With-the-rule astigmatism. In this type the two principal meridia are placed at right angles to one another but the vertical meridian is more curved than the horizontal. Thus, correction of this astigmatism will require the concave cylinders at 180° ± 20° or
convex cylindrical lens at 90° ± 20°. This is called 'with-the-rule' astigmatism, because similar astigmatic condition exists normally (the vertical meridian is normally rendered 0.25 D more convex than the horizontal meridian by the pressure of eyelids).
2. Against-the-rule astigmatism refers to an astigmatic condition in which the horizontal meridian is more curved than the vertical meridian. Therefore, correction of this astigmatism will require the presciption of convex cylindrical lens at 180° ± 20° or concave cylindrical lens at 90° ± 20° axis.
3. Oblique astigmatism is a type of regular astigmatism where the two principal meridia are not the horizontal and vertical though these are at right angles to one another (e.g., 45° and 135°). Oblique astigmatism is often found to be symmetrical (e.g., cylindrical lens required at 30° in both eyes) or complementary (e.g., cylindrical lens required at 30° in one eye and at 150° in the other eye).
4. Bioblique astigmatism. In this type of regular astigmatism the two principal meridia are not at right angle to each other e.g., one may be at 30o and other at 100°.
Optics of regular astigmatism
As already mentioned, in regular astigmatism the parallel rays of light are not focused on a point but form two focal lines. The configuration of rays refracted through the astigmatic surface (toric surface) is called Sturm’s conoid and the distance between the two focal lines is known as focal interval of Sturm. The shape of bundle of rays at different levels .
REGULAR ASTIGMATISM
The astigmatism is regular when the refractive power changes uniformly from one meridian to another (i.e., there are two principal meridia).
Etiology
1. Corneal astigmatism is the result of
abnormalities of curvature of cornea. It constitutes the most common cause of astigmatism.
2. Lenticular astigmatism is rare. It may be:
i. Curvatural due to abnormalities of curvature of lens as seen in lenticonus.
ii. Positional due to tilting or oblique placement of lens as seen in subluxation.
iii. Index astigmatism may occur rarely due to variable refractve index of lens in different meridia.
3. Retinal astigmatism due to oblique placement of macula may also be seen occasionally.
Types of regular astigmatism
Depending upon the axis and the angle between the two principal meridia, regular astigmatism can be classified into the following types :
1. With-the-rule astigmatism. In this type the two principal meridia are placed at right angles to one another but the vertical meridian is more curved than the horizontal. Thus, correction of this astigmatism will require the concave cylinders at 180° ± 20° or
convex cylindrical lens at 90° ± 20°. This is called 'with-the-rule' astigmatism, because similar astigmatic condition exists normally (the vertical meridian is normally rendered 0.25 D more convex than the horizontal meridian by the pressure of eyelids).
2. Against-the-rule astigmatism refers to an astigmatic condition in which the horizontal meridian is more curved than the vertical meridian. Therefore, correction of this astigmatism will require the presciption of convex cylindrical lens at 180° ± 20° or concave cylindrical lens at 90° ± 20° axis.
3. Oblique astigmatism is a type of regular astigmatism where the two principal meridia are not the horizontal and vertical though these are at right angles to one another (e.g., 45° and 135°). Oblique astigmatism is often found to be symmetrical (e.g., cylindrical lens required at 30° in both eyes) or complementary (e.g., cylindrical lens required at 30° in one eye and at 150° in the other eye).
4. Bioblique astigmatism. In this type of regular astigmatism the two principal meridia are not at right angle to each other e.g., one may be at 30o and other at 100°.
Optics of regular astigmatism
As already mentioned, in regular astigmatism the parallel rays of light are not focused on a point but form two focal lines. The configuration of rays refracted through the astigmatic surface (toric surface) is called Sturm’s conoid and the distance between the two focal lines is known as focal interval of Sturm. The shape of bundle of rays at different levels .
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