ASTIGMATISM

Astigmatism is a type of refractive error where the refraction varies in which the incident parallel rays don't come to a point focus upon the retina, due to refraction varies in different meridians of the eye.

Etiology:

1. Corneal astigmatism is the result of abnormalities of curvature of the cornea. 

2. Lenticular astigmatism is rare. It may be:

 - Curvature due to abnormalities of curvature of lens as seen in lenticonus.

 - Positional due to tilting, or oblique placement of lens as seen in subluxation.

 - Index astigmatism occurs rarely due to the variable refractive index of lens in different meridian. 

3. Retinal astigmatism, due to the oblique placement of macula, is seen occasionally.

Types:

Broadly, there are two types of astigmatism, 

- Regular astigmatism 

- Irregular astigmatism 

Regular astigmatism: The astigmatism is regular when the refractive power changes uniformly from one meridian to another. Depending upon the axis and the angle between the two-principle meridians, regular astigmatism can be classified into the following three types. 

1. Horizonto-vertical astigmatism: In this type the two principal meridians are placed at right angles to one another and those are in the horizontal (180 +/- 20) and vertical planes (90+/- 20). It is of further two types 

• With the rule astigmatism: With the rule astigmatism When the vertical meridian is more curved than the horizontal meridian, this is known as with the rule astigmatism. Sometimes it is physiological due to pressure of the eyelid on the cornea. Up to 0.50D we can ignore the error, as it does not cause many symptoms. This can be corrected either by a – (minus) cylinder x 180 or + (plus) cylinder x 90. 
• Against the rule astigmatism: The horizontal meridian is more curved than the vertical meridian. This is known as inverse astigmatism or against the rule astigmatism. It cause more symptoms and should be corrected for minimal error. This can be corrected either by – (minus) cylinder x 90 or + (plus) cylinder x 180.

2. Oblique astigmatism: Here the two principal meridia are not horizontal or vertical but still they are placed at right angle to each other, e.g. the two meridia can be 45° and 135° or 10° and 100°. C. 

3. Bioblique astigmatism: Here the two principal meridia are oblique and at the same time, they do not intersect at right angle to each other, e.g. one meridian may be at 10° and other at 30°. This type of astigmatism cannot be corrected by spectacles.

Refractive types of regular astigmatism:

Depending upon the position of the focal lines in relation to the retina, regular astigmatism is further classified into three types,

1. Simple myopic astigmatism: One meridian is focused on retina and the other is focused in front of retina, e.g. –1.00 DC at 180° prescription shows that it is a case of simple myopic astigmatism. 

2. Simple hypermetropic astigmatism: One meridian is focused at retina and the other is focused behind the retina, e.g. +1.00 DC at 180° prescription shows that it is a case of simple hypermetropic astigmatism. 

3. Compound myopic astigmatism: Both the principal meridia are focused in front of retina, e.g. –1.00DS/–1.00DC at 180° prescription shows that it is a case of compound myopic astigmatism. 

4. Compound hypermetropic astigmatism: Both the principal meridia are focused behind the retina, e.g. +1.00DS/+1.00DC at 180° prescription shows that it is a case of compound hypermetropic astigmatism. 

5. Mixed astigmatism: One meridian is focused in front of retina and the other is focused behind the retina, e.g. +1.00DS/ –2.00DC at 180° prescription shows that, it is a case of mixed astigmatism. 

Depending upon etiology, astigmatism can be: 

1. Corneal astigmatism: Abnormality lies in the anatomy of cornea 

2. Lenticular astigmatism: It may occur due to tilting of lens or abnormal curvature of lens (lenticonus). 

3. Retinal astigmatism: Due to oblique placement of macula as may occur in scarring of retina. 

Clinical Features

1. Patient may complain of eyeache, headache, tiredness of eyes and sometimes nausea if the error is small. These are known as asthenopic symptoms. 

2. Blurring of vision and distorted appearance of objects may be complained of by some persons.

Tools used in evaluation of astigmatism

 - Keratoscopic examination by placidos disc.

 - Retinoscopy by concave mirror/streak

 - Use of astigmatic fan or Maddox rod

 - Stenopaeic slit or pinhole

 - Keratometry

 - Jackson cross cylinder.

 Astigmatic fan: Astigmatic fan is a construction of vertical lines at a different angle meridian from 0 to 180 degrees. When a patient with astigmatic error looks at the fan some lines seem to be clearer than others, which help to detect the axis of astigmatism. 

Jackson’s cross cylinder: The cross cylinder is a sphero-cylinder lens in which the power of the cylinder is twice the power of the sphere and of the opposite sign. It is used to refine power and axis of cylindrical power.

Stenopic slit: The stenopaeic slit can be used to determine the refraction and principle axis in astigmatism. The slit aperture acts as an elongated ‘pin hole’, only allowing light in the axis of the slit to enter the eye. 

Placido’s disc: This is a flat disc bearing concentric black and white rings. A convex lens is mounted in an aperture in the center of the disc. 

Treatment: 

Regular astigmatism-- 

Optical correction 

- Spectacle (concave – convex cylinder lens) 

- Contact lens (toric lens - prism balastic lens) 

Surgical 

- Keratomiluesis 

- Excimer laser 

Irregular astigmatism 

Hard/semi-soft contact lens 

- Keratoplasty in central area of conical cornea 

- Excision of scar + replacement by graft 

Summary

•  Astigmatism is a type of refractive error where no point focus is formed on the retina. 
• Two main types of astigmatism are regular and irregular astigmatism‘With the rule’{ – (minus) cylinder x 180 or + (plus) cylinder x90} and ‘against the rule’ {– (minus) cylinder x 90 or + (plus) cylinder x 180} astigmatism are the types which are of more interest as we can understand patient symptoms. 
• Refractive types includes simple, compound and mixed astigmatism. 
• Optics of astigmatism can be explained by sturm’s conoid. 
• To find the axis of astigmatism we can use stenopic slit, placido’s disc, astigmatic fan and some techniques of retinoscopy. 
• We can use JCC to refine power and axis of astigmatism. 
• Cylindrical and spherocylindrical lenses can be used to correct astigmatism. 
• Contact lens and surgery are the other options. 

Key points to remember 

- Minimum cylinder, maximum comfort. 

- Give minus cylinder for more comfort. 

- For high cylinder, it is better to do keratometry 

- Refine axis and power before finalizing prescription 

- Check binocular comfort subjectively

• AK- ASTIGMATIC KERATOMY
• LRI- LIMBAL RELAXING INCISION
• CRI- CORNEAL RELAXING INCISION
• PARK- PHOTO ASTIGMATIC REFRACTIVE KERATOTOMY 
• LASIK- LASER INSITU KERATOMILESIS
• SMILE- SMALL INCISION LENTICULAR EXTRACTION.