Astigmatism

Astigmatism is an eye defect in which the vision is affected by an irregularly shaped cornea. The cornea rather being in the shape of a sphere is like an ellipse and decreases the capacity of the cornea to focus light. This defect is a refractive error of the eye where there is a difference in the degree of refraction in various meridians. The eye has varying focal points in different planes. The image may be correctly focused on the retina in the horizontal plane, but not in front of the retina in the vertical plane.

Types of astigmatism
There are basically two types of astigmatism that are based on axis of the principal meridians and based on focus of the principal meridians. On the axis basis, the different types are regular astigmatism in which the principal meridians are perpendicular and irregular astigmatism where the principal meridians are not perpendicular. This is also called Murdoch Syndrome. In the axis basis, the sub-types are as follows:

• with-the-rule astigmatism : axis is between 0 and 30 or 150 and 180 degrees
• against-the-rule astigmatism : axis is between 60 and 120 degrees
• oblique astigmatism : axis is between 30 and 60 or 120 and 150 degrees

On the focus basis, the various types are simple astigmatism, compound and mixed astigmatism. Simple astigmatism is sub-divided into the following:

• simple hyperopic astigmatism : retina coincides with first focal line
• simple myopic astigmatism : retina coincides with second focal line

The different types of compound astigmatism are:

• compound hyperopic astigmatism : both focal lines are ahead of the retina
• compound myopic astigmatism : both focal lines are behind the retina

In mixed astigmatism, the focal lines are on either side of the retina (straddling the retina).

Cause
This occurs when the cornea has the shape of an oblong football rather than the normal shape of a spherical baseball. In many astigmatic eyes, the oblong or oval shape effects in light rays to focus on two points in the back of the eye instead of only one. The reason is that similar to a football, the astigmatic cornea has a steep and flat curve. In regular astigmatism, the meridians in which the two different curves are located are 180 degrees apart. This is easy to correct. In case of irregular astigmatism, the two meridians may be placed at else than 180 degrees apart. Otherwise, there may be more than two meridians. This is complicated and more hard to correct and depends on the extent of the irregularity and the cause. Generally, astigmatism is hereditary . Most of the individuals get birth with an oblong cornea. The resultant viewing problem worsens with time. However, this may also be caused by an eye injury that has been caused by scarring of the cornea or eye surgery or keratoconus (this is a disease that leads to gradual thinning of the cornea).

Treatment of Corrective lenses
Contact lenses : These can treat both corneal and lenticular astigmatism. Different types of contact lenses like hard, soft, disposable, bifocal, rigid gas permeable and extended wear are available in the market. These can also be used in a process called Orthokeratology or Ortho-K. Rigid contact lenses can be used for a long time during the day till the curvature of the eye improves. Then the lenses are used often to retain the novel shape. If the treatment is stopped, the eyes return to the initial shape.

Eyeglasses : This is also a good substitute for contact lenses.

Treatment of Refractive surgery
LASIK surgery : Laser-assisted in-situ keratomileusis involves the use of an instrument called a keratome in which the doctor uses a keratome to make a thin, circular hinged cut in the cornea. A similar cut can be made using a special cutting laser. The surgeon lifts the flap and uses an excimer laser to carve the shape of the cornea below the flap. This type of laser differs from other lasers in the fact that it cannot produce heat.

Laser-assisted subepithelial keratomileusis : A very thinner layer of cornea is folded due to which the eye is less susceptible to damage if an injury takes place. This is a good option for people with thin corneas or those having a high risk of an eye injury at work or playing sports.

Photorefractive keratectomy : The outer protective layer of the cornea is removed before using an excimer laser to modify the curvature of the cornea.