Astigmatism

The term 'astigmatism' refers to a refractive error of the eye which is characterized by an irregular curvature of the cornea or the lens. Rather than being in the shape of a sphere, the cornea is shaped like an ellipse. When the cornea is curved more in one direction than the other, the light rays that fall on the eye are not refracted properly. When there is a difference in the degree of refraction in various meridians, the capacity of the cornea to focus light is adversely affected. This defect may cause the light rays to focus on two points in the back of the eye instead of only one. 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. This results in blurred vision. This optical defect is generally hereditary. Most of the affected individuals may have an oblong cornea at birth. This vision problem worsens with time. A person who is diagnosed with this refractive error often has difficulty is seeing things clearly. People who have imperfections in the curvature of the cornea or the lens are diagnosed with corneal and lenticular astigmatism respectively.

Types of Astigmatism

This refractive error is categorized into subtypes based on different criteria. It may be classified on the basis of axis of the principal meridians (meridians of greatest and the least refracting powers) or on the focus of the principal meridians. When the principal meridians are perpendicular to each other, one is diagnosed with regular astigmatism. The irregular type, which is also referred to as Murdoch Syndrome, is said to occur when the principal meridians are not perpendicular to each other. The regular type is categorized into the following subtypes:

• With-the-Rule refers to the subtype in which the eye has more refractive power along the vertical axis. The axis stays between 0 to 30 or 150 to 180 degrees.
• Against-the-Rule is another subtype in which the eye has more refractive power along the horizontal axis. Such people face problems in focusing on objects that are oriented vertically. The axis varies between 60 to 120 degree range.
• Oblique refers to another subtype in which the axis is between 30 and 60 or 120 and 150 degrees.

Another classification is based on the focus of the principal meridians. Under this classification, this refractive error is classified into simple, compound and mixed astigmatism. In simple type, only one of the two principal meridians is focused on the retina. This type is further divided into the following:

• In simple hyperopic type, one of the principal meridians is focused on the retina while the other is focused behind the retina.
• In simple myopic type, one of the principal meridians is focused on the retina while the other is focused in front of the retina.

In case of compound type, both the principal meridians are either focused behind the retina or in front of the retina. This type is further divided into the following

• In compound hyperopic type, both the principal meridians are focused behind the retina.
• In compound myopic type, both the principal meridians are focused in front of the retina.

In mixed type, one of the meridians is focused behind the retina while the other is in front of it. The focal lines are on either side of the retina (straddling the retina).

Treatment Options

The treatment depends on the extent of the irregularity and the underlying cause. Keratoconus, which refers to the thinning or degeneration of the structure of the cornea, can cause the cornea to turn from the normal round shape to a conical shape. This can give rise to irregular astigmatism. Toric contact lenses can help in treating this condition. These lenses can treat irregular curvature of the cornea as well as the lens. Various types of contact lenses are available in the market. Rigid gas-permeable contact lenses, which are also known as Ortho-K, can be used to reshape the cornea for improving vision. These lenses are also available as soft lenses. One can also wear spectacles.

One can also undergo LASIK (Laser-assisted in-situ keratomileusis) surgery. This surgical procedure involves the use of an instrument called keratome. A keratome is used 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.

People who have a thin cornea or those who are at a greater risk of suffering from eye injury at work can opt for Laser-assisted subepithelial keratomileusis. In this procedure, a very thin layer of cornea is folded. Thus, the eye is less susceptible to damage in event of an injury. Photorefractive keratectomy (PRK) is another procedure wherein the outer protective layer of the cornea is removed before using an excimer laser to modify the curvature of the cornea.

An irregularly curved cornea or lens can cause headaches, strain, blurred vision, difficulty in focusing and seeing fine details. Visual acuity test, keratometry, retinoscopy and autorefraction tests can help in diagnosing this defect. If one is diagnosed with this defect, one can either wear spectacles or contact lenses or undergo a surgical procedure to correct astigmatism.