Astigmatism

Astigmatism is a condition whereby the cornea is more curved along one meridian and less along another, resulting in a cornea that is rugby ball-shaped. Consequently, the eye cannot resolve images into sharp focus, no matter what distance an object is viewed from or how much the viewer accommodates.

The focussing ability of an eye is determined by the curvature of the cornea (front surface of the eye), the power of the crystalline lens in the eye, and the overall length of the eye.

Whereas myopia and hyperopia are normally errors caused by the length of the eye, astigmatism is normally caused by errors in the shape of the front surface of the eye or cornea. Less frequently, the astigmatism is caused by the crystalline lens (referred to as lenticular astigmatism).

In a normal eye, the cornea is spherical in shape and so the curvature is the same in the vertical, horizontal, or any other meridian. This means that light entering the eye will be focussed at the same point, no matter where it enters the eye. In astigmatism this isn’t the case. As with the rugby ball example, the curvature from point to point is less than around the centre.

Light entering the eye is be focussed at a different point, depending upon where it strikes the cornea. Light entering along the steeper curve will be focussed nearer and light entering along the flatter curve further away. Light entering anywhere between these will be focussed at points between the two. As a result, the eye simply cannot focus properly.

The irregularity of shape is classified as “with the rule”, where the flatter curve lies along the horizontal, “against the rule” where it lies along the vertical, and “oblique” if it lies along 45o or 135o.

Astigmatic, or toric, contact lenses can be made to correct all forms of astigmatism.

Because corneal astigmatism is a function of the shape of the cornea, this may need to be taken into consideration when fitting contact lenses, but many designs exist to correct all forms of astigmatism.

Lenticular astigmatism, where the irregularity in shape is due to the crystalline lens, requires a slightly different fitting technique, but can be corrected with a variety of lens designs.

Normal, healthy eyes exhibit regular astigmatism where the maximum curve is at 90o to the flattest curve – as in the rugby ball example.

Occasionally, an irregular astigmatism can occur which may be caused by various other factors. This is discussed elsewhere.

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