Corneal irregularity (astigmatism)

What is astigmatism?

Astigmatism is caused by the fact that the surface of the eye – i.e. the cornea – or the eye lens behind it has an oval shape (similar to a rugby ball) rather than a round shape. As a result, the light that hits the eye is not correctly focused on the retina, which in turn causes the image perceived by the eye to appear blurred.

Causes

The reason for astigmatism is that the cornea of the eye is not exactly spherical, as is the case with a healthy eye.

Astigmatism

The term ‘astigmatism’ is not only used to refer to corneal irregularity. This is because the lens of the eye can also be unevenly curved and therefore result in astigmatism. Regardless of the origin of the condition, a distinction is made in medicine between regular and irregular astigmatism, with regular astigmatism occurring most commonly. Corneal irregularity results in two focal lines, which are aligned perpendicular to one another. The irregular curvature therefore still follows certain rules, which is why it can usually be easily compensated for with a visual aid. In the case of irregular astigmatism, however, several corneal areas have different refractive power, whereby the light is refracted in a variety of ways; in extreme cases, focal lines are no longer even visible. Irregular astigmatism is therefore much more difficult to treat than regular astigmatism.

Although the terms ‘corneal irregularity’ and ‘astigmatism’ are often used to mean the same thing, there is actually something more hidden behind the concept of ‘astigmatism’: Irregularities of the lens (= lenticular astigmatism) or the back of the eye can also lead to astigmatism. However, corneal irregularity is the most common reason for astigmatism.

Other causes

Corneal irregularity is often congenital and may have genetic causes. A slightly uneven cornea up to 0.5 dioptres is quite common and is even considered normal. Causes such as ulcers and scarring on the cornea (caused by injuries, inflammations and infections of the cornea) or eye surgery (e.g. to treat cataract) are often responsible for astigmatism that only occurs later in life. Eye diseases such as keratoconus (= an inflammatory corneal condition that results in a cone-shaped deformation of the cornea) can also cause irregular astigmatism. This is rather rare, however.

Treatments

If the angle and refractive error of a corneal irregularity are known, it is possible to attempt to compensate for the visual defect with suitable visual aids. Other treatment options include surgery and corneal transplantation.

Glasses

In most cases, lenses polished into a cylindrical shape (known as cylindrical lenses) or contact lenses can correct astigmatism.

Contact lenses

The type of contact lens that is most compatible with astigmatism depends on the individual requirements. Hard contact lenses are often a good choice for astigmatism with slight refractive error. A tear film forms under the hard lenses, which automatically compensates for the corneal irregularity. Soft, correspondingly curved contact lenses can also correct astigmatism. These are known as toric contact lenses, which are shaped differently to conventional lenses.

Laser eye surgery

In some cases, it is also possible to treat astigmatism with a laser. The hot light beam removes the unevenness in the cornea and therefore creates an even surface. However, the ophthalmologist treating the patient decides whether a laser procedure is a suitable treatment method for a patient.

Artificial lens

Another surgical treatment method is the correction of astigmatism by inserting a new lens. The cornea is not altered; instead, the eye lens is removed and replaced by an artificial lens, known as the intraocular lens. It is shaped in such a way that it can compensate for astigmatism in the best possible way. However, this procedure is usually only used in cases of severe astigmatism.

Corneal transplantation

In rare cases, however, neither visual aids nor the surgical procedures mentioned above are of any help. The last resort is corneal transplantation. The unevenly curved cornea is removed and an intact donor cornea is implanted as a replacement.