What is astigmatism?

Astigmatism is also referred to as corneal irregularity and comes from the Greek α- (a-) meaning “without” and “stigma”, “a mark, spot, puncture”. It is a type of refractive error that is caused by an irregularity in the curvature of the cornea that leads to distorted and blurred vision at any distance. It is not age-related and affects short-sighted and long-sighted people.

The curvature of the cornea is measured in dioptres (dpt). This value can be negative or positive and correlates to short- and long-sightedness. If the curvature is 0.75 dpt or greater, it should be corrected. Curvatures over 2.25 dpt are considered more serious.

In astigmatism, the cornea is not curved in the same way as a normal cornea and the surface of the eye is not round, but oval. As a result, light rays entering the eye do not converge at a single point on the retina, but are focused at more than one place in the eye. This is why astigmatism is a type of refractive error. As a result, the vision of a person with an irregular corneal curvature will be blurry or distorted.

Types of astigmatism

Ophthalmologists distinguish between two types of astigmatism:

• Regular astigmatism, which is the most common type and is when the cornea is more curved in one particular direction than the other. In that case, the light rays entering the eye still form a certain pattern, despite converging in the wrong place on the retina. As a result, there will be two focal lines that are perpendicular to one another. 
• Irregular astigmatism: this is when the curvature of the cornea is uneven at multiple points across the surface of the eye. In that case, the focal lines will be at completely different angles to one another. As a result, the cornea refracts light rays in many different directions, which makes correcting this error much more difficult.

Astigmatism is largely congenital and very common. Nearly half of all spectacle wearers in Switzerland have some form of astigmatism. If it develops later in life, it could be the result of any of the following:

• Injury to the eye
• Infection or scarring of the eye as the result of an operation
• Eye diseases such as keratoconus (thinning of the cornea and surface irregularities), which usually develops between the ages of 15 and 30.

Quite frequently, the main symptom of astigmatism is not impaired vision, but things like headaches, eye strain and dizziness. They are the result of the eye’s constant attempts to compensate for the distorted vision by making the lens focus better. This constant effort is tiring for the ciliary muscles, which are responsible for focusing the eye. When astigmatism is more severe, the affected person’s vision will generally be blurry and distorted regardless of distance. This differs from the blurry vision experienced in short- and long-sightedness, where distance always plays a role. In children, there is a risk that, if not corrected, astigmatism will lead to decreased vision in the affected eye over time.

Your ophthalmologist or optician will be able to test you for astigmatism. They will also be able to precisely measure and create a three-dimensional map of the surface curvature of your corneas with a corneal topographer. Regular astigmatism can usually be easily corrected with glasses or contact lenses. It is not usually possible to correct more severe cases of irregular astigmatism with spectacle lenses, in which case hard, custom-made contact lenses will be used. Alternatively – just as with other vision problems – laser surgery is also always an option.