Conditions Corneal irregularity (astigmatism)

With astigmatism, the cornea of the eye, and sometimes the lens too, is irregularly curved. As a result, incoming light beams do not form a dot on the retina, but lines. This distorts the image perceived so that it is blurred. For example, in the dark you would not see a red traffic light as a red point, but rather as a blurry, short red line. Astigmatism is also known as a corneal irregularity.

The term “astigmatism” itself comes from the Greek and means something like “lacking dots”. This type of visual impairment is usually congenital and is often combined with short-sightedness or far-sightedness.

A mild corneal irregularity often causes no problems and goes unnoticed. If it is less mild, the following symptoms may occur:

• objects are seen blurred or distorted
• eyes are strained or painful
• headaches

A feature of astigmatism is that vision is blurred both close-up and in the distance – in contrast to short- or far-sightedness, with which vision is blurred with objects that are either far away or up close.

The cornea and the lens of the eye refract (bend) the light that enters our eye. The light is thereby focused and converges at a single point. This focal point normally lies on the retina – specifically in the central area of sharpest vision (macula). A particularly large number of visual cells are located there. They convert the light rays into nerve impulses and transmit them to the brain via the optic nerve. This is how a sharp image is created.

The situation is different for astigmatic eyes: in this case, the light rays do not converge on the macula. Instead, they spread out more widely across the retina and form a line. A blurred or distorted image is produced.

The reason for this is usually a corneal irregularity: The cornea covers the iris and pupil and is normally curved evenly in all directions, like a sphere. In the case of astigmatism, however, it is misshapen. In this case, it is more oval – like a ball that someone presses lightly against the ground with their foot. As a result, it does not refract incident light rays uniformly.

Depending on how irregularly the cornea is curved and refracts light, a distinction is made between two forms of astigmatism: 

• regular astigmatism: The cornea refracts light more strongly in the vertical than in the horizontal, for example. This form is more common and is normally congenital. Exactly how it develops is not known. The causes appear to be primarily genetic. 
• irregular astigmatism: The cornea has a completely uneven curve, causing light to refract at widely varying degrees across many points. This tends to occur later in life, for instance due to certain eye conditions such as a progressive deformation of the cornea into a cone shape known as keratoconus. Injuries or scars – for example, after a corneal inflammation or eye surgery – can also cause it.

Rarely, astigmatism is not caused by a curvature of the cornea, but by a deformed lens or retina.

As many people do not have a perfectly curved cornea, astigmatism is quite common. However, it is often so mild that people do not notice it at all. 

An estimated one third of adults in Germany has a noticeable astigmatism of more than 0.5 diopters. It is more common in older people and people whose parents have astigmatism.

The severity of astigmatism does not usually change over the course of a lifetime. However, sometimes it gets worse – especially if it is caused by certain eye diseases such as a progressive corneal deformation (keratoconus). In addition, a cataract can alter the refractive power of the lens and thereby intensify or alleviate astigmatism.

If astigmatism in children is left untreated, this can lead to permanent visual impairment. If the eyes are affected to varying degrees, the brain suppresses the information from the weaker eye so as not to have to process different images. Vision does not then develop properly and a condition known as lazy eye (amblyopia) develops. With most children, this lazy eye can be corrected with glasses or temporarily patching the stronger eye, for example.

Simple vision tests are part of the routine pediatric developmental check-ups for children. Some pediatric practices can also check the refractive power of the eyes using special devices. If abnormalities are detected, the doctor will refer the child to an ophthalmology specialist practice for a more comprehensive examination.

Parents who are astigmatic themselves are advised to have their child examined by an ophthalmologist by the age of two at the latest.

Employers must offer regular eye examinations by the company physician to people who work at a screen.

To diagnose astigmatism, opticians use a range of methods and equipment:    

• Placido disc: This is a round disc with colorful circles that you look at. The ophthalmologist checks how the circles are reflected on the cornea: If they are distorted, this is a sign of astigmatism.
• Ophthalmometry: Using a device called an ophthalmometer, the doctor projects two images onto the cornea. The spacing between the images is then adjusted so that they lie exactly on top of each other on the cornea. This is repeated from various angles. The results show how severely the cornea is curved.
• Corneal topography: The surface of the cornea can be measured very precisely using a computer-controlled device (keratograph) and represented like a topographic map featuring mountains and valleys. This is especially necessary in the case of irregular astigmatism or before eye surgery.

Further investigations determine the severity of the visual defect. This is done using special instruments, or through a vision test where the patient reads characters from an eye chart or screen through lenses of varying strengths.

Mild astigmatism of up to 0.5 diopters is rarely perceived as bothersome – treatment is therefore usually not necessary. More pronounced astigmatism can usually be corrected with glasses or contact lenses. However, glasses do not help with irregular astigmatism – in this case, only rigid (gas-permeable) contact lenses are an option.

There are also various surgical procedures for treating this sight defect. The best method depends, for example, on the severity of the astigmatism and the optician needs to judge this. 

Laser beams are often used in corneal surgery. Laser treatment is used to change the curvature of the cornea. Artificial lenses can also be used in the eye to correct the refraction.

Surgical procedures are normally only used with adults and must be paid for out of pocket. With children, an operation is only an option in specific cases that cannot be treated in a different way. 

If children have poor vision and do not wear vision aids, this can lead to problems at school. Studies have been able to demonstrate this, at least in the case of nearsighted and farsighted children. For instance, children have more difficulty seeing what is on the board and in books. However, their academic performance improved when they received glasses. 

If the astigmatism is sufficiently corrected, it usually doesn’t cause any problems in everyday life. However, children may be particularly reluctant to wear glasses – for instance, if they are teased by other children because of them. It can help to let children help choose their glasses, so that they can choose a “cool” model, for example. It is also important that the glasses are fitted as well as possible and sit comfortably.