Corneal irregularity (astigmatism)
Astigmatism is a refractive error whereby an irregularly shaped cornea leads to a blurred and distorted vision. In contrast with near-sightedness or far-sightedness, this can cause problems with seeing either near or far away.
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.
Symptoms
Astigmatism is only noticeable when it is more pronounced.
Mild forms of this vision defect, on the other hand, have hardly any noticeable effects on those affected and can be found in many people. Pronounced astigmatism can manifest itself in blurred vision in the near- and far-distance (by contrast, in the case of near-sightedness or far-sightedness only distant vision or only near vision is impaired) and symptoms such as dizziness, eye pain, and headaches. Then the ciliary muscles of the eye (i.e. the ocular muscles that are responsible for the tensing or squinting of the eye) try to compensate for the refractive error of the cornea by focusing the lens.
However, the frequent squinting of the eye leads to overstrain, which is why the aforementioned symptoms occur. Vision is also blurred and distorted. This is due to the fact that there is no focal point on the retina, but rather focal lines, which is why affected people see point-shaped structures more as stripes or bars.
People affected by astigmatism see other people and objects in the near- and far-distance as blurred. This is due to the fact that the brain does not receive any clear information from the retina, which would, however, be necessary to perceive a sharp image. In addition, objects may be distorted and circles may be seen as more oval.
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.
Diagnosis
If the existence of astigmatism is suspected, the ophthalmologist can clarify this using various examination methods and steps. These tests will also allow the type and manifestation of astigmatism to be determined more precisely.
The examination steps in detail:
Objective refraction: An infrared image is projected onto the patient’s back of the eye; at the same time, the sharpness of the image is measured. If it is not sharp, different lenses are placed in front of it until a sharp image is achieved. This allows the doctor to draw conclusions about the type of visual disturbance.
Ophthalmometry: If it is clear that there is astigmatism, the cornea can be measured more precisely and the nature of astigmatism can be determined more precisely. This is done, for example, using an ophthalmometer. This device, reminiscent of a microscope, projects an outline of a cross and a solid cross onto the affected person’s cornea: on an ideally shaped cornea, both crosses would line up exactly on top of each other. In the case of astigmatism, however, the more pronounced the astigmatism is, the more displaced the crosses. The values for the radius of curvature and the refractive power of the cornea can be determined very precisely in this way.
Corneal topography: However, the ophthalmometer reaches the limits of its use in the case of irregular astigmatism. A computer-controlled device (known as the keratograph) is used to analyse the refractive power of the entire corneal surface. This examination provides the most accurate data on the type and manifestation of astigmatism.
Subjective refraction: Once astigmatism has been defined using various devices, this is followed by what is known as subjective refraction. The active cooperation of the patient is required here: while the patient is looking at eye charts, the ophthalmologist holds various visual aids in front of his/her eyes one after the other. The patient must now say which visual aid allows him/her to see the eye charts most clearly. Once this has been clarified, there is nothing standing in the way of treating astigmatism.
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.
Aftercare
Astigmatism is a complicated visual defect compared with near-sightedness or far-sightedness, making it more difficult to treat. What is more, the available visual aids do not offer a completely satisfactory result.
Normally, astigmatism does not progress, but remains constant. An exception to this rule is keratoconus (the inflammatory corneal condition that results in a cone-shaped deformation of the cornea): with this variant, astigmatism continues to progress.
Regular visits to an ophthalmologist and adherence to all necessary check-up appointments are therefore essential in cases of astigmatism.
FAQs
What is the function of the cornea?
The cornea refers to the foremost part of the eyeball that lies in front of the pupil. It is slightly oval-shaped, slightly smaller than a 1-euro-cent coin and about half a millimetre thick. As it rests on the round eyeball, it is – similar to a contact lens – itself spherically curved. In a healthy eye, the cornea – together with the eye lens – ensures that the rays of light entering the eye in parallel are concentrated together and focused on a single point on the retina, known as the focal point. This makes clear vision possible.
Can astigmatism also occur in children?
Children need to be able to see clearly so that their sense of sight can develop fully and their brain can properly learn perception. Otherwise, even a visual aid will no longer be of any help in later life, as the brain is unable to process the information from the retina accordingly. Therefore, it is absolutely necessary to detect refractive errors such as astigmatism in children in good time and then to have them treated. In addition to glasses, contact lenses can also be a suitable visual aid for children.
Can astigmatism regress?
In very rare individual cases, astigmatism may slightly improve. As a rule, however, a corneal irregularity does not grow out. Regular monitoring of visual acuity is very important in cases of astigmatism, as other visual disturbances – such as near-sightedness – may also occur that frequently change over time.
From how many dioptres is astigmatism considered severe?
As soon as astigmatism is 0.75 dioptres and more, it should be compensated for with glasses and/or contact lenses. If the astigmatism is greater than 2.25 dioptres, it is considered to be more severe.
