astigmatism

Corneal curvature: description

The cornea (cornea) is the foremost part of the eyeball that lies in front of the pupil. It is slightly oval in shape, slightly smaller than a 1-cent piece and about half a millimeter thick. Because it rests on the round eyeball, it is itself spherically curved, much like a contact lens.

What is a corneal curvature?

A corneal curvature (inaccurate: “corneal curvature”) is when the cornea is not uniformly curved. This anomaly is also called “astigmatism.” The medical term is “astigmatism”, which comes from the Greek and means something like “pointlessness”. These terms already indicate the effect on vision of astigmatism.

Normally, the cornea, together with the lens of the eye, causes the parallel incident light rays to focus and focus on a single point of the retina (focus). As a result, sharp vision is possible.

However, when the cornea is bent, the cornea is unevenly curved, which means that the light can not be properly bundled. In some places, incident light rays are focused more tightly than others. They do not unite on the retina in the sequence in a single point: on the retina is not a single clear point displayed – the view is blurred.

What types of astigmatism are there?

In regular astigmatism, incident light rays are projected onto perpendicular focal lines (“rod”). This form of astigmatism can be further subdivided, but this is mainly relevant for the optician, so that he can make a perfectly fitting visual aid.

Irregular is a curvature of the cornea whose optical planes are not perpendicular to each other. To put it simply, while there is still some order in regular astigmatism, in an irregular one sometimes no system is recognizable. In extreme forms, such as scarring of the cornea, the light is deflected in different directions, so that there are hardly any focal lines. The treatment of irregular astigmatism is therefore more difficult.

Astigmatism can also be judged by where the focal lines are in relation to the retina. Often, one is in the retinal plane but the other is in front of it (astigmatism myopicus simplex) or behind (astigmatism hyperopicus simplex). It can also be a focal line in front of it and the other behind it (astigmatism mixtus). Sometimes, in addition to the astigmatism, hyperopia or myopia (astigmatism compositus) is what the specialist calls “hyperopia”.

Astigmatism is possible even without astigmatism

Although the same is often meant with astigmatism and astigmatism, the term “astigmatism” is actually broader. For even irregularities of the lens (lenticular astigmatism) and even the fundus can lead to astigmatism. Astigmatism is the most common reason for astigmatism.

Astigmatism: symptoms

Corneal curvature is only noticeable when it is more pronounced. Mild forms of this vision defect, on the other hand, have hardly noticeable effects for those affected and can be found in many people. However, pronounced astigmatism shows the following symptoms:

• Blurred vision near and far. In contrast, in the case of short-sightedness or farsightedness, only near-vision or only far-sightedness is impaired.
• Headache and eye pain
• in children, possibly a permanent decrease in vision

Many patients complain primarily of headache and eye pain with a slight astigmatism. On the other hand, symptoms of impaired vision often appear later or not at all. This is because the eye constantly tries to correct the blurred picture of the change in the lens shape, which in the long run exerts certain eye muscles and thus eventually causes headaches and eye irritation.

If vision problems occur, then the affected person not only appears blurred, but also distorted. Because there is no focal point on the retina, but focal lines, they see punctate structures rather than stripes or rod. This explains the term “astigmatism”.

If it is more pronounced, an innate astigmatism in children without treatment leads to a so-called low-vision (amblyopia). This is what people call a visual impairment in medicine, because vision is not properly “learned”. Since a sharp image never falls on the retina, no correct sensory perceptions can be transmitted via the optic nerve to the brain, where processing of these data takes place. If a congenital vision defect persists for too long, the brain adjusts to it and then makes normal vision impossible, even if the actual visual defect is finally corrected.

Corneal curvature: causes and risk factors

In many cases, astigmatism is congenital. It is occasionally hereditary – the astigmatism then shows up in several family members. An example of congenital astigmatism is the so-called keratoglobus, in which the cornea is arched and thinned forward.

Under certain circumstances, a corneal curvature also occurs only in adulthood. Then it arises, for example, through:

• Ulcers and scars on the cornea (caused by injuries, inflammation and infections of the cornea)
• Corneal cone (keratoconus): In this case, the cornea bulges in several spurts to a cone, which is usually noticeable between the 20th and 30 years of age.
• surgical procedures on the eye, such as operations to treat a glaucoma (glaucoma)

Astigmatism: examinations and diagnosis

A suspected corneal curvature can clarify the ophthalmologist using various examination methods. It is also possible to determine the type and severity of astigmatism.

So one can the Sehfehler for example by the so-called objective refraction determine. An infrared image is projected onto the patient’s fundus and at the same time it is measured to see if this image is in focus. If this is not the case, different lenses are placed in front of each other until a sharp image is obtained. This allows the examiner to draw conclusions about the type of vision defect.

If it is clear that a corneal curvature is present, one can measure the cornea more accurately and thus determine the astigmatism in more detail. This happens, for example, with a Ophthalmometer, Here, the device, remotely reminiscent of a microscope, projected a hollow and a line cross on the affected person’s cornea: on the ideally shaped cornea, both would lie exactly on top of each other. In the case of astigmatism, on the other hand, the crosses move against each other, and more so the more pronounced the astigmatism of the cornea. Values ​​for the radius of curvature and the refractive power of the cornea can be determined very precisely in this way.

In the case of irregular astigmatism, however, this diagnostic device reaches its limits. Here comes a computer-controlled Corneal topography (Videokeratoscopy) used to analyze the refractive power of the entire corneal surface. This study provides the most accurate data on the nature and severity of astigmatism.

For sufferers who want to get an idea of ​​their own astigmatism, there are special graphics that make this possible. To do this they must look at a so-called astigmatism sun wheel or ray figures from different distances. In the case of astigmatism then typical distortions arise. Such graphics are not a reliable astigmatism test.

If the astigmatism has been specified by different devices, then the subjective refraction: Here the cooperation of the patient is required. While he looks at the charts, the ophthalmologist shows him various visual aids in succession. The patient now has to say with which visual aid he sees the charts the sharpest. When this is clarified, nothing stands in the way of a treatment.

Astigmatism: Treatment

If the angle and the refractive error of a corneal curvature are known, one can try to compensate the visual defect with suitable visual aids:

• Cylindrical lenses with cylindrical cut (cylindrical lenses)
• soft, correspondingly curved contact lenses, which align themselves on the curved cornea
• Hard contact lenses that bend the cornea properly

The first view through eyeglass lenses is blessing and shock for most people with astigmatism. Although they now see sharp points, but the world seems unusually curved. And the later the astigmatism is corrected, the slower the eye gets used to the visual aid. Not infrequently, the habituation is accompanied by headaches.

Astigmatism: Laser surgery

In addition to visual aids, it is also possible to have a corneal curvature lasered. The hot light beam removes the bumps in the cornea and thus creates a uniform surface. Whether a laser procedure comes into question, the ophthalmologist must decide on a case-by-case basis.

Corneal curvature: lens replacement

Another operative treatment approach is the correction of astigmatism by a new lens. The cornea is left as it is, instead the eye lens is removed and replaced with an artificial lens (intraocular lens). It is shaped to balance astigmatism as best it can. This procedure is usually used only with a pronounced astigmatism.

Corneal curvature: Corneal transplantation

In rare cases, neither visual aids nor the aforementioned surgical procedures help further. As a last resort, then the corneal grafting remains. The curved cornea is removed and an intact donor cornea is implanted as a replacement.

Corneal curvature: disease course and prognosis

Compared to short or long sightedness, astigmatism is a relatively complicated visual defect that can not be treated as easily. Unfortunately, the existing vision aids do not offer a 100 percent satisfactory result. Normally, astigmatism does not progress but remains constant. An exception is the Keratokonus: In this variant takes the astigmatism always on to.