Corneal curvature / Top clinic in Germany, Friedrichshafen

The cornea of the eye

The cornea (eye) is the transparent, clear and translucent section of the sheath of the eyeball in front of the pupil. It is wetted by tear fluid and contributes two thirds of the light power of the visual system

The cornea of ​​the eye (cornea) is the translucent, anterior part of the outer eye-skin. The much larger part of this eye skin is the dermis (sclera), which can be seen as the whites in the eye.

The cornea of ​​the eye is as hard as horny substance and so thin that one can see through it as through wafer-thin horny disks: in the center the cornea is only half a millimeter thick, in the edge area about one millimeter. Through the cornea you can see the parts of the eye that are behind it, such as the iris (iris).

The cornea of ​​the eye is constantly kept in a state of swelling by the tear fluid from the outside and the aqueous humor from the inside, both of which have a high salt content - it has a water content of only 76 percent.

As the cornea, like a convex mirror, reflects part of the light that strikes it, the eye shines. After death, the cornea becomes cloudy and becomes dull and opaque.

The cornea (eye) is traversed by nerve fibers, but contains no blood vessels. It is therefore fed via the marginal loop network (a vascular system) at the transition to the sclera.

The five layers of the cornea (eye)

The cornea (eye) consists of five layers:

The outer layer is the anterior corneal epithelium, which passes into the conjunctiva without a clear border. Inside, the cells of this epithelial layer sit on a cell-free glass skin, the Bowman membrane. It heals in case of injuries only with a scarring.

Corneal lamellae, which have different course directions and are held together by a cement substance, follow as the next layer. This is followed by a second glass skin, the Descemet membrane, which consists of a simple cell layer.

The last innermost layer is a single-layered endothelium, which delimits the cornea (eye) against the anterior chamber filled with aqueous humor.

Descemet's membrane

Of particular importance is the Descemet membrane, because it is particularly resistant. Even if the cornea of ​​the eye is injured or is destroyed by disease, the Descemet membrane is usually preserved, thus preventing the outflow of aqueous humor from the anterior chamber of the eye.

If the Descemet membrane were damaged, this condition would be disturbed and the cornea of ​​the eye would swell - with the result that it would no longer be transparent.

Injury to the Descemet membrane would end in scarring, as in the Bowman's membrane.

Corneal function

The cornea (eye) is like a watch glass embedded in the sclera (dermis) and more arched than its surroundings. It has a very high refractive power of 43 diopters - the entire visual system has 60 dioptres. This high refractive power is related to the underlying aqueous humor, which is also highly refractive as a liquid.

This high refractive power is crucial for the function of the cornea: the cornea takes over the majority of the refraction of light, which leads to the focusing of the rays of light on the retina.

What problems can the cornea (eye) cause?

When the marginal loop network is destroyed, the nutrition of the cornea suffers - it comes to cloudiness and ulcers.

In astigmatism, the surface of the cornea (eye) is not spherically curved, but the meridians that go through the center of the cornea are curved differently. Astigmatism is also called astigmatism or astigmatism.

A keratoconus is said to be when the cornea (eye) progressively conically deforms in the middle and thins out at the margin.

Injuries, burns or burns can cause corneal opacities. Foreign bodies can also injure the cornea (eye).

Astigmatism

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.

In the case of astigmatism, an uneven shape of the cornea of ​​the eye causes the incident light to be distorted on the retina. The persons with this illness see in the consequence blurred. The causes of astigmatism are manifold, but often it is innate. With special visual aids, it can often be compensated.

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 impact 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). This makes sharp vision possible.

However, in astigmatism, the cornea is unevenly curved, which means that the light can not be properly focused. 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 he can make a perfectly fitting visual aid.

Irregular is a curvature of the cornea whose optical planes are not perpendicular to one another. To put it simply, while regular astigmatism still has some degree of order, sometimes an irregular system is no longer recognizable in an irregular one. In extreme forms, such as scarring of the cornea, the light is deflected in different directions, so that hardly any focal lines are available. 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: 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 usually makes itself noticeable between the 20th and 30th year of life. - Surgery on the eye, such as operations to treat a green cataract (glaucoma).

Corneal curvature: examinations and diagnosis

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

So you can determine the vision defect, for example, by the so-called objective refraction. 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 them until a sharp image is obtained. This allows the examiner to draw conclusions about the nature of a visual defect.

If it is clear that a corneal curvature is present, one can measure the cornea more accurately and thus determine the astigmatism more closely. This happens, for example, with an ophthalmometer. In this case, the device, remotely reminiscent of a microscope, projects a hollow and a line cross on the patient'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, a computer-controlled corneal topography (videokeratoscopy) is used, which analyzes the refractive power of the entire corneal surface. This study provides the most accurate data on the nature and severity of astigmatism.

For people who want to get an idea of ​​their own astigmatism, there are special graphics that make it 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 of the cornea has been specified by different devices, subjective refraction finally follows: the patient's cooperation is required. While he looks at the charts, the ophthalmologist shows him various visual aids one after the other. 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 glasses)
  • soft, correspondingly curved contact lenses, which align themselves on the curved cornea
  • Hard contact lenses that bend the cornea properly

The first glance through spectacle lenses causes blessing and shock for most patients 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 lase. The hot light beam removes the irregularities 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

Astigmatism is a relatively complicated visual defect that is not as easy to treat as short or long-sightedness. 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 keratoconus: In this variant, the astigmatism of the cornea continues to increase.

Corneal curvature

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MD Martina Bombelli-Huber

MD Martina Bombelli-Huber
Head of the Ophthalmic Department

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