What causes myopia and astigmatism
The cornea is the foremost section 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. Since it rests on the round eyeball, it is itself spherically curved, somewhat like a contact lens.
What is astigmatism?
Curvature of the cornea (inaccurately: "corneal curvature") is when the cornea is not evenly curved. This anomaly will too Astuteness called. Doctors speak of astigmatism astigmatism, which comes from the Greek and means something like "pointlessness". Both terms already indicate the effects that astigmatism has on vision:
Normally, the cornea, together with the lens of the eye, ensures that the parallel incident light rays are bundled and are focused on a single point of the retina (focal point). This enables sharp vision.
With astigmatism, however, the cornea is unevenly curved, which means that the light cannot be bundled properly. In some places, incident light rays are bundled more strongly than in others. As a result, they do not unite in a single point on the retina, but on a line (focal line): No single clear point is depicted on the retina - the view is blurred.
What types of astigmatism are there?
At the regular astigmatism incident light rays are mapped onto vertical focal lines (“rods”). This form of astigmatism can be further subdivided. However, this is mainly relevant for the optician in order to produce a custom-fit visual aid.
At a irregular astigmatism the optical planes are not perpendicular to one another. To put it more simply: while a certain order still prevails with regular astigmatism, with an irregular astigmatism, sometimes no system is recognizable at all. With extreme shapes (e.g. due to scarring of the cornea), the light is deflected in various directions, so that there are hardly any focal lines. Treatment of irregular astigmatism is therefore more difficult.
A corneal curvature can also be judged by where the focal lines lie in relation to the retina. Often one lies in the retinal plane, but the other in front of it (Astigmatism myopicus simplex) or behind (Astigmatism hyperopicus simplex). There can also be one focal line in front of it and the other behind it (Astigmatism mixtus). Sometimes, in addition to astigmatism, there is farsightedness or nearsightedness (hyperopia or myopia): Astigmatism compositus calls this the expert.
Astigmatism is also possible without astigmatism
Although astigmatism and corneal curvature are often meant the same thing, the term “astigmatism” is actually broader. This is because irregularities in the lens (lenticular astigmatism) and even in the fundus can lead to astigmatism. However, astigmatism is the most common reason for astigmatism.
A corneal curvature is only noticeable if it is more pronounced. On the other hand, slight forms of this visual defect have hardly any noticeable effects on those affected and are found in many people. However, pronounced astigmatism shows the following symptoms:
- Blurred vision near and far (in contrast, with nearsightedness or farsightedness, only distant vision or only near vision is impaired)
- Headache and eye pain
- a permanent decrease in eyesight in children
Many patients complain primarily about a slight astigmatism Headache and eye pain. Symptoms of a impaired vision In contrast, they often only appear later or not at all. This is because the eye is constantly trying to correct the blurred image by changing the shape of the lens, which in the long term puts a strain on certain eye muscles and ultimately leads to headaches and eye irritation.
When vision problems occur, the environment does not only appear to those affected blurred, but mostly too distorted. Because there is no focal point on the retina, but rather focal lines, you see point-like structures more as stripes or rods. This also explains the term “astigmatism”.
If it is more pronounced, a congenital astigmatism in children, if left untreated, leads to a so-called Weak-sightedness (Amblyopia). This is what medicine calls weak eyesight that arises because seeing is not "learned" properly. Since a sharp image never falls on the retina, correct sensory perceptions cannot be transmitted via the optic nerve to the brain, where this data is processed. If a congenital visual defect lasts too long, the brain adapts to it and then makes normal vision impossible, even if the actual visual defect is finally corrected.
Astigmatism: Causes and Risk Factors
In many cases there is an astigmatism innate. It is then occasionally hereditary - the astigmatism then shows up in several family members. An example of a congenital astigmatism is the so-called keratoglobus, in which the cornea is arched forward and thinned.
Under certain circumstances, astigmatism does not appear until adulthood. Then it arises, for example, through:
- Ulcers and scarring on the cornea (caused by injuries, sores, and infections of the cornea)
- Corneal cone (Keratoconus): Here the cornea bulges into a cone in several phases, which is usually noticeable between the ages of 20 and 30.
- surgical interventions on the eye, such as surgery to treat glaucoma
Astigmatism: examinations and diagnosis
The ophthalmologist can use various examination methods to clarify any suspicion of astigmatism. The type and extent of the astigmatism can also be determined in more detail.
For example, the visual defect can be determined using what is known as objective refraction. An infrared image is projected onto the patient's eye fundus and at the same time it is measured whether this image is in focus. If this is not the case, various lenses are placed in front of it until a sharp image is obtained. This allows the examiner to draw conclusions about the type of visual defect.
If it is clear that there is a curvature of the cornea, you can measure the cornea more precisely and thus determine the astigmatism more precisely. This is done, for example, with a Ophthalmometer. This device is vaguely reminiscent of a microscope. It projects a hollow and a reticule onto the patient's cornea:
On the ideally shaped cornea, both would lie exactly on top of each other. In the case of an astigmatism, on the other hand, the crosses move relative to one another, the more pronounced the astigmatism is. The values for the radius of curvature and the refractive power of the cornea can be determined very precisely in this way.
If the vision is irregular, the ophthalmometer reaches its limits. A computer-controlled device (keratograph) is used here, which analyzes the refractive power of the entire corneal surface. This examination provides the most precise data on the type and extent of astigmatism.
If the astigmatism has been specified by various devices, the subjective refraction finally follows. The cooperation of the patient is required here. While he is looking at eye charts, the ophthalmologist holds various visual aids in front of his eyes one after the other. The patient now has to say with which visual aid he can see the vision charts most clearly. Once this has been clarified, nothing stands in the way of treatment.
Special graphics help those affected to get an impression of their corneal curvature: the so-called astigmatism sun wheel and certain ray shapes are perceived with typical distortion when viewed from certain distances due to astigmatism. However, these graphics are not a reliable test of astigmatism!
Read more about the examinations
Find out here which examinations can be useful for this disease:
If the angle and the refractive error of a corneal curvature are known, one can try to compensate for the visual error with suitable visual aids. Other treatment options include surgery and corneal transplants.
Astigmatism: visual aids
The following visual aids can compensate for astigmatism:
- Glasses with cylindrical cut (cylinder lenses)
- soft, curved contact lenses that align themselves on the crooked cornea
- hard contact lenses that bend the cornea properly
For most of those affected with astigmatism, the first look through glasses is both a blessing and a shock. You can see clearly now, but the world appears unusually curved. And the later the astigmatism is corrected, the slower the eye gets used to the visual aid. Getting used to it is often accompanied by headaches.
Corneal curvature: correction by surgery
In some sufferers, there is a possibility of astigmatism laser allow. The hot light beam removes the unevenness in the cornea and thus creates an even surface. The attending ophthalmologist decides whether a laser procedure is an option for a patient.
Another surgical approach to treatment is to correct the astigmatism with 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 compensate for astigmatism as much as possible. This procedure is usually only used if the corneal curvature is very pronounced.
Corneal curvature: corneal transplant
In rare cases, neither visual aids nor the surgical procedures mentioned will help. The last resort is the corneal transplant. The crooked cornea is removed and an intact donor cornea is implanted as a replacement.
Read more about the therapies
Read more about therapies that can help here:
Astigmatism: course and prognosis
Compared to nearsightedness or farsightedness, astigmatism is a relatively complicated visual defect that is accordingly not so easy to treat. Unfortunately, the existing visual aids do not offer a 100 percent satisfactory result.
Usually astigmatism does not progress, but remains constant. Keratoconus is an exception: in this variant, the Astigmatism always on.
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