Corneal curvature: description
The cornea is the foremost portion 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, much like a contact lens.
What is astigmatism?
A corneal astigmatism (inaccurately, “corneal curvature”) is when the cornea is not evenly curved. This anomaly is also called astigmatism. Physicians speak of astigmatism in the case of corneal curvature, which comes from the Greek and means something like “spotlessness”. Both terms already indicate the effects that astigmatism has on vision:
In astigmatism, however, the cornea is curved unevenly, which means that light cannot be focused properly. Incoming light rays are focused more strongly at some points than at others. As a result, they unite on the retina not in a single point, but on a line (focal line): No single clear point is imaged on the retina – vision is blurred.
What types of corneal astigmatism are there?
In regular astigmatism, incident light rays are imaged on perpendicular focal lines (“rod”). This form of corneal curvature can be further subdivided. However, this is mainly relevant for the optician in order to make a precisely fitting visual aid.
One can also judge a corneal curvature by where the focal lines lie 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 it (astigmatism hyperopicus simplex). It is also possible that one focal line is in front and the other behind (astigmatism mixtus). Sometimes, in addition to astigmatism, there is farsightedness or nearsightedness (hyperopia or myopia, respectively): Astigmatism compositus is what the specialist calls this.
Astigmatism is also possible without astigmatism
Although astigmatism and corneal curvature are often used to mean the same thing, the term “astigmatism” actually has a broader meaning. This is because irregularities of the lens (lenticular astigmatism) and even of the back of the eye can also lead to astigmatism. However, corneal astigmatism is the most common reason for astigmatism.
Astigmatism: Symptoms
- blurred vision at near and far (in contrast to myopia or hyperopia, where only distance vision or only near vision is affected)
- headache and eye pain
- in children, possibly a permanent loss of vision
Many patients complain primarily of headaches and eye pain with mild astigmatism. Symptoms of reduced vision, on the other hand, often show up later or not at all. This is because the eye constantly tries to correct the blurred image by changing the shape of the lens, which in the long run strains certain eye muscles, eventually causing headaches and eye irritation.
When vision problems occur, the environment not only appears blurred to those affected, but usually also distorted. Because there is no focal point on the retina, but rather focal lines, they see point-like structures rather as stripes or rods. This also explains the term “astigmatism.
Astigmatism: Causes and risk factors
In many cases astigmatism is congenital. It is then occasionally hereditary – the corneal curvature then shows up in several family members. An example of a congenital corneal curvature is the so-called keratoglobus, in which the cornea is curved forward and thinned out.
Under certain circumstances, a corneal curvature does not appear until adulthood. Then it arises for example by:
- Ulcers and scarring on the cornea (caused by injuries, inflammations and infections of the cornea)
- Corneal cone (keratoconus): In this condition, the cornea bulges into a cone in several episodes, usually becoming noticeable between the ages of 20 and 30.
- Surgical interventions on the eye, such as operations for the treatment of glaucoma.
astigmatism: examinations and diagnosis
Objective refraction
For example, the visual defect can be determined by so-called objective refraction. This involves projecting an infrared image onto the back of the patient’s eye and simultaneously measuring whether this image is sharp. 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 nature of a visual defect.
Ophthalmometry
If it is clear that a corneal astigmatism is present, the cornea can be measured more precisely and the astigmatism can thus be determined more accurately. This is done, for example, with an ophthalmometer. This device is remotely reminiscent of a microscope. It projects a hollow cross and a reticle onto the cornea of the person affected:
Corneal topography
In the case of irregular astigmatism, the ophthalmometer reaches its limits. In this case, a computer-controlled device (keratograph) is used to analyze the refractive power of the entire corneal surface. This examination provides the most accurate data on the type and degree of corneal astigmatism.
Subjective refraction
Once the corneal curvature has been specified by various instruments, subjective refraction finally follows. Here, the patient’s cooperation is required. While the patient is looking at vision charts, the ophthalmologist holds various vision aids in front of the patient’s eyes one after the other. The patient must now say which visual aid he or she uses to see the charts most clearly. Once this has been clarified, nothing more stands in the way of treatment.
Astigmatism: Treatment
Once the angle and refractive error of a corneal curvature are known, attempts can be made to compensate for the visual defect with appropriate visual aids. Other treatment options include surgery and corneal transplantation.
Corneal curvature: vision aids
The following vision aids can compensate for astigmatism:
- lenses with cylindrical cut (cylindrical lenses)
- soft, appropriately curved contact lenses that self-align on the curved cornea
- hard contact lenses, which bend the cornea correctly
For most people with astigmatism, the first look through eyeglass lenses is both a blessing and a shock. Although they see now point-sharp, but the world appears unusually curved. And the later the astigmatism is corrected, the slower the eye gets used to the visual aid. It is not uncommon for the change to be accompanied by headaches.
Astigmatism: correction through surgery
Another surgical treatment approach is the correction of corneal curvature with a new lens. The cornea is left as it is, and instead the crystalline lens is removed and replaced with an artificial lens (intraocular lens). It is shaped in such a way that it compensates for astigmatism as well as possible. This procedure is usually used only in cases of severe astigmatism.
Astigmatism: corneal transplantation
In rare cases, neither visual aids nor the above-mentioned surgical procedures can help. As a last resort, corneal transplantation remains. The curved cornea is removed and an intact donor cornea is implanted as a replacement.
Corneal curvature: course and prognosis
Normally, astigmatism does not progress but remains constant. An exception is keratoconus: In this variant, the corneal curvature continues to increase.