Astigmatism (eye)

Astigmatism ( gr A = not, stigma = point, ie the Punktelosigkeit ), also known as astigmatism or astigmatism, referred to in the ophthalmic optics a special refractive error of the eye. Here, the object being viewed from an outgoing light beams will not be focused at one point on the retina level, but shown in a focal line, which has led to the term astigmatism. A light beam incident parallel to the optical axis of the eyeball is refracted to different degrees, depending on its formed with the optical axis plane of incidence. In contrast, the term spherical describes an optical arrangement, the refractive power of the plane of incidence of the light beam is independent (see also Figure errors / astigmatism).

When astigmatism can be usually a plane of incidence with maximum and determine with minimal power. This results in a greater curvature of the corneal surface ( corneal radius less ) to a higher power and a smaller curvature ( a higher corneal radius ) at a lower power. The difference between these two sections is referred to as a so-called main power of the astigmatism. The strength can be described either as the difference in corneal radii, or as the difference in refractive power. The angle of the plane of incidence to the horizontal plane is called axis. In the refraction and on a spectacle prescription, the extent ( refractive index ) is given an astigmatism in diopters ( Dpt. ), the type of correction in cylinder ( Cyl. ) and its position by means of its axial position ( axis). A difference in radii of 0.1 mm corresponds approximately to a power difference of 0.5 Dpt.

In the human eye, astigmatism applies up to 0.5 diopters than normal and is merely the physiological deviation from an ideal shape (circle ) dar. It can be congenital or may develop only with time, but subject as opposed to axial refractive errors such as short - or farsightedness less growth changes.

Causes

Different refractive media of the eye can be involved in the development of astigmatism. This is usually the cornea, which is not exactly circular, but due to a toric not rotationally symmetrical curvature is formed, and thereby created two perpendicular meridians having different refractive power. Hence the slang term " astigmatism ".

Moreover, there is the rarer lens astigmatism. It can be triggered either by a surface which is not purely spherical, or may be due to an accommodative differential contraction of the ciliary muscle. There are differences in the optical density inside the lens layers, there may be a refractive lens astigmatism. In addition, one knows a rare astigmatism of the eye fundus, such as high myopia. The Gesamtastigmatismus of an eye is made up of the aforementioned Einzelastigmatismen. The greatest influence on the Gesamtastigmatismus has due to the high refractive index difference, the cornea.

Corneal diseases such as keratoconus and the Keratoglobus, often lead to strong and irregular astigmatism and moreover to other aberrations. Various ophthalmic surgical procedures such as glaucoma, cataract and strabismus surgery can also lead to astigmatism. Also, astigmatism can be hereditary.

Molding

If a focal line of the astigmatism in the retina plane, while the other is behind it, it is called an astigmatism hyperopicus simplex, it lies in front of a myopic astigmatism simplex. Located in front of a firing line, the other behind the retina level, there is an astigmatism mixtus. Can be found next to the astigmatism still a wide - or short-sightedness, called this state compositus astigmatism.

One distinguishes the regular astigmatism, in which the maximum and minimum refractive planes are perpendicular to each other from the irregular astigmatism, in which the two layers have a different from 90 ° angle to each other.

Regular astigmatism

• Astigmatism according to the rule (A. rectus): the level at maximum power, the axis 90 ° (± 15 °), the minus cylinder a pair of glasses would be at 180 ° (± 15 °); the most common form of astigmatism
• Astigmatism against the rule (A. inversus ): the level at maximum power, the axis 0 ° ( ± 15 °), the minus cylinder a pair of glasses would be at 90 ° (± 15 °)
• Oblique astigmatism (A. obliquus ): all other regular astigmatism

Irregular astigmatism

• The two optical levels are due to an irregular curvature of the cornea is not perpendicular to each other.

Astigmatic Figure

An astigmatic imaging dioptric apparatus of the eye produces a blurred image of the environment on the retina. Whereas in the spherical mapping of a point light source, a focal point is generated, however, results in two focal lines in the astigmatic diagram. A is in the focal distance of the main section with the maximum power ( see figure T1 ) and the other at the focal point distance of the main section to the minimum power ( S1). The axes of these two focal lines are generally perpendicular to each other, respectively. Between the two focal lines (T1 and S1) there is an area in which a point is imaged as a blurred circle (see also: oblique astigmatism ). It is the smaller, the closer the focal lines of the two to each other and thus the lower is the extent of the astigmatism. One calls it the circle of least confusion also. The astigmatic imaging can be illustrated by means of a so-called Sturm's conoid.

A visual acuity reduction occurs in particular when the cylinder axis of the cylinder does not exactly perpendicular (90 °) or horizontal (0 °) is located, but in an oblique plane.

Diagnostics

The determination of astigmatism by means of objective refraction. An accurate measurement of the corneal curvature and topography can also be done with a keratometer or Keratograf. The values ​​determined in this way are the basis for the so-called subjective refraction. With regard to the determination of astigmatism two steps are required for this: the strength and balance of the axis adjustment of the cylinder. For both, a so-called cross cylinder, possibly with the use of a special investigation procedure, the cylinder fogging, zuhilfe taken with the degree of astigmatism and its position can be accurately determined. A cross cylinder balancing can also be done in a refractor.

Using a suitable graphics, such as an astigmatism - sun ( see photo at right ), or a radiating figure Snellen, which is used for the cylinder fine tuning, you can make a rough impression of the own astigmatism himself. This purpose we consider the uniformly shaped graphics with an eye perpendicular to the image plane at different distances, in which you can see a fine picture, and subjectively assessed whether all beams have uniform weights and distances. In the presence of astigmatism, the opposite in the corresponding direction rays will look different from the pairs of rays perpendicular to it. If the picture or the head is rotated around the viewing axis, this direction is in relation to the eye obtained, and does not rotate with the image.

Clinical significance

Astigmatism represents a refractive error of the eye and is usually not as morbid. However, he then developed a certain disease value, if it has a more pronounced, which also allows in a corrected state, no adequate visual acuity, it leads to asthenopic complaints, or if its causes pathological nature ( lens opacities, keratoconus, corneal scars, etc.), and it therefore is an irregular occurrence. A congenital, higher grade astigmatism can cause in early childhood, an irreversible, functional (amblyopia ), if it is not treated early.

A scientific study showed an approximately 85 %, significantly higher prevalence of astigmatism > 1.25 diopters in individuals with a congenital nystagmus than in normal persons.

Possibilities of correction

Regular astigmatism can be up to a certain strength balanced with so-called cylindrical lenses that have an astigmatism and initially accompanied often with a certain habituation neediness. Since the correction consists of a combination of spherical and toric lens, called this also sphero - toric lens. It has two main sections mutually perpendicular, with the arithmetic mean of which is referred to as spherical equivalent refraction. It is calculated using the formula:

This combination allows also represent the value of a cylinder using a conversion formula, either positively or negatively. Here, however, changes in the refractive effect nothing. The following values ​​for example glasses have an identical effect: 1.00 -0.50 / 0.50 0.50 0 ° / 90 °. The reading of these two values ​​is:

• 1.0 -0.5 Sphere combined with cylinder axis 0 degrees or
• 0.5 0.5 Sphere combined with cylinder axis 90 degrees.

Simple spherical rigid contact lenses are to a certain degree correct both regular and irregular Hornhautastigmatismen because between them and the corneal surface is a toric shaped tear film is produced, which compensates for the astigmatism. A balance can be achieved with rigid or soft toric contact lenses, which is maintained by an asymmetric weight distribution of their orientation on the cornea. Finally, refractive surgery procedure can be a way to correct all Astigmatismusformen.

There are rare cases in which an optical correction of astigmatism is satisfactorily reached neither by sphero - toric lenses, nor by toric contact lenses or surgery or lead to significant side effects. In such cases, a correction can with the value of the spherical equivalent offer by appropriate eyeglasses or contact lenses which, although itself does not consider the astigmatism, but visually seen often the best compromise.

Statutory health insurance ( GKV) in Germany, the cost of appropriate corrections only in narrowly defined exceptional cases must take. Is there a general right to a supply of optical appliances, so the costs are covered for the following medically necessary exceptions for contact lenses:

• In an irregular astigmatism with contact lenses if one at least 20 percent improvement in visual acuity compared with glasses is achieved
• In a astigmatism rectus / inversus from 3.00 dpt.
• At an oblique astigmatism from 2.00D.

The cost of refractive procedures may not be covered by statutory health insurance with very few exceptions.

It is from the early 1920s to the present day now and then propagates that various forms of eye training or relaxation exercises could have a positive impact on the objective degree of astigmatism and would this even completely eliminate in some cases. Such statements, however, have not supported until now by a scientific proof of the effectiveness of such procedures or even occupied. Serious work on this subject are not available even after decades. These kinds of practices are therefore rejected by the evidence-based medicine with regard to their alleged effects.

Some people with an astigmatism pinching his eyes and so achieve better visual acuity. They utilize the principle of stenopaic gap advantage by creating a kind of pinhole which interfering edge radiation and peripheral aberrations eliminated and so contributes to a higher depth of field. In general, however, this leads to various troubles ( asthenopia ) and represents no permanent replacement for an optical correction

Etymology

Astigmatism is derived from the ancient Greek word στίγμα stigma " point ", " stigma " from which a reversal of the importance Alpha privative is preceded by corresponding to the prefix "un - " or the suffix " -less" in German. Thus literally means astigmatism Punktlosigkeit or Brennpunktlosigkeit, in which there is no point-like image.