Astigmatism

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In optics, astigmatism is a monochromatic aberrationin which an optical system has different focalplanes for raysin different planes. Lenses and mirrors which are partly cylindrical show this behaviour already for on-axis rays, parabolic mirrors and uncorrected lens systems only proportional to the squared angle between rays and optical axis. Optical systems which are corrected to have no, or only higher order, astigmatism are called anastigmats.

In ophthalmology, astigmatism is a refraction errorof the eyein which there is a difference in degree of refractionin different meridians. It is typically characterized by an aspherical corneain which one axis of corneal steepness is greater than that of the perpendicular axis. Astigmatism causes difficulties in seeing fine detail, and can be often corrected by glasseswith a cylindricallens(i.e. a lens that has different radii of curvature in different planes), contact lenses, or refractive surgery.

Astigmatism occurs when either the cornea or the lensof the eye is not perfectly spherical. As a result, the eye has different focal points in different planes. For example, the image may be clearly focused on the retinain the horizontal plane, but in front of the retina in the vertical plane.

In some cases vertical lines (e.g., walls), may appear to the patient to be leaning over like the tower of Pisa.

A simple eye test

A simple eye test

• If you wear glasses, do this test both with and without wearing your glasses, actually 4 tests.
• Do this test one eye at a time, keeping the other eye covered, actually 2 tests if you don't wear glasses.
• Take a white sheet of blank paper, then using a pen or a pencil put a very black dot in the centre of the paper.
• Moving the paper to and fro find the "best" distance between the paper and the eye, where the dot is blackest and sharpest. Note that distance.
• Observe the dot while moving that paper with its dot to other distances. Do you see more than one dot? Do you see a line instead of a dot?
• Take another blank white paper, but this time put near the middle of it a very black, long straight line.
• Move the paper to the distance from the eye where that line is sharpest and blackest. Rotate the line and observe the difference. There may be an angle of the line that will be "best". Note the angle of the line; if there is an angle that is "best", then this angle is an indication of Astigmatism

Types of astigmatism

Image:Astigmatism text blur.png

Based on asymmetry of structure

• Corneal astigmatism - astigmatism due to an irregularly shaped cornea
• Lenticular astigmatism - astigmatism due to an irregularly shaped lens

Based on axis of the principal meridians

• Regular astigmatism

• Against-the-rule astigmatism
• With-the-rule astigmatism
• Oblique astigmatism

• Irregular astigmatism

Based on focus of the principal meridians

• Simple astigmatism
  • Simple hyperopic astigmatism
  • Simple myopic astigmatism
• Compound astigmatism
  • Compound hyperopic astigmatism
  • Compound myopic astigmatism
• Mixed astigmatism

Prevalence

According to an American study published in Archives of Ophthalmology, nearly 3 in 10 children between the ages of 5 and 17 have astigmatism [1]. A recent Brazilian study found that 34% of the students in one city were astigmatic [2]. Regarding the prevalence in adults, a recent study in Bangladesh found that nearly 1 in 3 (32.4%) of those over the age of 30 had astigmatism[3].

A recent Polish study revealed that "with-the-rule astigmatism" may lead to the onset of myopia[4].

A number of studies have found that the prevalence of astigmatism increases with age[5].

Treatment

Astigmatism may be corrected with eyeglasses, contact lenses, or refractive surgery. Various considerations involving ocular health, refractive status, and lifestyle frequently determine whether one option may be better than another. In those with keratoconus, rigid gas permeable contact lenses often enable patients to achieve better visual acuitiesthan eyeglasses.

Other uses of the term

Compact Discplayers use an astigmatic lens for focusing. When one axis is more in focus than the other, dot-like features on the disc project to oval shapes. The orientation of the oval indicates which axis is more in focus, and thus which direction the lens needs to move. A square arrangement of only four sensors can determine this bias without being fooled by long pits.

Related conditions

• Hyperopia
• Keratoconus
• Myopia
• Presbyopia

See also

• Eyeglass prescription
• Lasik
• Lens (optics)
• Ophthalmology
• Optician
• Optometry

External links

• Astigmatism
• MedlinePlus article
• AllAboutVision.com article
• VisionSimulations.com | What the world looks like to people with various diseases and conditions of the eyede:Astigmatismus

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This article is licensed under the GNU Free Documentation License.
It uses material from the http://en.wikipedia.org/wiki/Astigmatism Wikipedia article Astigmatism.