astigmatism

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astigmatism

(əstĭg`mətĭz'əm), type of faulty vision caused by a nonuniform curvature in the refractive surfaces—usually the cornea, less frequently the lens—of the eye. As a result, light rays do not all come to a single focal point on the retina. Instead, some focus on the retina while others focus in front of or behind it. The condition may be congenital, or it may result from disease or injury; it can occur in addition to nearsightednessnearsightedness
or myopia,
defect of vision in which far objects appear blurred but near objects are seen clearly. Because the eyeball is too long or the refractive power of the eye's lens is too strong, the image is focused in front of the retina rather than upon it.
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 or farsightednessfarsightedness
or hyperopia,
condition in which far objects can be seen easily but there is difficulty in near vision. It is caused by a defect of refraction in which the image is focused behind the retina of the eye rather than upon it, either because the eyeball is
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. The spherical lenses used to correct nearsightedness and farsightedness must be specially adapted to correct the out-of-focus plane of vision of the astigmatic eye. When the patient observes a pattern of straight lines placed at various angles, those running in one direction appear sharp while those in other directions (particularly at right angles to the sharp lines) appear blurred. A special cylindrical lens is placed in the out-of-focus axis to correct the condition. In many cases contact lenses are the most effective means of correcting astigmatism.
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Astigmatism: astigmatism due to concave mirrorclick for a larger image
Astigmatism: astigmatism due to concave mirror

astigmatism

(ă-stig -nă-tiz-ăm) An aberration of a lens or mirror system that occurs when light falls obliquely on the system and is focused not as a single point image but as two perpendicular and separated lines. In the reflecting system shown in the illustration rays from points A and B on the mirror converge to the vertical line image ab; rays from C and D converge to the horizontal line image cd. The pencil of reflected rays, elliptical in cross section, cannot produce a sharp image anywhere along its path; the plane of optimum focus occurs between ab and cd where the pencil has its smallest cross section. Astigmatism is not as severe an aberration as coma.

Collins Dictionary of Astronomy © Market House Books Ltd, 2006

astigmatism

[ə′stig·mə‚tiz·əm]
(electronics)
In an electron-beam tube, a focus defect in which electrons in different axial planes come to focus at different points.
(medicine)
A defect of vision due to irregular curvatures of the refractive surfaces of the eye so that focal points of light are distorted.
(optics)
The failure of an optical system, such as a lens or a mirror, to image a point as a single point; the system images the point on two line segments separated by an interval.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.

astigmatism

, astigmia
1. a defect of a lens resulting in the formation of distorted images; caused by the curvature of the lens being different in different planes
2. faulty vision resulting from defective curvature of the cornea or lens of the eye
Collins Discovery Encyclopedia, 1st edition © HarperCollins Publishers 2005
References in periodicals archive ?
The patient included in the study had significant cataract, pre-existing corneal astigmatism of more than 1.25 D.
AL, corneal curvature, and corneal astigmatism were measured using an IOL Master (Carl Zeiss Meditec, Jena, Germany) or an auto refract keratometer (Nidek ARK700A).
Corneal astigmatism was measured by Bausch and Lomb manual keratometer preoperatively and at the 1st, 4th and 6-8th weeks post-operatively.
The corneal astigmatism of the leprosy patients and non-leprosy patients were classified as mild (< 2 D), moderate (2-<4 D), severe (4-< 6 D), or very severe ([greater than or equal to] 6 D).
Conclusion: Intralesional 5-Fluorouracil injection improved cosmesis of primary as well as recurrent pterygia, but did not have statistically significant effect on corneal astigmatism.
Unlike conventional IOLs, the TECNIS Toric 1-Piece IOL can correct a patient's loss of focus due to pre-existing corneal astigmatism of one diopter or greater.
Corneal astigmatism of >1.25 D is prevalent is approximately 30% of eyes with cataract, 16-18 patients with regular astigmatism.
The effect of scleral buckling surgery on corneal astigmatism, corneal thickness, and anterior chamber depth.