astigmatism


Also found in: Dictionary, Thesaurus, Medical, Legal, Financial, Wikipedia.

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.
..... Click the link for more information.
 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
..... Click the link for more information.
. 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.
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.

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.

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
References in periodicals archive ?
The new study can't explain the sources of internal astigmatism.
The incision site was taken in the steepest meridian on the magnitude of pre-existing astigmatism which was evaluated by keratometric reading.
In our study, 150 cases of headache had astigmatism in which 114 cases had astigmatism less than 1 D and 16 cases had astigmatism between 1.
Astigmatism comes from Greek origin ("a" meaning absence and "stigma" meaning point).
Different factors affecting post-operative astigmatism include incision size and shape3, 4,5, location relative to the limbus6, suture technique and material.
The surgical management of patients with astigmatism presents both diagnostic and surgical challenges.
75D or more astigmatism in at least one eye; (2) this suggests that some patients are not wearing their optimal refractive correction in contact lenses, although others will have been fitted with an aspheric design to mitigate for low astigmatism.
Alcon, the global leader in eye care and a division of Novartis, will present a real-world data analysis that looks at the impact of not treating astigmatism during cataract surgery, and results from the More to See survey that reveal a widespread lack of awareness about cataract disease and available treatment options1,2.
Today, cataract surgery is regarded as a refractive surgery, promises rapid visual rehabilitation and pseudophakic emmetropia, which makes eliminating corneal astigmatism critical.
The keratometry readings determine the nature and extent of corneal astigmatism.
Objective: To compare the mean change in corneal astigmatism and clinical appearance after intralesional injection of 5-Fluorouracil in primary and recurrent pterygia.