astigmatism

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Related to mixed astigmatism: myopic astigmatism, hyperopic astigmatism, regular astigmatism

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.
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 Eye-Q mixed astigmatism procedure was approved by the FDA for the reduction or elimination of naturally occurring mixed astigmatism of up to 6.
In related matters, the Company expects that during this month it will submit to the FDA a PMA supplement requesting the Agency's approval to expand the approved indications for the LaserScan LSX to include the treatment of myopia, hyperopia, astigmatism and mixed astigmatism utilizing the LASIK procedure.
Approval of the premarket approval application is for the WaveLight ALLEGRETTO WAVE(TM) Excimer Laser System to perform LASIK treatments in patients 21 years of age or older for the reduction or elimination of naturally occurring mixed astigmatism of up to 6.
In the international market, the LaserScan LSX is utilized to treat the entire range of refractive errors including myopia, hyperopia and mixed astigmatism.
The wavefront-optimized algorithm is applied to all ALLEGRETTO WAVE treatments, including mixed astigmatism.
People with hyperopia (farsightedness) and astigmatism or eyes with mixed astigmatism are now capable of having FDA approved laser vision correction (LASIK) in the United States.
Davila continued, "Our recent new product introductions, which include Fourier software, CustomVue hyperopia, CustomVue mixed astigmatism, and Iris Registration, are also driving conversion to CustomVue procedures.
people with natural or induced mixed astigmatism had no approved treatment
Food and Drug Administration (FDA) to market and sell CustomVue treatments for mixed astigmatism.