astigmatism

astigmatism
/euh stig"meuh tiz'euhm/, n.
1. Also called astigmia /euh stig"mee euh/. Ophthalm. a refractive error of the eye in which parallel rays of light from an external source do not converge on a single focal point on the retina.
2. Optics. an aberration of a lens or other optical system in which the image of a point is spread out along the axis of the system.
[1840-50; ASTIGMAT(IC) + -ISM]

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Lack of symmetry in the curvature of the cornea or, rarely, the lens of the eye.

The unequal curvatures spread light rays, preventing them from being sharply focused at a point on the retina, causing blurring of part of the image. The effect of astigmatism can also be produced by misalignment of the lens. Astigmatic vision is corrected by means of lenses (see contact lens, eyeglasses) that refract the light rays to the proper degree in the opposite direction of that produced by the defects in curvature.

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▪ eye disorder
 nonuniform curvature of the cornea (the transparent, dome-shaped tissue located in front of the iris and pupil) that causes the eye to focus images at different distances, depending on the orientation of light as it strikes the cornea. The effect of astigmatism can also be produced by abnormalities or misalignment of the crystalline lens (sometimes called lenticular astigmatism), although this is rare. The portions of the image that are not focused on the retina (the light-sensitive tissue lining the inside of the eyeball) appear blurred. Astigmatism occurs independent of the existence of nearsightedness (myopia) or farsightedness (hyperopia). This condition usually can be corrected through a precisely designed lens ( eyeglasses or some forms of contact lenses (contact lens)) that counteracts the underlying corneal asymmetry. Refractive surgery is also capable of correcting limited amounts of astigmatism in some situations.

      A troublesome form of astigmatism, “irregular” astigmatism, can be caused by corneal scars and certain corneal diseases and is sometimes seen after corneal transplant. If significant, visual impairment from irregular astigmatism can be difficult to treat, although rigid contact lenses may help.

Daniel M. Albert David M. Gamm
 

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Universalium. 2010.

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  • astigmatism — ASTIGMATÍSM s.n. Aberaţie a unui sistem optic, care formează o imagine întinsă pentru un obiect. ♦ Defect al lentilelor sau al corneei şi cristalinului ochiului omenesc, care constă într o abatere de la forma sferică, ele având razele de curbură… …   Dicționar Român

  • Astigmatism — A*stig ma*tism, n. [Gr. a priv. + ?, ?, a prick of a pointed instrument, a spot, fr. ? to prick: cf. F. astigmatisme.] (Med. & Opt.) A defect of the eye or of a lens, in consequence of which the rays derived from one point are not brought to a… …   The Collaborative International Dictionary of English

  • astigmatism — 1849, coined by the Rev. William Whewell (1794 1866), English polymath, from Gk. a without + stigmatos gen. of stigma a mark, spot, puncture (see STICK (Cf. stick) (v.)) …   Etymology dictionary

  • astigmatism — ► NOUN ▪ a defect of the eye or a lens, resulting in distorted images. DERIVATIVES astigmatic adjective. ORIGIN from Greek stigma point …   English terms dictionary

  • astigmatism — [ə stig′mə tiz΄əm] n. [< Gr a , without + stigma (gen. stigmatos) a mark, puncture (see STICK) + ISM] 1. an irregularity in the curvature of a lens, including the lens of the eye, so that light rays from an object do not meet in a single focal …   English World dictionary

  • Astigmatism — An optical system with astigmatism is one where rays that propagate in two perpendicular planes have different foci. If an optical system with astigmatism is used to form an image of a cross, the vertical and horizontal lines will be in sharp… …   Wikipedia

  • Astigmatism — A common form of visual impairment in which part of an image is blurred, due to an irregularity in the curvature of the front surface of the eye, the cornea. The curve of the cornea is shaped more like an American football or a rugby ball rather… …   Medical dictionary

  • astigmatism — Ametropia Am e*tro pi*a, n. [Gr. ? irregular + ?, ?, eye.] (Med.) a visual impairment resulting from faulty refraction of light rays in the eye. Subtypes include {myopia} {astigmatism} and {hyperopia}. {Am e*trop ic}, a. [1913 Webster +PJC] …   The Collaborative International Dictionary of English

  • astigmatism — noun Date: 1846 1. a defect of an optical system (as a lens) causing rays from a point to fail to meet in a focal point resulting in a blurred and imperfect image 2. a defect of vision due to astigmatism of the refractive system of the eye and… …   New Collegiate Dictionary

  • astigmatism — [[t]əstɪ̱gmətɪzəm[/t]] N UNCOUNT If someone has astigmatism, the front of their eye has a slightly irregular shape, so they cannot see properly …   English dictionary

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