/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]
* * *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.
* * *▪ eye disordernonuniform 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.
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