corneal astigmatism


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astigmatism

 [ah-stig´mah-tizm]
an error of refraction in which a ray of light is not sharply focused on the retina, but is spread over a more or less diffuse area; it is due to differences in curvature in the refractive surfaces (cornea and lens) of the eye. adj., adj astigmat´ic. Its exact cause is not known; some common types of astigmatism seem to run in families and may be inherited. Probably everyone has some astigmatism, since it is rare to find perfectly shaped curves in the cornea and lens, but the defect is rarely serious. If the refractive error is troublesome, corrective lenses may be needed.
compound astigmatism that in which both principal meridians are either hyperopic (compound hyperopic astigmatism, with rays coming into focus behind the retina) or myopic (compound myopic astigmatism, with rays coming into focus in front of the retina).
corneal astigmatism that due to the presence of abnormal curvatures on the anterior or posterior surface of the cornea.
hypermetropic astigmatism hyperopic astigmatism.
hyperopic astigmatism that in which the light rays are brought to a focus behind the retina.
irregular astigmatism that in which the curvature varies in different parts of the same meridian or in which refraction in successive meridians differs irregularly.
lenticular astigmatism astigmatism due to defect of the crystalline lens.
mixed astigmatism that in which one principal meridian is hyperopic and the other myopic.
myopic astigmatism that in which the light rays are brought to a focus in front of the retina.
regular astigmatism that in which the refraction changes gradually in power from one principal meridian of the eye to the other, the two meridians always being at right angles; this condition is further classified as being against the rule when the meridian of greatest refractive power tends toward the horizontal, with the rule when it tends toward the vertical, and oblique when it lies 45 degrees from the horizontal and vertical.

cor·ne·al a·stig·ma·tism

astigmatism due to a defect in the curvature of the corneal surface.

cor·ne·al a·stig·ma·tism

(kōr'nē-ăl ă-stig'mă-tizm)
Astigmatism due to a defect in the curvature of the corneal surface.
References in periodicals archive ?
Providing the ocular astigmatism is greater than the corneal astigmatism, it is possible to reduce the difference between the two principal meridians, and this will reduce the effect of the induced astigmatism.
In the example above, the ocular astigmatism was equal to the corneal astigmatism, so the option here is to fit a bitoric RGP lens, where the front surface would be 'worked' to eliminate the induced astigmatism, known as a compensated bitoric lens.
Measured in the spectacle plane Ocular astigmatism Taken directly from the spectacle astigmatism, after adjusting for the BVD Corneal astigmatism Astigmatism arising just from the cornea, and can be measured using a keratometer Residual astigmatism Residual astigmatism = ocular astigmatism-- corneal astigmatism When a spherical RGP lens is placed on the cornea, it will correct the corneal astigmatism, but not lenticular astigmatism.
An assumption that can be made here is that: Total ocular astigmatism = corneal astigmatism + lenticular astigmatism.
Spherical RGP lenses will only correct corneal astigmatism, and will have no effect on the correction of lenticular astigmatism.
Here, the corneal astigmatism corresponds to the ocular refraction, therefore, spherical RGP lenses would provide a suitable option.
In this instance, fitting a spherical RGP lens would neutralise the corneal astigmatism, leaving the lenticular (residual) astigmatism of -1.
c) Surgical removal of the cause is likely to reduce the level of corneal astigmatism
Toroidal RGP's are useful when there is lenticular astigmatism present, or when there is a high degree of corneal astigmatism.
Back surface toric--this is to correct high degrees of corneal astigmatism and achieve a stable fit