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 ?
This helped us to eliminate the patients with posterior corneal astigmatism. Aberrometry was done for all the patients.
In terms of the minor criteria, we determined the proportions of patients in the different age groups with increased AL, flat cornea, and high corneal astigmatism, which were defined as AL> 23.5 mm, Km < 41.5 D, and AST > 1.0 D, respectively (Figure 2).
Biometry and corneal astigmatism in cataract surgery candidatesa from southern China.
Table-1: Mean value of corneal astigmatism in both groups.
The magnitude and direction of the IOL cylinder were based on total corneal astigmatism (TCA) measurement, as calculated by the Sirius with ray-tracing over a 3 mm diameter.
Results: On 12th post-operative week corneal astigmatism of group A (1.46+-0.377) was significantly (p less than 0.05) less than pre-operative astigmatism (2.151+-0.404).
Although combined multifocal tories represent a modality to correct both presbyopia and astigmatism caution does need to be applied for high astigmatism, that is to say, more than 2.50D of corneal astigmatism. The reason being is that some small degree of toric misalignment is relatively common and whereas monofocal toric IOL patients will be unlikely to be bothered by small amounts of residual or consecutive astigmatism--multifocal IOL patients most likely will be
Comparing the corneal astigmatism amongst the various grades of pterygium, a statistically significant increase in astigmatism was noted with the increase in the Grade from I to IV.
Finally, in the corneal refractive astigmatism calculated from autoref, the results were rather coarse; it is better to use some advanced machine to measure corneal astigmatism, such as IOL master [16].
Progressive keratoconus was defined as an increase in central corneal astigmatism of >1.00D or an increase of 1.00D in maximum cone apex curvature documented over a 6 months duration.
Changes in corneal astigmatism and high order aberrations after clear corneal tunnel phacoemulsification guided by corneal topography.