The cornea is aspheric and normally prolate: the central cornea is approximately spherical in front of the pupil and then flattens peripherally through the paracentral and peripheral zones towards the limbal zone, but asymmetrically in different meridia. The centre of the cornea can be defined geometrically (intersection of horizontal and vertical meridia), at the apex, aligned with the pupil centre, or on the visual axis.
Patients with spherical refractions tend to have round or oval patterns. Corneas with regular astigmatism have symmetric or asymmetric bowtie patterns, where as those with irregular astigmatism tend to have an irregular pattern. The right and left eyes of an individual demonstrate enantiomorphism, with mirror image symmetry.
Astigmatism tends to be minimal at birth and then develops “with the rule” (steep in the vertical meridian) during adolescence, possibly due to pressure from the lids. This tends to reduce during middle life and may even reverse to be “against the rule” (steep in the horizontal meridian) later in life.
Contact lens-induced corneal warpage is more common with rigid lenses than soft and is likely to be due to sustained pressure from the lens. Artefacts of corneal shape may arise from errors of alignment and focusing and from tear film drying which manifests as small areas of focal flattening or irregularity.
Corneal topography Corneal shape Normal cornea Aspheric Prolate Oblate Corneal centre Central zone Mid-peripheral zone Limbal zone Corneal meridia Corneal patterns Spherical refraction Meridian Astigmatism Bow tie Enantiomorphism With-the-rule Against-the-rule Contact lens Corneal warpage Artefact Alignment errors Corneal drying
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