A normal topographic image of a cornea without astigmatism demonstrates a relatively uniform color pattern centrally with a natural flattening in the periphery (Fig 1-9). Regular astigmatism is uniform steepening along a single corneal meridian that can be fully corrected with a cylindrical lens. Topographic imaging of regular astigmatism demonstrates a symmetric “bow-tie” pattern along a single meridian with a straight axis on both sides of center (see Fig 1-9). The bow-tie pattern on topographic maps is an artifact of Placido-based imaging: because the Placido image cannot detect curvature at the central measurement point, the corneal meridional steepening seems to disappear centrally and become enhanced as the imaging gets farther from center.
Irregular astigmatism is nonuniform corneal steepening from a variety of causes that cannot be corrected by cylindrical lenses. Irregular astigmatism decreases best spectacle-corrected visual acuity and may reduce contrast sensitivity and increase visual aberrations, depending on the magnitude of irregularity. Rigid gas-permeable and hard contact lenses can correct visual acuity reductions resulting from corneal irregular astigmatism by bridging the irregular corneal surface and the contact lens with the tear film. For more information on irregular astigmatism, see BCSC Section 3, Clinical Optics.
Corneal topography is very helpful in evaluating eyes with irregular astigmatism. Topographic changes include nonorthogonality of the steep and flat axes (Fig 1-10). Asymmetry between the superior and inferior or nasal and temporal halves of the cornea may also be seen on corneal topography, although these patterns are not necessarily indicative of corneal pathology. In contrast, wavefront analysis can demonstrate higher-order aberrations (such as coma, trefoil, quadrafoil, or secondary astigmatism). The ability to differentiate regular from irregular astigmatism has clinical significance in keratorefractive surgery. Traditional excimer laser ablation can treat spherocylindrical errors but does not effectively treat irregular astigmatism. Topography-guided ablation may be useful in treating irregular astigmatism not caused by early corneal ectatic disorders.