The asymmetric character of central and peripheral corneal astigmatism as determined by photokeratoscopic measurements

A new photokeratoscopic system of analysis which involved the computation of the radii of curvature of the anterior surface of the cornea was used to achieve an improved description of corneal astigmatism. It was hypothesized that the photokeratoscopic measurements would reveal the existence of two axis semi-meridians and two power semi-meridians which did not have an angular separation of 90[degrees]. It was also hypothesized that an evaluation of the peripheral corneal astigmatism would demonstrate that its magnitude was different than the amount of central corneal astigmatism. Measurements of photokeratographs of 30 corneas, selected for central keratometric astigmatism greater than 0.750 D., showed that there actually were four principal semi-meridians with angular separations other than 90[degrees] instead of two principal meridians 90[degrees] apart. The angular extents of each of the four corneal sectors formed by adjacent principal semi-meridians ranged from 55.0[degrees] to 122.5[degrees] for the 30 corneas. The astigmatism varied among the four sectors of each cornea, and it changed from center to periphery within each sector. The astigmatism was greater in the superior corneal sectors than in the inferior, and it was larger nasally than temporally. The general trend peripherally was an increase in astigmatism in the two nasal corneal sectors and a decrease in astigmatism in both temporal sectors. The variations in astigmatism were due to differential rates of corneal flattening. The slope of the central 2.000 mm of each principal semi-meridian was 0.00 which indicated the presence of an optic zone. The slopes of the principal semi-meridians progressively increased peripherally but at different rates. Theoretical and clinical implications of the asymmetric nature of corneal astigmatism were discussed.