Asymmetry of Central and Peipheral Corneal Astigmatism Measured by Photokeratoscopy

The radii of curvature of 10 astigmatic corneas (1.125 to 3.625 D) were measured by photokeratoscopy in 4 semimeridians: 2 axis semimeridians and 2 power semimeridians. Most of the angular separations between adjacent semimeridians differed from 90 deg by more than 2.5 deg; the separations ranged from 55 to 120 deg. Corneal astigmatism, as specified by the difference in curvature between adjacent axis and power semimeridians at corresponding distances from the corneal apex, was also highly asymmetric with decreasing astigmatism in the corneal periphery. An interesting revelation was the interchange of the axis and power semimeridians in the periphery of 3 subjects' corneas.