Surgical Correction of Astigmatism by Jean-Luc Febbraro Hamza N. Khan & Douglas D. Koch

Author:Jean-Luc Febbraro, Hamza N. Khan & Douglas D. Koch
Language: eng
Format: epub
Publisher: Springer International Publishing, Cham

(b)I–S index

This index is based on Placido topography data. It consists of averaging keratometric values obtained at various points of the superior hemicornea (I) and subtracting them from the mean of the points corresponding to the inferior hemicornea (S) (Fig. 8.6). I is calculated by the mean of the five keratometric values on the 3 mm central ring, at the intersection of the 210, 240, 270, 300, and 330 meridians. S is calculated according to the same principle on the 30, 60, 90, 120, and 150 meridians. Rabinowitz initially proposed a cutoff value of 1.2 D to suggest the possible presence of subclinical keratoconus, but a cutoff value of 1.4 D was subsequently adopted to more specifically distinguish between physiological asymmetry and the presence of subclinical keratoconus. An I–S value greater than 1.9 D is commonly associated with clinical keratoconus (visible thinning on the slit lamp, Fleischer ring, etc.). In order to increase the sensitivity of this index, it is recommended to place one of the points used to record keratometric values on the steepest inferior semimeridian.

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