LAUSR

corneal dimensions and the tick sign, we analysed topographic data available with the Galilei system from 19 eyes with keratoconus in 11 patients (Steinberg et al. 2015). In the affected eyes, the tick sign was present for 26/38 Purkinje patterns. For topographic correlation with our Purkinje patterns, we determined 1 mm temporally and nasally from the pupil centre the difference in corneal thickness between corresponding locations 0.5 mm above and below the horizontal meridian. We found that if this difference exceeded 20 lm (corresponding to 1.15° angle of tilt between anterior and posterior surface), the tick sign was always positive (20 patterns) and where the tick sign was missing, the pachymetric difference was always <20 lm.