Scheimpflug Characterization of Intrasubject Symmetry in Corneal Thinnest Point Location in an International Normal Cohort

Abstract

Purpose: : To characterize the degree of symmetry in corneal thinnest point location between fellow eyes among normal subjects in an international cohort.

Methods: : International, multicenter, cross-sectional study. Using the Pentacam Eye Scanner (OCULUS GmbH, Wetzlar, Germany), we examined both eyes of 506 normal adults from eight countries spanning six continents. The location of the corneal thinnest point (x, y) was measured relative to the corneal apex (0, 0) and alternatively expressed in polar coordinates (r, theta). Locations were compared between paired eyes. Normative thresholds in intrasubject variation were defined according to the Tukey method.

Results: : The overall thinnest point was located at a median r (interquartile range) of 0.59 (0.44-0.76) mm from the apex in the inferotemporal quadrant, 33 degrees below horizontal. Subgroup medians by country ranged from 0.49-0.69 mm from the apex in the inferotemporal quadrant, 23-54 degrees below horizontal. The median difference in thinnest point locations between paired eyes was 0.31 (0.19-0.47) mm. Separation of greater than 0.89 mm would be expected in fewer than 0.35% of normal corneas. Subgroup thresholds by country ranged from 0.64-1.12 mm.

Conclusions: : Corneas with more than approximately 1 mm of asymmetry in thinnest point location may be suspicious for ectasia. Future studies comparing normal and keratoconus populations are needed to further evaluate this hypothesis.

Keywords: cornea: clinical science • imaging/image analysis: clinical • clinical (human) or epidemiologic studies: risk factor assessment