LIMBUS VERSUS PUPIL CENTER FOR OCULAR ALIGNMENT MEASUREMENT WITH CORNEAL REFLEXES

Purpose. To investigate the accuracy of ocular misalignment measuremen t, using corneal reflections. Methods, Corneal reflex positions were m easured relative to two landmarks, the limbus center and the entrance pupil center, using high-resolution digital images for cyclopean gaze angles from 0 degrees to 18.8 degrees (34.04 prism diopters; PD) to th e right and to the left in 10 subjects. Distance h from the center of the corneal curvature to each landmark was determined from linear regr essions and showed significant differences between both conditions: me an h(limbus) was 5.243 mm, and mean h(pupil) was 4.884 mm, From these, limbus-and entrance-pupil-center-related Hirschberg ratios were deter mined as 11 degrees/mm (19.43 PD/mm) and 11.82 degrees/mm (20.92 PD/mm ), respectively; and ocular alignment was calculated for both conditio ns. Simulated angles of strabismus were calculated as the binocular di fference between ocular alignment of the right and left eyes (conditio n 1), and as the monocular difference between the ocular alignment of right eyes and left eyes separately (condition 2). Results. Condition 1: Errors in simulated angles of strabismus were approximately twice a s large for entrance pupil center compared with those in limbus-center -based evaluation; in the primary position, the 95% pupil-related conf idence interval of the binocular difference was +/-5.217 degrees (9.1 PD), compared with +/-3.174 degrees (5.5 PD) for the limbus-related op tion. Condition 2: Errors were approximately equal. Conclusions. The e ntrance pupil center is a less reliable landmark than is the limbus ce nter for measuring ocular alignment by using corneal reflections, beca use of unreliable positions of the entrance pupil center; different me an Hirschberg ratios should be used in both conditions.