Limbus versus pupil center for ocular alignment measurement with corneal reflexes

To investigate the accuracy of ocular misalignment measurement, using corneal reflections.

Corneal reflex positions were measured relative to two landmarks, the limbus center and the entrance pupil center, using high-resolution digital images for cyclopean gaze angles from 0 degree to 18.8 degrees (34.04 prism diopters; PD) to the right and to the left in 10 subjects. Distance h from the center of the corneal curvature to each landmark was determined from linear regressions and showed significant differences between both conditions: mean hlimbus was 5.243 mm, and mean hpupil was 4.884 mm. From these, limbus- and entrance-pupil-center-related Hirschberg ratios were determined 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 conditions. Simulated angles of strabismus were calculated as the binocular difference between ocular alignment of the right and left eyes (condition 1), and as the monocular difference between the ocular alignment of right eyes and left eyes separately (condition 2).

Condition 1: Errors in simulated angles of strabismus were approximately twice as large for entrance pupil center compared with those in limbus-center-based evaluation; in the primary position, the 95% pupil-related confidence interval of the binocular difference was +/- 5.217 degrees (9.1 PD), compared with +/- 3.174 degrees (5.5 PD) for the limbus-related option. Condition 2: Errors were approximately equal.

The entrance pupil center is a less reliable landmark than is the limbus center for measuring ocular alignment by using corneal reflections, because of unreliable positions of the entrance pupil center; different mean Hirschberg ratios should be used in both conditions.