Fixation-free assessment of the Hirschberg ratio

Abstract

PURPOSE. To describe a novel methodology by which to measure the Hirschberg ratio (HR) in infants. The methodology does not require fixation on specific points, and measurements are made while infants look naturally at a display. METHODS. The HR is calculated automatically from measurements of the direction of the optical axis, the position of the pupil center, and corneal reflexes in video images from an advanced two-camera eye-tracking system. The performance of the novel fixation-free procedure (FFP) was evaluated in 43 adults by measuring the average difference and the 95% limits of agreement with the standard fixation-based procedure (FBP). Repeatability of the HR measurements was evaluated by assessing the 95% limits of agreement between two independent measurements. Performance of the FFP was also evaluated in five infants. RESULTS. In adults, the average HR was 12.89 ± 1.22°/mm for FFP and 12.81 ± 1.22°/mm for FBP. FFP and FBP measurements were highly correlated (r = 0.95; P < 0.001). The 95% limits of agreement between FFP and FBP were ±0.86°/mm. The 95% limits of agreement of repeated measurements were ±0.66°/mm for FFP and ±0.77°/mm for FBP. In infants, the 95% limits of agreement of repeated measurements by FFP were ±0.63°/mm. CONCLUSIONS. In adults, the FFP provides accurate measurements of the HR that are in excellent agreement with measurements by FBP. In infants, measurements of HR by FFP show the same repeatability and consistency. © Association for Research in Vision and Ophthalmology.