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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 degrees/mm for FFP and 12.81 +/- 1.22 degrees/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 degrees/mm. The 95% limits of agreement of repeated measurements were +/-0.66 degrees/mm for FFP and +/-0.77 degrees/mm for FBP. In infants, the 95% limits of agreement of repeated measurements by FFP were +/-0.63 degrees/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.