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Abstract
PURPOSE: The Hirschberg ratio has been defined as the ratio of the change in angular position of the line of sight to the change in location of the first Purkinje image relative to the pupil center. This study was designed to determine empirically whether the adult value of the Hirschberg ratio (approximately 22 prism diopters/mm) is a suitable value to use for all ages. Because many structures in the eye are growing during the 1st year, it is possible that there could be some change in the Hirschberg ratio during this period. METHODS: A photographic technique was used to calculate the Hirschberg ratio for 323 infants between the ages of 27 and 365 days and for ten adult subjects. The study also measured angle lambda, the angle between the line of sight and the pupillary axis, in these populations. RESULTS: The Hirschberg ratio did not change with age during infancy and was similar for adults and infants both in value and in variance. Angle lambda, however, declined rapidly, from an extrapolated value of about 8.4 degrees at birth to near 5 degrees at 5 months, a change that is assumed to reflect axial growth of the eye. CONCLUSIONS: The results suggest that the same average Hirschberg ratio can be used to estimate angle of strabismus across age. The change in angle lambda is important to consider when evaluating angle of deviation from measures using the Hirschberg test.