Biometric confirmation of the Hirschberg ratio in strabismic children.

PURPOSE In the Hirschberg eye position test, the ratio of strabismic angle to decentration of the corneal reflex is dependent on two biometric parameters of the eye: the radius of the corneal curvature and the depth of the anterior chamber. This study was designed to confirm whether the Hirschberg conversion ratio (HR) previously determined for adults can be used for children of various ages despite structural growth of the eye. METHODS For 262 eyes of 131 children with strabismus (age range, 6 months to 11 years), the radius of the corneal curvature was measured with an auto-keratometer and the anterior chamber depth with an A-scan ultrasound unit under general anesthesia before the surgery. Using these measurements, the HR was computed on the basis of a geometric model. RESULTS The calculated HR was constant across the age range, and the mean+/-SD was 19.9+/-1.9 prism diopters/mm (95% confidence interval, 16.1-23.6 prism diopters/mm). The ratios for the two eyes in each subject showed good correlation (R = 0.854, P = 0.0001). Neither of the biometric measurements was significantly correlated with age, although considerable scatter of the measurements was observed. CONCLUSIONS These results indicate that the averaged HR can be applied in children regardless of the patient's age, although intersubject variance of the ratio should be taken into account.