Staying away from the optic nerve: a formula for modifying glaucoma drainage device surgery in pediatric and other small eyes

PURPOSE To provide guidelines for safe implantation of glaucoma drainage devices (GDDs) in small and pediatric eyes to avoid contact between the optic nerve (ON) and the posterior edge of the GDD plate. METHODS We developed a formula for calculating limbus-to-ON distance to estimate the available real estate for GDD placement in small eyes. The formula was validated using eyes of pediatric decedents undergoing clinical autopsy, with axial lengths (AL) of 15-24 mm. For each autopsy eye, we measured AL, anterior chamber depth, corneal diameter, and limbus-to-ON distances for the four eye quadrants. The main outcome measure was the degree of agreement between measured and calculated limbus-to-ON'distances. RESULTS A total of 15 autopsy eyes were divided into derivation (n = 10) and validation (n = 5) groups. A formula was derived to estimate superotemporal limbus-to-ON distance (D-sT) using AL and corneal diameter data. Linear regression showed excellent correlation between the measured DsT and AL (R-2 = 0.98). There was excellent agreement between measured and calculated limbus-to-ON values for all four eye quadrants (R2 range, 0.92-0.98). CONCLUSIONS Our formula accurately predicts limbus-to-ON distances across a wide range of clinically relevant ALs. Based on this information, GDD surgery in small eyes can be adjusted by positioning the GDD closer to the limbus or by trimming the posterior edge of the GDD plate. To our knowledge, this is the first set of guidelines developed to promote safe implantation of GDDs in small eyes.