Longitudinal Changes in Nystagmus Following Late Treatment for Congenital Blindness

Purpose: The purpose of this study was to quantify longitudinal changes in nystagmus following sight surgery for congenital blindness using a low-cost smartphone-based technique, in order to gain insight into the plasticity of oculomotor control late in childhood. Methods: Thirteen children aged 6 to 16 years (mean = 9.5 years, 9 girls) were surgically treated for bilateral congenital cataracts as part of Project Prakash in India. They were examined at 5 longitudinal time points: presurgery and 1 week, 1 month, 6 months, and 1 year postsurgery. Eye movements were recorded with a smartphone camera and macro lens. Pupil trajectories were extracted using computer vision techniques, analyzed in the frequency domain, and a nystagmus magnitude metric was defined as the mean magnitude of horizontal (0-5 hertz Hz) movements. This metric was examined longitudinally, and additional comparisons were made with 14 separate patients examined 4 years after surgery. Results: Nystagmus magnitude in the longitudinal group decreased significantly (1.7-fold, P = 0.006) during the first postsurgical year. Cross-sectional comparisons indicate stabilization by approximately 1 year postoperatively. Variability across individuals was not explained by visual acuity, age at surgery, or eye axial length. Binocular recordings revealed strong interocular coordination despite persistent nystagmus. Conclusions: Sight restoration after prolonged congenital blindness yields significant but incomplete reductions in nystagmus, indicating notable but circumscribed neural plasticity in the oculomotor system beyond infancy. In addition to demonstrating the feasibility of smartphone-based quantification of nystagmus in resource-limited settings, these results reinforce the importance of early visual input for calibrating eye movements and highlight important paths for rehabilitation.