Duration of binocular decorrelation predicts the severity of latent (fusion maldevelopment) nystagmus in strabismic macaque monkeys

Purpose. Infantile esotropia is linked strongly to latent fixation nystagmus (LN) in human infants, but many features of this comorbidity are unknown. The purpose of this study was to determine how the duration of early-onset strabismus (or time-liness of repair) affects the prevalence of LN in a primate model. Methods. Optical strabismus was created in infant macaques by fitting them with prism goggles on day 1 of life. The goggles were removed after 3 (n = 2), 12 (n = 1) or 24 weeks (n = 3), emulating surgical repair of strabismus in humans at 3, 12, and 24 months of age, respectively. Eye movements were recorded by using binocular search coils. Results. Each animal in the 12- and 24-week groups exhibited LN and manifest LN, normal spatial vision (no amblyopia), and constant esotropia. The 3-week duration monkeys had stable fixation (no LN) and normal alignment indistinguishable from control animals. In affected monkeys, the longer the duration of binocular decorrelation, the greater the LN: mean slow- phase eye velocity (SPEV) in the 24-week animals was three times greater than that in the 12-week monkey (P = 0.03); mean LN intensity in the 24-week monkeys was three times greater than that in the 12-week monkey (P = 0.03). Conclusions. Binocular decorrelation in primates during an early period of fusion development causes permanent gaze instability when the duration exceeds the equivalent of 3 months in humans. These findings support the conclusion that early correction of infantile strabismus promotes normal development of cerebral gaze-holding pathways.