Relationship between form-deprivation myopia and amblyopic deficit

This study investigated the relationship between form-deprivation myopia and amblyopic deficit. Five rhesus monkeys underwent monocular form-deprivation beginning at 24 ± 2 days of age. Lightweight helmets held a light-perception only Bangerter filter over the right eye and a plano lens over the left eye. Refractive error and axial length were measured biweekly using streak retinoscopy and noncontact biometry. After 135 ± 2 days of treatment, visual function in each eye was assessed under sedation by recording Visual Evoked Potentials (VEP) elicited by monocular stimulation with square wave gratings (spatial frequencies 0.25-16 cycles per degree). An Ocular Dominance Index (ODI), the difference in VEP amplitudes between form-deprived (amblyopic) eye stimulation and fellow (control) eye stimulation divided by the sum, quantified amblyopic deficits. At the end of treatment, mean spherical equivalent refraction and axial length were -0.43 ± 4.41 D and 16.66 ± 0.70 mm in form-deprived eyes and +1.75 ± 2.71 D and 16.07 ± 0.61 mm in fellow eyes, respectively. Anisometropia was highly variable, ranging from -6.13 to +2.75 D (mean -2.18 ± 3.60 D). All monkeys demonstrated a significant amblyopic deficit, with a mean ODI of -0.66 ± 0.11 (range: -0.43 to -0.81). Correlations between ODI and anisometropia and ODI and axial length were not significant in this small cohort. Monocular form-deprivation in young rhesus monkeys produced a robust amblyopic deficit as measured by VEPs, even in the absence of myopia. Findings indicate that form-deprivation induces central visual pathway deficits may not be correlated with the amplitude of form-deprivation myopia.