The artifact of retinoscopy revisited: Comparison of refractive error measured by retinoscopy and visual evoked potential in the rat

Purpose. The validity of retinoscopy in small eyes has not been clearly established due to uncertainty regarding the source of the ocular reflections assessed during this procedure. A widely cited model which proposes that their origin is the inner limiting membrane of the retina was evaluated in the rat by comparing refractive errors measured by retinoscopy to those measured by visual evoked potentials (VEPs). Methods. Ten rats were refracted both by cycloplegic streak retinoscopy and by VEP while viewing 0 05 to 0.15 cpd square-wave gratings reversed at 1.875 Hz. Spherical aberration of the rat eye was assessed as a potentially confounding variable in VEP refraction by sequential retinoscopic refractions across the rat's natural pupil through a 1.5-mm pinhole. Results. All animals were moderately to highly hyperopic by both methods (range = +4.5 to +18.5 D). Spherical aberration was minimal (median = 3.5 D of overcorrected aberration). The median difference between retinoscopic refractions and those by VEP was not significant (+1.94 D more hyperopia by retinoscopy; p = 0.062, Wilcoxon signed rank) but was significantly less than the +9.64 D difference predicted by an inner limiting membrane model (upper 95% limit = +3.76 D). Conclusion. This suggests that the origin of the retinoscopic reflex is located in the outer retina rather than at the inner limiting membrane. Correction factors for retinoscopy in small eyes may be smaller than previously assumed.