Amblyopia in unilateral congenital ptosis: early detection by sweep visual evoked potential

Abstract

•Background: Fixation preference assessment is a clinical tool widely used to determine amblyopia in young infants and children. It is our clinical experience that this tool underestimates amblyopia. The purpose of this study was to compare the results of sweep visual evoked potentials to fixation preference assessment in cases of unilateral ptosis. • Methods: Sweep visual evoked potentials were performed in 17 children with unilateral ptosis thought to have equal acuity by fixation preference asessment. Binocular and monocular sweep visual evoked potentials were recorded to square-wave gratings of 80% contrast counterphase modulated at 6 Hz. A range of spatial frequencies from 1 to 30 cycles per degree were presented over a 10-s period. Resolution acuity was determined as the zero-microvolt intercept of linear regression analysis on the visual evoked potential amplitude versus spatial frequency. • Results: Nine of the 17 children had interocular resolution acuity differences ranging from 0.8 to 2 octaves by sweep visual evoked potential testing. This correlates to a Snellen equivalent interocular difference of 2 to 7 lines and clinical amblyopia. • Conclusion: This study confirms our clinical impression that children who are unable to preform recognition acuity tasks and are thought to have equal vision by fixation preference assessment often have 2 or more lines of Snellen acuity difference (amblyopia) when they are finally old enough to be tested by Snellen methods. It also implies that amblyopia precedes refractive errors and strabismus in unilateral ptosis cases. Clinical methods to determine amblyopia other than fixation preference assessment need to be explored with a view to earlier detection, better definition and treatment of amblyopia.