PURPOSE: To learn whether electrophysiological changes indicating amblyopia 
occur even in the absence of clinically recognizable amblyopia.
DESIGN: Prospective study.
METHODS: Four consecutive infants between 7 and 19 months of age with unilateral 
periocular vascular lesions that intermittently obstructed vision in the 
affected eye and no clinical evidence of amblyopia were evaluated. No child had 
anisometropia greater than 0.50 diopter in the greatest meridian or strabismus. 
Sweep visual evoked potential vernier acuity was measured under monocular 
viewing conditions with the fellow eye tested as the control.
RESULTS: Response amplitudes and acuity thresholds were significantly diminished 
in the affected eyes. A phase analysis showed slowing of the response in the 
affected eyes compared with the control eyes.
CONCLUSIONS: An amblyopia-like effect on vernier acuity occurred in infants with 
unilateral periocular vascular birthmarks when the lesion caused intermittent 
occlusion of the eye. Whether long-term effects will occur is unknown, but 
children with no clinically apparent amblyopia in the setting of a vascular mark 
or other cause of intermittent occlusion of the visual axis should be followed, 
since these electrophysiology findings suggest amblyopia may be present.