The Lancaster red-green test before and after occlusion in the evaluation of incomitant strabismus

Patients with incomitant strabismus can often fuse in a limited area of gaze. Prolongation of neurologically learned fusional vergence tone ("vergence adaptation") in and near this area can result in misleading measurements with standard clinical measures of strabismus. Monocular occlusion for at least 30 minutes eliminates most of the effect of vergence adaptation. The Lancaster red-green test provides an elegant and convenient map of incomitant strabismus. We investigated the efficacy of the Lancaster red-green test before and after monocular occlusion for the investigation of incomitant strabismus.

We retrospectively studied the results of the Lancaster red-green test in 6 patients with incomitant vertical strabismus in whom we suspected that vergence adaptation might be distorting the pattern of deviation. The test was performed before and after monocular occlusion for 30 to 60 minutes, and the preocclusion and postocclusion results were compared.

In the 6 cases studied, the Lancaster red-green test showed at least a 5-PD increase in the hyperdeviation, after monocular occlusion. The increases were mostly in primary gaze and downgaze, which tended to regularize the pattern of deviation.

The combination of monocular occlusion and the Lancaster red-green test is useful for uncovering the effect of vergence adaptation. Such results may often simplify the planning of surgical correction because the incomitance usually decreases after monocular occlusion, making it less likely that surgery will worsen the alignment in the area previously fused. We recommend that monocular occlusion should be considered when planning surgery or even prism correction for incomitant deviations, especially when the initial Lancaster red-green test shows an unexpected incomitant pattern where there is fusion in 1 direction of gaze but not in others.