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

Background: Patients with incomitant strabismus can often fuse in a limited area of gaze. Prolongation of neurologically learned fusional vergence ton e ("vergence adaptation") in and near this area can result in misleading me asurements with standard clinical measures of strabismus. Monocular occlusi on for at least 30 minutes eliminates most of the effect of vergence adapta tion. The Lancaster red-green test provides an elegant and convenient map o f incomitant strabismus. We investigated the efficacy of the Lancaster red- green test before and after monocular occlusion for the investigation of in comitant strabismus. Methods: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 patte rn of deviation. The test was performed before and after monocular occlusio n for 30 to 60 minutes, and the preocclusion and postocclusion results were compared. Results: In the 6 cases studied, the Lancaster red-green test sh owed at least a 5-PD increase in the hyperdeviation, after monocular occlus ion. The increases we re mostly in primary gaze and downgaze, which tended to regularize the pattern of deviation. Conclusions: The combination of mon ocular occlusion a nd the La nc aster red-green test is useful for uncoveri ng the effect of vergence adaptation. Such results may often simplify the p lanning of surgical correction because the incomitance usually decreases af ter monocular occlusion, making it less likely that surgery will worsen the alignment in the area previously fused. We recommend that monocular occlus ion should be considered when planning surgery or even prism correction for incomitant deviations, especially when the initial Lancaster red-green tes t shows an unexpected incomitant pattern where there is fusion in 1 directi on of gaze but not in others.