A novel test for predicting fusional potential in patients with torsional diplopia.

We introduce the novel "white disk" test to evaluate fusional potential in patients with torsional diplopia. In this test, a white disk is presented on a black background in a darkened room to eliminate perception of torsion. Horizontal and vertical misalignment is neutralized with prisms when necessary. The absence of diplopia on the white disk test is used to establish fusional ability. Nine patients with torsional diplopia were assessed using this test. All had excyclotorsion (mean, 9.9 degrees +/- 4.2 degrees; range, 5 degrees-15 degrees) and could achieve single binocular vision on testing. Postoperatively, 9 patients reported no torsional diplopia in the preferred gaze position, with a mean excyclotorsion of 0.4 degrees +/- 2.8 degrees (range, 5 degrees to 4 degrees). The test appeared to predict fusional potential in all patients.