Reversed Fixation Test as a Means to Differentiate between Dissociated and Non-Dissociated Strabismus

Dissociated vertical deviation (DVD) depends on the balance of visual inputs coming through the right and left eyes. In the classical Bielschowsky Filter Test the balance of visual inputs is altered by darkening the retinal image of the fixing eye with a filter. In the Reversed Fixation Test (RFT) the balance of visual inputs is altered by not only darkening the fixing eye (occlusion put on) but also by brightening the squinting eye (occlusion removed) and, at the same time, switching the fixation to the other eye. In a series of 30 patients the RFT revealed a DVD component of 16.3 ± 4.9± while the Bielschowsky Filter Test revealed 3.4 ± 3.2± only. In seven cases the value of the Bielschowsky Filter Test was even zero while the RFT revealed a DVD component between 8 and 20±. The RFT allows a quantitative differentiation between DVD and esotropia with elevation in adduction (= upshoot in adduction or strabismus sursoadductorius or overaction of the inferior oblique muscle), even if both types of vertical deviation occur in the same patient.