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.
In a controlled study, the amblyopic eye of a group of 12 patients was stimulated in accordance with the original method of Campbell et al. (1978), using gratings, and a control group of 10 patients with black and white pictures. As in other controlled studies, no specific benefits ensued from the use of gratings. In both groups only slight visual improvements resulted, probably due to the repeated testing of visual acuity and nonspecific effects of short-term occlusion. A clear improvement in visual acuity and fixation behaviour was only observed after subsequent long-term occlusion.
We assume that our two patients had a "motor memory" that stores the muscle innervation for parallel eyes, i.e. the innervation suitable for binocular vision, and that the patients made use of or neglected this memory at will. In general, our observations suggest that a variable recourse to such a motor memory explains why, in patients with unilateral elevation in adduction, the angle of squint often changes. A voluntary access to this memory may, however, be exceptional.
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