Facilitation of vertical vergence by horizontal saccades, found in a patient with dissociated vertical deviation

The authors examined vertical vergence in a 15-year-old girl with dissociated vertical deviation, a 6d̀ convergent strabismus, no binocular vision, latent nystagmus, and a minimal left amblyopia. Eye movements were recorded during 4s-periods of (1) both eyes open, alternated with 4s-periods of (2a) right eye covered, (2b) left eye covered or (2c) both eyes closed. The patient preferred fixation with the right eye; when this eye was covered, the left eye took over fixation with an almost completely monocular, downward saccade (a horizontal saccade occurred at the same time). The right eye only made a very small saccade, and then started to drift upward with an exponentially decreasing velocity. Enright has shown that fast convergence or divergence may occur when a horizontal saccade is performed at the same time. Similarly, the authors suspected that, in their patient, vertical vergence was facilitated by the horizontal saccade that resulted, in this case of horizontal strabismus, from alternation of fixation. To examine whether this was the case, they compensated for the horizontal strabismus by placing a 30 prism-dioptre base-out prism in front of the left eye (the patient had no binocular vision). With the prism in place, the horizontal saccade that occurred when the right eye was covered was smaller, and only little vertical vergence occurred together with the horizontal saccade. It seems possible that vertical vergence is also facilitated by horizontal saccades.