Vertical fusional vergence - The key to dissociated vertical deviation

Objectives: To test the previous findings of Enright that disparity-induced vertical vergence is mediated primarily by the oblique muscles, and to rel ate this normal eye movement pattern to the eye movement pattern seen in su bjects with dissociated vertical deviation. Methods: Sixteen normal volunteers underwent 55 measurements of the cyclove rsion associated with prism-induced vertical vergence using an afterimage a pparatus. A Vernier scale measured the direction and magnitude of the torsi onal shift that occurred with recovery of fusion on removal of a 3- or 4-pr ism diopter prism. Results: Of the 55 trials, the directions of torsional shift were consisten t with the oblique muscles being the primary mediators of vertical fusional vergence in 51 (93%) (P =.03 using a binomial distribution). The mean +/- SD value of torsional shift was 1.15 degrees +/- 0.76 degrees in the expect ed direction. Conclusions: Vertical fusional vergences in this study were produced primar ily by the oblique extraocular muscles. The eye movement patterns of these vertical vergences in normal subjects are qualitatively similar to those se en in recordings of patients with dissociated vertical deviation. Dissociat ed vertical deviation thus seems to be an exaggeration of a normally occurr ing eye movement pattern. The cyclovertical component of dissociated vertic al deviation may help stabilize the fixing eye by damping vertical nystagmu s, while the accompanying hypertropia is an incidental and undesirable side effect.