Vertical Fusional Vergence

Abstract

Objectives: To test the previous findings of Enright that disparity-induced vertical vergence is mediated primar- ily by the oblique muscles, and to relate this normal eye movement pattern to the eye movement pattern seen in subjects with dissociated vertical deviation. Methods:Sixteen normal volunteers underwent 55 mea- surements of the cycloversion associated with prism- induced vertical vergence using an afterimage appara- tus.AVernier scale measured the direction and magnitude of the torsional shift that occurred with recovery of fu- sion on removal of a 3– or 4–prism diopter prism. Results: Of the 55 trials, the directions of torsional shift were consistent with the oblique muscles being the pri- mary mediators of vertical fusional vergence in 51 (93%) (P = .03 using a binomial distribution). The mean ± SD value of torsional shift was 1.15° ± 0.76° in the expected direction. Conclusions: Vertical fusional vergences in this study were produced primarily by the oblique extraocular muscles. The eye movement patterns of these vertical vergences in normal subjects are qualitatively similar to those seen in recordings of patients with dissociated vertical deviation. Dissociated vertical deviation thus seems to be an exaggeration of a normally occurring eye movement pattern. The cyclovertical component of dis- sociated vertical deviation may help stabilize the fixing eye by damping vertical nystagmus, while the accompa- nying hypertropia is an incidental and undesirable side effect.