Mechanisms of Vertical Fusional Vergence in Patients With Congenital Superior Oblique Paresis Investigated With an Eye-Tracking Haploscope

PURPOSE. To determine the mechanisms of vertical fusional vergence in patients with congenital unilateral superior oblique paresis (SOP) and to discuss the implications of these mechanisms. METHODS. Eleven patients were examined with our eye-tracking haploscope. RESULTS. Three different fusion mechanisms were found, producing significantly different cyclovergence to vertical vergence ratios (P < 0.05): primary use of the vertical rectus muscles in seven patients (ratio: 0.36 +/- 1.6), primary use of the oblique muscles in one patient (0.04), and use of the superior oblique muscle in the higher eye and the superior rectus muscle in the lower eye in three patients (1.15 +/- 0.32). Lancaster red-green testing showed alignment differences among these groups, primarily differences in amount of subjective extorsion between the two eyes in straight-ahead gaze: The patient with oblique-muscle-mediated fusion showed essentially no subjective extorsion (0.5 degrees), the patients with vertical-rectus-muscle-mediated vertical fusion showed a mean +/- SD subjective extorsion of 3.6 degrees +/- 1.4 degrees, and the patients with the mixed (oblique/rectus) fusion mechanism showed 7.0 degrees +/- 1.7 degrees (P < 0.05). CONCLUSIONS. The choice of fusion mechanism may be a function of how much intorting effect is needed. Use of the oblique muscles bilaterally causes the least intorting effect, use of the vertical rectus muscles bilaterally adds more intorting effect, and activation of the paretic'' superior oblique muscle in the higher eye and the superior rectus muscle in the lower eye provides the greatest intorting effect. Subclassifying congenital SOP in this way (in which the paretic'' muscle may remain functional in many cases) may help guide its optimal surgical correction.