VERGENCE FACILITY - ESTABLISHING CLINICAL UTILITY

Purpose. Vergence facility testing attempts to assess the ability of t he fusional vergence system to respond rapidly and accurately to chang ing vergence demands over time [defined as the number of cycles per mi nute (cpm) that a stimulus can be fused through, alternating base-in ( El) and base-out (BO) prisms], Decisions to use vergence facility as a clinical test are hampered by a lack of systematically gathered norma tive data. Methods. Twenty symptomatic and 20 control subjects with ag es between 18 to 35 years of either sex and any race were pooled, base d on vision-symptom level determined by a self-report questionnaire. I nclusion/exclusion criteria included vision correctable to 6/6 (20/20) Snellen acuity or better in each eye and normal phorias, Vergence fac ility response was tested over a 1-min period, using 16 combinations o f BI/BO flip prisms at 4.0 and 0.4 m, based on Morgan's norms and pilo t data. Results. Horizontal vergence facility responses were not the s ame among those with and without symptoms, and not all magnitudes of B I/BO flip prisms produced the same response difference, A single flip prism, 3 Delta BI/12 Delta BO, was found to differentiate optimally be tween groups at distance and near. Repeatability of test results (with the 3 Delta B1/12 Delta BO prism) was poor at distance and good at ne ar. Conclusions. In addition to providing valuable normative data, thi s study indicates that the vergence system nearly resets its ''zero po int'' at any distance and sheds further light on the results of dynami c convergence and divergence stimulation on the accommodative-vergence system. From a clinical standpoint, the results improve the diagnosis of binocular vision abnormalities, The recommended near vergence faci lity test is easily implemented, using a commonly available flip prism (3 Delta B1/12 Delta BO) and having a clinical failure criterion that is easily recalled (15 cpm, sum of the BI and BO magnitudes).