Vergence facility: establishing clinical utility

Vergence facility testing attempts to assess the ability of the fusional vergence system to respond rapidly and accurately to changing vergence demands over time [defined as the number of cycles per minute (cpm) that a stimulus can be fused through, alternating base-in (BI) and base-out (BO) prisms]. Decisions to use vergence facility as a clinical test are hampered by a lack of systematically gathered normative data.

Twenty symptomatic and 20 control subjects with ages between 18 to 35 years of either sex and any race were pooled, based on vision-symptom level determined by a self-report questionnaire. Inclusion/exclusion criteria included vision correctable to 6/6 (20/20) Snellen acuity or better in each eye and normal phorias. Vergence facility response was tested over a 1-min period, using 16 combinations of BI/BO flip prisms at 4.0 and 0.4 m, based on Morgan's norms and pilot data.

Horizontal vergence facility responses were not the same among those with and without symptoms, and not all magnitudes of BI/BO flip prisms produced the same response difference. A single flip prism, 3 delta BI/12 delta BO, was found to differentiate optimally between groups at distance and near. Repeatability of test results (with the 3 delta BI/12 delta BO prism) was poor at distance and good at near.

In addition to providing valuable normative data, this study indicates that the vergence system nearly resets its "zero point" at any distance and sheds further light on the results of dynamic 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 facility test is easily implemented, using a commonly available flip prism (3 delta BI/12 delta BO) and having a clinical failure criterion that is easily recalled (15 cpm, sum of the BI and BO magnitudes).