vergence facility

Vergence facility is the number of cycles per minute that a patient can fuse a target through alternating base-in and base-out prism. The test captures how rapidly and accurately the fusional vergence system responds to changing vergence demands over time.¹ Fusional reserves measure the maximum vergence a patient can exert, and vergence facility measures the rate of change of vergence.² A patient can have normal fusional vergence amplitudes and still have a problem with vergence dynamics, and smooth vergence testing alone would miss it.³

The flipper carries more base-out than base-in prism because the ability to converge is usually greater than the ability to diverge.² The usual combination is 3Δ base-in and 12Δ base-out, which best separates symptomatic from asymptomatic patients at near.^2,3 At near, the patient views a vertical line of small letters at 40 cm, and the examiner flips the prism as soon as the print becomes single and clear.⁴ A full cycle includes both the base-in and base-out lenses, and the result is the number of full cycles completed in 60 seconds.⁴ The expected finding at near is 15 cycles per minute ± 3 cpm.⁴ Norms set at 1 SD below the mean of an asymptomatic control group are 12 cpm at distance and 15 cpm at near.²