Convergence accommodation to convergence (CA/C) ratio: stability with different levels of convergence demand

Aim: To measure the convergence accommodation to convergence (CA/C) ratio over a range of levels of convergence demand.

Methods: Convergence accommodation was measured in 26 subjects with normal binocular single vision. The CA/C ratio was calculated by measuring the accommodative changes induced with base-out prisms of 5^Δ to 20^Δ in 5 ^Δ steps whilst the participant maintained binocular fixation on a pseudo-Gaussian target at 40 cm. Accommodative change was measured using the open-field view Shin-Nippon SRW-5000 autorefractometer [Grand Seiko Company, Fukuyama, Japan].

Results: The mean age of the participants was 20 ± 2.5 years (range 18–29 years). The mean (± SD) CA/C ratio for the 5 ^Δ, 10 ^Δ, 15 ^Δ and 20 ^Δ prisms was 0.13 (±0.07) D/PD, 0.12 (±0.05) D/PD, 0.13 (±0.05) D/PD and 0.13 (±0.05) D/PD, respectively. The overall mean (±SD) CA/C ratio was 0.13 (±0.04) D/PD (range 0.04–0.20 D/PD). One-factor repeated measures ANOVA found that the CA/C ratio did not change significantly with an increase in convergence (F = 0.202, d.f. = 3, 69, p = 0.8947).