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

Abstract

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). Conclusion: The CA/C ratio did not change significantly with increasing induced vergence. The data suggest that some degree of non-linearity/variation should be expected in the normal, healthy population of the same age group.