Convergence accommodation to convergence (CA/A) 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. Method: Convergence accommodation was measured in 26 subjects with normal binocular single vision (BSV). The CA/C ratio was calculated by measuring the accommodative changes induced with base-out prisms of 5Δ to 20Δ in 5 Δ steps whilst the participant maintained binocular fixation on a pseudo- Gaussian target at 40cm. 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 to 29 years). The mean (± S.D.) CA/C ratio for the 5Δ, 10Δ, 15Δ and 20Δ 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 (±S.D.) CA/C ratio was 0.13 ± 0.04 D/PD (Range: 0.04-0.20 D/PD). A linear relationship between accommodative response (D) and prism strength was found. One factor repeated measures ANOVA found that the CA/C ratio did not significantly change with an increase in convergence (F=0.202, d.f. =3, 69, p=0.8947). Conclusion: The CA/C value ratio did not change significantly with increasing induced vergence. The data suggest that some degree of nonlinearity/ variation should be expected in normal, healthy population of the same age group.