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Original article

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

Authors:

Komal J. Hirani ,

Academic Unit of Ophthalmology and Orthoptics, University of Sheffield, Sheffield
About Komal J.
BMedSci (Hons)
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Alison Y. Firth

Academic Unit of Ophthalmology and Orthoptics, University of Sheffield, Sheffield
About Alison Y.
MSc DBO(T)
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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 5Δ to 20Δ 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.
DOI: https://doi.org/10.22599/bioj.11
How to Cite: Hirani, K.J. and Firth, A.Y., 2009. Convergence accommodation to convergence (CA/C) ratio: stability with different levels of convergence demand. British and Irish Orthoptic Journal, 6, pp.60–64. DOI: http://doi.org/10.22599/bioj.11
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Published on 01 Aug 2009.
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