Stimulus and response AC/A ratios in an orthoptic student population

Abstract

Aim: To measure the stimulus and response AC/A ratios using the distance prism cover test gradient method in young adults without strabismus or ocular abnormality. Methods: AC/A ratios were measured in a group of orthoptic students. The distance deviation (at 3.8 m) was measured with an alternate prism cover test using a Gulden prism bar with and without À3.00DS lenses. A Shin-Nippon SRW-5000 autorefractor [Grand Seiko Company, Fukuyama, Japan] (open view) was used to measure the refractive state with and without a À3.00DS lens. The stimulus and response AC/A ratios were calculated. Results: Data were analysed for 16 participants. The mean and standard deviations (SD) of the distance stimulus gradient AC/A ratio were 1.98:1 (1.30) and the mean (SD) distance response gradient AC/A ratio were 2.43:1 (1.60). A paired t-test found a significant difference between the distance response and stimulus gradient AC/A ratio values (t = 3.58, p = 0.003). A Bland-Altman plot suggested that the difference increased as the size of AC/A ratio increased. Conclusion: The response gradient AC/A ratio when measured using the prism cover test at a distance of 3.8 m was found to be higher than the stimulus AC/A ratio in a group of normal participants who were orthoptic students. This is due to the accommodative lag which normally occurs when viewing through minus lenses.

Quantifying interactions between accommodation and vergence in a binocularly normal population

“…Previous studies have found the response AC/A ratio is between 8% and 24% larger than the stimulus ratio (Alpern, Kincaid, & Lubeck, 1959;Gratton & Firth, 2010;Rosenfield, Ciuffreda, & Chen, 1995), which compares well with the current findings. This systematic difference between the stimulus and response AC/A ratios could be explained by the increased lag at higher accommodation stimulus levels (Fig.…”

“…This tendency has been shown in a previous study which examined the response AC/A and stimulus AC/A using a distant target (Gratton & Firth, 2010).…”

“…7. Other studies have also found a significant correlation between stimulus and response AC/A ratios with r 2 values of 0.79 and 0.96 Gratton & Firth, 2010).…”

“…Several studies have investigated the stimulus and response AC/ A ratios concurrently in the same subjects and found that the two measures are well correlated, with the response ratio being 8-24% higher than the stimulus ratio (Alpern, 1962;Gratton & Firth, 2010;Rainey et al, 1998;Rosenfield, Ciuffreda, & Chen, 1995). The close proportional relationship between the stimulus and response AC/A ratios makes the stimulus AC/A a clinically useful measure of accommodative vergence interaction .…”

Convergence accommodation in orthoptic practice

“…1D that textbooks tell us to expect [4][5][6] and ratios obtained using different methods, which should be the same, rarely correlate. 5,7,8 Importantly, most laboratory research, which is often used to extrapolate to clinical situations, uses response AC/A ratios.…”