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

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 -3.00DS 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 -3.00DS 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.