The effects of myopia and AC/A measuring methodology on AC/A ratio outcome

Abstract

Purpose: This study aims (1) to understand the extent of myopia on the AC/A ratio, (2) to examine whether the calculated AC/A ratio would be different from the gradient AC/A ratio in a clinical setup, (3) to investigate the difference between positive and negative spherical lens stimuli on AC/A ratio, and (4) to unveil the mutual effects of different amounts of myopia and AC/A measuring methodology on AC/A ratio outcome.Methods: volunteered participants (30 males and 42 females) were refractive examined and divided into two groups : 40 low myopes (- 0.50D＜ spherical equivalents(SE) ＜ - 3.00D) and 32 high myopes (SE≥ - 3.00D). The Von Graefe Technique (VG) was used to measure the distance lateral phoria at 6 m and near lateral phoria at 0.4 m. Binocular additions of positive and negative spherical lenses were used for lateral phoria measurement. The calculated AC/A ratio and the gradient AC/A ratio were then correlated and analyzed by two-way ANOVA.Results: The average AC/A ratios of the high myopes were higher than those of the low myopes. The average calculated AC/A ratios were higher than the average gradient AC/A ratios. In gradient AC/A ratios, the average value using positive spherical addition was higher than when using negative spherical addition. No significant difference of AC/A ratios was found between the bi-factorial inter-relations of myopia and AC/A ratio determination methods (F = 0.75, P= 0.48＞ 0.05). The extent of myopia did not affect AC/A ratio (F = 3.14, P= 0.081＞ 0.05). There was a significant difference among measuring methods and the outcomes of AC/A ratios (F = 6.16, P= 0.003＜ 0.05). The calculated AC/A ratio was significantly different from the gradient AC/A ratio (both P=0.003＜ 0.005). However, no significant difference was found between the gradient AC/A ratios using positive lenses and those using negative lenses (P=0.46＞ 0.05).Conclusions: This study shows that the calculated AC/A ratio is significantly greater than the gradient AC/A ratio, probably due to proximal convergence and accommodative lag. A greater amount of myopia has a higher AC/A ratio than that of lower myopia in all AC/A ratio measurement, though the difference was insignificant. This finding suggests that there are deviations between stimulus AC/A ratio and response AC/A ratio. Our data suggest that anomalous binocular factors and accommodative lag should be considered for myopia research, apart from stimulus AC/A ratio.