Purchase this article with an account.
C. Murray, D. Newsham; Normative Values for the Accommodative Convergence to Accommodation Ratio (AC/A). Invest. Ophthalmol. Vis. Sci. 2010;51(13):801.
Download citation file:
© ARVO (1962-2015); The Authors (2016-present)
Measurement of the AC/A ratio is important in the diagnosis and treatment of strabismus. The AC/A ratio can indicate the cause of strabismus and help to guide the management. Whether a ratio is considered to be high or low is often based on a so-called normative range that lies between 3-5:1. However, this "normal" range is not based on data collected from normal subjects. Some older studies have reported the mean ratio to be slightly lower at around 3:1 in normal subjects. No studies have compared all current clinical methods in normal subjects, therefore the aim of this study is to calculate normative AC/A ratio data and compare these norms across the different clinical techniques.
Data was collected on 15 normal subjects. All subjects had near and distance vision tested along with an orthoptic examination and auto-refraction (Plusoptix) to determine suitability for inclusion criteria for the study. The AC/A ratio was measured using the Gradient method with convex lenses (NG) and concave lenses (DG), the Gradient with the Synoptophore (SG) and the Heterophoria (H) method. Subjects were retested by the same examiner on two separate occasions. (maximum three weeks apart).
Mean (±SD) AC/A ratio for DG was (1.22±0.86) and SG (1.33±0.98). The mean for NG (2.86±2.40) was higher than the other gradient methods and H had a much higher mean value (5.96±1.13). Comparison of all methods with ANOVA showed that there was a statistically significant difference between the methods (p<0.001). Post hoc analysis showed that the differences were between H and NG (p<0.001), H and DG (p<0.001), H and SG (p0.05).
Mean ratios for DG and SG were lower than currently quoted normal values with NG in the lower end of the current normal range. H tended to give much larger AC/A ratios in the same subjects, possibly due to the affects of proximal convergence. Interestingly, the difference between NG and DG just reached statistical significance. With such a difference between the results from methods that are currently in clinical use, this data suggests that there is not a single normal range of AC/A ratio and that the normal values must be viewed in relation to the method used to measure it. Early indications from this data suggest that our perception of the normal AC/A ratio should be lowered to aid proper diagnosis and treatment of certain types of strabismus.
This PDF is available to Subscribers Only