Validity of clinical measures of the AC/A ratio

The gradient stimulus and heterophoria stimulus measures of the accommodative convergence/accommodation (AC/A) ratio are simple and widely-used techniques. We compared these two clinical methods with the more accurate and objective gradient response AC/A ratio method. Twenty-three subjects, with ages ranging from 17 to 42 years and with normal binocular vision, were assessed. The response AC/A ratio was determined using a Canon AutoRef R-l to measure accommodation and an IRIS eye movement monitor to measure eye movements. Stimulus AC/A ratios were determined by taking phoria measures with the presentation of lenses (gradient method) and by varying the working distance (heterophoria method), with the accommodation taken as dioptric value of each stimulus. The mean AC/A ratios were 4.37 ± 1.31 ΔA/D (mean ± standard deviation), 3.49 ±2.17 Δ/D, and 5.81 ± 1.03 Δ/D, for the gradient response, gradient stimulus and heterophoria stimulus AC/As, respectively. The gradient stimulus AC/A showed only moderate validity, as measures corresponded with the ratios of the response method mainly when identifying very high and very low AC/A ratios (r²= 0.388). Inaccuracies in the gradient stimulus AC/A arose because the accommodation often varied markedly from the value of the stimulus lenses. The heterophoria stimulus AC/A appeared to be invalid for the subjects tested, as no correlation was evident between it and the response AC/A (r²= 0.003). Since the heterophoria technique is susceptible to both errors of focus and the influence of proximal vergence, it may reliably detect only very high AC/A ratios. (Clin Exp Optom 1995; 78: 1: 3–10)