AC/A ratio, age, and refractive error in children

PURPOSE. To examine how the response AC/A ratio (the amount of accommodativ e convergence per unit of accommodative response) varies as a function of r efractive error and age, to determine whether it is a risk factor for the o nset of myopia, and to examine the relation between ocular structural featu res and the AC/A ratio. METHODS. Accommodation was stimulated by a letter target presented in a Bad al system at 0.00, 2.25, and 4.37 D to 828 children aged 6 through 14 years in 1996. Of these, 726 had no myopia in 1996 and were available for examin ation the following year, Accommodative response and cycloplegic refractive error were measured by autorefraction and convergence by monitoring the re lative movement of Purkinje images I and TV. Lens radii of curvature were m easured by video phakometry, corneal radius of curvature by topography, and ocular axial dimensions by A-scan ultrasonography. RESULTS. Adjusted for age, the response AC/A ratio was highest in myopes (6 .39 Delta/D), intermediate in emmetropes (3.94 Delta/D), and lowest in hype ropes (3.40 Delta/D; P < 0.0001; two-way analysis of variance [ANOVA]). The stimulus AC/A ratio did not vary with refractive error. Adjusted for refra ctive error, the response AC/A ratio did not change as a function of age. I n non-myopic children, having a response AC/A ratio of 5.84 Delta/D or more elevated the risk of development of myopia within 1 year by 22.5 times (95 % CI = 7.12-71.1). In a subsample of children without myopia who had refrac tive errors less than +0.75 D, having a response AC/A ratio of 5.84 Delta/D or more elevated the risk of development of myopia within 1 year by 3.21 t imes (95% CI = 1.14-9.07). The AC/A ratio was associated with all measured ocular features except lens spherical volume. Only the negative correlation s with refractive error and the shape of the crystalline lens (Gullstrand l ens power) were significant in a multiple regression model (adjusted R-2 = 0.16). CONCLUSIONS. An elevated response AC/A ratio was associated with myopia and a-as an important risk factor for its rapid onset. The association between higher AC/A ratios and flatter crystalline lens shapes, as well as other r eported features of accommodation in myopia, may be explained by pseudocycl oplegia," which the authors define as tension on the crystalline lens that increases the level of effort needed to accommodate. Accommodative deficits in myopia may be the functional consequences of the underlying anatomy of the enlarged eye.