The effect of positive-lens addition and base-in prism on accommodation accuracy and near horizontal phoria in Chinese myopic children

The effect of positive-lens addition (0, +0.75, +1.50, +2.25, +3.00 D each eye) and base-in prism power (0, 1.5, 3 Delta each eye) on both near focusing errors and latent horizontal deviations was evaluated in 29 Chinese myopic children (age: 10.3 +/- 1.9 years, refractive error: -2.73 +/- 1.31 D). Accommodation response and phoria were measured by the Shin-Nippon auto-refractor (right eye) and Howell-Dwyer near phoria card at 33 cm with each of the 15 lens/prism combinations in random order. The initial accommodative error was -0.96 +/- 0.67 D (lag) and near phoria was -0.8 +/- 5.0 Delta (exophoria). The positive-lens addition decreased the accommodative lag but increased the exophoria as the power increased (e.g. up to -9.1 +/- 4.1 Delta with +3 D). A 6-Delta base-in prism totally controlled the exophoria induced by a +1.50 D addition (-0.3 +/- 4.3 Delta), but the accommodative lag was still considerable (-0.69 +/- 0.54 D). In the graphical analysis of the data, a lens addition of +2.25 D combined with a 6-Delta base-in prism minimized both the lag and lens-induced exophoria to -0.33 D and -2.4 Delta respectively (regression analysis). This lens and prism combination decreased the lens-induced exophoria by 4.5 Delta compared with that measured with +2.25 D alone (-2.4 Delta vs -6.9 Delta). The results suggest that incorporating near base-in prism when prescribing bifocal lenses for young progressing myopes with exophoria could reduce the positive lens-induced oculomotor imbalance.