Effect of near-vision addition lenses on the accuracy of the accommodative response.

BACKGROUND: Addition lenses (adds) are frequently used to increase the accuracy of the near-vision focusing response, and may also play a role in the prevention or retardation of refractive error development. However, following the introduction of such a lens, if the accommodative response is reduced by an amount equal to the magnitude of the near add, then the resulting accommodative error (and degree of retinal defocus) would remain unchanged. METHODS: This study measured the accommodative response in 28 subjects while they viewed a near target binocularly, either through their distance refractive correction alone or this correction combined with a +0.75 D, +1.50 D, +2.00 D, or +2.50 D near add. RESULTS: The mean findings demonstrated a small (0.03 D) lead of accommodation to the near target through the distance correction, and monotonically increasing leads of accommodation with larger adds. Further, the additional lens power required to reduce the accommodative error to zero correlated significantly with the initial accommodative error. CONCLUSIONS: Near addition lenses may actually increase the degree of retinai defocus for individuals who manifest small accommodative errors.

Vergence adaptation and the clinical AC/A ratio.

The accuracy of the gradient technique for measuring the clinical accommodative convergence to accommodation (AC/A) ratio is dependent upon obtaining veridical heterophoria measurements. However, previous studies have demonstrated that the sustained output of slow fusional vergence, which may take several minutes or even hours to decay, can bias heterophoria assessment. In the clinical setting, it is usual to estimate the AC/A ratio after just a few seconds of dissociation. This study investigated whether the slow fusional vergence response alters this crosslink ratio by comparing values of AC/A measured both before and immediately after a 1-hr period of continuous monocular occlusion. Sustained occlusion produced a significant change in near heterophoria in 10 out of the 21 subjects examined, but no significant change in AC/A was observed in either the adapting or non-adapting subgroups. Accordingly, while the sustained output of slow fusional vergence will influence clinical measurements of heterophoria, its presence does not alter the stimulus AC/A ratio significantly.