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V. Sreenivasan, R. Suryakumar, E.L. Irving, W.R. Bobier; Adaptation to Near Addition Lens . Invest. Ophthalmol. Vis. Sci. 2006;47(13):1183.
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Near additions lenses have been prescribed to attenuate the progression of myopia in children on the basis of eliminating lags of accommodation that may signal excessive axial elongation of the eye. However, slowing myopia in this way has met with limited success. Studies investigating the effect of these adds on adults consistently show a reduction in the lag of accommodation which is maintained over time. These lenses also induced significant exophoria1. Could there be difficulty in binocularly adapting to these adds? This study explores the vergence and accommodative responses of 5 adult subjects to near addition lenses over a 30 minute period.
Subjects fixated a near target at a distance of 33 cm for 30 minutes with and without a +2D add. A PowerRefractor (Multichannel Co) took monocular and binocular "plane of focus measures" (sum of accommodation and lens power) at 3, 6, 9, 20 and 30 minute intervals. Near phoria measures were taken concomitantly using a modified Thorington technique. Tonic accommodation was measured at the outset and upon completion of the testing.
Initially, the +2D lens increased the subjects’ plane of focus by 0.97D binocularly which was reduced to 0.58D monocularly. Over time, only the binocular measures of the plane of focus showed a significant reduction after 6 minutes (0.33 D and p=0.02). The induced exophoria of 4.2 ± 2.2 Δ with the +2D lens showed a significant (p=0.00) reduction (3.4Δ) over 6 minutes. No change in phoria over 30 minutes was found without a near add. Tonic accommodation increased significantly (0.50D and p = 0.01) following reading without the +2D add.
The vergence and accommodative measures suggest that +2D near lenses initially invokes increased exophoria which is compensated by increased convergence and convergence– accommodation. Vergence adaptation occurs over time reducing convergence– accommodation. However, the plane of focus remained higher than 3D. It would appear that +2D lenses are capable of reducing accommodative lag and allowing comfortable binocular vision, but only if vergence adaptation occurs. (1) BC Jiang et al, Invest. Ophthalmol. Vis. Sci., May 2005; 46: E–abstract 5594.
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