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Yohann Benard, Anne Seidemann, Helmut Altheimer, Andrea Welk, Gregor Esser; Reducing prismatic imbalance at near in progressive addition lenses. Invest. Ophthalmol. Vis. Sci. 2018;59(9):2967.
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© ARVO (1962-2015); The Authors (2016-present)
In anisometropia the prismatic power at near is different in the left and right spectacle lens for Progressive Addition Lenses (PALs) wearers which may hinder the adaptation to this type of lenses. In this case, a prism that compensate for the difference in prismatic power only at near could improve acceptance. Also, people with convergence insufficiency may benefit from a change of near prism to support binocular vision. Yet, creating a different prism in the near portion of the lens also creates a cylindrical error which grows twice as fast as the prismatic difference. The main question was thus: what are the wearers more sensitive to, prism or astigmatic error?
Two measurements were performed: 1) we induced a vertical prismatic difference of 3cm/m and measured the effective prismatic difference perceived by four subjects over 15 minutes using the Maddox cross. Two measurements were also performed after the prisms were removed. 2) we measured the effect of a cylinder at 0° up to 1.5D in 0.25D steps on the near visual acuity. The cylinder was introduced either monocularly (the fellow eye occluded) or binocularly.
Subjects already perceived a prism slightly lower than the induced one directly after its introduction. The perceived prism decreased with time, down to half of the introduced prism after 15 minutes. After removing the prism, the subjects first showed prismatic difference in the opposite direction of the induced prism which was compensated after five minutes. Also, subjects reported diplopia only directly after introducing or removing the prism. For both monocular and binocular conditions, the effect of astigmatism was similar to previous findings, with a drop of one line of visual acuity with 0.75 D of astigmatism.
In PALs, if the prismatic difference at near is too high (e.g. in case of anisometropia), it is possible to reduce it by introducing astigmatic error. However, because astigmatic error grows twice as fast as prism introduction in PALs, the introduction of cylinder at near seems to be too much of a cost in comparison to the plasticity of the visual system in term of prismatic compensation.
This is an abstract that was submitted for the 2018 ARVO Annual Meeting, held in Honolulu, Hawaii, April 29 - May 3, 2018.
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