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Sheila Rae, Holly Price, Stephanie Wong, Shahina Pardhan; Accommodative Accuracy In The Presence Of Astigmatic Defocus In Myopes. Invest. Ophthalmol. Vis. Sci. 2012;53(14):2304. doi: https://doi.org/.
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Accuracy of accommodation has been investigated in different refractive error groups as accommodative lag has been proposed as a source of blur which could stimulate ocular growth and myopia development. Recent reports of differences in visual acuity or depth of focus dependent on orientation of astigmatic defocus raises questions as to how astigmatic defocus is processed by the visual system.
Accommodative accuracy was assessed using the Shin Nippon SRW5000 auto-refractor whilst subjects fixated an N10 size printed text target. Eleven emmetropes, ten low and twelve moderate myopes were fully corrected using trial lenses for both spherical and astigmatic refractive error. Accommodative responses were then recorded with subjects fixating the text target with either no additional defocus or with + / - 1.0D astigmatic defocus at either 90 or 180°, with the five conditions presented in a random order. Accommodative lag was calculated as the difference between stimulus and response values.
Comparing positive to negative astigmatic defocus for all subjects, significantly higher accommodative lag was found for negative compared to positive astigmatic defocus at 90 ° (paired t-test: t (32) = -3.168; p = 0.03) and 180 ° (paired t-test: t(32) = --7.613; p < 0.001). When split for refractive error groups, an effect of sign of defocus was seen for 180 ° for all groups (p < 0.005) but only for moderate myopes at 90 ° (p = 0.001). Accommodative lag differed significantly between 90 and 180 ° axes for +1.0DC (paired t-test: t(32) =2.292; p = 0.018) but the difference did not reach significance between 90 and 180 for -1.0DC (p = 0.089). When split for refractive error groups, an effect of orientation was shown for both +1.0DC and -1.0DC for low myopes (p = 0.05).
Astigmatism which is under-corrected may exists in a variety of clinical situations including contact lens wear. Negative astigmatic defocus gives greater lag of accommodation (greater distance from retina to circle of least confusion) especially at orientation of 180. Least lag (closest distance from retina to circle of least confusion) was found for positive astigmatic defocus at orientation 180. In myopes with negative astigmatic defocus, one focal line would be close to the retina and the circle of least confusion and other focal line imaged behind the retina. This would lead to a difference in clarity of vertical and horizontal detail in an image. For myopes, leaving uncorrected astigmatism in a positive direction would minimize accommodative lag.
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