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Thomas Callan, Patricia Sha, Jennifer Luu, John G. Flanagan; Upper Limits of the Best Sphere Correction for Astigmatism in Threshold Visual Fields. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):1059.
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© ARVO (1962-2015); The Authors (2016-present)
To investigate the amount of spherical equivalent power that can be used in lieu of an astigmatic trial lens up to a decrease in mean deviation (MD) of 1 decibel (dB). The Humphrey Field Analyzer (HFA) advocates correction of astigmatism at 1.25 diopters of cylinder (DC).
Six normal subjects with previous perimetry experience and known spherical refractive errors were tested with the HFA 24-2 SITA Standard program. Two baseline visual fields were performed with the spherical refraction corrected for each subject. Additional visual fields were performed with myopic astigmatic refractive errors of 1.50, 2.00, 2.50, 3.00 and 3.50 DC induced using trial lenses. For these tests, the spherical equivalents of the refractive errors were corrected with the appropriate power spherical lens.
The difference from the Baseline MD (average of the first two tests) was calculated for each visual field. The results of the different amounts of astigmatic blur corrected by utilizing the spherical equivalent and the effect on the MD are shown in Table 1. It was not until the astigmatic refractive error reached 3.50 D astigmatism did the mean deviation decrease by more than 1 dB.
Spherical equivalent lens correction of astigmatism is effective for refractive errors greater than the manufacturer’s recommendation of correction at 1.25 DC. In this study, astigmatic refractive errors up to 2 DC were corrected with the spherical equivalent before the mean deviation changed more than 0.5 dB, and 3DC for 1 dB.
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