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Editors Selection IGR 17-4
Structure Measurement: Astigmatism and RNFL imaging
Comment by Tae-Woo Kim on:
46736 Astigmatism and optical coherence tomography measurements, Hwang YH; Lee SM; Kim YY et al., Graefe's Archive for Clinical and Experimental Ophthalmology, 2011;
It has been recognized that refractive errors may affect the measurement of retinal nerve fiber layer thickness using optical coherence tomography (OCT). As ra etinal image can be distorted and vary in eyes with astigmatism, it is possible that the astigmatism may also affect the RNFL measurement by OCT. Hwang et al. (1338) investigated this issue by evaluating the changes in the RNFL and macular thicknesses as measured by OCT after induction of astigmatism by wearing toric soft contact lenses. Toric lenses of +1.5 -3.25Dx90° and +1.5-3.25D x180° were used, which induced a mean with-the-rule (WTR) astigmatism of -2.92D x171° and an against-the-rule (ATR) astigmatism of -3.18Dx 88°, respectively.
Astigmatism of moderate degree does not influence the assessment of the RNFL thickness by OCT substantially
They found that WTR astigmatism led to decreases of RNFL thickness measurements in average, superior quadrant, and 6 and 12 o'clock hour sectors, and ATR astigmatism led to decreases of RNFL thickness in average, nasal, and temporal quadrant, and 2, 3, and 9 o'clock hour sectors in signal strengthadjusted RNFL analysis. Macular thickness did not show a significant change after induction of the astigmatism. The authors speculated that the RNFL thickness measurement was affected by the regional increase of the scan distance from the optic disc after induction of the astigmatism. As RNFL becomes thinner with increasing distance from the optic disc, changes in such distance should affect RNFL thickness. Although the changes in the RNFL measurement was statistically significant, the amount of changes were less than 2 µm for the average RNFL thickness, less than 3 µm for the quadrant thicknesses, and less than 7 µm for the clock hour thicknesses. Thus, it appears that astigmatism of moderate degree does not influence the assessment of the RNFL thickness by OCT substantially. However, it may be warranted to consider the effect of astigmatism when evaluating RNFL thickness measured by OCT in eyes with higher degrees of astigmatism.
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