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X Huang, R Vessani, JM Liebmann, R Ritch; Effect of Refraction on the RNFL Thickness Measurement by Scanning Laser Polarimetry . Invest. Ophthalmol. Vis. Sci. 2002;43(13):1005.
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Purpose: Scanning laser polarimetry (SLP) assesses the retinal nerve fiber layer (RNFL) thickness by measuring its birefringence. The assumption in SLP is that the reflection that carries the birefringence information of the RNFL arises deep to the retina. Refraction data entered in the GDx Access (Laser Diagnostic Technologies, San Diego) changes the focal position of the measured beam in the eye. The purpose of this study is to evaluate the effect of refraction on the RNFL thickness measurement by SLP. Methods: Sixteen eyes of 8 normal subjects were examined with each refraction setting available in the GDx Access (+5, 0, -5, and -10 diopters). The best corrected refraction errors (RE) for the subjects ranged from -6.5 to 0 diopters with a mean of -2.1 2.4(s.d.) diopters. Percentage variations of the parameters superior and inferior maximum, and temporal and nasal median were calculated to evaluate the effect of refraction on the measurements. Results: For each eye, all refraction settings gave qualitatively similar RNFL thickness maps. For all 16 eyes, the superior and inferior maximums varied less than 17% with the refraction changes. For 14 eyes of 7 subjects either temporal or nasal median had at least a 25% variation; and for 2 eyes of 1 subject both temporal and nasal medians had more than 30% variations. The minimum temporal or nasal median occurred when the refraction was set to closest to the subject’s best corrected RE. Conclusion: The RNFL thickness measured by SLP was affected by the refraction setting. Superior and inferior maximums were less affected than temporal and nasal medians. These results suggest that GDx parameters that relate to temporal or nasal medians are very sensitive to the refraction setting.
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