Error in refractive correction and its impact on scanning laser polarimetry

Purpose: The GDx VCC is a scanning laser polarimeter which measures thickness of the retinal nerve fiber layer (RNFL). A perpendicular incident laser beam in the RNFL is a prerequisite of a correct measurement of the thickness of this layer. An error in refractive correction could cause an obliqueness of the laser beam and consequently a deviation of the RNFL thickness reading. The present study investigated the influence of error in refractive correction on the estimation of the RNFL thickness and the nerve fiber index (NFI). Methods: Totally 32 persons were included in this study, 17 without ocular diseases and 15 with glaucoma. The measurement of the RNFL was carried out first routinely with correction of the spherical equivalent refractive error. Then the measurement was repeated 4 times with excessive refractive correction of + 1, + 2, - 1, or - 2D, respectively. The deviation in RNFL thickness and NFI caused by each overcorrection was calculated. An univariate ANOVA analysis was applied to analyze the influence of the error in refractive correction on the result of thickness of RNFL and the NFI. Results: The error in refractive correction caused a significant deviation in the RNFL thickness reading and the NFI in both the group without ocular diseases and the group with glaucoma. The influence was different between these 2 groups. Conclusion: A careful correction of refractive error is important for glaucoma diagnosis and follow-up with GDx VCC.