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NT Choplin, Q Zhou; The Effect Of Individualized Compensation For Anterior Segment Birefringence On Retinal Nerve Fiber Layer Assessments As Determined By Scanning Laser Polarimetry . Invest. Ophthalmol. Vis. Sci. 2002;43(13):993.
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© ARVO (1962-2015); The Authors (2016-present)
Purpose:Scanning laser polarimetry is a method designed to estimate retinal nerve fiber layer (RNFL) thickness through measurement of birefringence, a physical property of the RNFL and the cornea. The commercial instrument, the GDx® (Laser Diagnostic Technologies, Inc., San Diego, California), uses an anterior segment compensator of fixed magnitude (60 nanometers) and slow polarization axis (15 degrees nasally downward) to eliminate the contribution of the anterior segment to the total signal. Previous studies have shown up to 30% of patients are not adequately compensated by this method based on axis alone. The aim of this study was to determine the effect of individualized anterior segment compensation using a newly designed anterior segment compensator upon estimates of retinal nerve fiber layer thickness compared to those as determined with the fixed compensator in the commercial device. Methods:The new anterior segment compensator consisted of two crossed polarizers mounted in a rotatable housing, capable of neutralizing birefringence of up to 120 nanometers at any axis, and was incorporated into the GDx®. Anterior segment birefringence was estimated by obtaining a scan of the macula using no compensation, and the compensator then set to neutralize it. Normal subjects (50 eyes) and patients with glaucoma (24 eyes) underwent RNFL measurements using the standard (fixed) compensator and the variable compensator. The results were compared using Hotelling’s generalized means test with Bonferroni’s adjustment for multiple comparisons. Results:All thickness values were statistically significantly (p < 0.05) lower as determined with the variable compensator, with no discernible differences in any of the modulation parameters in both normal subjects and those with glaucoma. Conclusion:On the average, individualized anterior segment compensation lowers the RNFL thickness values as determined by scanning laser polarimetry while preserving modulation. This may prove useful in identifying RNFL damage in eyes not properly compensated by the fixed method.
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