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Editors Selection IGR 23-2

Structure and function measurements: Corneal compensation in GDx

Comment by David Greenfield on:

20448 A Defined Change of Polarization Axis is Detected by Variable Corneal Compensation of the GDxVCC, Schmidt E; Boehm AG; Pillunat LE, Journal of Glaucoma, 2008; 17: 19-23

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Corneal compensation strategies with scanning laser polarimetry have evolved considerably over the last decade. The first generation scanning laser polarimeter (NFA I) became commercially available in 1992 and was equipped with a single detector, which was later replaced by a double detector (NFA II). It incorporated a fixed corneal compensator (FCC) to adjust for the corneal retardation and assumed all individuals have a slow axis of corneal birefringence 15 de-grees nasally downward and a corneal birefringence magnitude of 60 nm. GDx with variable corneal compensation (VCC) was introduced in 2002 and is the current commercial iteration of this technology. GDxVCC provides individual customized anterior segment birefringence compensation using retardation measurements obtained in the macula, based on the form birefringence shown by the Henle's fiber layer. Enhanced corneal compensation (ECC) is a research platform that has been described to improve the signal-to-noise ratio and eliminate artifacts associated with atypical birefringe patterns. Schmidt et al. (184) evaluated the effect of extreme head tilt upon corneal compensation using the GDxVCC. Fifteen normal right eyes underwent imaging in standard positions, and repeat scanning was performed turning the head position upside down 180 degrees; images were subsequently saved as left eyes. The authors found insignificant differences in retardation parameters after correction for the 180 degree alignment shift and conclude that a defined change in polarization axis was correctly compensated by the VCC of the polarimeter.

This study further strengthens the body of literature that supports the effi cacy of the commercial VCC for neutralizing anterior segment birefringence

This study further strengthens the body of literature that supports the efficacy of the commercial VCC for neutralizing anterior segment birefringence. Clinicians should recognize, however, that under certain scenarios including corneal or macular pathology this compensation strategy may fail. Furthermore, a subset of GDxVCC images are characterized by atypical birefringence patterns with alternating bands of high and low retardation throughout the parapapillary region. Enhanced corneal compensation (GDxECC) is a promising tool to further enhance signal and reduce noise associated with atypical birefringence artifact.

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