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Editors Selection IGR 23-2

Examination methods: Compensated polarimetry

Comment by David Greenfield on:

14041 Enhanced imaging algorithm for scanning laser polarimetry with variable corneal compensation, Reus NJ; Zhou Q; Lemij HG, Investigative Ophthalmology and Visual Science, 2006; 47: 3870-3877

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Corneal compensation with the scanning laser polarimeter (SLP, GDx-VCC, Carl Zeiss Meditec, Dublin, CA) continues to evolve. The present strategy employs an integrated variable corneal compensator (VCC, which determines and neutralizes eye-specific corneal polarization axis (CPA) and magnitude (CPM) using the concept of the macula as an intraocular polarimeter. The user must select one of two compensation strategies depending upon the presence or absence of macular pathology in order to successfully neutralize corneal compensation. Despite advances, atypical retardation patterns exist, particularly in elderly patients with myopia and eyes with reduced retinal pigment epithelial pigmentation. The study by Reus, Zhou and Lemij (758) describes a new methodology to further reduce image artifact associated with current corneal compensation strategies and evaluates the discriminating power among a cross-sectional population of 177 normal, glaucoma suspect, and glaucomatous eyes with variable stages of visual field loss (average mean deviation -12.5 dB).

Indeed, the Enhanced Corneal Compensation (ECC) algorithm is a robust strategy for eliminating atypical birefringence patterns that may be identified and measured using the typical scan score parameter. As previously reported with fixed corneal compensation, atypical birefringence artifacts observed with VCC are largely oriented nasally and temporally around the optic disc. Though such locations are not as relevant for glaucoma diagnosis and monitoring as the superior and inferior peripapillary regions, ECC significantly improves the discriminating power for glaucoma diagnosis as judged by receiver operator characteristic curves. There are limitations to switching to a new corneal compensation platform. Retardation parameters undergo a significant change when converting from VCC to ECC therefore creating a new baseline image is necessary. Additionally, a new normative database will need to be collected using the ECC algorithm.

Glaucoma diagnosis has been enhanced. Reus and colleagues nicely demonstrate the utility of this software algorithm. This is a well designed and executed study and represents a welcome addition to our understanding of birefringence artifact using SLP.

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