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Editors Selection IGR 16-4
Clinical examination methods: Polarimetry problems
Comment by David Greenfield on:
13077 Enhanced corneal compensation for scanning laser polarimetry on eyes with atypical polarisation pattern, Toth M; Hollo G, British Journal of Ophthalmology, 2005; 89: 1139-1142
It has been reported that in a subset of eyes, GDx-VCC scans have atypical birefringence patterns (ABP) such that the brightest areas of the retardation maps are not consistent with the histologically thickest portions of the peripapillary RNFL located along the superior and inferior arcuate bundles. Eyes with ABP are characterized by variable areas of high retardation arranged in a spoke-like peripapillary pattern, or 'splotchy' areas of high retardation nasally and temporally. Proposed mechanisms for atypical images include older age, myopia and thinner retinal pigment epithelium. An enhancement module (enhanced corneal compensation, ECC) has been recently described to improve the signal-to-noise ratio and eliminate artifacts associated with ABP. The ECC algorithm introduces a predetermined birefringence bias to shift the measurement of the total retardation to a higher value region to remove noise and circumvent the problem of atypical patterns. The amount of birefringence bias is determined using the birefringence pattern of the macular region, and then is mathematically removed point by point from the total birefringence pattern of the VCC to improve the signal and obtain a retardation pattern of the RNFL with least noise. The study by Toth and Hollo (801) investigates the ability of ECC to improve the magnitude of image artifact, as measured by the typical scan score (0-100 with lower scores associated with greater image atypia) based on a support vector machine analysis. The authors found that ECC improved the TSS scores but was associated with significant differences in RNFL assessments as compared with commercially available GDx-VCC software. This study adds further evidence that atypical birefringence patterns can confound the interpretation of GDx images. Eyes with atypical images have an apparent increase in RNFL thickness and may result in falsely normal RNFL assessments in glaucomatous eyes. Clinicians and investigators should be aware of this phenomenon and exhibit caution when interpreting the retardance parameters generated in such eyes. Software strategies to reduce and or correct ABP images, such as ECC, would be expected to further improve the diagnostic accuracy of this technology and are currently under investigation.
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