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Editors Selection IGR 17-4

IOP, VF, Imaging and Electrophysiology: Atypical birefringence patterns

Comment by Tae-Woo Kim on:

22331 Clinical characteristics of eyes demonstrating atypical patterns in scanning laser polarimetry, Orlev A; Horani A; Rapson Y et al., Eye, 2008; 22: 1378-1383

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In a subset of eyes, GDx-VCC scans provide 'atypical birefringence patterns (ABP)'. Such images deviate from the normal pattern of birefringence generally characterized by the presence of high peripapillary retardation superiorly and inferiorly that corresponds histologically to the distribution of the superior and inferior arcuate nerve fiber bundles. Current GDx-VCC software includes an exportable parameter called typical scan sore (TSS) that provides a numerical representation of the degree of typicalness in each scan. Although several hypotheses have been raised, the underlying cause of ABP image is yet to be established. Orlev et al. (1461) investigated the relationship between the magnitude of ABP and various clinical parameters including age, gender, refractive error, corneal polarization axis and magnitude, GDx global parameters (TSNIT average and NFI), and the visual field status (glaucoma hemifield test, mean deviation, and the pattern standard deviation). Of the ten clinical parameters studied, age, corneal polarization magnitude and TSNIT average were found to be significantly correlated with TSS on bivariate analysis. However, the magnitude of correlation was too low (r² = 0.086, 0.069 and 0.16, respectively) to suggest a potential causal link between these factors and ABP. Moreover, the authors noted that the correlation of TSNIT average with ABP may represent a confounding effect, rather than an underlying association since eyes showing ABP demonstrated a blotchy pattern of peripapillary nerve fiber layer thickening which would likely translate into artifactiously increased TSNIT value. Thus, the result of this study does not add much insight into the pathophysiology of ABP. The underlying cause of ABP still remains to be shown.

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