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Editors Selection IGR 9-4

Scanning Laser Polarimetry: Atypical retardation pattern in SLP

Comment by Christopher Bowd on:

24498 Atypical retardation pattern in measurements of scanning laser polarimetry and its relating factors, Yanagisawa M; Tomidokoro A; Saito H et al., Eye, 2009; 23: 1796-1801

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Several studies have shown that the discrimination between healthy and glaucomatous eyes using scanning laser polarimetry (i.e., GDx) is difficult when eyes with atypical retardation patterns (ARP) are included and it has been reported that as many as 40% of eyes (healthy and glaucomatous combined) show ARPs.¹ To address the ARP issue, a new software algorithm called enhanced corneal compensation (ECC) has been introduced. This software essentially boosts the signal to overcome low sensitivity that can make retardation measurements susceptible to optical and electronic noise. It is assumed that ECC decreases the number of ARPs compared to the older variable corneal compensation (VCC) algorithm, thus improving the chances of successful discrimination.² Yanagisawa et al. (1090) described the percentage of healthy and glaucoma eyes from samples of 105 and 82, respectively, that showed ARPs (defined somewhat arbitrarily as those with a Typical Scan Score < 80) using both GDx ECC and GDx VCC. These authors showed that the overall percentage of ARPs decreased from approximately 14% to approximately 2% when using ECC compared to VCC. This result indicates that using ECC instead of VCC should improve the overall ability of GDx to discriminate healthy from glaucomatous eyes. In addition, their study suggests that when using VCC, a decreasing Typical Scan Score (TSS) is associated with increasing age and increasing myopia. Also, excluding eyes with ARPs strengthened the association between average retinal nerve fiber layer thickness and standard automated perimetry mean deviation (see also^3-4).

The definition of clinically relevant TSS cut-off defining ARPs has not been determined empirically

The definition of clinically relevant TSS cut-off defining ARPs has not been determined empirically. For instance, using an ROC regression model, we found a decrease in ROC curve area for discriminating between healthy and glaucoma eyes of only 10% when TSS was 70 compared to 100.⁵ Therefore it may be that a TSS cut-off lower than the cut-off of 80 used in the study by Yanagisawa and colleagues is a more clinically relevant cut-off. If this is the case, ECC may show an even greater improvement over VCC for classifying eyes as healthy or glaucomatous.

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