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Editors Selection IGR 16-4

Scanning Laser Polarimetry: Impact of Atypical Retardation Patters on detection of progression

Comment by David Greenfield on:

24015 Impact of atypical retardation patterns on detection of glaucoma progression using the GDx with variable corneal compensation, Medeiros FA; Alencar LM; Zangwill LM et al., American Journal of Ophthalmology, 2009; 148: 155-163

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First described in 2005, eyes with atypical retardation pattern (ARP) have plagued the clinician seeking to use the GDx with variable corneal compensation (GDxVCC) to facilitate glaucoma diagnosis. The clinical consequence of ARP is that the artifact results in a falsely greater measured RNFL thickness and may mask glaucomatous RNFL loss. Such eyes are often myopic with regions of RPE atrophy, and generate considerable scleral reflectance and scatter that is thought to contribute to the underlying mechanism. GDxVCC scans with ARP are characterized by a typical scan score (TSS, a continuous variable ranging from 0-100) below 80 and radial spokelike patterns of birefringence particularly in the nasal and temporal parapapillary regions. In the present study, Medeiros et al. (1089) highlight additional concern regarding the impact of GDxVCC images containing ARP. The authors longitudinally studied 377 eyes (glaucoma suspects and glaucomatous eyes) with a median follow-up of four years and demonstrated that eyes with ARP had a significant effect on detection of progressive RNFL loss. ARP was common with almost 20% of all eyes characterized by TSS values below 80. Lower TSS values at baseline were significantly associated with greater change in TSS scores over time; each unit of TSS change was associated with a 0.14 micron difference in TSNIT average over time. Eyes with baseline TSS below 80 had a 50% likelihood of undergoing a ten-unit change in TSS, compared to eyes with TSS of 80 or more that had a 14% likelihood.

All technologies may falsely identify glaucomatous change and identifying sources of artifact are critical

The high frequency of eyes that underwent large variability in the TSS score raises both concern and questions the underlying mechanism of ARP. Why would the magnitude of scleral scatter within a given eye markedly change in a relatively short time frame of four years? To what degree does eye position or testing conditions contribute to the source of variability? Does instrument calibration play a role? Will newer corneal compensation strategies such as enhanced corneal compensation (ECC) reduce the prevalence of TSS variability and therefore reduce the bias contributed to longitudinal RNFL thickness assessment? The results of this study have a number of important clinical consequences. Most importantly, clinicians should measure TSS values in all eyes that undergo GDxVCC imaging. Although a Q-score is provided on the GDxVCC printout that represents a quality score (based on image focus, motion artifact, centration, illumination, etc) the quality of corneal compensation must be measured using the TSS score and eyes with values below 80 should be avoided due to artifact that will bias not only the diagnosis of glaucoma but also the judgment of progression. Moreover, the assessment of glaucomatous progression in eyes with a ten-unit change in TSS value should be avoided due to significant bias. This occurred in 50% of eyes with TSS values below 80, but also occurred in 14% of eyes with normal retardance patterns characterized by TSS values ≥80. In other words, a high quality GDxVCC scan without birefringence artifact may have a one in seven chance of undergoing a ten-unit change in TSS value that will bias the determination of progression. This study emphasizes the importance of image quality when interpreting GDxVCC images in clinical practice, and adds to our understanding of RNFL thickness changes that may occur in eyes with progressive and non-progressive glaucomatous optic neuropathy. All technologies may falsely identify glaucomatous change and identifying sources of artifact are critical. Medeiros and colleagues are to be congratulated for this excellent contribution to the literature.

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