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Editors Selection IGR 11-3

Posterior Segment Imaging: GDx VCCversus GDx ECC

Comment by Marco Vizzeri on:

26248 Rates of progressive retinal nerve fiber layer loss in glaucoma measured by scanning laser polarimetry, Medeiros FA; Zangwill LM; Alencar LM et al., American Journal of Ophthalmology, 2010; 149: 908-915

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At any given time, while striving to provide appropriate treatment for patients with glaucoma, clinicians are challenged by the progressive nature of the disease. Evaluating the rates of change over time for one of the major indicators of glaucomatous damage, such as RNFL thickness, can help identify patients likely to develop significant visual impairment due to glaucoma during their estimated lifespan. The assumption being made that glaucoma progresses in a linear fashion. Scanning laser polarimetry with variable corneal compensation (GDxVCC) seems capable of evaluating the rates of RNFL loss in glaucoma.¹ Medeiros et al. (703) compared the performance of GDxVCC and the newer GDx with enhanced corneal compensation (GDxECC) for detecting glaucomatous progression using calculated rates of change. While both techniques showed greater rates of loss in eyes progressed by visual field and/or optic disc stereophotographs compared to eyes that did not, GDxECC showed better ability to distinguish progressors from non-progressors. These results are not surprising, as ECC was designed to improve neutralization of atypical retardation patterns (ARP) that can mask true changes in RNFL over time. One wonders whether GDxVCC may still be useful for detecting change in eyes with no apparent ARPs and a typical scan score (TSS) greater than 80. The results of this study are inconclusive with regard to this aspect. When excluding from the analysis images with TSS lower than 80, GDxECC still performed better than GDxVCC. However, 196 GDxVCC images had to be excluded from the analysis versus none for GDxECC. Although a 'weighted' method for calculating the rates of change was used to account for estimates from eyes with fewer available images, the large number of GDxVCC images excluded might have biased the results. The authors do not specify the total number of images evaluated with the two techniques. However, the number of observations during follow-up along with length of followup likely remains a critical factor to obtain accurate estimates of the rates of change. It should also be reminded that GDx appears less sensitive to detect change in eyes with more advanced disease. Future studies will evaluate whether novel technology, such as SD-OCT, is better suited to monitor disease progression throughout the entire glaucoma continuum. Nevertheless, careful clinical examination and visual function testing should always be considered when making patient management decisions.

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