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Editors Selection IGR 17-4
Imaging Techniques: Time-domain OCT and progression
Comment by David Greenfield on:
24906 Detection of glaucoma progression with stratus OCT retinal nerve fiber layer, optic nerve head, and macular thickness measurements, Medeiros FA; Zangwill LM; Alencar LM et al., Investigative Ophthalmology and Visual Science, 2009; 50: 5741-5748
In the present study, Medeiros et al. (1498) compare the ability of timedomain (Stratus) OCT to detect glaucoma progression using structural parameters obtained from three scanning locations: the optic nerve head (ONH), macula, and RNFL thickness. This study consisted of 253 eyes (glaucomatous eyes and glaucoma suspects) longitudinally followed for a median of four years, and annual imaging was performed. Overall, 13% of eyes showed progression over time, with only 20% of progressing eyes having both a structural and functional endpoint. This finding continues to be a repeated observation in almost all prospective glaucoma clinical trials. Further, as demonstrated in other studies, eyes judged to have progression using visual fields and/or optic disc photography had significantly higher rates of RNFL loss.
RNFL parameters performed significantly better than ONH and macular thickness parameters
What is most interesting in this study is that RNFL parameters performed significantly better than ONH and macular thickness parameters in discriminating progressors from nonprogressors. ONH and macular thickness maps obtained with Stratus OCT are generated from only 6 radial scans with extensive interpolation between each scan. The limited data generated from scans obtained within these regions are therefore less sensitive to detection of progressive glaucomatous change and are likely to have greater test-retest variability. Although eyes with progressive glaucoma certainly have a greater rate of retinal ganglion cell atrophy, there are limitations to using the macular thickness as a clinical endpoint for glaucoma monitoring. Stratus OCT measures total retinal thickness in the macular region and therefore represents a surrogate of RGC loss, and a large proportion of persons with glaucoma have co-existing macular disease that may introduce artifact.
This study emphasizes the importance of examining the RNFL in clinical practice. Fourier-domain OCT continues to dominate the imaging market. Quantitative measurements generated with these instruments are not backwards compatible and many clinicians no longer obtain time-domain OCT in practice. Questions will undoubtedly be raised with regard to the role of Fourier-domain OCT for glaucoma detection and monitoring, and whether three-dimensional scans obtained using high resolution Fourier-domain OCT within the macular region and ONH will be more robust as compared to time-domain.
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