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Editors Selection IGR 23-3

IOP, VF, Imaging and Electrophysiology: Longitudinal variability in imaging

Comment by Eytan Blumenthal on:

22205 Longitudinal variability of optic disc and retinal nerve fiber layer measurements, Leung CK; Cheung CY; Lin D et al., Investigative Ophthalmology and Visual Science, 2008; 49: 4886-4892

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Retinal nerve fiber layer (RNFL) thickness, as well as neuroretinal rim area can each serve as an objective structural parameter for following glaucoma patients, as well as glaucoma suspects. Change greater than the longitudinal variability (LV) can justifiably be considered disease progression, while change smaller than LV might be dismissed as measurement variability (fluctuations that occur in the absence of disease progression). Hence, LV is the 'yard stick' by which a series of imaging tests are analyzed for progression. Since LV might vary by disease state (stable glaucomatous eyes often present higher variability over time than healthy eyes) it might be advisable to quantify this value separately for healthy, and for diseased eyes.The present study by Leung et al. (1334), a joint effort of the University of Hong Kong and the Hamilton Glaucoma Center, UCSD, set out to quantify LV for OCT (StratusOCT), SLP (GDx VCC) and CSLO (HRT 3). Eighty-eight subjects (half healthy, half glaucomatous) were scanned on each of these imaging devices, in each of three office visits spanning a nine-month period.

Longitudinal variability is the 'yard stick' by which a series of imaging test are analyzed for progression

For imaging tests to qualify as progression tools, they should ideally be both reproducible and sensitive to change. 'Reproducibility' is defined as the ability of a device to provide consistent measurements. In contrast, 'sensitivity to change' is a useful parameter indicating the number of progression events that can be detected along the spectrum of the disease (from complete health to end-stage glaucoma), thus reflecting the ability of an instrument to be clinically useful for monitoring glaucoma progression. In this study, all three devices showed low LV and thus appear useful for studying progression. In addition, LV was not found to correlate with the severity of glaucoma. This finding is reassuring, as stable LV throughout the glaucoma continuum would enhance the usefulness of a device for studying progression. Most importantly, data collected in this study may serve to develop progression algorithms aimed at detecting change in serial imaging studies of RNFL thickness and neuroretinal rim area.

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