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Editors Selection IGR 12-2
Clinical examination methods: Structural progression
Comment by Douglas Anderson on:
12377 Use of progressive glaucomatous optic disk change as the reference standard for evaluation of diagnostic tests in glaucoma, Medeiros FA; Zangwill LM; Bowd C et al., American Journal of Ophthalmology, 2005; 139: 1010-1018
See also comment(s) by Paul Healey •Medeiros et al. (545) report a group of patients with impending glaucoma who showed a change in the disc configuration over time, compared to another group with good ocular health who did not show change. They found that about half of those with changed discs also had a visual field defect, but the others did not. The GDx VCC scanning laser polarimetry was able to discriminate rather well those with emerging glaucoma (changing discs) from those who were healthy.
Two conclusions are reached by the authors: that GDx is able to pick up some cases of emerging glaucoma before field defects are recognized, and that a changing disc, even if not yet definitively abnormal, can be used as a sign of early glaucoma to validate any instrument's ability to recognize early glaucoma.
Early diagnosis of glaucoma may be enhanced by using progression as a criterionThe paper highlights that there are two ways to recognize glaucoma. The first is the traditional approach to define 'abnormality' (disease) as being outside the bounds of findings in 95% of a healthy population - whether it be disc configuration, nerve fiber measurements, or visual field parameters. The problem with any measurement has been that the range of normal overlaps that found in patients with early glaucoma, so that any cut-off point has false-positives and false-negatives. One idea behind improved technology is to find some measurement that has less overlap between eyes with early glaucoma and those in perfect health. As cases of glaucoma are heterogeneous, it may be inescapable that some will show one abnormal feature first and others another feature, for example disc abnormality before field abnormality, or field abnormality before disc abnormality (a cohort not included in this particular study). The second way to diagnose emerging glaucoma is to recognize a 'change' over time, perhaps while the feature being observed or measured is still 'in the normal range'. This principle of early diagnosis by documenting a change over time is demonstrated by the authors for disc configuration, and needs study with regard to visual fields and new technologies being developed to quantify nerve fibers. This method requires repeat examinations over some period of time, is potentially most useful for early diagnosis, and is a rationale for images or quantified baseline measurements in those at high risk of developing glaucoma. It does not replace the strategy of diagnosing a well established case of glaucoma on a single examination by virtue of being outside normal limits.
References
- Felipe A. Medeiros, MD, Linda M. Zangwill, PhD, Christopher Bowd, PhD, Pamela A. Sample, PhD, Robert N. Weinreb, MD. Use of Progressive Glaucomatous Optic Disk Change as the Reference Standard for Evaluation of Diagnostic Tests in Glaucoma. Am J Ophthalmol 2005;139:1010–1018.
- Nathan Congdon, MD, MPH. Progression of the Optic Disk as a Reference Standard in Glaucoma. Am J Ophthalmol 2005;139:1099-1200.
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