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Editors Selection IGR 11-3

Imaging: GDx RNFL parameters versus OD topography

Comment by David Greenfield on:

21705 Comparison of retinal nerve fiber layer and optic disc imaging for diagnosing glaucoma in patients suspected of having the disease, Medeiros FA; Vizzeri G; Zangwill LM et al., Ophthalmology, 2008; 115: 1340-1346

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Examination and documentation of the optic disc and retinal nerve fiber layer (RNFL) is essential for diagnosis and monitoring of glaucoma. With a high-power convex lens and stereoscopic slit lamp biomicroscopy, a clinician can observe and document the salient characteristics of glaucomatous optic neuropathy and identify subtle structural changes consistent with progression that include focal or diffuse narrowing of the neuroretinal rim width, loss of peripapillary RNFL, and increased vertical excavation of the optic cup. Stereoscopic optic disc photography allows such features to be permanently recorded for future reference. Recommendations for imaging of the optic disc and RNFL in glaucoma have emerged during the past few years. Digital imaging is recommended to facilitate assessment of the optic nerve and RNFL in the Consensus Initiative of the World Glaucoma Association. Moreover, it is recognized in this consensus report that different technologies may be complementary, and may detect different abnormal features in the same patient. Further support has been provided by a recent comprehensive review by the Ophthalmic Technology Assessment Committee Glaucoma Panel of the American Academy of Ophthalmology. In the present study, Medeiros et al. (1045) compare glaucoma detection using RNFL thickness generated using GDxVCC with optic disc topography measurements generated with the HRT. Forty eyes with progressive glaucomatous optic disc change noted on stereophotographic review with normal standard automated perimetry were compared with 42 normal eyes. RNFL parameters performed significantly better than optic disc topography parameters; the nerve fiber indicator (NFI) parameter had the best overall performance for glaucoma detection (83% sensitivity, 70% specificity). The authors conclude that RNFL imaging using the GDxVCC may provide added benefit to the clinician when combining clinical examination with the optic disc appearance.

Other studies have similarly found the NFI parameter to exhibit robust performance for glaucoma diagnosis. Clinicians should be cautious when using a single parameter, or a single technology, to facilitate glaucoma diagnosis. The range of normal optic disc topography and RNFL thickness values is large, and study results often vary across populations.

RNFL parameters performed significantly better than optic disc topography parameters

In the present study, the investigators were cautious to exclude poor quality scans or scans characterized by birefringence artifact. Imaging may produce false identification of glaucoma and its progression. Imaging also may fail to detect a glaucomatous optic disc or RNFL. In the study by Medeiros et al., 53% of glaucomatous eyes had a normal Moorfields Regression Analysis (MRA) global value, and 31% of normal eyes had an abnormal MRA classification. Thus, clinicians should not make clinical decisions based solely on the results of one single test or technology. The clinician who successfully integrates imaging in practice complements their clinical evaluation with adjunctive diagnostic testing. It is interesting to note that although the best parameter generated an area under the receiver operator characteristic (ROC) curve of 0.8, optic disc stereophotography which was utilized by experts for assessment of glaucomatous optic disc change would have provided a ROC value of 1.0. Why have stereophotographs not been widely embraced in clinical practice, and what limitations exists towards their use in glaucoma clinical trials?

The nerve fiber indicator (NFI) parameter had the best overall performance for glaucoma detection

Glaucoma progression occurs slowly and changes are often subtle and easily missed on review of stereophotographs. Further, fewer optic disc endpoints exist in eyes with moderately advanced glaucoma with considerable neural rim atrophy complicating the ability to detect structural change. In ocular hypertensive eyes with normal optic disc appearance enrolled in the European Glaucoma Prevention Study (EGPS) and OHTS, isolated optic disc endpoints were observed in 40% and 55% of progressing eyes respectively. In contrast, 0.8% and 11% of progressing eyes with established glaucomatous optic nerve damage demonstrated detectable structural endpoints in the Early Manifest Glaucoma Trial (EMGT) and Collaborative Normal-Tension Glaucoma Study, respectively. In addition, unresolved issues exist regarding viewing methods and confirmation of suspected optic disc progression. The authors are to be congratulated for providing a very useful addition to the literature. Computer-assisted imaging of the optic disc and RNFL facilitates glaucoma diagnosis and monitoring. Other issues need to further addressed. How does imaging using fourier-domain optical coherence tomography compare? Will the present results translate to newer GDx platforms that employ enhanced corneal compensation? How do HRT and GDx compare in their ability to detect progressive glaucomatous structural change? It is clear that as technologies evolve, some questions become answered yet new ones always emerge.

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