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Editors Selection IGR 13-2

Structure Tests and Anatomy: Progression analysis by GDx

Comment by Robert Fechtner on:

25244 Agreement for Detecting Glaucoma Progression with the GDx Guided Progression Analysis, Automated Perimetry, and Optic Disc Photography, Alencar LM; Zangwill LM; Weinreb RN et al., Ophthalmology, 2010; 117: 462-470

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Glaucoma patients and suspects are followed for progression of disease with functional and structural testing. The ability to identify progression depends on the skills of the examiner and the tools available. The subjects for this study by Alencar et al. ( 247), were 252 individuals (453 eyes) enrolled in the Diagnostic Innovations in Glaucoma Study at the Hamilton Glaucoma Center, University of California, San Diego. This ongoing study has collected prospective longitudinal data on a large cohort of subjects. It provides a unique opportunity to evaluate diagnostic strategies. This report evaluates new progression detection software for scanning laser polarimetry (GDx GPA) and compares it with standard automated perimetry and stereo optic disc photography as reference standards for progression.

Different technologies measure different aspects of glaucoma

Of the subjects in this report, 22 had normal examinations, 129 had abnormal visual fields and 302 were classified as glaucoma suspects. By standard automated perimetry (SAP) or masked optic nerve evaluation by expert readers 34 of 431 eyes showed progression (13 based on SAP alone, 16 based on photos alone and 5 based on both.) The GDx GPA identified 17 of these 34 (50% sensitivity); this included 12 of 21 that progressed by optic nerve photographs and 7 of 18 that progressed by SAP. Fourteen of the 431 suspect and glaucoma eyes were identified as progressing by GDx GPA, but not by SAP or photos. None of 22 healthy eyes showed progression by GDx GPA. Sensitivity, specificity, and likelihood ratios were calculated and reported.

This is the first study to compare GDx GPA with SAP and stereo optic nerve photography. While the GDx GPA was able to identify some of the eyes with progression on SAP or photography, the agreement was not excellent. This is not surprising; in only 5 of 34 eyes with progression was there agreement between SAP and photographs. Further, we do not know whether the 14 eyes identified as progressing by GDx alone represented false positives or improved diagnostic ability. Different technologies measure different aspects of glaucoma. That is why we need multiple tools for glaucoma evaluation and management. GDx GAP allowed automated detection of some glaucoma or suspect eyes that progressed by SAP and/ or photography and did not identify any of the known normal eyes as having progressed. While we should not expect this nerve fiber layer measurement tool to supplant visual function testing and optic nerve examination it seems substantial progress has been made with new software.

The scanning laser polarimetry (SLP) examinations used for this study were performed with the commercially available hardware and new software. This research group and others have reported on enhanced corneal compensation techniques that may improve the performance of SLP.

Validated computer assisted nerve fiber layer analysis will extend our senses and offer opportunities for improved care

I would hope that further refinements in SLP that have been reported in research settings will be introduced in the commercial instruments. Validated computer assisted nerve fiber layer analysis will extend our senses and offer opportunities for improved care. However, there will always be a several year delay between the development of a new progression tools and validation studies. By the time one strategy for progression is validated a new one may be introduced.

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