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Editors Selection IGR 13-2

Examination methods: Laser scanning: Structure-function relationship

Comment by Robert Fechtner on:

18171 American Chinese glaucoma imaging study: A comparison of the optic disc and retinal nerve fiber layer in detecting glaucomatous damage, Leung CK; Medeiros FA; Zangwill LM et al., Investigative Ophthalmology and Visual Science, 2007; 48: 2644-2652

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Leung et al. (436) used data from an American database (University of California, San Diego) and a Chinese database (The Chinese University of Hong Kong) to explore the structure function relationship in glaucoma. Nerve fiber layer measures (StratusOCT version 3) and optic nerve measures (Heidelberg Retina Tomograph 2) were compared with visual function (Humphrey Field Analyzer II) expressed as mean defect using linear and non-linear regression analyses. Normals had no abnormal visual fields and no elevated IOPs. Subjects with glaucoma were included who had visual field defects, regardless of optic disc appearance or documented progressive glaucomatous change in the optic disc (prior to study imaging) regardless of visual field status. To better compare the imaging technologies, the structural parameters were standardized and expressed in fractions of 'structural integrity'. The relationships between structure and function were best described by nonlinear regression models. At the same, proportional loss of structural integrity the RNFL was associated with poorer visual function than the neuroretinal rim parameters. The change in visual sensitivity per unit change in structural integrity was higher in the RNFL than neuroretinal rim.

This study showed that the RNFL had a stronger structure function relationship with higher diagnostic sensitivity than the neuroretinal rim. The authors suggest RNFL may be a better surrogate measure for monitoring glaucoma progression. There are several interesting implications from this observation. The authors choseOCT and HRT as representative methods for measuring RNFL and neuroretinal rim. These observations will not necessarily translate to other technologies. The cross-sectional correlations here suggest that RNFL may be a better surrogate for monitoring but that must remain speculative until there are validated hardware and software approaches for measuring progression of RNFL parameters that are tested in large prospective studies. Still, anyone planning a study of glaucoma progression would do well to pay attention to this important observation that RNFL measures may be an important surrogate.

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