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Editors Selection IGR 24-3

Combined Structure and Function Measurement: SD-OCT + SAP combined

Comment by Gustavo de Moraes on:

46476 Structure-function relationship in glaucoma using spectral-domain optical coherence tomography, Rao HL; Zangwill LM; Weinreb RN et al., Archives of Ophthalmology, 2011; 129: 864-871

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Rao et al. (1351) reported a prospective, cross-sectional study investigating the structure-function relationship in glaucoma using spectral-domain optical coherence tomography (SD-OCT) and standard automated perimetry (SAP). The relevance of this study lies in understanding the efficacy of structural and functional technologies in detecting glaucomatous damage. Although previous reports have investigated this relationship using other imaging devices ‐ including time-domain optical coherence tomography (TD-OCT) ‐ this is the first study to describe this relationship using a novel technology that can potentially be more accurate than conventional ones and to employ topographic correlations (i.e., spatial consistency) between visual function and retinal nerve fiber layer thickness. In the present study, the linear model described by Hood and Kardon¹ was able to fit the data comparing retinal nerve fiber layer thickness and visual field sensitivity and revealed a stronger association between the infero-temporal and superotemporal arcuate regions with the corresponding visual field sectors when compared to other topographies. Some of the reasons for an imperfect structure-function relationship using any technology are the high-variability of measurements, how structure and function variables are measured and compared, and inherent limitations of each technique. One of the advantages of the SD-OCT technique is the high speed of scans which provide high-resolution images of retinal and optic nerve head structures and reduces the need of data interpolation. Given these technical improvements, one would expect a stronger structure-function relationship when compared to previous reports. Although the authors did not aim to directly compare this relationship between SD- and TD-OCT, there seems to be little or no meaningful difference in how a linear model fits the data using these technologies. The authors also included a sub-analysis testing the association between inner retinal layer measurements (retinal nerve fiber + ganglion cell + inner plexiform layers) and functional loss which added new information to the current knowledge. Nevertheless, some of the limitations of using this combined measurement and to correlate it with SAP results are 1) the relatively small area of the SAP printout that corresponds to the macula, 2) the fact that the nerve fibers from areas outside the macular region may cross and overlap with those measured in the macula, and 3) the oblique relationship between ganglion cells and photoreceptors in the macula. These points may have contributed to a decrease in the strength of the relationship in this area which despite representing a small proportion of the 24-2 SAP printout corresponds to over 60% of the visual cortex. To overcome these limitations, Hood et al.² proposed a new model that suggests using 10-2 SAP results and taking into account the misalignment between the retinal location being stimulated and the corresponding structure to be measured.³ Therefore, it is possible that the strength of the relationship between macular structures and central field sensitivities may be even stronger than currently reported. This is an important study that provides clinically useful information for glaucoma management and the development of new methods that combine structure and function to diagnose glaucoma and detect progression.

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