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Editors Selection IGR 15-3

Clinical Examination Methods: Structure and function

Comment by Alfonso Anton Lopez on:

54489 The influence of intersubject variability in ocular anatomical variables on the mapping of retinal locations to the retinal nerve fiber layer and optic nerve head, Lamparter J; Russell RA; Zhu H et al., Investigative Ophthalmology and Visual Science, 2013; 54: 6074-6082

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This is a very interesting study that brings more light and details to the limited knowledge on the topographical relationship between structure and function in glaucoma. Lamparter et al. Used digitally enhanced RNFL red-free photographs (blue filter of 495 µm wavelength) and manual drawing of RNFL bundles. Authors evaluated the potential influence of several parameters on structure-function mapping: spherical equivalent, axial length, disc area (HRT), ellipticity ratio (Spectralis OCT), orientation of the optic nerve head ([ONH]Spectralis HRA), tilting of the ONH (Spectralis OCT) and the position of the ONH in relation to the fovea (RNFL photos). The authors used a thorough, complex and laborious method to delineated actual RNFL bundles (more than 6000 in the study) on the red-free photographs of 100 subjects. They used at least three marker points per bundle to build mathematical functions (cubic splines) and to automatically identify the entry angle of each bundle into the ONH. A scaled Humphrey 24-2 Visual Field grid pattern was then superimposed on the red-free photographs centered on the macula. Complex mathematics allowed the study of the relation between the visual field points and the RNFL bundles. Thirty-three out of 52 field test points, from all subjects, had ten or more intersecting bundles and were used to build the optimal linear model for each visual field location based on the structural parameters mentioned above. The location at which the RNFL bundles, from different subjects, enter the ONH is very variable and the model built in this study can explain part, but not all, of such variability. The position of the ONH in relation to the fovea was the most important predictor for variations in the map, followed by disk area. Additionally, axil length, spherical equivalent, ellipticity ratio, variation in orientation and tilt of ONH influenced mapping on nine, nine, nine, eight and six visual field positions, respectively. The authors conclude that individualized structure-function maps could be built and may assist clinicians detecting glaucoma and monitoring glaucomatous progression.

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