Symbolic representation of optic nerve head cup presents difference in patients with Primary Open Angle Glaucoma

The aim of this study was to semantically represent the shape feature of the surface of the optic nerve cupping, that cannot be represented by a single number, using Symbolic Aggregate Approximation (SAX). For this purpose, 114 reports of the Heidelberg Retina Tomograph (HRT) from clinical studies of the Laboratory of Research and Applications in Ophthalmology of Aristotle Univ. Thessaloniki, corresponding to 97 normal subjects and 97 patients with Primary Open Angle Glaucoma (POAG) classified by glaucoma experts after full clinical examination, were used. These reports were exported as TIFF images including the results from HRT analysis. By use of an in-house MATLAB application, optic cup contour was unwrapped with respect to the centroid of the cup surface and thus the 2-d shape of the cup surface contour was transformed to 1-d data series. The min symbolic distance distance(mindist) of the 1-d data series was then calculated for each pair of reports and the results were arranged into three groups Glaucoma-Glaucoma, Normal-Normal and Glaucoma-Normal (GG,NN,GN) depending on the initial classification of the reports. Comparing the means of mindist between the 3 groups, statistically significant differences were found (ANOVA, p<;.05). Statistically significant difference among the three distributions of the mindist was confirmed by non parametric Independent-Samples Kruskal-Wallis Test (p<;.001). These findings suggest that the symbolic representation of the optic cup surface could be used to unveil features of the phenotype of glaucoma that cannot be spotted by the currently measured quantitative stereometric parameters.