Measurement of a novel optic disc topographic parameter, "spikiness", in glaucoma

Background: Structural changes in the lamina cribrosa have been implicated in the pathogenesis of glaucomatous optic atrophy, but not observed. This p aper presents a novel parameter of topographic variability within the optic disc, termed "spikiness", which may reflect glaucoma-related changes in th e lamina. Methods: Four age-matched groups of normal patients (n=12, mean a ge 64.8 years) and patients with ocular hypertension (n=14, mean age 63.1), primary open-angle glaucoma (n=11, mean age 70) and low-tension glaucoma ( n=15, mean age 66.3) were recruited. Images of normal and glaucomatous eyes from the Heidelberg Retina Tomograph were imported into ERDAS image proces sing software where the spikiness data (30 consecutive mean surface height values across the base of the optic cup in both the Vertical and horizontal meridians) were extracted in a format that facilitated further statistical analysis. Results: Significant differences in topographic variability (spi kiness) existed in the vertical (F=3.64, P=0.01) but not the horizontal mer idian (F=1.25, P=0.3) through the optic disc. Spikiness was inversely relat ed to Humphrey mean deviation (P<0.05), and cup-disc ratio (P<0.004) and wa s directly related to nerve fibre layer thickness (P<0.005). Of particular interest was the finding that the spikiness measure was the only optic disc parameter to significantly discriminate low tension glaucoma from primary open angle glaucoma. Conclusion: A new measure of surface variability (topo graphy) at the floor of the optic cup has been described. The new index of spikiness may represent a measurement of lamina cribrosa fragility which ha s been implicated, but not previously estimated, in glaucomatous eyes.