MORPHOMETRIC ANALYSIS OF THE OPTIC DISC SURFACE - THE LEVEL OF SMOOTHNESS AS A DIAGNOSTIC PARAMETER FOR GLAUCOMA

Purpose: Since the glaucomatous loss of nerve fibers changes the appea rance of the optic disc, we evaluated the morphology of the surface of the optic disc in normal and glaucomatous eyes by using a computerize d system to provide the reciprocal position of a large number of point s placed on its surface in order to study the clinical significance of differences in the 'smoothness' of optic disc surface. Methods: The m orphology of the optic disc surface was evaluated by means of simultan eous stereoscopic videographic pictures (IMAGEnet X Rev-3.5lb Topcon E urope, The Netherlands): the reciprocal distribution of a large number of points located on the surface of one eye of 100 subjects randomly chosen (45 normal and 55 glaucoma patients) was studied. In order to d efine the level of 'smoothness' of the optic disc surface, the differe nces of the relative position of each surface point were studied by me asuring the standard deviation (SD) from the average heights of the po ints (n. ranging from 623 to 1916 depending on the size of the disc ar ea) that identify the optic disc surface. Results: The coefficient of variation of the reciprocal location of the points, placed on the opti c disc surface at the different measurements performed by a single ope rator was 10.4%. The differences in Optic Disc Surface Smoothness (ODS S) between glaucoma and normal group were statistically significant (p < 0.0001 using Mann-Whitney U test). No correlation was detectable be tween age and standard deviation. The best threshold value, calculated using ROC methodology, able to separate the two groups was: normal gr oup: SD less than or equal to - 17.79 (-1 x 10(-2) mm); glaucoma group : SD > - 17.79 (-1 x 10(-2) mm). Such threshold value had a sensitivit y of 82.1%, a specificity of 92.2% and a diagnostic precision (DP) of 86.5% in dividing the glaucoma group from the normal group. Conclusion : ODSS is a global index of optic disc conditions based on quantitativ e measurements of the morphology of the optic disc surface. As such it does not provide information about the location and the characteristi cs of optic disc damage. Nevertheless, ODSS measurement is able to sep arate normal from glaucomatous optic disc with a rather interesting se nsitivity, specificity and diagnostic precision (DP). As such it could be useful both for research and clinical applications.