Photogrammetric analysis of the three-dimensional geometry of normal and glaucomatous optic cups

Using photogrammetrically generated maps and measurements, the geometric configuration of the optic cups from a group of normal, hypertensive, and glaucomatous eyes were analyzed in detail. Six distinct three-dimensional shapes were found to characterize the range of normal cups. These forms closely resemble a cone, a cylinder, a hemisphere, a cone-cylinder, a cone-hemisphere, and a cylinder-hemisphere. A composite picture of glaucomatous change was produced by collating the data obtained from different nerve heads which were thought to represent serial stages of disease. Among the many topographic features identified by this analysis is the early deepening of the optic cup which seems to take place only in disc in which the lamina was not visible before glaucoma began. In cups in which this process has occurred and in those in which the lamina was already exposed during the physiologic state, no further increase in the maximum value of this dimension appears to take place until late in the course of the disease. On the surface of the disc, the pattern of erosion is more vertical than horizontal in direction, creating a vertically oriented ellipse of the cup orifice. Its enlargement occurs in a concentric fashion and usually leads to the disappearance of the superior or inferior disc margins or both before losing the rest of the rim. In spite of the precision with which the optic cup morphology was mapped and measured, the end result still lacks the specificity which could be achieved by actually following the same optic discs while they are undergoing progressive atrophy. Such a study is now underway.