Photogrammetric Measurement of the Optic Disc Cup in Glaucoma

Cupping of the optic disc associated with the atrophy of the optic nerve is one of the earliest recognizable signs of glaucoma . Measurements of the optic cup changes would provide objective evaluation in early diagnosis and therapy of glaucomatous eyes, by following the course of the patient in relation to the loss of visual field and medical or surgical control . A semi-analytical photogrammetric technique, the radial section method, has been devised to provide general references so that any differences in optic discs can be compared objectively. The coefficient of variation of the cup parameters is about 4% for cup diameter, and abour 5% for cup depth and volume . Due to the small size of the dilated pupil and refractive index of the eye , the accuracy of photogrammetric measurements of the optic disc is limited . However, optimal geometry of fundus stereophotography can be obtained by convergent stereophotography . On model experiments with 15 degrees of convergent angle, the new method improves the accuracy eight times in cup depth and more than two times i n cup diameter measurements over that of non-convergent method .