The alteration and asymmetry of cup and disc diameters.

Increases in cupping with time and differences in cup size between the two eyes of one person were studied in a material derived from a general ophthalmic population survey. Cup and disc diameters were measured on fundus photographs. The effect of refraction on the magnification of the eye-camera system was compensated for by the use of a simple correcting factor. The disc diameter increased slowly but steadily with advancing years but the rim area remained unaffected and as a consequence the rim breadth diminished. The increase in disc diameter and the decrease in rim breadth combined to cause marked increase in cup diameter with ageing. A multiple regression analysis showed cup diameters to be independent of refraction as well as of systemic blood pressure and only weakly associated with intraocular pressure. Side differences in cup diameter were strongly dependent on side differences in disc diameter. Some implications of these findings were briefly discussed.