Also found in: Acronyms.
ratio, cup-disc (C/D)
The ratio of the horizontal diameter of the physiological cup to that of the horizontal diameter of the optic disc. It should be less than 0.5. If it exceeds that value, or if there is a difference in ratio between the two eyes, or if there is a progressive enlargement of the cup, glaucoma may be suspected (Fig. R1). See glaucomatous cup.
Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann