Component evaluation of cup/disk ratio estimation

Cup/disk (C/D) ratio estimation is frequently used to evaluate cupping within the optic nerve head. Two components are involved in the determination of this value: (1) the cup and disk margin determination, and subsequently (2) the magnitude estimation of the cup size relative to the disk size. This study investigated the magnitude estimation component by asking fourth year optometry students to estimate C/D ratios on optic nerve head photographs and drawings. The importance of the magnitude estimation was demonstrated by the high degree of variability among subjects on the nerve head drawings upon which cup and disk margins were presented unambiguously. A subsequent survey of faculty clinicians indicated a large percentage of practitioners (41%) have no method they consciously use to help determine their C/D ratio estimate. A specific method is presented which allows clinicians to simply and logically deduce this magnitude estimation.