Traditionally, bilateral asymmetry in the cup-to-disc ratios has been considered to be an early sign of clinical glaucomatous damage, and a predictor of future damage in patients with ocular hypertension.
13,22 All large epidemiologic studies on glaucoma prevalence have used cup-to-disc ratio asymmetries of greater than or equal to 0.2 or greater than or equal to 0.3 to define glaucoma or to trigger referral for definitive diagnosis.
22–26 A 0.2 difference in cup-to-disc ratio is the upper limit of approximately 95% of healthy populations, and a higher value was thus considered to be a clinical sign aiding early diagnosis of glaucoma.
27 An increase in disc cupping represents axonal loss of retinal ganglion cells. Examination of a localized RNFL defect by red-free fundus photography or the thickness map of OCT reveals the lost RNFL relative to surrounding RNFL.
28 The most commonly used structural parameters for glaucoma diagnosis, such as RNFL thickness or rim area, measure residual retinal ganglion cells, not a “loss” caused by glaucoma. Earlier reports have shown that diagnosis of glaucoma by measuring such “loss,” rather than measuring the residual retinal ganglion cells and comparing it with the normative database, afford better sensitivity.
8,9 The Thickness map of Cirrus OCT exhibited higher sensitivity in this context than did the Deviation, Quadrant, or Clock-hour maps.
10 The total number of retinal ganglion cells in an eye ranges from approximately 700,000 to 1.2 million, RNFL thickness varies among individuals.
29 The OCT normative database contains a large range of variations, decreasing the sensitivity of glaucoma detection when comparisons with normative database values are performed. A representative case is shown in
Figure 4. This patient had early-stage glaucoma in the right eye, and the utility of comparing the two eyes is evident. The extent of disc cupping is greater in the right than the left eye, but other maps derived by Cirrus OCT showed near-normal findings. However, diffuse RNFL atrophy is evident in the red-free fundus photograph and the Thickness map of the right eye. Comparison of the two eyes clearly revealed the loss of RNFL in the superior and inferior region of the eye with early-stage glaucoma. Comparing the two eyes, with identifying of intereye difference, may be a valuable means by which to detect “loss” associated with glaucoma, because the onset of glaucoma in either eye is asymmetric.
11,12 Usually, the two eyes are symmetric in terms of optical and anatomical characteristics, unless anisometropia is present.
30 The opposite eye may serve as a useful reference for the glaucomatous eye in an individual patient. Such examination may enhance our ability to detect early-stage glaucomatous changes. Our approach is better than a simple comparison to the mean values of healthy eyes differing in terms of race, age, axial length, and disc size with the examined eye.