Local Comparison of Cup-to-Disc Ratio in Right and Left Eyes via Optical Coherence Tomography B-Scans of Optic Nerve Head

The symmetricity of even organs of body may help clinicians in the early detection of diseases. So, we estimated the cup-to-disk ratio (CDR) in left and right eyes and checked the symmetricity between the two eyes using the data of optical coherence tomography (OCT).
In this study, data from 40 normal subjects were used. This data was taken from the 3D-OCT1000 Tapcon model. For estimating CDR in each B-scan, at first the internal limiting membrane (ILM) and retinal pigment epithelium (RPE) layers were extracted; then using the end-point of RPE layer and the depth of cup, the boundary of disk and cup were determined. Finally, CDR of each B-scan was compared point-by-point and the symmetricity parameters were evaluated.
Findings: CDRs in the left and right eyes of 40 normal data were investigated locally. We divided optic disk area into three regions named upper, middle and lower. Then, CDR of each region was calculated for left and right eyes of each patient. The local CDR were 0.433 ± 0.0548, 0.432 ± 0.061, and 0.432 ± 0.0635 for the right eyes and 0.441 ± 0.0635, 0.443 ± 0.0731, and 0.417 ± 0.675 for the left eyes in upper, middle and lower regions, respectively. The differences between local CDRs in different regions of the two eyes were 0.0483± 0.042, 0.0420 ± 0.039, and 0.067 ± 0.045, respectively.
The results of local comparison of CDRs between the left and right eyes indicated that the level of asymmetricity in the lower region of disk was higher than upper and middle regions, and the middle region has the most symmetricity. In addition, in point-by-point symmetricity evaluation of CDRs, the highest level of symmetricity is seen in the second B-scan, near the center of optic disk.