Adjusted peripapillary retinal nerve fiber layer thickness measurements based on the optic nerve head scan angle

To verify the effect of the scan angle of the optic nerve head (ONH) on measurements of peripapillary retinal nerve fiber layer (RNFL) thickness by using spectral-domain optical coherence tomography (OCT). Both eyes of 64 healthy volunteers were scanned by the optic disc cube 200 x 200 scan of a spectral-domain OCT system. Ultra-high resolution OCT images of the ONH were used to determine the horizontal, vertical, and three-dimensional scan angles of the ONH. The adjusted clock-hour RNFL thicknesses generated by the three-dimensional scan angle of the ONH were then compared with the original clock-hour RNFL thicknesses. The mean horizontal, vertical, and three-dimensional scan angles of the ONH were 12.62 +/- 5.17 degrees, 4.17 +/- 3.30 degrees, and 13.62 +/- 5.13 degrees, respectively. In 125 (97.66%) eyes, the scanned ONH image was tilted temporally; in 89 (69.53%) eyes it was tilted inferiorly. The adjusted clock-hour RNFL thicknesses generated by the three-dimensional scan angle of the ONH were significantly different from the original values (P = 0.009); the mean difference was 13.26 +/- 14.95 microm, and the overall correlation and agreement were not excellent, especially in the inferior quadrant. Current intraretinal imaging devices such as OCT assume that the ONH is positioned exactly in front of the scanning beam; however, this assumption appears to be inaccurate. Because the scan angle of the ONH varies and also influences RNFL thickness measurements, it may be better to consider using the ONH scan angle as an adjustment factor when peripapillary RNFL thicknesses are calculated by OCT.