Clinical science Figure 2 Correlation between the retinal nerve ﬁbre layer (RNFL) as measured with GDx enhanced corneal compensation (ECC) and the superior angle. The slopes of the regression lines (K) together with the limits of the 95% CI of the slopes are plotted against the 64 sectors (plots). Statistical signiﬁcance of the slope is reached when the 95% CI does not include the zero line. The sections of the TSNIT (temporalesuperiorenasale inferioretemporal) graph marked with a bold line symbolise signiﬁcance of Pearson R. In the clinical situation the problem described above is reduced to some extent by interocular comparison. At least in false- negative patients with glaucoma, asymmetry in the RNFL distribution is displayed. As a high degree of symmetry is expected in normal eyes, asymmetry is a useful clinical sign of early glau- coma. However, symmetry is sometimes limited in normal eyes and the disease itself may also be symmetric in some patients. When interpreting the results of this trial a number of intrinsic limitations have to be discussed. We had the aim to test a simple indicator of the retinal vessel distribution for the postulated effect on the RNFL distribution. While we found that even our simple parameter of retinal vessel location had a signiﬁcant correlation with the thickness of RNFL over one- third to a half of the circumference, we did not test the effect of other retinal vessels. We also did not take into account split bundles that might also inﬂuence RNFL thickness.4 Split bundles primarily occur in the superior region and only rarely in the inferior region. Our data demonstrate, however, a similar effect of superior temporal and inferior temporal blood vessel location on the RNFL. Split bundles thus do not appear to have greatly inﬂuenced our study results. Furthermore, we cannot exclude the possibility that the automatic exclusion of major blood vessel positions from analysis by the GDx software might have inﬂuenced our ﬁndings. Another limitation might be the assumption of a linear asso- ciation between blood vessel position and RNFL thickness. To test a possible non-linear relationship we additionally performed a stepwise multiple regression analysis including a quadratic term of the angle. This analysis demonstrated a non-linear relationship for 25% of the plots with signiﬁcant results. However, the effect of non-linearity was small (data not shown). For clinical use of our ﬁndings an automated analysis of the blood vessel position would be necessary. Since the GDx auto- matically excludes the major blood vessels from the RNFL measurement, the data of blood vessel locations should also be available for analyses similar to ours, although the present software does not offer access to these data. Finally, the axon density seems to be well coupled to the location of the superior and inferior temporal arteries and veins. A better understanding of the relationship between the distri- bution of the major blood vessels and GDx RNFL thickness helps us to understand interindividual variation. Reduction of Figure 3 Correlation between the retinal nerve ﬁbre layer (RNFL) as measured with GDx variable corneal compensation (VCC) and the inferior angle. The slopes of the regression lines (K) together with the limits of the 95% CI of the slopes are plotted against the 64 sectors (plots). Statistical signiﬁcance of the slope is reached when the 95% CI does not include the zero line. The sections of the TSNIT (temporalesuperiorenasale inferioretemporal) graph marked with a bold line symbolise signiﬁcance of Pearson R. Br J Ophthalmol 2011;95:680e684. doi:10.1136/bjo.2009.170068 683