Clinical science Table 2 TSS (VCC) TSS (ECC) Superior angle Inferior angle 95.05611.39 98.6564.67 72.1611.08 77.0669.96 95.5e100.0 100.0e100.0 65.0e78.5 71.0e85.0 ECC, enhanced corneal compensation; TSS, typical scan score; VCC, variable corneal compensation. DISCUSSION The RNFL thickness proﬁles measured with OCT vary across individuals, even when refractive error and scan quality are taken into consideration.6 This variation in RNFL proﬁles is large, relative to that found owing to measurement error,14e16 age17 18 or the size of the optic disc.17 19 Reference values for RNFL thickness of different ages, ethnic groups and refractive stages are needed to deﬁne normality of RNFL thickness. In the present study we could ﬁnd a correspondence between the location of retinal blood vessels and the RNFL thickness around the optic disc (TSNIT graph) as measured with SLP. We investigated whether the angle of speciﬁc retinal blood vesselsdthe most temporal major retinal blood vessels superi- orly and inferiorlyddiverging from the optic nerve head has an impact on the RNFL thickness around the TSNIT graph. We report that smaller superior or inferior angles (oblique retinal vessel course) were associated with thicker RNFL in the super- otemporal and inferotemporal regions, but thinner RNFL in the superonasal and inferonasal regions. A more oblique course of the most temporal major retinal blood vessels is thus associated with a more oblique location of the arcuate bundles of retinal nerve ﬁbres. We are the ﬁrst to demonstrate a statistical correlation of retinal blood vessel position with RNFL distribution around the optic disc using SLP. This is in correspondence with the results of the original article of Hood et al,10 where a correlation was reported between the position of large retinal vessels and the position of RNFL thickness maxima as measured with GDx and OCT. However, this analysis was qualitative not quantitative. Why should there be an association of retinal blood vessels and the thicker branches of the RNFL? It has been established for several organs and also for the retina20 21 that axon cues offer Superior angle Inferior angle r Signiﬁcance r Signiﬁcance GDx ECC GDx VCC NFI TSNIT TSNIT SD Average superior Average inferior TSS NFI TSNIT TSNIT SD Average superior Average inferior TSS 0.287 0.066 0.369 0.351 0.079 0.069 0.212 0.348 0.452 0.317 0.030 0.204 0.052 0.779 0.224 0.014 0.617 0.800 0.129 0.132 0.068 0.027 0.839 0.060 0.114 0.038 0.278 0.232 0.011 0.159 0.025 0.058 0.132 0.173 0.009 0.122 0.396 0.859 0.309 0.075 0.940 0.519 0.845 0.781 0.558 0.185 0.947 0.213 ECC, enhanced corneal compensation; NFI, nerve ﬁbre indicator; TSNIT, temporalesuperiorenasaleinferioretemporal, average; TSNIT SD, ellipse SD; TSS, typical scan score; VCC, variable corneal compensation. guidance for sprouting and developing vasculature. Vasculature may even share common guidance signals with axons.22 23 Considering these ﬁndings one should expect some correlation between the location of the superior temporal and inferior temporal arteries and veins, for example, and the distribution of RGC axons (especially at the localisation of the temporal arcuate nerve ﬁbre bundles). As a consequence, the location of the blood vessels may help to predict the variation in the proﬁles of RNFL thickness because they mark regions of increased axonal density. Our ﬁndings may be of relevance for the diagnostic accuracy of RNFL measurements. Normal eyes with a straight course of RNFL may be more prone to false-positive RNFL defects at the temporal superior or inferior regions, right beside the edge of the arcuate RNFL bundles. On the other hand in glaucomatous eyes with an oblique RNFL course, a small RNFL bundle defect located temporal superior or inferior might be missed by the GDx. This patient would have started off with high RNFL values in these regions and even after a partial loss of RNFL these values might remain within normal limits. The opposite problem might occur in the nasal superior and nasal inferior sectors. These regions, however, are believed to be of less clinical importance for the diagnosis of glaucoma. Mean values of TSS VCC and ECC, superior and inferior angle Mean values±SD IQR Table 3 Correlation of GDx VCC and ECC parameters with the superior angle and the inferior angle Figure 1 Correlation between the retinal nerve ﬁbre layer (RNFL) as measured with GDx variable corneal compensation (VCC) 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. 682 Br J Ophthalmol 2011;95:680e684. doi:10.1136/bjo.2009.170068