measurement of IOP by Goldmann applanation tonometry and standard automated perimetry (SAP). Subjects eligible for participation in the study according to the inclusion/exclusion criteria were included. One eye was selected randomly for the GDx VCC and ECC measurements. The study was performed at the Department of Ophthal- mology, Allgemeines Krankenhaus, Vienna. Methods Automated visual ﬁeld testing was performed with the Humphrey ﬁeld analyser II (program 30-2). Visual ﬁeld eligibility criteria were <33% false-positive responses, <33% false-negative responses and <33% ﬁxation losses. SLP GDx VCC was performed using a commercial GDx VCC system Version 5.5.0 (NDB Version 1.05.00, Carl Zeiss Meditec, Dublin, California, USA). Additionally we used the GDx ECC method, which provides individualised corneal compensation with enhanced SLP measurement sensitivity. Details of the set-up are described elsewhere.12 13 Brieﬂy, SLP assesses RNFL thickness in the peripapillary retina by measuring the RNFL birefringence with a near-infrared diode laser. With GDx ECC software, the corneal polarisation compensator is automatically adjusted so that the combined retardation magnitude from the cornea and the compensating retarder is w55 nm with a vertical slow axis of polarisation. This adjustment bias serves to boost the signal to overcome low sensitivity that can make retardation measurements susceptible to optical and electronic noise. After image acquisition, the bias is subtracted to yield the RNFL retardation values. In our study, each subject with pupils undilated had scans on the same day performed by an experienced operator. The spherical equivalent refractive error was tested subjectively and entered into the software to allow the GDx VCC and ECC to focus on the retina. All selected images were of high quality (quality scan score of $8) with a centred optic disc, were well focused and illuminated throughout the image, and were without motion artefacts. A ﬁxed concentric measurement band with 27 pixels (w2.4 mm) inner diameter and 35 pixels (w3.2 mm) outer diameter was centred on the optic disc, after which the measurements of peripapillary retardation were conducted. Areas of blood vessels are a source of noise and are therefore automatically excluded for analysis by the GDx soft- ware.4 Retardation was converted to an estimate of RNFL thickness by the software. The GDx data were exported to a personal computer for data analysis. To examine the retinal blood vessel position we measured the angle between a line through the centre of the optic disc and the most temporal major retinal blood vessel at a particular peri- papillary location at 35 pixel distance from the optic disc centre (¼ outer margin of the measurement band) and between a horizontal line through the optic disc centre. This was performed separately on the GDx printout for the upper and the lower retinal blood vessels using a setsquare. The respective values are referred to as superior angle and inferior angle. All analyses were done for GDx VCC and ECC. The param- eters investigated in this study were RNFL thickness values of 64 sectors (‘plots’) around the optic disc on the measurement band, and the parameters of the GDx standard printout: TSNIT average, superior average, inferior average, ellipse standard deviation (TSNIT SD), as well as nerve ﬁbre indicator (NFI) and typical scan score (TSS). Statistical methods We calculated linear regression analyses with the superior angle and the inferior angle as independent variables and the RNFL Br J Ophthalmol 2011;95:680e684. doi:10.1136/bjo.2009.170068 Clinical science thickness values of 64 sectors of the TSNIT graph as dependent variables. To depict the slopes of the regression lines of the 64 sectors according to the formula (y¼Kx + d) and their signiﬁ- cances, we plotted a modiﬁed TSNIT graph using K values and the upper and lower limits of the 95% CI of the K values. Statistical signiﬁcance of the slope was reached when the 95% CI did not include zero. While this analysis yields the clinically relevant information about the effect size (¼ slope) it is not identical to an analysis of the strength of the correlation. We thus additionally checked the signiﬁcance of Pearson R to test the strength of correlation of the superior or inferior angle with each of those sectors. To test the linearity of regression we additionally performed a stepwise multiple linear regression analysis using the inferior angle and its square and the superior angle and its square. Statistical analyses were done with the SPSS software package (SPSS, Chicago, Illinois, USA) release No. 16.0.2. RESULTS Fourty-four female and 37 male subjects were included in the study. Subjects’ baseline characteristics, IOP and visual ﬁeld mean deviation (MD) are given in table 1. We were only able to obtain a sufﬁcient measurement quality of 81 out of 83 patients. Hence all data are from 81 subjects. There was a signiﬁcant correlation between the superior angle and the inferior angle (r¼0.344, p¼0.002). The refractive error correlated signiﬁcantly with the superior angle (r¼0.29, p¼0.01) but not with the inferior angle (r¼0.18, p¼0.112). TSS VCC and ECC, superior and inferior angle mean values are summarised in table 2. The correlations of GDx VCC and ECC parameters with the superior angle and the inferior angle are shown in table 3. For these parameters the main ﬁnding was a signiﬁcant negative correlation between the superior angle and the superior average of RNFL in GDx VCC and GDx ECC. The slopes of the regression lines of the superior and the inferior angle with the RNFL thickness values of the 64 sectors around the optic disc are displayed in modiﬁed TSNIT graphs (ﬁgures 1e4). These ﬁgures clearly demonstrate a statistically signiﬁcant negative correlation between the superior angle and the temporal superior and temporal inferior RNFL thickness values, and a (less pronounced) positive correlation between the superior angle and the nasal superior and nasal inferior RFNL thickness values, for both GDx VCC and ECC (ﬁgures 1 and 2). The same correlation was found between RNFL thickness values and the inferior angle (ﬁgures 3 and 4). The steepest slope of the regression line was located in the superotemporal and inferotemporal regions ( 0.7 to 1.0 mm/8). Using this slope, the difference in RNFL thickness for the IQR of the superior angle is 13 mm. For the 10e90% range of superior angle (55e898) this difference amounts to 34 mm. The maximum R values were found superotemporally for the superior angle and inferotemporally for the inferior angle ( 0.497 to 0.627). Table 1 Subjects’ baseline characteristics, IOP and visual ﬁeld MD Age (years) 27.369.5* Sex (female/male) 44/37 Refractive error (dioptres) 0.2361.55* IOP (mm Hg) 14.862.4* MD (dB) 0.1360.59* *Results are presented as means 6 SD, (n¼81). dB, decibel; IOP, intraocular pressure; MD, mean deviation. 681