Intra- and inter-observer variation of optic nerve head measurements in glaucoma suspects using disc-data

Abstract

The aim of this study was to determine the intra- and inter-observer variation in the use of a system designed for exact measurements from standard optic nerve head photographs. The commercially available system consisted of a colour CCD Videocamera, a dedicated frame grabber and customised software run on a IBM AT compatible computer. Masked measurements were made 3 times by 2 observers, from stereophotographs of the optic nerve head of 56 eyes from 30 glaucoma suspects. The cup was defined on the basis of contour, not pallor and the disc area was defined as the area inside Elschnig's ring.Intraobserver variances were 0.001±0.001 mm² for cup area (mean±SD), 0.002+0.002mm² for disc area and 0.002±0.003 mm² for rim area. These values for intra-observer variance were comparable with the results obtained using manual planimetric techniques. Intra-observer variance for disc area was significantly larger for the less trained of the two experienced observers.Interobserver variances were 0.004±0.009 mm² for cup area, 0.008±0.013 mm² for disc area and 0.009±0.014mm² for rim area. These inter-observer variances were significantly larger than those previously reported for manual planimetry. The absolute differences between the two observers ranged from −0.35 to + 0.20 mm² (−0.08±O.llmm²) for cup area, from −0.38 to + 0.15 mm² (− 0.08±0.11 mm²) for disc area and from − 0.29 to + 0.34 mm² (− 0.06±0.12 mm²) for rim area. There was a tendency for greater differences between the two observers in the measurement of smaller neuroretinal rim areas, whereas no size dependence of differences existed for the other two measurements. This study emphasises the subjective nature of optic nerve head assessment: despite good intra-observer variance there is significantly more inter-observer variance, precluding its usefulness in routine clinical work.