Variance components for PIMD-2π estimation of the optic nerve head and consequences in clinical measurements of glaucoma.

PURPOSE: To estimate the sources of variation for Pigment epithelium central limit-Inner limit of the retina Minimal Distance averaged over 2π (PIMD-2π), and further to analyse their consequences for clinical measurements of glaucoma. METHODS: Forty subjects with early to moderate stage glaucoma were included. Three SD-OCT volumes of the optic nerve head (ONH) were captured at two occasions. Each volume was segmented three times for PIMD-2π. The magnitude of the sources of variation for PIMD-2π measurements was estimated with an analysis of variance. RESULTS: A 95% confidence interval for mean PIMD-2π was estimated to 215 ± 12 µm (df = 38). The estimated variance for subjects was 1280 µm2 . The within-subject estimated variance for occasions, volumes and segmentations was 10 µm2 , 30 µm2 and 40 µm2 , respectively. The within-subject variances were used to model follow-up of PIMD-2π over time. A linear loss rate of 0.05 of baseline PIMD-2π/year was assumed. A significant PIMD-2π change could be detected in approximately 16-18 months with evenly spaced visits every 4 or 6 months. CONCLUSIONS: Due to the small within-subject estimated variances, a clinically undesirable PIMD-2π change from baseline can be detected in approximately 18 months. Detection of significant PIMD-2π loss in a subject requires knowledge of normal age loss and measurement variability.

A strategy for OCT estimation of the optic nerve head pigment epithelium central limit-inner limit of the retina minimal distance, PIMD-2π.

PURPOSE: To develop a semi-automatic algorithm for estimation of pigment epithelium central limit-inner limit of the retina minimal distance averaged over 2π radians (PIMD-2π) and to estimate the precision of the algorithm. Further, the variances in estimates of PIMD-2π were to be estimated in a pilot sample of glaucomatous eyes. METHODS: Three-dimensional cubes of the optic nerve head (ONH) were captured with a commercial SD-OCT device. Raw cube data were exported for semi-automatic segmentation. The inner limit of the retina was automatically detected. Custom software aided the delineation of the ONH pigment epithelium central limit resolved in 500 evenly distributed radii. Sources of variation in PIMD estimates were analysed with an analysis of variance. RESULTS: The estimated variance for segmentations and angles was 130 µm2 and 1280 µm2 , respectively. Considering averaging eight segmentations, a 95 % confidence interval for mean PIMD-2π was estimated to 212 ± 10 µm (df = 7). The coefficient of variation for segmentation was estimated at 0.05. In the glaucomatous eyes, the within-subject variance for captured volumes and for segmentations within volumes was 10 µm2 and 50 µm2 , respectively. CONCLUSION: The developed semi-automatic algorithm enables estimation of PIMD-2π in glaucomatous eyes with relevant precision using few segmentations of each captured volume.

Variance components in confocal scanning laser tomography measurements of neuro-retinal rim area and the effect of repeated measurements on the power to detect loss over time.

PURPOSE: To estimate the variation in measurements of neuro-retinal rim area (NRA) determined by confocal scanning laser tomography and consequences for clinical follow-up. METHODS: Altogether, 24 healthy subjects were randomized on -320 µm, Moorfields and Standard NRA plane strategies. Additionally, NRA was measured in 32 glaucoma subjects. Variance components for subjects, visits and measurements were estimated with analysis of variance. Sample sizes required to detect a 6.0 × 10-2  mm2 NRA change were estimated assuming a significance level of 0.05 and a power of 0.8. Consequences for independent group, and paired comparison design, respectively, were analysed. Further, precision in estimates within subjects over time was investigated. RESULTS: The variation of NRA among subjects was considerably larger than the variation among visits and measurements. For glaucoma subjects, the variation among visits and measurements were of the same order but larger than in healthy subjects. It was found that independent group comparisons require inconveniently large sample sizes. Within-subject paired comparisons over time require sample sizes of below 15 subjects. The estimated variations for glaucoma subjects imply that 54 months of follow-up is required for detection of change from baseline. CONCLUSIONS: The variance for subjects is substantial in relation to those for visits and measurements. Cross-sectional independent group comparisons of levels of NRA are unsuitable, due to considerable subject variation. Levels of NRA differences within subjects between visits can be estimated with acceptable precision. Neuro-retinal rim area (NRA) measurement can be used for long-term follow-up of glaucoma progression.