Eccentricity and measurement variability and repeatability with the retinal thickness analyser

Abstract

Aims: To define the variability and repeatability of retinal thickness measurements using the retinal thickness analyser (RTA) and to elucidate any interaction between eccentricity (that is, position relative to the fovea) and variability and repeatability.

Methods: The sample comprised 20 normal subjects of mean age 33 years. Each subject attended for two visits. Repeated RTA scans were acquired centred on the fovea and for any one of the four possible non-foveal scan areas. The mean retinal thickness (+SD) was calculated for a series of concentric circular bands centred on fixation. A repeated measures analysis of variance (ANOVA) was used to determine any significant interaction between the variability of RTA thickness values and eccentricity.

Results: The group mean coefficient of variation and coefficient of repeatability were highest at the fovea. The repeated measures ANOVA revealed that the within test variability of RTA measurements varied significantly with eccentricity (p<0.0001). Similarly, the between test repeatability varied significantly with eccentricity (p = 0.045).

Conclusion: The significantly elevated within test variability and between test repeatability in the foveal area need to be considered when using the RTA to evaluate patients with macular disease.

Figure 1

Schematic diagram showing the default RTA scan locations (f  =  fovea, SN  =  superonasal, ST  =  superotemporal, IN  =  inferonasal, IT  =  inferotemporal) and the radii used for the circular band analysis (ranging from 200–3000 μm radii, centred on the fixation target, in 200 μm steps).

Figure 2

(A) Group mean profile of retinal thickness (0–3000 μm) derived using the circular band analysis for visits 1 (solid circle) and 2 (open square). (B) Group mean profile of COV derived using the circular band analysis for visits 1 (solid circle) and 2 (open square). (C) Bar chart showing group mean COR derived using the circular band analysis. For graphs A and B, the error bars represent plus or minus 1 standard error of the mean (COV  =  coefficient of variation. COR  =  coefficient of repeatability).