Between-Subject Variability in Asymmetry Analysis of Macular Thickness

Purpose To investigate the use of asymmetry analysis to reduce between-subject variability of macular thickness measurements using spectral domain optical coherence tomography. Methods Sixty-three volunteers (33 young subjects [aged 21 to 35 years] and 30 older subjects [aged 45 to 85 years]) free of eye disease were recruited. Macular images were gathered with the Spectralis optical coherence tomography. An overlay 24- by 24-degree grid was divided into five zones per hemifield, and asymmetry analysis was computed as the difference between superior and inferior zone thicknesses. We hypothesized that the lowest variation and the highest density of ganglion cells will be found approximately 3 to 6 degrees from the foveola, corresponding to zones 1 and 2. For each zone and age group, between-subject SDs were compared for retinal thickness versus asymmetry analysis using an F test. To account for repeated comparisons, p < 0.0125 was required for statistical significance. Axial length and corneal curvature were measured with an IOLMaster. Results For OD, asymmetry analysis reduced between-subject variability in zones 1 and 2 in both groups (F > 3.2, p < 0.001). Standard deviation for zone 1 dropped from 12.0 to 3.0 m in the young group and from 11.7 to 2.6 m in the older group. Standard deviation for zone 2 dropped from 13.6 to 5.3 m in the young group and from 11.1 to 5.8 m in the older group. Combining all subjects, neither retinal thickness nor asymmetry analysis showed a strong correlation with axial length or corneal curvature (R-2 < 0.01). Analysis for OS yielded the same pattern of results, as did asymmetry analyses between eyes (F > 3.8, p < 0.0001). Conclusions Asymmetry analysis reduced between-subject variability in zones 1 and 2. Combining the five zones together produced a higher between-subject variation of the retinal thickness asymmetry analysis; thus, we encourage clinicians to be cautious when interpreting the asymmetry analysis printouts.