Fig 1.
Presence of myopia, tilt, oblique insertion, and beta PPA in examined optic discs.
Of all 209 investigated optic discs 69 were myopic, 111 tilted, 39 obliquely inserted and 80 presented with beta PPA, while 45 were not affected by any of these features. Distribution of these characteristics is displayed in a Venn diagram with 69 eyes characterised by a single change, 65 afflicted by two changes, 20 eyes displaying three and 10 eyes presented with all four of the assessed features. The combinations of myopia and oblique insertion as well as tilted discs and beta PPA were found to be significantly associated in pairwise comparison of the complete data set (*).
Table 1.
Descriptive statistics for 209 patients included in the current study.
Fig 2.
Influence of optic disc characteristics on average disc/BMO sizes measured with four different imaging modalities.
Optic disc sizes of 209 participants were measured using KOWA stereoscopic photography and HRT3 topographic imaging and corresponding BMO measurements obtained with the Spectralis and Cirrus OCT. Differences in average size measurements was examined for each imaged modality based on patient ethnicity (A), absence or presence of optic disc tilt (B), oblique optic disc insertion (C), absence or presence of beta PPA (D), refraction error of equal or less than -1 defined as myopia (D). Of these, myopia as well as oblique insertion and/or presence of beta PPA were associated with significant and instrument specific differences in average measurement sizes (E). Bars: 95% confidence interval; Asterisks: Significant difference in average measurement size using ANOVA analysis, * = p<0.05, ** = p<0.01.
Table 2.
Average optic disc area, standard deviation (SD) and 95% confidence interval (CI) for 209 optic discs measured with different modalities.
Fig 3.
Pairwise comparison of optic disc/BMO measurements between investigated imaging modalities.
Bland Altman analysis was applied to examine differences in optic disc/BMO measurements between each pair of imaging devices by plotting the absolute difference in measurements over the average measurement (both mm2). The largest differences independent of absolute measurement size were observed with comparisons of either HRT3 (A), Spectralis OCT (B), or Cirrus OCT (C) to those derived from theKOWA. Absolute measurements are closer between HRT3 and either Spectralis OCT (E) or Cirrus OCT (F), while BMO measurements obtained from the two OCT instruments (G) resulted in the smallest absolute difference with size measurements as well as narrowest confidence interval. For each comparison, the average difference (solid horizontal line) and 95% confidence interval (dashed horizontal lines). Displayed regression lines (dashed line) and associated regression coefficient (R2) are indicative of a potential bias in comparative measurements with absolute size, which is most prominent in comparisons with Cirrus OCT BMO measurements (C, E, F). Additionally, optic disc features determined to significantly influence the average difference in size measurements using ethnicity, myopia, optic disc tilt, oblique insertion, and presence of beta PPA as independent variables in a multivariate regression analysis.
Table 3.
Pairwise correlation of optic disc or BMO area measurements respectively from different modalities and impact of optic disc features.
Table 4.
Optic disc variables impacting on differences in optic disc size measurements between different imaging modalities.
Fig 4.
Representative examples of optic discs leading to significant difference in size interpretation with different imaging modalities.
(A) Comparative images of the KOWA, HRT, Spectralis and Cirrus highlighting the larger disc sizes on the OCTs in comparison to the KOWA and HRT. The discrepancy is located supero-temporally and related to oblique insertion resulting in the location of BMO (white arrows) not correlating to the location of the visible disc margin. (B) Comparative images of the KOWA, HRT, Spectralis and Cirrus highlighting the difference in the two OCTs automated localisation of BMO. The higher resolution Spectralis line scan enables automated detection closer to a trained observer’s manual assessment (white arrows). (C) Comparative images of the KOWA, HRT, Spectralis and Cirrus highlighting an outlier in the Spectralis measurement of disc size due to incorrect delineation of BMO (white arrow) related to the presence of overlying vasculature.