Background: Precise optic disc size measurements based on anatomically exact disc margins are fundamental for a correct assessment of glaucoma suspects. Computerized imaging techniques, such as confocal-scanning-laser-tomography (CSLT), which applies operator defined boundaries and optical-coherence-tomography (OCT), which incorporates an alternative detectable landmark (Bruch's-membrane-opening (BMO)), have simplified the planimetry of the optic disc and BMO-area, respectively. This study's objectives are to compare both modalities for area and to define a threshold for macro-BMO using BMO-OCT.
Methods: Retrospectively, patients that simultaneously received CSLT and BMO-OCT scans were included. Their images were correlated and agreement was determined using Bland-Altman-analysis. The diagnostic power of a macro-BMO threshold using OCT was derived after creating a receiver-operating-characteristics-curve using the well-established analogous CSLT threshold (2.43 mm2).
Results: Our study included 373 eyes with a median optic disc area by CSLT/ BMO-area by OCT of 2.56 mm2 and 2.19 mm2 respectively. The Bland-Altman-analysis revealed a systematic deviation with a diverging tendency with increasing area, which enabled the creation of the following mathematical relation: disc-area (CSLT)*0.73 + 0.3 = BMO-area (OCT). BMO-area of 2.19 mm2 showed the best diagnostic power for identifying macro-BMOs using OCT (sensitivity: 75%, specificity: 86%).
Conclusions: Area measurements (CSLT optic disc area vs. BMO-area by OCT) showed a systematic deviation with a divergent tendency with increasing size. Our mathematical equation offers an estimated comparison of these anatomically diverse entities. Considering BMO-OCT´ anatomical accuracy, the 2.19 mm2 threshold may improve discernment between glaucoma suspects and norm variants.
Keywords: BMO-OCT; CSLT; Glaucoma; Macrodiscs.