A comparison of optic disc area measured by confocal scanning laser tomography versus Bruch’s membrane opening area measured using optical coherence tomography

Abstract

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.

The Diagnostic Value of Optical Coherence Tomography for Detecting Glaucoma in High Myopia

“…Although the thickness parameters measured by OCT are valuable in diagnosing glaucoma among high myopia, several aspects need to be considered when interpreting the results of OCT. First, due to the magnification effect of longer axial length in high myopic eyes, OCT will scan larger area than that in emmetropic eyes. The actual size of fundus dimension can be expressed as t = P * q * s, where t is the actual fundus dimension, s is the measurement obtained by OCT, P is the magnification factor for the imaging system, q is the magnification factor related to the eye 50. Note that RNFL becomes thinner to distant to optic disc in healthy eyes, thinning due to glaucoma or magnification will be challengeable to distinguish in this situation.…”

“…The actual size of fundus dimension can be expressed as t = P * q * s, where t is the actual fundus dimension, s is the measurement obtained by OCT, P is the magnification factor for the imaging system, q is the magnification factor related to the eye. 50 Note that RNFL becomes thinner to distant to optic disc in healthy eyes, thinning due to glaucoma or magnification will be challengeable to distinguish in this situation. Also, it is more difficult to measure a thinner layer accurately in OCT. 51 Previous study indicated that the measurement of the RNFL thickness was significantly affected by ocular magnification and the measured value may be less than the actual thickness.…”

Evaluation of Retinal Nerve Fiber Layer and Macular Ganglion Cell Layer Thickness in Relation to Optic Disc Size

“…While some authors report no correlation between HRT and OCT optic disc measurements [ 38 ], Cazana et al have recently demonstrated transferability of HRT measurements to an OCT-based BMOA assessment. They report a BMOA of ≥2.19 mm 2 to resemble the adequate threshold value for optic discs to be considered macrodiscs; however, they did not include microdiscs in their analysis [ 15 ]. Therefore, as definitive BMOA reference values for micro-, norm- and macrodiscs remain elusive, this study adhered to the traditional HRT division for optic discs size classification and added a 5% quantile approach in order to examine extreme optic nerve heads.…”

“…Traditionally, this attribution of optic discs to one group has been based on fundoscopic assessment or on measurement results of confocal scanning laser tomography (CSLT). In recent years, studies using OCT for the characterization of optic disc morphology have demonstrated that approaches based on Bruch´s membrane opening area (BMOA) can be used instead [ 7 , 15 , 16 , 17 ]; however, categorization and thresholds for micro- and macrodiscs are still not universally defined.…”