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Editors Selection IGR 13-3

OCT: CSLT, SLP and OCT and detection-localized RNFL defects

Comment by Marco Vizzeri on:

22582 Comparison between confocal scanning laser tomography, scanning laser polarimetry and optical coherence tomography on the ability to detect localised retinal nerve fibre layer defects in glaucoma patients, Windisch BK; Harasymowycz PJ; See JL et al., British Journal of Ophthalmology, 2009; 93: 225-230

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Localized retinal nerve fiber layer (RNFL) defects are a typical feature of glaucomatous optic neuropathy. These defects can be difficult to detect on clinical examination and additional testing to confirm their presence is often required. Windisch et al. (48) evaluated the ability of confocal scanning laser tomography (CSLT), scanning laser polarimetry (SLP) and optical coherence tomography (OCT) to detect the presence of localized RNFL defects in glaucomatous eyes. Although all observers generally agreed on the presence of a localized defect as evidenced by the printout available for each imaging technique, approximately 20% of eyes with localized defects visible on color fundus photographs were missed by at least one instrument. In addition, another 20% of eyes were missed by all three instruments. The reader is left wondering whether a possible explanation can be suggested for these findings. A post-hoc analysis of the results might have helped in achieving this task. Image quality, for example, is a critical factor that can greatly affect the outcome of any imaging technique. The occurrence of atypical scan patterns can limit the ability of SLP to accurately depict any characteristic localized loss of axons. Parapapillary misalignment of the Stratus OCT scan circle also can produce significant changes in the RNFL assessment, characterized by an increase in measured RNFL thickness in the quadrant in which the scan is closer to the disc, and a significant decrease in RNFL thickness in the quadrant in which the scan is displaced further from the optic disc, thus interfering with the accurate detection of a localized RNFL defect. In clinical practice, confirmation of the presence of a localized RNFL defect often derives from the evidence of a corresponding visual field defect on standard automated perimetry (SAP). SAP results from this study were not reported. However, stablishing a correlation between structure and function in glaucoma is an essential task for the examining clinician, particularly when it is suspected that a localized RNFL defect is present. By utilizing color fundus photographs only as the gold standard, the authors failed to provide a more robust reference standard to which imaging instruments should have been compared with regard to their ability to detect the presence of localized RNFL defects.

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