The OCT RNFL Probability Map and Artifacts Resembling Glaucomatous Damage

Abstract

Purpose The purpose of this study was to improve the diagnostic ability of the optical coherence tomography (OCT) retinal nerve fiber layer (RNFL) probability (p-) map by understanding the frequency and pattern of artifacts seen on the p-maps of healthy control (HC) eyes resembling glaucomatous damage. Methods RNFL p-maps were generated from wide-field OCT cube scans of 2 groups of HC eyes, 200 from a commercial normative group (HC-norm) and 54 from a prospective study group, as well as from 62 patient eyes, which included 32 with early glaucoma (EG). These 32 EG eyes had 24-2 mean deviation (MD) better than −6 dB and perimetric glaucoma as defined by 24-2 and 10-2 criteria. For the HC groups, “glaucoma-like” arcuates were defined as any red region near the temporal half of the disc. Results Seven percent of the 200 HC-norm and 11% of the 54 HC RNFL p-maps satisfied the definition of “glaucoma-like,” as did all the patients’ p-maps. The HC p-maps showed two general patterns of abnormal regions, “arcuate” and “temporal quadrant,” and these patterns resembled those seen on some of the RNFL p-maps of the EG eyes. A “vertical midline” rule, which required the abnormal region to cross the vertical midline through the fovea, had a specificity of >99%, and a sensitivity of 75% for EG and 93% for moderate to advanced eyes. Conclusions Glaucoma-like artifacts on RNFL p-maps are relatively common and can masquerade as arcuate and/or widespread/temporal damage. Translational Relevance A vertical midline rule had excellent specificity. However, other OCT information is necessary to obtain high sensitivity, especially in eyes with early glaucoma.

Within‐eye and between‐subject variability for reflectance of the retinal nerve fibre layer

“…This means that en face RNFL reflectance maps show little effect of biological factors that are known to affect RNFL maps, making it likely that maps of reflectance could be a useful clinical addition to measurements of RNFL thickness. For instance, what appears as an arcuate defect on an RNFL thickness deviation map in a healthy eye [9][10][11] would not appear as an arcuate defect on a reflectance map. On the other hand, when an arcuate pattern in a thickness deviation map truly reflects glaucomatous damage, it is very likely that it would appear as an arcuate defect in an en face reflectance map.…”

“…Another aspect is variability caused by artefacts due to choice of measurement, such as the effects of axial length for global circumpapillary RNFL thickness and location of the major blood vessels for sector thicknesses. A related artefact from the choice of measurement is in en face RNFL thickness deviation maps, where arcuate defects occur in about 20% of eyes free of glaucoma 9 due to normal between‐subject variability in the distance from the fovea to the arcades 10,11 …”

“…A related artefact from the choice of measurement is in en face RNFL thickness deviation maps, where arcuate defects occur in about 20% of eyes free of glaucoma 9 due to normal between-subject variability in the distance from the fovea to the arcades. 10,11 Several laboratories have reported that en face maps of RNFL reflectance show good potential for detecting glaucomatous damage, [12][13][14][15][16][17][18][19] and that by using fixed distances below the inner limiting membrane (ILM) the problems with normal between-subject variability in the distance from the fovea to the arcades can be avoided. 20,21 Measured reflectance will be affected by factors that reduce the amount of light that reaches the tissue, such as shadows from blood vessels or floaters and changes in the position of the eye, but not by factors that affect RNFL thickness such as axial length.…”

Identifying and understanding optical coherence tomography artifacts that may be confused with glaucoma

“…(12) However, in some cases, normal variation in the location of the major nerve fiber bundles can create a pattern on the RNFL probability map that resembles an arcuate defect. (13,15,16) We have recently described a simple anatomical model to help understand the basis for artifacts due to misalignment of RNFL bundles. (14) Our simple model is based upon the average RNFL thickness map of healthy eyes.…”

“…In a recent publication, La Bruna et al found that the prevalence of such arcuate-like artifacts in healthy eyes in a normative dataset was about 5%. (16) In order to differentiate these artifacts from actual RNFL defects, they proposed the "vertical mid-line rule," according to which an artifact arcuate-like defect would not be extended beyond the vertical mid-line of the retinal posterior pole (i.e. through the fovea).…”