Pearls

• Make sure the scan is high quality

  • signal strength above 7/10, centered tracings, accurate segmentation

  • beware when comparing signal strength 9/10 to prior 7/10 - a “stable” RNFL thickness of same value may actually be masking progression

• Look at the RNFL thickness maps

  • these look like sideways red/yellow butterflies against a blue sky - the wings should be symmetric within each map and between the right and left maps

  • each thickness map is created from 200 B-scans each comprising 200 A-scans, and all information on the OCT is derived from these maps

• Don’t skip the thickness plots

  • helpful for identifying and localizing focal rim thinning and asymmetry

  • consider 2 main parameters: thickness and contour. An abnormal contour is more suspicious for glaucomatous RNFL loss than abnormal thickness; if both are present, suspicion is even higher

• Always correlate OCT images with exam

  • optic nerve assessment and visual field findings

  • in advanced disease, HVF is usually more useful for monitoring progression

• Look at the ganglion cell complex

  • GCC = ganglion cell layer + inner plexiform layer

  • can help identify early damage before RNFL thinning

  • Fun fact: in cases of retro-axonal degeneration (from intracranial pathology including prior stroke, demyelination) the GCC may show loss respecting the vertical midline with a relatively preserved RNFL

  • GCC can help follow progression in late disease when RNFL is “at the floor” and only able to measure vessels/glial tissue thickness (the floor is reached around average thickness <60 microns)

• Don’t rely only on the colors

  • false positives and false negatives exist - don’t miss “green disease”

• Watch out for artifacts

  • errors in capture or segmentation (examine the extracted horizontal, vertical, and circular tomograms for accuracy)

  • macular edema, VMT masking thinning; resolution of edema or focal traction appearing as acute progression

• Don’t forget about other etiologies of RNFL and/or ganglion cell loss

  • high myopia/myopic degeneration, other optic neuropathies, retinal disease including vascular occlusions, intracranial pathology (stroke, MS)