OCT interpretation
The OCT is a useful tool for assessing and quantifying optic nerve structure but we need to understand its limitations, avoid common pitfalls, and use the information wisely. Keep in mind that the OCT supplements rather than replaces careful optic disc assessment.
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)