OCT Interpretation
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Direct answer
Optical coherence tomography is ultrasound's optical cousin: low-coherence interferometry times the echoes of infrared light back-scattered from the retinal layers, resolving detail of a few micrometres — cross-sectional structure in vivo, quantified. Reading a report follows a fixed drill: check signal strength and centration, then the map numbers against device norms (central macular thickness roughly 250-280 micrometres depending on platform; average retinal nerve fibre layer about 90-100 micrometres), then the normative colours (green within the database, yellow borderline, red below the first percentile), then inter-eye symmetry, and finally correlation with the disc appearance and visual field. The colours are an argument, not a verdict — high myopes flag red falsely ("red disease") and early glaucoma hides inside green ("green disease") — which is why the optometrist interprets the OCT, never the reverse.
What you must remember
- Physics in a line: near-infrared low-coherence light, echo delay encoded by interferometry, axial resolution of a few micrometres; spectral-domain dominates, swept-source at longer wavelengths images choroid better.
- Macular map: ETDRS nine-sector grid with the central 1 mm subfield — device-dependent normal central thickness roughly 250-280 micrometres; know your machine's normative database and never compare across platforms.
- RNFL norms: average thickness about 90-100 micrometres in healthy adults, thinning with age; the quadrant clock-hour map and ganglion cell complex analyses refine glaucoma detection, respecting the ISNT pattern of healthy nerves.
- Reading order that never varies: image quality (signal strength, centration, segmentation lines) before numbers, numbers before colours, colours before conclusions; inter-eye asymmetry exceeding roughly 5-10 micrometres of RNFL is meaningful even when both eyes sit "green".
- Red and green disease: normative databases skew to older, non-myopic eyes — high myopes and large discs flag falsely red; early glaucomatous thinning can sit within green — disc appearance and fields arbitrate.
- Artefact catalogue: blink and motion lines, decentration, epiretinal membrane or large vessels dragging segmentation, shadowing from vitreous floaters and dense cataract dropping signal strength below acceptance.
- Pathology signatures: subretinal fluid in central serous chorioretinopathy, intraretinal cysts in macular oedema, drusen and pigment epithelial detachments in age-related degeneration, full-thickness defect in macular hole, taut posterior hyaloid in vitreomacular traction, and focal RNFL notches in glaucoma.
- Progression discipline: compare like scans with like (same protocol, same machine), guided progression analysis beats single-scan intuition, and structural change must march in step with functional change before management escalates.
Reading an OCT report without the colours deciding
A 58-year-old glaucoma suspect: pressure 26 mmHg right, disc excavated inferiorly. The OCT shows signal strength 9, good centration, segmentation hugging the right layers — trustworthy. Average RNFL 68 micrometres, red on the normative chart, the 7 o'clock hour at 42; the ganglion cell map shows matching inferior-temporal loss. The left eye averages 92 micrometres, green throughout. Asymmetry of 24 micrometres dwarfs any ageing explanation; the 24-2 field confirms a superior nasal step. Conclusion: glaucomatous damage, right eye — the colours merely decorate a conclusion the numbers and symmetry had already drawn.
Now the counter-case: a 26-year-old high myope at −9.00 D flags red diffusely on RNFL, average 78 micrometres, healthy discs, full fields, normal pressure. The database simply lacks many −9 D eyes whose stretched retinas lay their fibres thin; this is red disease. Declaring her a glaucoma patient on colour alone starts a decade of unnecessary drops and anxiety. The optometrist's value is precisely this adjudication — scan quality, then numbers, then the eye they came from.
Where students slip
Quoting universal cut-offs: there is no "normal central thickness" independent of machine — platforms differ systematically and are never interchangeable. Reading the summary colour first, ignoring segmentation errors riding under a clean-looking printout, and skipping artefact interrogation — a decentred macular cube mislabels sectors, and a low-signal scan through cataract thins RNFL spuriously. Treating OCT as diagnostic of glaucoma alone: macular cysts, holes and diabetic oedema referrals are optometric bread and butter, and central thickness thresholds drive anti-VEGF referral letters. And the classic examination answer: OCT is a structural test; function still needs a field, and the pair must agree before the diagnosis does.
Frequently asked questions
What normal central macular thickness should be quoted?
Roughly 250-280 micrometres for the central subfield — but strictly the device's own normative value, since platforms differ systematically and are never interchangeable.
What is red disease on OCT?
Falsely "abnormal" colour coding in eyes outside the normative database — typically high myopes or large discs — read as glaucoma despite healthy discs and fields.
Which quality checks precede interpretation?
Signal strength within acceptance, correct centration, intact segmentation lines, and absence of blink or motion artefact — before any number or colour is believed.
Which OCT signs indicate macular oedema versus central serous chorioretinopathy?
Intraretinal cystic spaces for oedema versus subretinal fluid beneath a detached neurosensory retina in central serous — different spaces, different referrals.
Why must RNFL asymmetry be checked even with both eyes green?
Inter-eye differences beyond roughly 5-10 micrometres suggest early glaucomatous loss that each eye's within-normal comparison conceals — symmetry is the early-warning channel.