Approach to the OCT macula

Optical coherence tomography is an ultrasound made of light. A beam of near-infrared light is scanned across the retina and the reflections from each depth are timed by interference, giving a cross section, the B-scan, with a resolution of a few microns. Bright means reflective: tissue that is dense, fibrous or lies across the beam. Dark means transparent or fluid. The image is a map of reflectivity, not of colour or of function, and that is the first thing to keep in mind: a dark space is fluid or a hole, a bright band is a layer that scatters light, and a shadow beneath something bright means the light did not get through.

Machines display the B-scan in greyscale or in a false colour scale where red and white are most reflective and blue and black least. Learn to read both. The scan is usually shown alongside an en face image of the fundus with the line of the section drawn across it, and often with a thickness map colour-coded against a normal range.

The fovea is the landmark of the whole scan. In a normal eye it is a smooth pit: the inner layers thin away to nothing at the centre so that the outer bands run beneath a shallow depression. Four things can happen to it. It can be filled in, so the pit is shallow or gone: that is thickening, from oedema or a membrane pulling the retina flat. It can be pulled up into a peak by something on the surface. It can be split, with cystic spaces in its walls and a gap in the middle: a hole. Or the whole fovea can be lifted as a dome on a pocket of fluid beneath it.

Say which of these it is before you go further. The contour is the summary of the scan, and it usually points at the diagnosis before the layers confirm it.

Walk down the scan in order and name what you see. At the top, the vitreous is black; a fine grey line lying on or near the surface is the posterior hyaloid, and where it is still attached at the fovea while separated elsewhere it can pull, which is vitreomacular traction. The retinal surface should be a smooth, continuous, faintly bright line, the internal limiting membrane. A bright irregular sheet on top of it, with the retina puckered beneath, is an epiretinal membrane.

Beneath the surface, the inner retina alternates bright and dark. The nerve fibre layer is bright and thickest towards the disc, the ganglion cell layer darker, the inner plexiform layer bright, the inner nuclear layer dark, the outer plexiform layer bright, the outer nuclear layer dark and thickest under the fovea. Then come the outer bands, which matter most for vision: the external limiting membrane, a thin bright line; the ellipsoid zone, a strong bright line that is the photoreceptor inner segments; and the interdigitation zone against the pigment epithelium. Beneath them the retinal pigment epithelium is the brightest band in the scan, with Bruch's membrane at its base, and under that the choroid, mottled by its vessels, ending at the sclera.

The ellipsoid zone is the line to look for when vision does not match the rest of the scan. Where it is disrupted, the photoreceptors are damaged and the acuity is poor whatever else has been treated; where it is intact under a lesion, vision can recover.

Fluid is black on OCT, and where it sits is the diagnosis. Intraretinal fluid forms round or oval cystoid spaces within the layers, mostly the inner nuclear and outer plexiform layers, separated by bright septa: the pattern of diabetic macular oedema, vein occlusion, uveitic oedema and the oedema after cataract surgery. Subretinal fluid is a single smooth pocket between the outer retina and the pigment epithelium, lifting the retina as a dome: central serous chorioretinopathy when the RPE is intact and the choroid thick, a neovascular membrane when there is bright material or a break in the RPE beside it, and retinal detachment when it extends beyond the macula. Sub-RPE fluid lifts the pigment epithelium itself off Bruch's membrane as a pigment epithelial detachment: a dome whose roof is the bright RPE line.

Say the compartment out loud in the report. It decides the treatment: intraretinal and subretinal fluid in age-related degeneration is injected; subretinal fluid under an intact RPE in a young patient is usually watched; a pocket of fluid with a tear in the retina is a surgical problem.

After the dark spaces, look for bright ones. Hard exudates are small, very bright dots in the outer plexiform layer, each casting a shadow, and mark chronic leakage. Hyper-reflective material under the retina or under the RPE, especially when it is irregular and beside fluid, is a neovascular membrane or its scar. Drusen are bumps of medium reflectivity under the RPE. A bright thickened inner retina with the outer layers obscured beneath is the acute phase of a retinal artery occlusion. Bright dots scattered through the vitreous are cells.

Shadowing is a sign in itself: something dense enough to shadow is a vessel, an exudate, a haemorrhage or pigment, and the shadow points to where it lies.