Retinal Layers and OCT Made Easy: Where Every Lesion Sits
Last updated: October 2026
Ten retinal layers, four outer OCT bands and a dozen lesion types: it is easy to get lost. But the retina is simply a three-neuron relay with two blood supplies. Once you see that, you can place any haemorrhage, exudate or fluid pocket in its layer, and read an OCT scan from top to bottom.
Four rules
1. Read the relay backwards from the lightLight passes through the whole retina to reach the photoreceptors at the back. The signal then travels forwards: photoreceptor (ONL) → synapse (OPL) → bipolar cell (INL) → synapse (IPL) → ganglion cell (GCL) → axon (NFL) → optic nerve. Three nuclear layers, two plexiform layers.
2. Two blood supplies, split at the OPLThe central retinal artery feeds the inner retina (NFL to INL). The choroid feeds the outer retina (ONL, photoreceptors, RPE). That is why a CRAO whitens the inner retina but leaves a cherry-red spot: the fovea has no inner layers.
3. The shape of a lesion follows the fibresIn the NFL the fibres run along the surface, so blood spreads into flames. In the INL and OPL the cells stand upright, so blood stays as dots and blots. In Henle's layer the fibres radiate from the fovea, so exudate forms a star and cysts look petaloid.
4. On OCT, fibres are bright and nuclei are darkLayers of horizontal fibres or membranes (NFL, IPL, OPL, RPE) reflect strongly; nuclear layers (GCL, INL, ONL) are dark. Fluid is black.
Where does it sit? Lesions by layer
| Finding | Layer | Why |
|---|---|---|
| Flame haemorrhage | Nerve fibre layer | Blood tracks along horizontal axons; hypertension, CRVO, disc oedema |
| Cotton wool spot | Nerve fibre layer | Axoplasmic stasis from a micro-infarct; diabetes, hypertension, HIV, lupus |
| Dot and blot haemorrhages | Inner nuclear / outer plexiform | Vertical cell columns confine blood; diabetic retinopathy, ocular ischaemia |
| Hard exudates | Outer plexiform (Henle) | Lipid leaks from capillaries; star at the macula in neuroretinitis and hypertension |
| Cystoid macular oedema | Inner nuclear + outer plexiform | Petaloid on FA because of radial Henle fibres |
| PAMM (paracentral acute middle maculopathy) | Inner nuclear layer band | Ischaemia of the intermediate and deep capillary plexuses |
| Preretinal (sub-hyaloid) blood | Between ILM / hyaloid and retina | Boat-shaped with a fluid level; Valsalva, PDR, Terson |
| Subretinal fluid or blood | Between photoreceptors and RPE | CSCR, neovascular AMD, RD |
| Drusen, PED | Between RPE and Bruch's membrane | Dry AMD, type 1 MNV |
| Subretinal drusenoid deposits (reticular pseudodrusen) | Above the RPE | High-risk AMD feature |
| Roth spot | White-centred haemorrhage, any inner layer | Platelet-fibrin plug; endocarditis, leukaemia, anaemia |
Memory hook: Flames in the Fibres; Dots in the Deep (INL); exudates make Henle's star.
Reading the outer OCT bands (IN•OCT consensus)
| Band | Name | What it is | If lost |
|---|---|---|---|
| 1 | External limiting membrane (ELM) | Junctions between Müller cells and photoreceptors | Poorer visual prognosis |
| 2 | Ellipsoid zone (EZ) | Mitochondria-rich photoreceptor inner segments | Photoreceptor damage: macular hole, hydroxychloroquine toxicity, MEWDS, RP |
| 3 | Interdigitation zone (IZ) | Cone outer segment tips meeting RPE microvilli | Often first to go in early disease |
| 4 | RPE / Bruch's complex | Pigment epithelium and its basement membrane | Atrophy (hypertransmission into the choroid) |
Memory hook: Read from top down: ELM, EZ, IZ, RPE: “Every Eye Is Readable”.
Fluid and neovascular membranes on OCT
| Where | OCT look | Typical causes |
|---|---|---|
| Intraretinal fluid (IRF) | Dark cysts in the INL and OPL | Diabetic macular oedema, CRVO, uveitis, post-cataract (Irvine–Gass) |
| Subretinal fluid (SRF) | Dark dome between the photoreceptors and RPE | CSCR, neovascular AMD, VKH, optic pit |
| Sub-RPE (PED) | RPE lifted off Bruch's membrane | Serous, drusenoid or fibrovascular PED; polypoidal vasculopathy |
| Type 1 MNV | Under the RPE | Occult CNV of AMD; polypoidal choroidal vasculopathy (aneurysmal type 1) |
| Type 2 MNV | Above the RPE (subretinal) | Classic CNV; myopic CNV, younger patients |
| Type 3 MNV | Within the retina, from the deep plexus | Retinal angiomatous proliferation in older patients |
On OCT angiography the retinal capillaries form a superficial plexus (in the GCL/NFL), an intermediate plexus (inner border of the INL) and a deep plexus (outer border of the INL). The choriocapillaris sits just under Bruch's membrane.
The fovea is special
- Only cones, packed tightly, with the inner layers pushed aside to form the pit; no retinal vessels in the central 0.5 mm (the foveal avascular zone).
- Its outer layers rely on the choroid, so it stays red in CRAO and swells first in choroidal disease.
- Xanthophyll (lutein, zeaxanthin) sits in the inner layers here, which is why the macula looks dark on FA and blocks blue light.
- Foveal hypoplasia (albinism, aniridia) shows as persistent inner layers across the centre on OCT.
Matching artery to damage
| Occlusion | Layer that dies | Sign |
|---|---|---|
| Central retinal artery | Inner retina | White oedema, cherry-red spot, later thin inner retina on OCT |
| Cilioretinal artery | Inner retina between disc and fovea | Can be the only vessel spared in CRAO, preserving central vision |
| Deep capillary plexus | Middle retina (INL) | PAMM: grey patches; band in INL on OCT |
| Choroidal vessels | Outer retina and RPE | Elschnig spots (hypertension), triangular sign of Amalric (posterior ciliary) |
Where residents slip
- Placing hard exudates in the NFL: they sit in the OPL (Henle's layer).
- Calling all dark spaces on OCT 'CME': check whether it is intraretinal, subretinal or under the RPE.
- Forgetting the choroid feeds the photoreceptors: it explains the cherry-red spot and why CRAO spares the fovea's colour.
- Reading the RPE band as the ellipsoid zone: the RPE is the brightest band at the bottom; the EZ is just above it.
- Mixing up type 1 and type 2 MNV: type 1 is under the RPE, type 2 above it.
Test yourself: flip cards
Quick quiz
Related: Retina hub, OCT macula reference, OCT interpretation tips, DME OCT biomarkers, Layers of the eye.
Key sources
- Staurenghi G, Sadda S, Chakravarthy U, Spaide RF; IN•OCT Panel. Proposed lexicon for anatomic landmarks in normal posterior segment SD-OCT. Ophthalmology. 2014;121(8):1572-1578
- Spaide RF, Jaffe GJ, Sarraf D, et al. Consensus nomenclature for reporting neovascular AMD data (CONAN). Ophthalmology. 2020;127(5):616-636
- American Academy of Ophthalmology. Basic and Clinical Science Course, Section 12: Retina and Vitreous (current edition)
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