Retinal anatomy, examination and multimodal imaging
Key Takeaways
Inner retinal circulation and choroidal support of the outer retina explain different patterns of ischaemia and fluid.
Fluorescein leakage, staining, pooling and window defects describe different mechanisms.
OCT angiography measures flow-related signal and does not demonstrate dye leakage.
Ultrasound is valuable in opaque media, but a negative scan does not exclude every peripheral tear.
Localize the tissue before naming the disease
The neurosensory retina converts light into neural signals, while the retinal pigment epithelium (RPE) supports photoreceptor outer segments, transports fluid and contributes to the outer blood–retinal barrier. The inner barrier is formed by retinal vascular endothelium. A lesion in either system can produce retinal fluid, but the cause and treatment differ. Optical coherence tomography (OCT), angiography and ultrasound answer complementary questions rather than supplying interchangeable diagnoses.
From vitreous to choroid, recognize the internal limiting membrane, nerve fibre and ganglion cell layers, inner plexiform and nuclear layers, outer plexiform and nuclear layers, external limiting membrane, photoreceptor bands and RPE–Bruch membrane complex. OCT band names are imaging descriptions; the ellipsoid-zone signal is not a literal histological line containing only one structure. At the fovea, displaced inner layers and specialized cones support high acuity. The foveal avascular zone depends on choroidal support for the outer retina.
Blood supply and vitreous attachments
The central retinal circulation supplies much of the inner retina, while the choriocapillaris supplies the outer retina. The choroid has high blood flow and fenestrated capillaries, with the RPE preventing uncontrolled entry into the subretinal space. This explains why arterial occlusion preferentially whitens the inner retina and why RPE disease can cause subretinal fluid without primary retinal vein leakage.
The vitreous gel develops from the embryonic vitreous system and changes with age. The vitreous base, disc, macula and major vessels are important adhesion sites. During posterior vitreous separation, persistent focal adhesion can cause a tear, vitreomacular traction or haemorrhage. Hyaloid remnants such as a Mittendorf dot or Bergmeister papilla differ from a persistent fetal vascular stalk with traction and cataract.
Examination strategy
Measure vision and ask about distortion, night blindness, flashes, floaters and a curtain. Examine pupils before dilation, then perform slit-lamp and dilated fundus assessment. Indirect ophthalmoscopy provides a broad view of peripheral retina; slit-lamp lenses give a detailed stereoscopic posterior view. Scleral depression helps examine peripheral breaks when clinically appropriate, but avoid pressure on a suspected open globe. A widefield photograph does not consistently exclude a small anterior tear.
Document lesion location relative to disc, fovea, vessels and ora serrata. Describe elevation, colour, haemorrhage, exudate and associated traction before offering a diagnostic label. Compare both eyes. Imaging requests should identify the question: for example, whether macular fluid is intraretinal, whether a lesion leaks, or whether an opaque eye contains a detachment.
| Modality | Main information | Limitation |
|---|---|---|
| OCT | Cross-sectional structure and fluid compartments | Artifacts, segmentation errors, limited peripheral coverage |
| Fluorescein angiography | Retinal perfusion and leakage over time | Leakage can obscure lesion detail |
| Indocyanine green angiography | Choroidal vascular patterns | Invasive dye study; interpretation is disease-specific |
| Autofluorescence | Distribution of endogenous fluorescence | A signal is not a direct count of surviving cells |
| OCT angiography | Flow-related signal without dye | Motion, projection and low-flow artifacts; no leakage measurement |
| Ultrasound | Anatomy despite opaque media | Operator dependence and limited microstructural detail |
Reading angiography
On fluorescein angiography distinguish window defects, staining, pooling and leakage. A window defect transmits more underlying fluorescence because RPE pigment is reduced; leakage spreads beyond its initial borders in later frames. Pooling accumulates within an anatomical space, while staining increases signal within tissue without the same expanding margin. Blocked fluorescence may result from blood, pigment or a mass. Delayed filling and capillary nonperfusion have different implications from a bright late image alone.
Indocyanine green is useful for choroidal lesions, including polypoidal disease and inflammatory choroidal abnormalities, because its spectral properties and protein binding differ from fluorescein. Before either dye, review prior reactions, systemic circumstances and local emergency procedures. Do not describe a dye as incapable of causing a serious reaction.
Echography and clinical application
B-scan shows cross-sectional echoes and movement. A detached retina generally forms a reflective membrane tethered at the optic disc; vitreous membranes, choroidal detachments and tumours have different attachments and motion. A-scan assesses amplitude along a line and can assist tissue characterization or biometry. Ultrasound biomicroscopy resolves anterior-segment structures rather than replacing posterior B-scan.
In a patient with dense vitreous haemorrhage, ultrasound can identify a detachment or mass, but a negative scan does not exclude every retinal tear. Arrange repeat examination and imaging as clinically indicated. In a patient with suspected choroidal neovascularization, OCT defines fluid and angiography may clarify the lesion type. Interpret image quality, examination and symptoms together; neither an automated thickness map nor one bright angiographic spot is sufficient.
Sources: EBO curriculum and ISCEV standards for complementary functional testing.
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