16.2 Structural OCT Interpretation: Retinal Fluid & Biomarkers

Structural OCT: Layers, Fluid, and Biomarkers

OCT displays optical reflectivity rather than a histology slide. Device, wavelength, signal, focus, scan angle, averaging, segmentation, motion, shadow, and ocular pathology affect appearance. Begin with patient and eye, scan pattern, orientation, centration, signal, and comparison registration. Then inspect every relevant B-scan before using automated maps.

Layer orientation

From vitreous inward, commonly recognized bands include the internal limiting membrane, retinal nerve fiber layer, ganglion-cell and inner plexiform complex, inner nuclear layer, outer plexiform and Henle-fiber region, outer nuclear layer, external limiting membrane, ellipsoid and interdigitation zones, and RPE/Bruch's complex. Reflectivity and layer boundaries vary at the fovea, optic disc, vessels, and pathology.

The ellipsoid zone is associated with mitochondria-rich photoreceptor inner segments. Its continuity can correlate with function, but no one band determines acuity or prognosis. Oblique scan angle and shadow can imitate attenuation.

Fluid compartments

• Intraretinal fluid appears as hyporeflective spaces within neurosensory retina. Cysts, cavitation, schisis, tubulation, or segmentation error can mimic one another.
• Subretinal fluid separates outer photoreceptors from the apical RPE. Its cause can include vascular leakage, inflammation, traction, tumor, central serous disease, or detachment mechanisms.
• Sub-RPE fluid or material elevates the RPE away from Bruch's membrane and forms a pigment epithelial detachment.

The technician describes location, extent, scan quality, and change. The clinician decides whether a space is active exudation and whether treatment is indicated.

Pigment epithelial detachments

A smooth dome with relatively homogeneous hyporeflective content may be described as serous-appearing. An irregular elevation with heterogeneous internal reflectivity can be fibrovascular or have another mixed cause. Drusenoid elevations tend to have more homogeneous reflective material, while blood can create dense anterior reflectivity and posterior shadow. These patterns overlap. Hemorrhage, type 1 neovascular tissue, central serous disease, inflammation, drusen, polypoidal lesions, and tumors require multimodal context.

Do not diagnose neovascular AMD or select anti-VEGF therapy from a PED shape alone. Check segmentation, adjacent fluid, hemorrhage, prior scans, angiographic or OCT-A data when ordered, and the clinical examination.

Descriptive biomarkers

DRIL describes inability to distinguish specified inner-retinal boundaries over a measured region. Research definitions and prognostic associations vary by disease and study.

Hyperreflective foci are small bright dots with several possible cellular or material sources. Size, shadow, layer, migration, and disease context affect interpretation.

Subretinal hyperreflective material is material between retina and RPE and can represent neovascular tissue, blood, fibrin, exudation, or fibrosis.

Outer retinal tubulation is a bordered round or branching tubular structure in degenerating outer retina. It can resemble a fluid cyst but should not be used alone as a treatment-activity marker.

Hypertransmission is increased penetration below an attenuated or absent RPE signal. It can support an atrophy pattern but must be confirmed with overlying layer loss, size criteria when a formal definition is used, scan quality, and serial context. Shadowing is the opposite pattern: reduced signal beneath blood, pigment, vessel, opacity, or other blocking material.

Artifacts and longitudinal comparison

Inspect automated boundaries through the entire lesion. Fluid, a steep PED, myopia, traction, atrophy, or low signal can pull a segmentation line into the wrong layer and create a false thickness value. Correct only within the authorized device workflow, retain the source data, and flag maps that remain invalid.

Use the same device, scan pattern, follow-up registration, and averaging when possible. Confirm that an apparent change is not caused by different centering, scan density, segmentation, or shadow. If a lesion is missed between sparse B-scans, acquire the ordered denser volume rather than interpolating from a map.

Scope

A CRT can identify layers, compartments, artifacts, and descriptive patterns needed for quality acquisition. The chart should distinguish observation from interpretation: “hyporeflective space beneath the neurosensory retina on a well-centered scan” is an observation; “active neovascular AMD requiring injection” is a diagnosis and treatment decision.

A repeatable review sequence

Use the same order every time: confirm eye and scan; judge signal and motion; inspect the vitreoretinal interface; follow retinal layers from inner to outer; localize any space or material; inspect the RPE and choroidal signal; check segmentation; compare registered prior scans; then record artifacts and objective change. Scroll beyond the single central B-scan, because a focal lesion may sit between lines or outside the displayed thumbnail. When en face and cross-sectional findings disagree, preserve both and route the discrepancy rather than choosing the more dramatic display.