OCT Scan Normal Eye vs. 8 Most Common Pathologies

Differentiating between an OCT scan of a normal eye vs. a pathological one is a practical skill gained after years and years of practice. However, educating yourself on the basic differences will speed up the process. Understanding the “why” and “how” behind any changes on the OCT scan, compared to a normal macula OCT, will speed up your learning curve and deepen your expertise as a retinal expert.

The article’s first part focuses on key OCT features and their meaning as a structural change for retinal architecture. The second part discusses the most recognizable OCT features of eight common pathologies.

OCT Scan: Normal Eye

When evaluating an OCT scan, the most logical step is to understand how a normal macula OCT should look. The most telling feature across all scans is the contrast between light and dark areas. Typically, the nerve fiber layer and the underlying ganglion cell layer appear brighter than the densely packed nuclear layers. This is followed by the inner plexiform layer interface, which presents as a bright, hyperreflective area.

The inner nuclear layer, composed of densely packed nuclei, appears dark. This is followed by the outer plexiform layer, the outer nuclear layer, and Henle’s layer. The external limiting membrane, an important landmark for assessing retinal health, is also visible. The ellipsoid zone (EZ) is another bright layer, while the interdigitation zone may not always be distinguishable from the underlying RPE layer, even in healthy eyes. Finally, the RPE and inner choroid appear hyperreflective.

Structure

The ELM and EZ are critical structures to assess. In a normal macula OCT, the distance between the EZ and ELM is shorter than between the EZ and the RPE. The apparent “elevation” of the EZ in the foveal center results from the elongated outer segments of the foveal cones.

It’s important to remember that not all retinal structures are readily visible on a normal macula OCT. For example, Henle’s fiber layer is more easily distinguished in the presence of retinal pathology, such as swelling or thinning. Similarly, Bruch’s membrane is usually not visualized unless there is a separation between the RPE and Bruch’s membrane, often indicative of disease.

Thickness

Choroidal thickness is another key factor in OCT assessment. A general rule of thumb is that the choroid (between the RPE and the outer choroidal boundary) is approximately as thick as the retina. Thinning of the choroid may be observed in myopic or older patients, while marked choroidal thickening can raise suspicion for diseases like central serous retinopathy.??

The OCT scan also provides information about laterality. The nerve fiber layer is characteristically thicker near the optic nerve head.? Conversely, if the nerve fiber layer is not visualized in its expected location on an otherwise OCT normal scan, it could signal significant nerve fiber layer loss, potentially due to glaucoma or other optic neuropathies.

Reflectivity

Specific OCT terminology helps describe scans and differentiate normal findings from pathology.

Two fundamental concepts in OCT interpretation are hyporeflectivity and hyperreflectivity, which form the basis for understanding the structural composition of the retina as visualized in an OCT scan.

Hyporeflectivity refers to the increased light transmission capacity of a structure. The OCT scanning laser beam passes through hyporeflective structures with minimal reflection. The quintessential example of a hyporeflective structure is the vitreous humor. It appears as a dark area in the uppermost portion of a normal OCT scan, situated above the retina.

But hyporeflectivity can also be pathological, deviating from the patterns observed in a normal macula OCT; in the retina, it manifests in three primary ways.

Like the vitreous, subretinal fluid exhibits high light transmission and appears black on OCT. A uniformly black region suggests the fluid lacks cellular debris or other inclusions.

Fluid can also accumulate within the retinal layers, for example, between the layers of the neuroepithelium. This intraretinal fluid also appears hyporeflective on OCT.

Following a degenerative process within the retina, a cavity or void may form where retinal tissue has been lost. These degenerative cavities lack the cellular components necessary to reflect light and thus appear as dark spaces on OCT.? It’s important to differentiate these cavities from cystic spaces, which may have distinct clinical implications.

One example is outer retinal tubulations. While associated with various diseases, outer retinal tubulations (ORTs) generally indicate outer retinal degeneration and atrophy.

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