OCT: Illuminating the retina layer by layer

May 22, 2020

When diagnosing posterior uveitis, three considerations are important when using optical coherence tomography (OCT), according to Kathryn Pepple, MD, PhD.

When diagnosing posterior uveitis, three considerations are important when using optical coherence tomography (OCT), according to Kathryn Pepple, MD, PhD.

OCT type 
The first is the type of OCT system: spectral-domain (SD) or swept-source (SS).  “Both are excellent for obtaining high-quality images of the retina and the retinal vasculature, but SS-OCT has some advantages over SD-OCT used for patients with uveitis,” Dr. Pepple commented.

One advantage is the longer imaging wavelength of the SS-OCT system that facilitates deeper evaluation into the choroid.

“In the white dot syndromes, the deep choroidal tissues may be the location from which these syndromes emanate, and deeper visualization can be beneficial,” she said. SD-OCT can provide images of the choroid with use of the enhanced imaging procedure, she pointed out. Dr. Pepple is assistant professor, University of Washington, Mercer Island, WA.

Related: OCT providing physicians with improved view of ocular surface

A second advantage is that SS-OCT allows better visualization of the vitreous, because SS-OCT is faster, wider images can be obtained, which may be important for diseases in locations other than the macula.

Structure versus flow
The second consideration is determined whether the image shows a structure or flow. OCT B-scans show structures; the newer flow imaging uses B-scan repeat imaging shows motion contrast to show retinal or choroidal blood flow.

This then can be segmented into the different vascular layers in the deep capillary plexus of the retina and the deep capillary flow, and the slabs can be seen in the en-face view.

Layer segmentation and en-face views
All previous en-face imaging and dye-based angiography provide a topside view of a structure; in the case of the retina with its seven layers, the technologies provided views of all layers but clinicians could not discern which layer the imaged structures were in.     

“OCT revolutionized that and permitted viewing the retina and choroid via a side B-scan images. This information about the various layers revolutionized what we knew about various diseases. Now we have en-face slab images that allow removal of individual ‘pancakes’ to determine which pancake contains the pathology under study,” Dr. Pepple said. 

Related: Understanding advantages, limitations of SS-OCT

The white dot syndromes
A close look at the images from the various white dot syndromes underscores the importance of being able to dissect the retinal layers and the information that that dissection provides about the various diseases.

Multiple evanescent white dot syndrome (MEWDS)
This disorder is characterized by multiple white dots that are accompanied by changes visible on OCT. The structural B-scans obtained from these patients when overlaid by slab images of the outer nuclear layer (ONL) or the ellipsoidal layer visualize the spots and corresponding dots seen in the two images.

Dr. Pepple described that the spots are discontinuities in the external limiting membrane, while the dots are hyperreflective spots in the ONL. “The dots overlaying the spots suggest the presence of a contiguous lesion that is likely at the photoreceptor layer,” she said.

This information is in contrast to that suggested by indocyanine green angiography (ICGA) images that showed hypocyanescent lesions that were thought to be flow voids in the choriocapillaris or choroid.

However, SS-OCT showed otherwise, i.e., there are no flow abnormalities and that MEWDS is a “photoreceptor-it is.” “The dark spots on ICGA might be the result of photoreceptor blockage.

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Multifocal choroiditis (MFC), punctate inner choroidopathy (PIC), choroidal neovascular membrane (CVMN)
In the next retinal layer, MFC and PIC seem to affect the area under the retinal pigment epithelium (RPE), Dr. Pepple noted.

“With active lesions, hyperactive dome-shaped hyperactive deposits can be seen under the RPE with discontinuities in the RPE resembling volcanos of inflammation in the RPE erupting into the outer retina,” she described.

Signal transmission through lesions increases into the choroid beneath. With resolution of the lesions and the dome-shaped deposit, small breaks are present in the RPE along with changes in the outer  layers near the lesions.

Related: Assessing quality of OCT scans impacts clinical-decision process

OCTA flow images can identify flow in the hyperreflective lesions in the outer retinal slabs and show choroidal neovascular membrane (CVMN). “Typically, these are associated with other findings of a CVMN on multimodal images but occasionally can identify flow in signals that were not apparent in typical modalities,” she advised. 

When viewing the choriocapillaris slab, evidence suggests that the choriocapillaris is a component of this disease with a decreased flow signal from acute lesions. Scars also can result in decreased flow.

Acute multifocal placoid pigment epitheliopathy (AMPPE)
Structural OCT visualization of acute lesions shows hyperreflectivity and thinning of the ONL, changes in the RPE, and subretinal fluid in the hyperacute lesions. SD-OCTA flow images show defects in the choriocapillaris; the choriocapillaris lesion is shown to be larger than the outer retina and research has shown lesions can resolve over time.

ICGA shows that during active AMPPE inflammation, the hypocyanescent lesions are more widespread. When the disease is quiescent, the lesions mostly resolve but the damage in the outer retinal layer remains, with thinning of the layer and loss of some outer retinal bands.

Related: OCT artifacts and pitfalls: In the eye of the beholder

Serpiginous chorioretinitis
“This disease is a much more damaging form of choriocapillary inflammation that seems similar to AMPPE, with thickening of the RPE, ONL hyperreflectivity, and loss of the ellipsoid bands on the retina and external limiting membrane,” Dr. Pepple said and noted that the most interesting findings in this disease concern choriocapillaris flow.

En-face choriocapillary imaging and quantitative monitoring have contributed substantially to the understanding of the disease. The lesions can respond to steroids. During an active flare of the disease, the lesions are shown to expand, but with treatment the lesions decreased. Dr. Pepple showed that “because the lesions are sharply demarcated, boundary analysis can actually detect the amount of damage inflicted by the flare and how much recovery was achieved with treatment.” 

Dr. Pepple speculated that AMPPE and serpiginous chorioretinitis may be the same disease but with degrees of severity.

Birdshot retinochoroidopathy
This disease occurs deeper in the choroid than the others. “The deep choroidal lesions can be visualized in some patients on color fundus imaging. Structural OCT images show many of the birdshot findings, including macular edema and thinning, loss of the outer retinal bands, and epiretinal membranes that cause complications over the long term,” she said.

Retinal OCTA can see changes in the retinal vasculature, including infrequent CVMN, capillary dilation and loops, and changes in the intercapillary distance.

Enhanced-depth imaging OCT and SS-OCT have illuminated the changes in choroidal structure in birdshot patients, including chronic choroidal thinning, active choroidal thickening, hyporeflective and hyperreflective foci, the controversial suprachoroidal space and fluid, and increased choroidal reflectivity. SD-OCT has identified longitudinal changes. Preclinical deep choroidal lesions can be observed with SD- and SS-OCTA.

The newer technologies have shown that the white dot syndromes are localized in the various retinal layers, with MEWDS concentrated in the photoreceptors, CVNM in the subretinal and sub-RPE space, MCP/PIC in the outer retina, RPE, and choriocapillaris; AMPPE and serpiginous in the choriocapillaris; and birdshot in the choroid. 

Read more by Lynda Charters

Kathryn Pepple, MD, PhD
E: kpepple@uw.edu
Dr. Pepper has no financial interest in this subject matter.