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Diagnostic and monitoring advances in glaucoma need fine-tuning
This article was reviewed by Robert L. Stamper, MD
Optical coherence tomography angiography (OCTA), a relatively new imaging procedure, can do many things, such as generate 70,000 B-scans per second, identify differences among them, block saccadic movements, and correct fixation loss.
“The user can derive the presence of movement in a vessel by looking at a static structure sequentially; the movement is detected by the algorithm. Motion detection makes the image sharper, and artificial intelligence applied provides a useful image,” according to Robert Stamper, MD, distinguished professor of clinical ophthalmology and director emeritus of the Glaucoma Service, University of California, San Francisco, and who described the advantages of the technology.
OCTA has become indispensable for managing macular degeneration and diabetic retinopathy. In fact, it is almost the standard of care, Stamper commented, because of its ability to differentiate the capillary plexi and their status in all of the retinal layers and their possible connections.
Stamper explained that in glaucoma, 2 retinal areas are considered. One in the macula is the superficial macular capillary plexus, which is the area that feeds the retinal nerve fiber layer or the axons of the ganglion cells. Another retinal area affected by glaucoma is the radial peripapillary capillaries. This comprises the vascular midlayer of the optic nerve, which is reduced in glaucoma.
Stamper demonstrated that initially, the capillaries are relatively evenly distributed. Early in the disease, the capillaries begin to drop out and this progresses to generalized capillary loss in advanced glaucoma in the peripapillary capillary plexus. The deep capillaries are present despite the advance of the disease.
The current imaging devices have the ability to quantify the capillary density by ocular sector and facilitate follow-up over time.
The hope for OCTA
Stamper explained that OCTA must become more sophisticated and evolved before it reaches its full potential for glaucoma specialists.
Considering that blood vessels are the first severely affected structures in glaucomatous eyes, Stamper pointed out, the hope is that OCTA might have better diagnostic sensitivity compared with standard OCT.
Clinicians also want OCTA to shed light on the pathophysiology of the disease. Specifically, is the source mechanical, such that the optic nerve is being pushed back, or are the vessels dropping out first, he speculated.
“The thought was that OCTA might provide a way of distinguishing between primary open-angle glaucoma and normal-tension glaucoma, which is often associated with some vascular issues,” he said.
Stamper recounted that previous OCTA studies have shown (1) that the vessel density is reasonably repeatable but more variable than the standard OCT thickness of the nerve fiber layer, (2) the presence of a posterior subcapsular cataract markedly reduces the ability to measure the vessel density, (3) the vessel density is correlated with the findings on indocyanine green angiography and declines with age, and (4) the decline seems to be accelerated in glaucoma.
“The most important technologic ability is that OCTA can detect movement. It knows where vessels are, but the shortfall is that it does not provide information about how much is flowing through those vessels or how rapid the flow is,” he explained.
In addition, the dropout of radial peripapillary capillary density and superficial macular capillary density is well correlated with standard OCT and visual field measurements. OCTA images can distinguish between the normal retinas and those with early glaucoma. When a defect is present in the macular area, this almost always shows up on the 10-2 visual fields. The hope was that these problems in the vessels would appear on OCTA images before they appeared on standard OCT images.
“Unfortunately, the diagnostic ability of OCTA in the macular and peripapillary areas is worse than the nerve fiber layer measurement in both the macular area and the peripapillary area,” Stamper said. “It appears that the ganglion cell complex or the thickness of the 3 anterior retinal layers declines before the changes are apparent in the macular density profile. The vessel, therefore, may not be the primary change.”
Interestingly and unfortunately, after successful glaucoma surgery, almost no change is seen in the OCTA images.
However, 1 positive finding that has become apparent is that a baseline reduction in the capillary density is associated with a higher risk of visual field progression.
Using OCTA, Stamper and colleagues evaluated the macular and peripapillary areas in 25 patients with open-angle glaucoma and 25 patients with normal-tension glaucoma. He reported that the vessels in those areas decrease with glaucoma severity.
“Unfortunately, in this preliminary study, we found no difference between the 2 groups,” he said.
Considering the hopes for the usefulness of OCTA, Stamper noted that OCTA does not have better diagnostic sensitivity. It does not show earlier changes compared with standard OCT images; it does not definitively provide information about the pathophysiology or evolution of glaucoma; and as of now, the technology does not differentiate among primary open-angle glaucoma, high-pressure glaucoma, and normal-tension glaucoma.
Stamper and his colleagues are now evaluating a large patient group, which may provide some additional information.
“OCTA offers an interesting new window into various layers of retinal circulation in a variety of conditions and is most useful in the conditions around the macula,” he said. “The technology still has not shed light on the long-suspected feeling that normal-pressure glaucoma has a different pathophysiology than primary open-angle glaucoma.”
It remains unclear whether OCTA offers improvement or additional information over standard OCT for glaucoma diagnosis or monitoring, Stamper added. He offered the caveat that the technology is new and with improvements in sensitivity and specificity along with better interpretation of findings, more information about glaucoma will be forthcoming.