OCTA: Potential early-warning system for glaucoma

Optical coherence tomography angiography (OCTA) may be as an important tool in the management of patients with glaucoma, according to Robert L. Stamper, MD.

The technology, which relies on the acquisition of sequential OCT B-scans and one cross-sectional layer of the retina, compares each of the sequential scans with each other.

Any observed differences between the scans are considered to have resulted from movement because the tissue structure does not change. Most of the commercially available devices can shorten the scanning time to acquire about 70,000 B-scans per second during the three seconds required to obtain a scan.

Following acquisition, computerized motion-correction and eye-tracking are performed to ascertain that eye movements are not responsible for any movement perceived by the computer, he explained.

How it works

The concept is simple. When repeated photographs are taken in the same scene in which something is moving, the only thing that changes from one photograph to the next is the thing that is moving.

“The clinician can then interpret this as what is assumed to be the presence of a blood-containing vessel,” explained Dr. Stamper, the distinguished professor of ophthalmology and director emeritus of the Glaucoma Service, University of California San Francisco.

In the retina, there are several capillary plexi in the different retinal layers, i.e., a superficial layer around the macula, a middle layer, a deep layer, and the choriocapillaris.

“The B-scans can scan each of these layers,” Dr. Stamper added. “Motion correction is applied to a blurry image and with further computer manipulation, the result is a much sharper image.”

He then demonstrated how tiny capillaries can be visualized using OCTA.

Wide acceptance

OCTA technology is used by retina specialists because it can visualize any retinal location. Certain forms of age-related macular degeneration are easily visualized with OCTA than with standard OCT, Dr. Stamper said.

In patients with diabetic retinopathy, the vascular changes in the macula can be ob-served before they are detected either clinically or by using standard OCT, he noted.

“This technology clearly identifies glaucomatous changes,” Dr. Stamper emphasized.

OCTA images show dropout of capillaries around the optic nerve head and in the peripapillary areas in the various stages of glaucoma, he explained. A recent advance in the technology is its ability to provide density measurements of the capillaries.

“We now can literally count the number of capillaries per unit of retinal area,” he said.

What is known

OCTA has reported previously advantages and has increased the knowledge of the effect of glaucoma on the eye, according to Dr. Stamper.

He noted that repeatable measurements can be obtained, the capillary density in the superficial macular area is known to be lower in glaucomatous eyes compared with age-matched controls, the peripapillary vessel density coincides with information provided on indocyanine green angiography images, the deep peripapillary microvascular dropout is correlated with the retinal nerve fiber layer thickness and location, and in the macular area the abnormalities observed in the foveal avascular zone correspond to paracentral visual field defects.

In addition, the optic nerve, peripapillary, and macular vessel densities can differentiate normal eyes from those with ocular hypertension.

“We are beginning to think we have an early-warning system for detecting early glaucoma,” Dr. Stamper said. “We can see decreases in capillary density that correspond with the locations of the visual field defects and this correlates with the findings on standard OCT images.”

Another interesting bit of information is that following trabeculectomy, the findings on OCTA do not change, which suggests that the ocular changes are permanent and do not improve with lowering of the intraocular pressure, he added.

Also, the deep layer of capillary dropout around the optic nerve is correlated with the subtle and not-so-subtle changes in glaucoma.

Under investigation

Now that quantitative analyses of the vascular density are possible, the next step is determining which is better, the macular or peripapillary density.

Dr. Stamper questioned, “Does one come before the other regarding identifying the earliest signs of glauco-ma?”

Another consideration is whether this knowledge provides information about patho-physiology, i.e., is capillary dropout the first event and then axonal death or vice versa. This remains to be determined.

“OCTA offers an interesting new way of looking at various layers of retinal circulation in a variety of conditions, not just in glaucoma and macular disease,” Dr. Stamper explained. “We need to determine what has long been suspected, that the primary prob-lem is not the intraocular pressure but a vasculopathy. “Is OCTA a good method for identifying those patients, and if so, should those patients be treated differently from the treatment given to patients with standard high pressure? Because those questions are not yet answered, it is unclear if OCTA offers improvement and if we are evaluating the correct parameters. More work is needed.”