ANNUAL DIAGNOSTIC TECHNOLOGY ISSUE

UNVEIL RETINAL VASCULAR DISEASE THROUGH OCTA

OPTICAL COHERENCE TOMOGRAPHY ANGIOGRAPHY AIDS IN DIAGNOSIS AND MANAGEMENT

Diana Shechtman, O.D., F.A.A.O., and Sherrol Reynolds, O.D., F.A.A.O.

A 35-YEAR-OLD diabetic patient presented for her six-month assessment of mild non-proliferative diabetic retinopathy OU. Her diabetes management changed to include both metformin and insulin due to an elevated hemoglobin AIC value three months prior. She denied any changes in her vision since last visit. Best correct visual acuities were 20/20 OD and OS. Pupils, ocular motilities, confrontation visual fields and anterior segment evaluation were unremarkable. Dilated examination revealed dot and blot hemorrhages, tortuous vessels, exudates and a suspicious area of retinal vessels, OD (See Fig. 1a). Knowing that the integrity of the retinal microvasculature is essential in the evaluation of diabetic retinopathy, an optical coherence tomography angiography (OCTA) was performed. A 6mm by 6mm OCT angiogram showed multiple scattered microaneurysms (MAs) and intraretinal irregular capillaries, denoted as dilated, looped and coarse, more likely early intraretinal microvascular abnormalities (IRMA). (See Fig. 1b).

Fig. 1a, top, is a retinal image of a 35-year-old diabetic patient.
Fig. 1b, bottom, is OCTA showing intraretinal microvascular abnormalities.

MAs are the earliest clinical signs of diabetic retinopathy and OCTA can show exquisite resolution of retinal microvasculature changes, by delineating them. These early changes often result in hemorrhage, leakage and exudates and, if found near the macula, may be associated with diabetic macular edema, which may lead to vision loss.

Here, we discuss how OCTA works, its benefits and limitations and what happened to the aforementioned patient.

HOW IT WORKS

OCTA utilizes motion contrast to construct detailed volumetric angiographic images, mapping the retinal and choroidal vasculature, in a matter of seconds, via the assessment of blood flow. Specifically, it compares the differences in intensity of either the amplitude or phase of reflected light contrast, between sequential OCT B-scans that are taken at the same retinal cross section. In essence, the device relies on the principle that the retina is static and that if we correct for gross eye movement, the differences between sequential scans would represent movement of erythrocytes (red blood cells “RBC”). Hence, the location of retinal or choroidal blood vessels (blood flow) can be mapped out, according to the 2015 International Journal of Retina & Vitreous. Since OCTA is cross-registered with SD-OCT, the precise location of the vascular anomaly on OCTA can be correlated with the exact structural change on the OCT B-scan. (See Fig 2, p.25.)

Fig. 2: Superficial segment OCTA, top, and corresponding OCT B-scan, bottom, of patient with history of laser for DME showing area of broken capillaries.

BENEFITS

OCTA can detect changes in blood vessel flow in a variety of retinal vascular diseases. This may include, but is not limited to, idiopathic macular telangiectasia and diseases associated with choroidal neovascular membrane (CNV), such as occult central serous choroidopathy, myopic degeneration and age related macular degeneration. Additionally, OCTA may be particularly useful in the evaluation of vascular ischemic diseases, such as retinal vascular occlusive disease (retinal vein and artery occlusion) and diabetic retinopathy.

Traditional angiography, fluorescein angiography or indocyanine green angiography, remain the gold standard, particularly for the evaluation of CNV, according to April 2012’s Ophthalmology and November 2010’s British Journal of Ophthalmology. OCTA is a quick, non-invasive technique that avoids dye-related complications, since it uses motion of RBC as intrinsic contrast. It simultaneously provides information on both the retinal and the choroidal microvasculature and generates a 3D high-resolution image set. This provides structural and functional (i.e. blood flow) information of the vascular plexuses at different depths, from the internal limiting membrane to the choroid, which is of particular importance in the evaluation of any retinal or choroidal vascular disease.

The capillary plexus is of particular interest in the assessment of retinal vein occlusion-associated collaterals (See Fig. 3, p.26), idiopathic macular retinal telangiectasia (IMT), and diabetic retinopathy. In fact, IMT was the first vascular condition described with OCTA, according to the Sept.-Oct. 2014 issue of Ophthalmic Surgery, Lasers & Imaging. The abnormal telangiectatic branching vessels are clearly noted in the deep capillary segmentation. Additionally, deeper segmentation makes the assessment of the choriocapillaris and larger choroidal vessels feasible. This is of particular importance in the evaluation of a CNV.

Fig. 3: Collaterals in a 47-year-old woman

Jai and associates, in the July 2014 issue of Ophthalmology, were the first to demonstrate high-resolution image visualization of a CNV using OCTA. The device can show the full extent of a CNV and easily delineate a CNV independently of associated leakage. This may be of value when assessing the response to particular anti-VEGF agents, according to the July 2015 issue of American Journal of Ophthalmology.

In addition to assessing retinal capillary non-perfusion (See Fig. 2), vascular anomalies (such as vascular loops, tortuosity vessels (See Fig. 1b) and IRMA, OCTA also reveals the presence of superficial neovascularization, which project forward and, thus, are easily seen in the vitreoretinal interface or superficial retinal slab.

LIMITATIONS

There are limitations of OCTA. Some MAs are not observed using OCTA alone. This is because the principle of OCTA is dependent upon normal blood flow movement, and MAs represent blood stasis. Diseases that cause blood flow stasis or cessation, such as fibrotic CNV, would not benefit from the technology. Yet, OCTA can detect other abnormalities associated with diabetic retinopathy, such as areas of retinal non-perfusion (reduced capillary density) (see Fig. 2) and increased vascular remodeling (see Fig. 1b).

Additionally, OCTA provides a limited field of view, and thus, peripheral retinal ischemia and neovascularization may not be assessed. However, the OCTA images mosaic may help increase, to some extent, the field of view, according to a December 2013 PLOS One article. OCTA is also prone to image artifacts, including projection artifacts (i.e. ghost vessels) and movement artifacts (i.e. gross eye movement and poor fixations). These factors may lead to poor quality images and pathology misinterpretation.

Also, because no exogenous dye is used, there is no ability to visualize dynamic leakage with OCTA. The evaluation of the concomitant HDOCT can help in the assessment of any associated edema. The absense of an exogenous dye may be an advantage, though, because the vascular abnormalities, like a CNV, would not be degraded by the associated leakage and, thus, exact size measurement and delineation can be performed. In addition, diseases that affect leakage can be assessed through a structural OCT B-scan obtained by the OCTA. Moreover, OCTA imaging may be further obscured by a hemorrhage, which decreases the ability of light to penetrate into the deeper layers of the retina or choroid.

PATIENT’S OUTCOME

The OCTA detected more MA changes, not clinically observed, as well as the intraretinal irregular capillaries, more likely IRMA. The patient, who lives in South America, was advised to see a retinal specialist for further evaluation and scheduled for a three-month follow-up OCTA evaluation. The patient was counseled on compliance to her medications, diet and exercise for better diabetes control. As diabetes require interdisciplinary management, a report was shared with her treating physicians.

With the evolution of OCT, the resulting OCTA is an important novel imaging modality in the management of retinal diseases. It provides concurrent information on both structural and blood flow changes at different layers of the retina and choroid. Alteration in retinal and choroidal vasculature is the primary cause of a number of posterior segment conditions, including diabetic retinopathy, mentioned above.

No CPT code exists solely for OCTA imaging, but it can be billed using the standard retinal OCT code: CPT code 92134-retina. The technology holds great potential in a clinical setting, given that it is a non-invasive image technology that can provide depth-encoded information of blood flow. Initial data in the use of vascular-related ocular disease have been promising and may have a wide utilization in retinal and choroidal vascular diseases. In the future, OCTA may help understand vascular histopathology, provide insight into vascular response to treatment management and provide insight in how to manage retinal diseases. OM

	DR. SHECHTMAN is a professor of optometry at Nova Southeastern University College of Optometry, where she serves as a clinician in the Eye Institute. She is also the coordinator of the macula diabetes service, the director of interdisciplinary education and a member of the Optometric Retinal Society. Email her at dianashe@nova.edu.
	DR. REYNOLDS is an associate professor at the Nova Southeastern University College of Optometry and clinical preceptor/attending in the college’s diabetes and macular clinic. She is a fellow of the Optometric Retina Society and chairperson for the Florida Optometric Association Healthy Eyes Healthy People Committee. Comment at tinyurl.com/OMcomment.