At the completion of this CPD activity, optometrists will understand the clinical utility of optical
coherence tomography angiography (OCT-A), including:
- Discern the benefits of OCT-A change analysis over time as a tool for earlier diagnosis of pathology of the retina and optic nerve.
- Recognise common conditions where OCT-A imaging is clinically beneficial.
- Understand the importance of choosing the appropriate OCT-A scan protocol and retinal layer appropriate to the suspected pathology.
\
Dr Georgina Clark
BA MA MBBS (Hons) FRANZCO
Sydney at Mosman Eye Centre and Bondi Eye Doctors
OCT angiography has emerged as a pivotal imaging technology but despite its utility for detailed assessment of the retinal and disc microvasculature, many optometrists have been slow to adopt it. With optometrists well placed to use it for early diagnosis and disease monitoring, DR GEORGINA CLARK shares practical tips for performing OCT-A scans.
Optical coherence tomography angiography (OCT-A) is a useful imaging tool that has become widespread in hospitals and private ophthalmology practices since its introduction in 2016. Uptake in optometry practices, however, has been slower, with research suggesting that optometry practices consider OCT-A less applicable to their practices than other imaging technologies.1 This isn’t surprising, given that initially, OCT-A was viewed as an alternative to fundus fluorescein angiograms (FFA) – tests traditionally performed by ophthalmologists.
Over the last five years, clinical research, driven by data science enhanced with artificial intelligence analysis, has expanded the clinical applications of OCT-A. There are now many valuable opportunities for optometrists to use OCT-A as a diagnostic and monitoring tool for many eye conditions. OCT- A can be used to identify eye pathology earlier and, in some cases, even before permanent vision loss occurs.2
No longer just an alternative to FFA, OCT-A can now be used to assess for various forms of optic neuropathy; from glaucomatous optic neuropathy to non-arteritic ischaemic optic neuropathy and even arteritic ischemic optic neuropathy.
OCT-A is used to identify cases of age-related macular degeneration (AMD) that are at high risk of transformation from ‘dry’ to exudative neovascular AMD. Additionally, OCT- A is used to identify ‘quiescent’ choroidal neovascularisation age-related macular degeneration (CNV ARMD), a new sub type of the neovascular AMD (nAMD) not identifiable with FFA or regular OCT. In short, the applications of this technology are wide and significant.
Advantages and disadvantages of OCT-A
OCT-A is easy to use and fast. Optometrists and their staff can perform the tests without need for nursing or medical staff. Also, it is a safe, non-invasive test, especially when compared to FFA and the associated risk of allergy and anaphylaxis. Also, compared to FFA, OCT-A is a relatively low-cost investigation, both for the patient and the medical system.
One drawback of OCT-A is that the process of learning the skill of interpreting OCT-A can be daunting, especially outside of a formal teaching program (most retinal fellowships now include dedicated teaching on the interpretation of OCT-A as well as FFA).
By virtue of its digital nature, the OCT-A gathers a lot of data. This, in turn, means that the clinician needs to make a series of informed decisions about what data to collect and analyse. OCT-A is most useful when the clinician has a particular disease in mind, and then performs the appropriate scans to look for that condition.
How OCT-A works
OCT-A enables capture of retinal and choroidal microvasculature images. OCT-A technology uses laser light reflectance of the surface of moving red blood cells to accurately depict vessels in different layers of the eye. The same tissue area is repeatedly imaged and differences are analysed. Differences in ‘scatter’, due to movement of red blood cells, allow imaging of the vasculature. OCT-A technology is used to provide insight about blood flow through the eye’s structures, picking up abnormalities such as reduced blood perfusion, or new abnormal vessels.
OCT-A provides high-resolution detail and also shows the vertical relationship between the vessels and the surrounding tissue architecture through the B-scan slice. Traditional FFA shows the superficial capillary plexus whereas OCT-A can show the deeper vascular layers right down to the choriocapillaris.
the appropriate scan protocol: where to look for suspected pathology
There is a lot of information gathered and presented on the OCT-A device. The clinician can elect to scan the following regions of the eye: fundus, macular and disc. Acquisition sizes range from 3 x 3 mm to 12 x 12 mm and even wider field using montage functions up to 50 degrees. Additionally, macular cubes with 3D representation can be rendered.
When scanning the retina, it’s advisable to start your assessment by going to scan ‘Acquire’, and then the ‘Angiography Analysis Screen’, as pictured in Figure 1.†
In Figure 1 (higher resolution version here), the following layers are imaged:
- VRI (vitreoretinal interface)
- Retina
- Deep capillary plexus
- Superficial capillary plexus
- Choriocapillaris
- Choroid
- RPE-RPE best fit
- ORCC (outer retina choriocapillaris)
The clinician clicks on the layer of the retina he or she wants to assess. As such, the OCT-A is largely driven by pattern recognition in the layer of the retina in which the clinician expects to find pathology.
It’s critical to choose the appropriate scan protocol depending on the indication and likely pathology. For example, for a suspected CNV in a patient with AMD, a macular scan of 3 x 3 mm can help detect smaller lesions and segmentation can be more finely tuned. However, for diabetic retinopathy assessment, montage or wide field scans of 12 x 12 mm provide a better view of peripheral retinal ischaemia, and will show capillary non-perfusion and neovascularisation.
Case 1. Enlarged foveal avascular zone
In Figure 2 (found at the top of this article) and Figure 3, this is the case of a 56-year-old male with type one diabetes who presented with blurry vision, worse in the right eye. Physical exam of the patient was unremarkable. There was no refractive error change; best- corrected visual acuity right eye was 6/9 and left eye is 6/7.5. There were no lens changes. On dilated fundus exam, mild NPDR was noted in both eyes, but no diabetic macular oedema.
Without an alternative explanation for his reduced vision, in this patient you may suspect capillary perfusion at the macular secondary to diabetes. This would appear as enlargement of the foveal avascular zone (FAZ), with irregular margins.
Studies have demonstrated that, compared with fluroscein angiogram, OCT-A allows better discrimination of the central and parafoveal macular microvasculature, especially for FAZ disruption and capillary dropout.3
To identify enlargement of the FAZ, it’s important to scan the macula in the highest resolution possible on your device. Enlargement of the FAZ (increased area of non perfusion at the fovea), with increased irregularity of the border, can be seen on the level of the superficial capillary plexus and the deep capillary plexus. Sometimes, relatively smaller spotty areas of non perfusion can also be identified in the choriocapillaris.
It is a valuable to also employ the change analysis software to look for change over time. Having a baseline scan in patients such as this can prove extremely useful down the track. Also, frequent imaging allows for pathology to picked up earlier, especially where the change analysis software is employed.
There is also an argument for scanning often as part of the education process. That is, the more scans of normal FAZ a clinician looks at, the more likely that clinician is to pick up the abnormal scans.
Case 2: Branch retinal vein occlusion
This is the case of a 70-year-old female. She is known to have suffered a right branch retinal vein occlusion nine months ago. Her best corrected visual acuity is 6/24. Her optometrist is concerned about possible ischaemia.
An OCT-A scan of her retina in performed. The imaging options at this visit were 6 x 6 mm, 8 x 8 mm, and Montage (which offers up to 50-degree field of view). The Montage images are shown in Figure 4. These reveal regions of peripheral ischaemia and peripheral capillary drop out. This patient was referred for consideration of sectoral argon retinal laser to reduce risk of neovascularisation. This fast and efficient scan can help pick up patients at risk of complication.
Figure 4B (higher res version here) demonstrates BRVO in another patient. This patient has peripheral retinal ischaemia with ‘drop out ‘ of vascular flow in the periphery. In addition, OCTA at the level of the superficial capillary plexus shows vascular tortuosity, dilation, telangiectasia and decreased vascular density.
Case 3: Quiescent nAMD
An 85-year-old male presented has known nAMD. An OCT-A scan reveals a ‘quiescent CNV’ in the fellow eye.
The role of OCT-A in the imaging and diagnosis of nAMD is now familiar to both optometrists and ophthalmologists alike. More recently, research has identified a new entity which is named ‘quiescent’ CNV. These are lesions detectable on OCT-A as abnormal vascular networks above Bruch’s membrane. However, they show no evidence of leakage (intraretinal fluid/subretinal fluid) on OCT – or FFA. As such, it’s a pre-clinical CNV that is only diagnosable using OCT-A.4
Figures 5A-5C (higher res version here) show quiescent or non-exudative choroidal neovascular membrane in a myopic patient.
This is relevant in patients with history of nAMD one eye. For these patients, the risk of conversion to exudative or active disease at one year was 15 times higher than those without subclinical lesions.5 Closer follow up and patient counselling for symptoms is recommended in this cohort.
It’s noteworthy that this patient’s CNV is most visible in the RPE – RPE Fit layer in Figure 5C. The RPE- to-RPE fit analysis compares the Anterior RPE border (interdigitation zone) to posterior RPE border. In healthy eyes, this is close to parallel. An increasing deviation between the anterior and posterior border is suggestive of developing pathology. Thus, this is a useful tool in monitoring those at risk of transitioning to an exudative CNV.6
Future trends
Overall, advancements in OCT and OCT-A technology are an active area of research. The digital nature of these images lend themselves to prospective and retrospective research. The image data sets can be stored and assessed by data scientists for subtle signals and trends that will help with the earlier diagnosis of disease.
Additionally, artificial intelligence is now actively employed in the segmentation and interpretation of images both for clinical and research purposes. This research is powering trends towards earlier identification of disease. and at risk individuals. This, in turn, increases the scope for closer monitoring in those patients. At this time, OCT-A clinical guidelines based on that research are being developed by the American Academy of Ophthalmology and the United Kingdom’s NICE guidelines group.
Summary
The applications of OCT-A technology are expanding rapidly. OCT- A can be used to identify eye pathology earlier, and, in some cases, even before permanent vision loss occurs.
As the first point of contact for many patients seeking eyecare, early consideration of disease has long been an inherent part of optometric practice. OCT-A can, and does, nicely complement that aspect of optometry as a useful tool in the early diagnosis and monitoring of eye pathology. In coming years, this exciting field of research will be increasingly streamlined with practical clinical guidelines in development by ophthalmology colleges around the world.
NOTE: † This article contains ZEISS images and nomenclature. However, equivalent analysis screens are applicable in other makes of OCT-A.
More reading
OCT-A in macular disorders: tips, tricks and practical clinical use
Twenty-first century collaborative glaucoma management in regional Australia
Unilateral glaucoma or historic non-arteritic anterior ischaemic optic neuropathy (NAION)?
References
1.Cheung R, Ho S, Ly A. Optometrists’ attitudes toward using OCT angiography lag behind other retinal imaging types. Ophthalmic Physiol Opt. 2023 Jul;43(4):905-915. doi: 10.1111/opo.13149. Epub 2023 Apr 21. PMID: 37082888.
2. Miguel, A.I.M., A.B. Silva, and L.F. Azevedo, Diagnostic performance of optical coherence tomography angiography in glaucoma: a systematic review and meta-analysis. Br J Ophthalmol, 2019.
3. Soares M, Neves C, Marques IP, et al. Comparison of diabetic retinopathy classification using fluorescein angiography and optical coherence tomography angiography. British Journal of Ophthalmology 2017;101:62-68.
4. Carnevali A, Cicinelli MV, Capuano V et al (2016) Optical coherence tomography angiography: a useful tool for diagnosis of treatment-naïve quiescent choroidal neovascularization. Am J Ophthalmol 169: 189–198
5. de Oliveira Dias JR, Zhang Q, Garcia JMB, et al. Natural History of Subclinical Neovascularization in Nonexudative Age-Related Macular Degeneration Using Swept-Source OCT Angiography. Ophthalmology 2018; 125: 255–266.
6. Parravano M, Borrelli E, Sacconi R, Costanzo E, Marchese A, Manca D, Varano M, Bandello F, Querques G. A Comparison Among Different Automatically Segmented Slabs to Assess Neovascular AMD using Swept Source OCT Angiography. Transl Vis Sci Technol. 2019 Mar 27; 8 (2):8. doi: 10.1167/tvst.8.2.8. PMID: 30941265; PMCID: PMC6438244.
