Eye care practitioners and surgeons gathered in Manchester recently for a day of lectures and workshops aimed at keeping them up to date on OCT and other developing technologies in retinal imaging. Hats off to the organisers Haag-Streit UK for assembling a line-up of internationally renowned speakers and researchers (each introduced by HS-UK clinical training manager Grant Duncan, figure 1) for a day of education that refreshingly stayed clear of being overtly commercial.
OCT angiography
Enthusiasm for your subject is important for any presenter so who better to give an overview of the progress of OCT angiography (OCTA) than Moorfields consultant ophthalmologist Pearse Keane, a leading expert with this technology. The technique allows clear visualisation of the retinal vasculature and also the choriocapillaris and shows many smaller vessels that would not be seen by other methods. Indeed, OCTA ‘is one of the hottest topics in ophthalmology’ as cited in a recent editorial paper1 and new developments, such as disc specific software (figure 2), steps to reduce artefacts, new ways of analysing data, are appearing all the time.
Figure 2
Typically, there are four image panes on screen after scanning (figure 3 – top row) relating to;
- Superficial retinal capillary plexus.
- Deeper retinal capillary plexus.
- Outer retina (as this should be avascular, it should appear dark).
- Choroid.
Figure 3
Keane suggested that ‘we are not quite ready yet for analysis of the choroidal image’ as it very likely to be influenced by artefacts. Of the currently available instruments, there is some variability in their assessment and so anyone wishing to buy one ‘should first try it out on one of your worst case patients’ to see how it performs.
Figure 4
Moorfields has used the Angiovue (UK supplier Haag Streit UK) since 2014, and the rest of Keanes talk summarised what he had so far learned from the assessment of several thousand patients. Firstly, there are several clinical conditions where OCTA can improve clinical assessment and care;
- Diabetic macular ischaemia – ‘in general, OCTA performs well with vascular disease and provides accurate and reproducible images.’ Figure 4 shows the vessels of such a patient, most superficial to the left and deeper to the right. Also, the latest analytical software allows the calculation of flow area, no flow area and flow density (figure 5).
Figure 5
- Vein occlusions – ‘OCTA helps to spot ischaemia and non-ischaemia signs, though if there is a lot of blood then this is difficult.’ It is excellent for spotting abnormal vessels (figure 6) and, where there appears vessels on the en face image but not in OCTA, ghost vessels too (figure 7).
Figure 6
Figure 7
- Retinal macroaneurysms (figure 8) – ‘much easier to make out subtle anatomical features than before.’
Figure 8
- Choroidal neovascularisation (CNV) – ‘beautiful visualisation of the new vessel complex (figure 9), though not so good in the elderly with poor fixation and media clarity.’ It performs best on early AMD changes and removes the need for fluorescein angiography, and can also be useful for non-AMD CNV conditions or when a fluorescein angiography is equivocal, ‘such as when you are not sure if there is inflammatory infiltration or a CNV membrane.’
Figure 9
Figure 10
- Others – ‘by simple adjustment of the plane of the image you can often identify pathology otherwise hidden.’ The examples shown included myopic CNV, inflammatory CNV, chronic central serous chorioretinopathy (figure 10) and dome-shaped macula.
- Geographic atrophy – ‘OCTA can show differences in features between dry AMD (figure 11) and Stargardt disease’.
Figure 11
- Anterior segment – though ‘still a work in progress,’ Keane showed some nice uses of OCTA in anterior conditions such as corneal neovascularisation (figure 12) and rubeosis.
Figure 12
There are some limitations Keane summarised as follows;
- Small field of view – typically scans are used in 2x2 or 3x3 degree area. Larger fields are possible but there is less detail.
- Motion artefacts – these are very common and result in a ‘patchwork quilt’ effect when fixation is not stable (figure 13).
Figure 13
- Projection artefacts – these show as vessels on deeper scans but are projections of vessels from other layers.
Finally, when looking to the future, Keane predicted the following developments will be likely;
- Faster acquisition speeds.
- Wider fields, with better resolution at 8x8 or greater.
- Better segmentation and image analysis.
- Increased options for assessing non-retinal and anterior structures.
Disc assessment
Dr Michel Puech runs a research facility in Paris looking at all aspects of ophthalmic imaging. His work has recently shown that some changes with advancing glaucoma are detectable with OCTA. These include enlargement of the pores of the lamina cribrosa, reduced blood flow in the nerve head and changes in peripapillary vascularisation (figure 14). Puech believes OCTA may become a useful technique in glaucoma work.
Figure 14
Diabetes
Consultant ophthalmologist and Moorfields researcher Dawn Sim began her discussion of diabetes with a clear illustration of how OCTA offers better visualisation of superficial vessels in retinal vascular oedema (figure 15 left) as compared with fluorescein angiography (figure 15 right). She went on to summarise some further findings from two recently published studies.2,3
Figure 15
Dry AMD and the 2RT
Professor John Marshall, he of the excimer laser fame, and co-researcher Ali Hussain (‘someone to keep an eye on, especially if he tries to visit America’ quipped the always entertaining Marshall). He explained how structures exposed to the environment, such as the skin, need to regularly replace cells in order to cope with the constant damaging influences. The retina is similarly vulnerable and has developed a system where the photoreceptors continually shed their debris into the retinal pigment epithelium (RPE) so disposing of waste materials without cell division, but inevitably leading to a gradual build-up of diffuse lipofuscin throughout the RPE which eventually may be visualised as drusen if it forms focal clumps. Unfortunately, it is the lipofuscin that interferes with the normal activity of the RPE and underlying Bruch’s membrane and, rather shockingly, ‘by the age of 40, half of the RPE’s ability to repair the receptor cells is lost.’
Marshall outlined the enzymes and control mechanisms that exist to help control this process but, unfortunately, as the body produces more enzymes with age to cope with waste products, these enzymes themselves become bound to debris and so contribute to the problem further.
The idea behind the 2RT project is, instead of looking at gene replacement to avoid disease in the first place, to attempt to ‘rejuvenate’ the existing system. The laser used for this is the Ellex 2RT which stands for ‘retinal rejuvenation therapy’ and the idea is to introduce very short duration (three-nanosecond) pulses of energy (532nm wavelength) to multiple small areas (400 micron diameter). This should increase the release of enzymes without damaging the neural retina but instead improve waste transport and therefore limit the impact of the degenerative process. The much anticipated trial results are not yet available but they are certainly something to keep an eye out for.
References
1 Puliafito CA. OCT angiography: the next era of OCT technology emerges. Ophthalmic surgery, lasers & imaging retina. 2014 Sep-Oct;45(5):360.
2 Bradley PD, Sim DA, Keane PA. The evaluation of diabetic macular ischemia using optical coherence tomography angiography. IOVS, 2016, 57, 626-631.
3 Sim DA, Keane PA et al. The effects of macular ischemia on visual acuity in diabetic retiniopathy. IOVS, 2013, 54, 2353-2360.