Future of imaging
A 3D reconstruction of the posterior eye curvature based on swept-source OCT. The complex shape of the posterior staphyloma can be appreciated in the 3D reconstruction. Image courtesy of Gemmy Cheung MBBS, FRCOphth, FAMS, MCI
How far can we see into the future? Or rather, can we catch a glimpse of what the future of imaging will look like for patients with pathologic myopia?
Gemmy Cheung MBBS, FRCOphth, FAMS, MCI, Singapore National Eye Centre, Singapore, sought to answer this question during her presentation at the EURETINA 2020 Virtual Conference. “Imaging in Myopia: Seeing Far into the Future” gave us insight into the present state of the art in myopia imaging techniques, and what we can expect soon.
Professor Cheung started her lecture by reviewing two important classification systems for high myopia. First, the META-PM (meta-analysis for pathologic myopia) international classification for myopic maculopathy, which was published in 2015. Second, the ATN classification, published in 2019. These classification systems are intended to clarify research into this highly complex disease and improve clinical follow-up of patients who suffer from it.
The META-PM incorporates both OCT and colour fundus photography to describe not only retinal changes, but also those in the choroid, Bruch’s membrane and the RPE. This classification system defines five categories of myopic maculopathy: “no myopic retinal degenerative lesion” (Category 0), “tessellated fundus” (Category 1), “diffuse chorioretinal atrophy” (Category 2), “patchy chorioretinal atrophy” (Category 3), and “macular atrophy” (Category 4). Three additional features to supplement these categories were defined as “plus” lesions: lacquer cracks, myopic choroidal neovascularisation, and Fuchs spot. Posterior staphyloma was considered as a further, important sign of myopic retinopathy. The intra-observer agreement for this system is ≥85%.
A more recently developed system, the ATN classification, takes three factors into account: atrophy, traction and neovascularisation. This system has led to the discovery that, for example, the pathogenesis of myopic tractional myopathy is different from that of myopic atrophic maculopathy and myopic neovascular maculopathy.
After completing this review of classification systems, Prof Cheung switched to the imaging systems themselves, where great advances have been made in terms of speed and resolution as well as scan width and depth. Swept-source OCT (SS-OCT) represents the latest such advance. It boasts longer wavelengths and higher scanning speed, which results in images that are longer in width and deeper in penetration than those of previous generations.
“These properties of SS-OCT are particularly useful in evaluating eyes with posterior staphyloma,” she said. “In some eyes, we can look even further, right at the sclera. We are now adopting these imaging techniques in children and teenagers undergoing atropine treatment, and the pattern of progression will no doubt bring new insights into the development of pathologic myopia and staphyloma.”
Prof Cheung then showed an example of an SS-OCT-derived 3D-reconstruction of the posterior eye wall, in which we can clearly visualise two deep staphylomas on either side of the optic nerve. “In the next few years, we can anticipate further advances in SS-OCT with increasingly faster scan speed and higher resolution,” she concluded.
After displaying a series of impressive SS-OCT images, Prof Cheung moved on to the advances in OCT angiography (OCT-A).
“OCT-A allows us to see and detect choroidal neovascularisation with high sensitivity and specificity, which will enable their incorporation into diagnostic algorithms used in clinical practices as a screening tool,” she said.
This might make fluorescein angiography unnecessary for this indication. One limitation, however, is that the flow signal produced by the neovascularisation may persist even with treatment. Hence retreatment decisions should not be made with OCT-A in isolation, but together with structural OCT.
OCT-A can also be used to evaluate changes in the choriocapillaris and choroid. “However, these very thin structures make it difficult to achieve quantification, particularly because of their susceptibility to inaccurate automated segmentation,” said Prof Cheung.
Despite this limitation, Prof Cheung referred to a paper in which a wide-field, whole-eye OCT system could not only scan the posterior pole with great resolution, but can produce a topographic curvature map of the posterior pole. With longitudinal data, these types of quantitative techniques will be extremely valuable in our ability to assess changes in the posterior eye wall.
Finally, using dynamic evaluation of the eye shape during different gaze positions, some groups have looked at the potential mechanical forces involved in the development of staphyloma, vitreomacular traction and intra-choroidal cavitation.
In summary, there have been many significant advances in imaging technologies, which have in turn expanded the possibilities for imaging in pathologic myopia. These improvements will enable clinicians to evaluate myopic pathology and its pathogenesis in new ways.
“The new imaging techniques allow us to see and think and imagine in a three-dimensional fashion, not only at one tissue layer but in multiple, specific layers,” she said.
These imaging data will further complement clinical data, longitudinal follow-up as well as both serum and ocular biomarkers to further our understanding of the development and progression of myopia and its complications.