Swept-source OCT: A new frontier in study and management of choroidal disorders
High-resolution imaging of the choroid allows visualization of previously unknown disease manifestations.
Swept-source OCT with the Topcon DRI Triton system provides high-resolution imaging of the choroid, allowing visualization of previously unknown disease manifestations, according to one researcher. The combination of high speed and longer wavelength ensures better, deeper penetration and fast, clear image acquisition, resulting in near-histologic resolution of the choroidal structures.
“It is a fast, noninvasive test that improves clinical efficiency and minimizes patient discomfort. We can therefore repeat it as many times as we want, monitoring the condition closely as we could never do before,” Danny Ng, MPH, FRCS, clinical assistant professor in the Department of Ophthalmology & Visual Sciences at The Chinese University of Hong Kong, told Ocular Surgery News.
Deeper into pachychoroid disorders
Several of the diseases belonging to the pachychoroid spectrum are highly prevalent in Southeast Asia. Among them, central serous chorioretinopathy (CSCR) is the fourth most common vision-threatening retinal pathology after age-related macular degeneration, diabetic retinopathy and branch retinal vein occlusion.
“One of the cardinal features of CSCR is the abnormally dilated choroidal vessels. In the past, we couldn’t always guarantee seeing them well and monitoring them closely because indocyanine green angiography involves infusion of a dye and is therefore an invasive method. Now we can see the dilated choroidal vessels, have a fast structural view of the choroid as well as an en face view of segmental layers,” Ng said.
In pachychoroid neovasculopathy, type 1 choroidal neovascularization (CNV) is another disease in which enhanced choroidal imaging helps to localize the neovascular lesion. Equally, subclinical CNV is detected in case of pachychoroid pigment epithelium detachment (PED).
“PED may not cause a lot of acute, severe symptoms, but when we do a screening with the Triton, sometimes we can detect subclinical CNV without associated subretinal or intraretinal fluid leakage. Then we know that these patients should be kept under observation. Treatment is not indicated, and we can sequentially monitor these patients in an easy, noninvasive manner,” Ng said.
Close monitoring of PCV
Polypoidal choroidal vasculopathy (PCV) is also associated with pachychoroid. It is a common occurrence in Asian patients with AMD, with a reported prevalence between 20% and 50%.
“On average, half of our AMD patients have PCV. The presence of polyps makes it more complex to treat as compared with type 1 or type 2 CNV,” Ng said.
Indocyanine green angiography (ICGA) was used in the past for monitoring regression of the polyps with treatment and showed that many of these patients were refractory to anti-VEGF monotherapy. Combination with photodynamic therapy became the gold standard for this pathology. Histology studies showed that PDT does have some remodeling effect on choroidal morphology.
“One of the cardinal features of PCV patients is that they have abnormally dilated choroidal vessels due to loss of smooth muscle cells and increased hyalinization of blood vessels. In the past, we could only look at a very small population because pathology samples are not easy to obtain. Only in a small number of cases we could look at the cross-section of the choroid and see what was going on in PCV patients. With the Triton, I can now look at the choroidal morphology of all my PCV patients and do it repeatedly. This is a huge advantage because we can truly monitor the effect of treatment,” Ng said.
“What we can see right now is that this choroidal vascular hyperpermeability is associated with big dilated vessels, clearly visible on the Triton OCT scans,” Ng said.
This information has high prognostic value and can guide treatment choices, he said. Several reports have shown that patients with choroidal vascular hyperpermeability are often poorly managed with anti-VEGF alone and might respond better with the adjunct of PDT.
“So what helps me a lot is that I can view the choroidal vessels using the Triton machine and easily recognize which cases may benefit more of a combined treatment with PDT,” he said.
Direct, precise view of myopic CNV
Pathologic myopia is another condition in which SS-OCT has significant advantages. In the presence of macular hemorrhage, for instance, it helps in the crucial step of differentiating between CNV and submacular hemorrhage secondary to lacquer crack lesions without CNV.
“CNV requires treatment with anti-VEGF, but when the CNV is not present you don’t need to treat and can wait for spontaneous resolution. The Triton machine helps me with an easy, noninvasive and fast scan when I would classically have to order a fluorescein angiography to look for the presence of myopic CNV,” Ng said.
Retinal hemorrhages block fluorescein and often prevent the detection of leakage on dye angiography. Because choroidal neovascularization is typically smaller and less exudative in pathologic myopia as compared with AMD, it is often obscured by hemorrhages.
“With the OCT angiography module and en face imaging, I can identify CNV more confidently. While fluorescein angiography gives indirect evidence of the presence of CNV, OCT angiography can give us the true impression of the choroidal new vessels, and it is not blocked by the presence of blood,” Ng said.
Headed in the right direction
There are a few limitations with this technology. Although minimized by the fast image acquisition, projection artifacts, often involving segmentation error, can still occur and mislead about the presence and location of new vessels on OCT angiography, Ng said. Sometimes shadows are seen in the more superficial layer of the choroid, and the exact layer of blood vessel is not always easy to discern. In pathologic myopia, the posterior staphyloma and consequent outpouching of the posterior wall of the eye may result in image distortion.
“In these cases the automated segmentation algorithm might fail, and we have to manually redraw the segmentation line in order to avoid the projection artifact,” Ng said. “One of the improvements I am looking forward to is a more accurate automated segmentation, which would make the use of the system more practical in the clinic.”
The technology is relatively young, but it is a step in the right direction, he said.
“It is a noninvasive tool. We don’t have to rely on the dye infusion and can look at the microvascular features. It improves our understanding of the disease and allows for better disease management, improving clinical efficiency and reducing patient discomfort,” he said. – by Michela Cimberle
- For more information:
- Danny Ng, MPH, FRCS, can be reached at Department of Ophthalmology & Visual Sciences, Faculty of Medicine, the Chinese University of Hong Kong, 4/F Hong Kong Eye Hospital, 147K Argyle St., Kowloon, Hong Kong; email: email@example.com.
Disclosure: Ng reports no relevant financial disclosures.