“If you do a corneal map scan, the measurements are automatically corrected for those distortions. “The Visante gives you a cross-sectional view—a different view from gonioscopy,” she continues. “Anterior segment OCT can be very helpful if you want to know whether an angle is going to close in the dark,” Dr. Schuman points out. A second important use for AS-OCT in LASIK patients is as a way to evaluate the cornea when a LASIK patient needs an enhancement. Gonioscopy is critical when deciding whether or not an iridotomy should be performed. “There are various mathematical formulas to determine the actual space between the iris and the trabecular meshwork and the shape of that space,” he says. I’ll perform UBM imaging in cases with persistent angle closure after iridotomy or cases in which I suspect an irido-ciliary mechanism such as cysts or plateau iris.”. “If you’re interested in looking at the palisades of Vogt, you can see that with anterior segment-OCT; in fact, our group recently received a patent for using anterior segment OCT to evaluate the stem cell niche. Not surprisingly, many optometrists now use it to help them … However, purchasing an OCT device was difficult to justify for most anterior segment surgeons. OCT technology has been utilized successfully for anterior segment evaluation, offering several clinically relevant applications. For example, if you want to diagnose forme fruste keratoconus with the existing corneal mapping software that’s available in the United States, you’ll be looking for epithelial and pachymetric thinning patterns that are different from the patterns you see on topography. “With gonioscopy, you have to use some slit-lamp illumination, which is a potential source of error. Ever since it first appeared, optical coherence tomography has been evolving. “It takes a few minutes to do, but there are things you can see with gonioscopy that you can’t see with anterior segment OCT, such as color, pigment and blood vessels. Learn how to apply this valuable information in the diagnosis and management of anterior segment conditions. “Ophthalmologists are very busy, and any time you do something outside of your routine it takes longer to do it and to interpret it. Histopathology (HP), however, remains the gold standard for the measurement of tumour thickness. “Anterior chamber OCT scans spot-by-spot, so until you have a very fast system, or a parallel system, it doesn’t cover as much area as quickly as some of the other technologies,” says Dr. Huang. I do think there will eventually be a paradigm shift. But if you take a cross-section by doing a line scan, perhaps to measure the depth of a corneal lesion, it will measure it as being much deeper than it really is unless you use the dewarping correction.”, • Start by working with healthy eyes. Reproduction in whole or in part without permission is prohibited. Show more on: IMAGING AND DIAGNOSTIC INSTRUMENTS, Femtosecond Cataract: Getting Up to Speed, A New Take on Allergy Diagnostics & Treatment, The Ongoing Role of Ophthalmic Ultrasound. It is easy to use, quick, and noninvasive . Furthermore, OCT usually shows more angle closure than gonioscopy, so treatment decisions can’t be based on imaging alone. Posterior segment OCT uses a lower wavelength of light at 830 nm 3. Optical coherence tomography (OCT) is a noncontact technology that produces high-resolution cross-sectional images of ocular tissues. The initial AS-OCT device that was introduced (Visante, Carl Zeiss Meditec) could be used only for anterior segment evaluation, and the wavelength for the scans (1,310 nm) was optimized for examination of the iridocorneal angle. “You can get beautiful images and extract a lot of useful information with it. Here, three experts share their advice on making the most of this technology in the clinic, and offer a glimpse into other uses for anterior chamber OCT that are being developed. Q What are the indications for SCODI of the anterior segment? Is it just appositional when it’s closed? It’s just that the utility of this technology hasn’t yet been fully exploited. Its capabilities allow for non-contact imaging, detailed visualization and analytics of anterior segment structures of … But I think OCT will be used a lot more because it’s non-contact and it’s versatile. Nevertheless, anterior segment OCT does have the advantage of being noncontact. We have an NEI grant to study the measurement of astigmatism and topography with corneal OCT, and we’re finding it’s more accurate than other technologies.”. “I also use corneal OCT before performing crosslinking on keratoconus patients, to make sure the cornea is thick enough,” Dr. Huang adds. Use of anterior segment OCT growing With this easy non-contact technique, the images are taken quickly, and the technician decides which axis to explore. Of course, it only provides a structural assessment, not a functional assessment—it doesn’t tell you anything about the consequences of an angle narrowing, such as how much the IOP rises as a result. Dr. Schuman makes a related point. Anterior segment OCT is a relatively new technology, and hardware and software updates continue to be unveiled. The 1310nm wavelength of AS-OCT has several advantages over an 820nm wavelength. • When evaluating the condition of the angle. At the practical level, the important factor is simply whether the angle is open, closed or occludable.”, However, Dr. Radhakrishnan admits that she uses the quantitative tools with a few patients. 2-4 Anterior segment imaging has a different set of requirements than posterior segment imaging. Of course, that’s just a matter of engineering. “This is one of the real advantages of AS-OCT technology. Those questions, for the most part, can be answered with imaging. Cross-sectional tissue images of these ocular structures can be generated in vivo, for detailed analysis. But OCTA could be useful in some situations, and there are a number of areas to be explored, including cases of neovascularization or tumors.”, Dr. Chopra agrees that it’s not clear yet how useful anterior segment OCT angiography will be. In particular, the scanning speed of commercially available OCT units has steadily increased, allowing the technology to be used for an ever-longer list of clinical tasks. Surgeons are now using anterior segment OCT for several purposes: • When performing LASIK. Anterior segment OCT has multiple potential uses relating to managing glaucoma: • Evaluating potential angle closure. “You have to learn what to look for in order to evaluate different things. “Those capabilities have to do with what you can see, rather than the openness or closedness of the angle,” he explains. “It also measures the anterior lens vault, which can help determine the level of phacomorphic cataract and the expected improvement.”, • When managing keratoconus. It can also evaluate iris vessels to some extent, unless the eye is heavily pigmented. “The iris bows forward. Background Optical coherence tomography (OCT) is a widely used imaging device in ophthalmology and has been evolving since its first publication in 1991 by Huang et al . It is used … “That’s probably because some pupillary constriction occurs during gonioscopy, since light is used to look at the angle, while an AS-OCT scan can be done in complete darkness. Several studies , , , , have described the use of anterior segment optical coherence tomography (AS-OCT) in evaluating punctal stenosis. Anterior segment OCT (AS-OCT) enables the precise visualization of anterior segment structure; thus, it can be used in various corneal and ocular surface disorders. To investigate the imaging capabilities of anterior segment optical coherence tomography (AS OCT) in microbial keratitis and to assess whether measurements of the quantitative parameters, infiltrate thickness and corneal thickness, were possible. “Some OCT instruments [outside the United States] are able to get a 360-degree view of the anterior segment, and some can give you essentially a video of the angle, as if you’re sitting inside the eye looking at it,” he continues. Patients have a much
better understanding when they’re looking at scans of their own eye rather than at an eye model or a diagram.”, “I also repeat gonioscopy after iridotomy, to see how much the angle has opened,” she continues. “In most cases, if an eye has enough neovascularization to be considered abnormal, you’ll be able to see it. Dr. Schuman says that one potential problem when trying to visualize the angle with OCT is that you can have shadowing from the scleral structures of the angle itself. The clinical applications of anterior segment OCT are extremely important and useful. A lot of the value of gonioscopy at this point lies in revealing things that imaging can’t reveal. Dr. Chopra says he routinely uses AS-OCT when patients are undergoing angle-based minimally invasive glaucoma surgery. Sunita Radhakrishnan, MD, research director of the Glaucoma Research and Education Group at the Glaucoma Center of San Francisco, and Joel S. Schuman, MD, Eye and Ear Foundation Professor and chairman of the department of ophthalmology at the University of Pittsburgh School of Medicine, and co-inventor of OCT, note that this technology can be useful in multiple ways: Dr. Schuman notes that in addition to the Visante, a number of spectral domain OCT devices currently avail-
able in the United States, including the Cirrus and RTVue, can be used to image the anterior segment. Some biometry measurements, like sulcus-to-sulcus width, need that kind of penetration. In plateau iris, you see a flat iris plane. Using time domain OCT technology (TD-OCT), the Visante® creates cross-sectional images of anterior segment structures (Figure 2). New code—92132 Scanning computerized ophthalmic diagnostic imaging, anterior segment, with interpretation and report, unilateral or bilateral. The drawback with UBM is that it requires contact. On Jan. 1, a new Level I code replaced a Category III code for anterior segment OCT, which is also known as scanning computerized ophthalmic diagnostic imaging (SCODI). And of course, one of the great powers of OCT is measuring the axial length.”, David Huang, MD, PhD, the Peterson Professor of Ophthalmology, and a professor of biomedical engineering at Oregon Health & Science University in Portland, notes that an important advantage of OCT is that it has excellent resolution compared to ultrasound or Scheimpflug imaging. This form of OCT eye test is performed to … “For example, it can measure the thickness of the epithelium—in some systems also the endothelium—which would be beyond the capability of other technologies,” he says. All rights reserved. OCTA utilizes phase variations, differences in signal amplitude or changes in full OCT signal in consecutive B-scans to detect blood flow [].As current OCTA systems are designed for retinal imaging, adaptor lens is needed to image the anterior segment [2, 4, 8].Current systems use … I also think—and this is anecdotal—that it provides a more physiologic assessment of the angle structure.”, Dr. Radhakrishnan agrees. “I think UBM should be limited to situations where you really need penetration, such as when you want to look through a uveal or iris tumor or evaluate a ciliary body problem. Of course, AS-OCT has other noteworthy limitations. Another general area of potential use is as a pre- and postsurgical evaluation tool for of AC procedures. Chopra and Huang note the pros and cons of AS-OCT compared to ultrasound biomicroscopy. Optovue uses optical coherence tomography (OCT) to perform SCODI-A with iVue ®, iScan™ and Avanti™. The ANTERION is a swept-source OCT device with a 1300 nm light source, which allows for the acquisition of high-contrast anterior segment images along a large image depth. “Studies have shown that about 25 percent of angles still look narrow after an iridotomy. I think it will rapidly progress as a technology.” REVIEW. • Measuring astigmatism for toric IOLs. “AS-OCT is helpful for determining the quality of the corneal surface, and can clearly delineate an existing LASIK flap,” notes Dr. Chopra. As today’s devices become more sophisticated, options designed to help those with limited vision are increasing. “Anterior segment OCT isn’t turnkey or routine in most offices at this point, unlike posterior segment OCT imaging,” Dr. Schuman points out. If the two IOL calculations disagree, often the best result comes from averaging them.”, Dr. Chopra agrees. • When treating corneal opacities or irregularities with PTK. Anterior segment OCT is rapidly becoming a valuable tool for managing some glaucoma patients. Dr. Chopra points out that AS-OCT and gonioscopy can be quite comparable when analyzing the angle. In 1994, Joseph Izatt, Ph.D., and associates first performed an anterior segment evaluation with OCT using a 1310nm wavelength. “The other advantage of gonioscopy is that you can visualize the angle 360 degrees,” he adds. It also requires some light to visualize the angle.”, Drs. Are there synechiae? Optometrists will have an enhanced understanding of the use of OCT for anterior chamber assessment in glaucoma monitoring and refractive surgical pathways Optometrists will have an evidence-based understanding of assessment of the dimensions and curvature of the cornea and other anterior chamber structures using OCT “In my opinion, this greater understanding allows a much-more-informed consent for proceeding with laser iridotomy, laser iridoplasty or cataract extraction to treat angle closure.”. Then you need a caliper that measures depth perpendicular to the anterior surface.”, • When looking at the angle, be as perpendicular to the tissue as possible. (Faster scanning rates means more sampliing and denser data per volume scanned.). It soon became the standard diagnostic tool for following many macular conditions. “The problem is that the topography capability hasn’t been FDA-approved and people aren’t familiar with the other maps, so there’s an educational process that needs to take place for clinicians to be comfortable using this relatively new technology. “AS-OCT can reveal the differences between anterior corneal curvature and posterior corneal curvature,” he says. “Anterior segment OCT is non-contact, which is even more important now due to the COVID-19 crisis limiting contact with the patient,” he explains. Dr. Chopra says he also uses AS-OCT to monitor corneal thinning in keratoconus patients. If anything doesn’t look wide open, you can decide to perform gonioscopy, or if you see apposition and don’t do iridotomies, you can refer the patient to a specialist.“, However, Dr. Schuman believes economic realities might make adding anterior segment OCT too much of a burden. The use of anterior segment OCT (AS‐OCT) in both clinical and research domains is increasing, 2, 3 particularly with respect to the use of this technology for diagnosing and monitoring conditions of the ocular surface and anterior segment. The principle of OCT is based on Michelson’s interferometry 2. If an angle looks occludable to me by gonioscopy, I do the Visante as a confirmation. But I also use the OCT corneal power measurement that includes the posterior surface refractive power. “UBM allows visualization of the ciliary body and structures posterior to the iris, which anterior segment OCT typically hasn’t been able to image well,” notes Dr. Chopra. “This is very helpful for eyes that have previously had LASIK,” says Dr. Huang. In addition, they offer some pearls for surgeons who are adding this to their list of clinical tools. There are newer UBM devices that overcome some of these limitations, but I have limited experience with those.”, Dr. Radhakrishnan also notes that anterior segment OCT can tell the doctor a little bit about the mechanism in angle closure. Background/Objective Anterior segment optical coherence tomography (AS-OCT) and ultrasound biomicroscopy (UBM) are two non-invasive imaging techniques used for the measurement of tumour thickness in corneal and bulbar conjunctival tumours. “In my practice I do transepithelial PTK and topography-guided PTK,” says Dr. Huang. “One of the things he suggested was that gonioscopy—currently the gold standard for judging things like how open or closed the angle is—probably shouldn’t be the gold standard. “Sometimes you can see almost the whole angle, but you lose the most posterior portion that’s really of interest,” he explains. Given the advantages of AS-OCT, it’s worth asking why it’s catching on so slowly. 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