EyeWorld Asia-Pacific March 2011 Issue

16 EW FEATURE March 2011 Imaging equipment takes on more roles by Rich Daly EyeWorld Contributing Editor Surgeons are identifying a growing number of uses for anterior and posterior segment imaging equipment O phthalmic surgeons have identified an expanding number of uses for both anterior and posterior imaging equipment. But the clinical applications for some equipment may not match their marketing pitch. Like many other ophthalmologists who treat glaucoma and retinal disease, Sanjay Asrani, MD, associate professor of ophthalmology, and director of education and head of glaucoma OCT reading center, Duke University Eye Center, Durham, NC, USA, relies on optical coherence tomography (OCT) systems for his anterior and posterior segment imaging. Both the Spectralis (Heidelberg Engineering, Vista, Calif., USA) and the Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, Calif., USA/Jena, Germany) allow him to measure the retinal nerve fiber layer thickness in the peripapillary region “in great detail”. The ability to image large areas of the posterior pole in high resolution and at high speed allows him to create retinal thickness maps. These maps can be created with eye tracking software, which gives “excellent reproducibility” of measurements. “Getting a global view of the posterior pole retinal thickness with a compressed color scale allows for detection of small abnormalities which has not yet been possible in glaucoma,” Dr. Asrani said. “This may permit objective measurement of progression in glaucoma as well.” The same equipment allows detailed imaging and visualization of the trabecular meshwork, scleral spur and Schlemm’s canal. Additionally, they are designed to allow visualization of the architecture of the angle without any incident light and without contact. “This permits the diagnosis of intermittent angle closure glaucoma and possible prevention of chronic angle closure,” Dr. Asrani said. New uses found The new OCT systems have allowed Dr. Asrani to add asymmetry analysis of retinal thickness maps in glaucoma. “One can subtract the retinal thickness of one eye from the other to pick up asymmetrical losses in glaucoma as well as subtract the thickness of the upper half of one eye from the lower half of the same eye to resemble the Glaucoma Hemifield Test,” Dr. Asrani said. New uses that the OCT systems allow in the anterior segment include monitoring filtering blebs, studying tear film, evaluating newer glaucoma procedures such as the trabectome (NeoMedix, Tustin, Calif., USA), and accurate diagnosis of open angles versus narrow angles, he said. Jay S. Pepose, MD, PhD, professor of clinical ophthalmology, Washington University School of Medicine, St. Louis, Mo., USA, said the Spectralis and the Visante AS-OCT (Carl Zeiss Meditec) have allowed better definition of the vitreo-retinal interface and identification of gossamer thin epiretinal membranes that are difficult to see on biomicroscopy alone. “This is very important information for surgeons implanting presbyopia-correcting IOLs,” Dr. Pepose said. Possible future uses for the Visante that Dr. Pepose anticipates include providing help in planning surgery for angle-supported phakic IOLs that may eventually become available. Additionally, the anterior segment function of the Spectralis may facilitate flap thickness measurement in LASIK patients considering retreatment or aid in the diagnosis of infectious keratitis. That last function could include identifying acanthamoeba in the cornea. Mark Packer, MD, clinical associate professor of ophthalmology, Casey Eye Institute, Oregon Health & Science University, Portland, Ore., USA, said systems like the Cirrus, which he uses, have opened “a whole new world of ‘optical pathology’ not visible on fundoscopic exam”. These include “angiographic” cystoid macular edema. “I am still sorting out the clinical relevance of some of these findings,” he said. Limitations included But the OCT systems also have their drawbacks and limitations, noted Douglas J. Rhee, MD, assistant professor, Massachusetts Eye and Ear Infirmary, Harvard Medical School, Boston, Mass., USA. Dr. Rhee said neither system offers improvements in detecting glaucoma, although they may eventually be shown by research to improve progression tracking. “It’s because, for one, the scan times are a lot faster, so there is less time for motion artifacts, such as blinking,” said Dr. Rhee, about their potential for improving progression detection. “The second factor is that a lot of them—instead of requiring you to manually aim the circle over the nerve—have software that does it for you. So the technologies in practical uses will have less issue with artifacts.” This should be an improvement over earlier OCT systems, which studies found were good at detecting progression but were “basically useless” in real-world clinical settings because patients often would move or blink, and produced unreliable data, he noted. Another caveat for the technology, Dr. Rhee said, is its limited use in the anterior segment. “I don’t believe that it is of value because I don’t see an epidemic of acute angle closure occurring under the general ophthalmologists’ watch,” he said. “Generally, if people have acute angle closure they have not been examined by an ophthalmologist for several years. So I don’t think our technique of gonioscopy is missing a lot.” The improved images of the angles provided by ultrasound biomicroscopy and anterior segment OCT has shown that the angles are much narrower than previously thought. But that insight does not appear to carry any clinical significance, Dr. Rhee said. The new OCT systems’ contact- free designs also appear to have little practical value, he said. Ophthalmologists using such devices will likely need to measure patients’ intraocular pressure, as well, during those office visits, so they will still need to have their eye anesthetized. The newer OCT systems also drew criticism regarding their image processing speed. Shannon Wong, MD, Austin, Texas, who uses the Cirrus for all of her glaucoma patients and presbyopia-correcting IOL recipients, as well as some patients with macular pathology, said faster speed would help. “As the images are fairly large I am told that the computer needs time to process the images,” Dr. Wong said. “If the unit processor were faster it would make imaging more time-efficient.” Dr. Asrani noted that the OCT systems are vulnerable to artifacts in their posterior segment images due to some media opacities, including posterior vitreous membranes and epiretinal membranes. And in the anterior segment, the systems are unable to visualize the ciliary body and ciliary processes. EW Editors’ note: Dr. Asrani has financial interests with Heidelberg Engineering (Vista, Calif., USA). Dr. Packer’s facility was named a “Center of Excellence” by Carl Zeiss Meditec (Dublin, Calif., USA/Jena, Germany). Drs. Pepose, Rhee, and Wong have no financial interests related to their comments. Contact information Asrani: sanjay.asrani@duke.edu Pepose: jpepose@peposevision.com Packer: mpacker@finemd.com Rhee: douglas_rhee@MEEI.harvard.edu Wong: shannon@austineye.com