Eyeworld

EW GLAUCOMA 72 February 2016 by Tony Realini, MD, MPH O ptical coherence tomog- raphy (OCT) has become the clinical standard for automated optic nerve im- aging in glaucoma. OCT has many advantages over other imaging platforms, including ease of use, no need for dilation, rapidity of scans, and excellent reproducibility to facilitate detection of change over time. To optimize the value of OCT imaging in glaucoma, clinicians should be adept at interpreting the test output. This includes evalua- tion of both the test quality and the structural data it provides. Scan quality assessment "As with visual field testing for glaucoma, obtaining an OCT scan of good quality significantly affects the ability to diagnose glaucoma and glaucoma progression," said Donald L. Budenz, MD, MPH, University of North Carolina at Chapel Hill, N.C. The first parameter that Dr. Budenz reviews is the signal strength. "One important pearl is that a low signal strength causes artifactual thinning of the retinal nerve fiber layer (RNFL)." Cataract is a major culprit that can lead to a low signal strength, he said. The minimum acceptable signal strength for the Cirrus OCT (Carl Zeiss Meditec, Jena, Germany) is >6, for the RTVue OCT (Optovue, Fremont, Calif.) is >30, and for the Spectralis OCT (Heidelberg Engineering, Heidelberg, Germany), the quality score should be at least 20, he said. Next, he looks at segmenta- tion. This is the process by which the OCT software identifies the borders of the RNFL so it can report its thickness. All of the commer- cially available OCT platforms have excellent segmentation software, but there can be errors in some patients. Reviewing the segmentation plot Pearls and pitfalls of OCT imaging in glaucoma W hen we evaluate a glaucoma patient, we yearn for objectivity. Our goals are to accurately determine whether the patient has glaucoma and if the patient is stable or showing signs of progression. Although the intraocular pressure (IOP) is objective, it is not a reliable indicator of glaucoma damage and cannot tell us anything about progression. The visual field is very helpful, but its subjectivity too often makes it a frustrating test for both patient and doctor. The development of optical coherence tomography (OCT) addresses the unmet need for objectivity in glaucoma. The OCT has brought much greater consistency to our evaluation of the optic nerve. There is enormous variability in this 1–2 mm of ocular anatomy, and so a standardized assessment is vital. But interpreting the OCT is not as simple as just seeing whether the test values fall outside the normal range. In this "Glaucoma editor's corner of the world," Tony Realini, MD, Donald Budenz, MD, Jullia Rosdahl, MD, and Yao Liu, MD, give us tips for evaluating this critical test. They remind us that OCT needs careful interpretation to be a useful clinical tool. I remember when the OCT first became available. At Emory University, we bought one of the first in the region, and there was even a story about it on the local nightly news. The reporter was aware that the visual field was a very unpopular test with patients and asked whether the OCT would make the field test unnecessary. There was a brief time when some were hopeful that an objective and automated assessment of the optic nerve might reduce the need for subjective visual field testing. However, this has turned out not to be true. The OCT greatly enhances our ability to follow patients, but it cannot replace the functional assessment of visual function provided by the field test. The role of OCT in glaucoma decision-making is evolving as both software and hardware improve. A worsening OCT would not usually lead to glaucoma surgery if the visual field were stable and the IOP acceptable. However, we are using the OCT more frequently to help with decisions about increasing medical and laser treatment—and we rely on it in making a glaucoma diagnosis. The OCT has been a giant step forward in our care of glaucoma patients. Reay Brown, MD, glaucoma editor Circumferential scan around the right optic disk shows glaucomatous thinning of the inferior bundle continued on page 74 Glaucoma editor's corner of the world can identify scans in which seg- mentation is inaccurate, resulting in erroneous scans. Artifacts Once scan quality is assessed, the output should be reviewed for evi- dence of artifacts. "Between 20 and 45% of OCT scans are reported to have at least one type of artifact," said Yao Liu, MD, Madison, Wis. These can occur with all of the commercially available OCT platforms, she added. Artifacts can lead to inaccurate OCT interpretations. Some artifacts arise from scan- ning technique. Head tilt, blink, Scanning laser ophthalmoscope image shows the optic nerve head of the corresponding OCT. There is an inferior notch corresponding to the loss of RNFL inferiorly. Source: Jullia Rosdahl, MD, PhD

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