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EW GLAUCOMA 64 August 2017 by Tony Realini, MD, MPH Dr. Chen said. As vitreomacular traction develops, the entire retina, including the RNFL, is stretched and thickens. When the PVD releases, the retina and the RNFL become thin again. "Failure to recognize traction on the segmentation scans, and the release of this traction on subsequent scans, can lead to the erroneous conclusion that RNFL became thinner over time due to glaucoma progression," Dr. Chen said. Number 1: Decentration affects 27.8% of scans. "Scans that are vertically off-center can artifactually demonstrate thinning of the sector in the direction of decentration," Dr. Chen said. If the nerve is decentered superiorly in the scan area, the supe- rior RNFL may appear artifactually thin, and likewise for inferior de- centrations. Decentrations are easily identified on the optic nerve image provided on scan printouts. Myopia: The great confounder In addition to these common causes of OCT artifacts, myopic optic nerves are also difficult to image ac- curately. "These are difficult cases," said Angelo Tanna, MD, North- western University, Chicago. "The discs are hard to interpret on clinical examination, and the visual fields are often abnormal as well. In many of these eyes, we rely on imaging to such as dry eye or media opacities such as cataract," Dr. Chen said. Each device has a different quality measure; the minimum acceptable quality levels for commonly used machines are given in Table 1. Scans with low quality values should be interpreted with caution. Number 3: Misidentification of the posterior aspect of the RNFL affects 7.7% of scans. "Depending on whether the error is too anterior or too posterior, the results can be an under- or overestimation of RNFL thickness," she said. A very thin RNFL thickness is a clue to a pos- sible segmentation error, she said. "Even in advanced glaucoma, the RNFL thickness does not go to zero. There is a floor effect." Even with complete loss of axons in the RNFL, some connective tissue remains and has measurable thickness. This minimum thickness varies between instruments and typically falls in the 50- to 65-micron range, she said. Michael Kook, MD, University of Ulsan, Seoul, Korea, agreed, say- ing, "In advanced cases, OCT may not be useful in detecting glaucoma progression due to this floor effect." Number 2: Errors associated with posterior vitreous detachment (PVD) affect 14.4% of scans. The evolution of PVD over time can mimic both progressive thickening and thinning of the RNFL on OCT, In 2015, she and colleagues published a study describing the common artifacts in RNFL scans obtained with the Spectralis spectral domain OCT (Heidelberg Engineer- ing, Heidelberg, Germany). 1 "These artifacts are common to all the com- mercial devices," she said. In order of increasing frequency, she described the five most common artifacts seen on RNFL OCT tests and explained how to identify these artifacts. Top five artifacts Number 5: Misidentification of the anterior aspect of the RNFL affects 3.2% of scans. As Dr. Chen explained, all of the commercially available OCT devices have software that analyzes the scan data and identifies the relevant tissue layers— in the case of glaucoma, the RNFL. The segmentation process is not perfect, and errors occasionally oc- cur. The RNFL segmentation map is presented in the OCT printout, and reviewing it should be a standard part of the systematic interpretation of OCTs. "This artifact is easy to identify on the segmentation map," she said, "because the anterior bor- der of the RNFL will appear to be in the vitreous cavity." This can often arise in eyes with anterior vitreous traction due either to vitreomacular traction or to epiretinal membranes. Number 4: Poor signal affects 5.1% of scans. "Low signal strength can arise from ocular surface conditions Knowing what to look for can aid interpretation of scans in glaucoma patients I maging of the optic nerve and retinal nerve fiber layer has become an important adjunc- tive test in the evaluation and management of glaucoma. Global and focal retinal nerve fiber layer (RNFL) thickness data obtained with optical coherence tomography (OCT) can help distinguish glauco- matous nerves from non-glaucoma- tous nerves, and serial scans over time can aid in identifying progres- sive disease. As with virtually all tests, OCT is not perfect and can be plagued with artifacts, some that can signifi- cantly mislead the clinician if not identified. At the 2017 American Glaucoma Society annual meeting in Coronado, California, several glaucoma specialists shared strate- gies for identifying artifacts on RNFL OCT scans. Artifacts are common "All spectral domain OCT machines have artifacts," said Theresa Chen, MD, Massachusetts Eye and Ear Infirmary, Boston. Based on published studies, "as many as 20–45% of RNFL OCT scans have at least one artifact," she said. "Fortunately, about 85% of them are easily identified by carefully examin- ing the device's printout." Don't be fooled by artifacts in retinal nerve fiber layer OCT Presentation spotlight " Knowing what these artifacts are and where to look to identify them can help clinicians avoid making clinical decisions based on incorrect information. " Machine Minimum acceptable scan quality level Cirrus SD OCT (Carl Zeiss Meditec) Signal strength >6 (max 10) RTVue (Optovue) Signal strength index >30 (max 100) 3D OCT (Topcon Medical Systems) Image quality >45 (max 160) Spectralis (Heidelberg Engineering) Quality >15 (max 40) Table 1

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