The advent of optical coherence tomography has fundamentally changed the management of many retinal diseases. Thanks to OCT, practitioners can now quantify details of the retinal anatomy more easily and accurately. Here are four tips for using OCT to assess macular scans.
1. Pay Attention to Scan Quality
The science behind OCT is complex, but at its core, OCT is based on the reflection of light off of tissue.
As a result, clear fluid or mostly fluid structures such as vitreous tend to reflect very little light — appearing black on an OCT scan. Media opacities such as cataract or vitreous hemorrhage will hinder the transmission of light and thus reduce scan quality. Look at the amount of noise in the front of the retina and any shadows that these media opacities might cause for clues about a patient’s vision loss.
Another important aspect of the scan quality is its centration. Most scans will be centered on the fovea, but in some cases, you’ll get an eccentric scan to visualize noncentral pathology or because the patient cannot fixate adequately. If the scan does not include the fovea, it’s important to know why — or whether or not it needs to be redone. Use the raster line over the composite image to assess if the scan is adequately centered.
2. Use Accurate Language
Some diseases have a characteristic or even pathognomonic appearance on OCT. It’s important to describe abnormalities using accurate terminology. For example, most hyporeflective pockets within or beneath the retina represent fluid. However, there are other instances where this may be due to tissue atrophy instead (e.g., macular telangiectasia).
When in doubt, you can still accurately describe a certain layer of the retina using the terms “hyperreflective” or “hyporeflective.” These terms won’t commit you to a diagnosis.
3. Familiarize Yourself With the Anatomy
Become familiar with the layers of the retina and what “normal” looks like. For an in-depth classification of retinal layers pertaining to OCT, be sure to check out Proposed Lexicon for Anatomic Landmarks in Normal Posterior Segment Spectral-Domain Optical Coherence Tomography.
Here are a few key areas to keep in mind:
- Inner segment/outer segment (IS/OS) junction — i.e., inner segment ellipsoid zone. Loss of this thin reflective line represents a defect in the photoreceptors and typically has a profound negative impact on the patient’s vision.
- Retinal pigment epithelium layer. You can see irregularity in primary RPE abnormalities such as pattern dystrophy. For example, it’s common to see hyperreflective focal elevation of the RPE in the case of abnormal accumulation of material such as drusen or blood. In addition, hyporeflectivity under the RPE can represent a pigment epithelial detachment. Hyporeflectivity between the RPE and retina is a common indication of subretinal fluid.
- Outer retina. Fluid often accumulates in this location in macular edema related to inflammation, diabetes and vascular abnormalities such as vein occlusion or choroidal neovascularization. Nuclear layers are generally hyporeflective, and plexiform layers are generally hyperreflective in both the outer and inner retina.
- Inner retina. The inner retinal anatomy’s integrity is a good reflection of the eye’s vascular status. In chronic ischemic situations, you’ll notice atrophy. More severe, acute situations, such as a retinal arterial occlusion, can present with a thickened, hyperreflective inner retina. Normally, the inner retina has a smooth contour, with a gradual depression at the fovea. When that normal contour is lost, be sure to account for it, whether it originates from the outer retina (e.g., macular edema) or a preretinal abnormality (e.g., an epiretinal membrane or vitreomacular traction).
4. Don't Forget the Subfield Analysis
One powerful OCT feature is the ability to produce a topographical map of retinal thickness. Retinal vessel registration further enhances the accuracy of this scan and allows for sequential comparison. This is very useful both for assessing how diseases respond to treatment and educating patients about their progress.
OCT is a rapid and accessible imaging modality that provides a very useful complement to the posteriorsegment exam. Its reproducibility also makes it a helpful way to serially follow chronic conditions like macular degeneration, diabetic macular edema and many others. By seeing changes over time, both clinician and patient can accurately understand the disease and its response to treatment. For those who manage retinal diseases, the ability to assess and describe OCT findings is a critical skill.
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About the authors: Brian Chan‐Kai, MD, is a vitreoretinal specialist at EyeHealth Northwest in Portland, Ore. Dr. Chan‐Kai completed his residency at the Cullen Eye Institute/Baylor College of Medicine, where he also served as chief resident. He trained in vitreoretinal surgery and diseases at the Casey Eye Institute/Oregon Health and Science University. He is a member of the YO Info editorial board.
D. Wilkin Parke III, MD, is a vitreoretinal specialist with VitreoRetinal Surgery, PA, in Minneapolis. He completed his residency and surgical retina fellowship at Bascom Palmer Eye Institute in Miami, where he served as chief resident in his final year. He is a member of the YO Info editorial board.