Introduction
Epiretinal membranes (ERMs) are a common vitreoretinal interface disorder.1 Around 10% of patients older than 60 years of age may experience an idiopathic ERM.2 ERMs can be asymptomatic, but their progression may result in macular distortions and abnormal central visual function. Routinely, pars plana vitrectomy and membrane peeling are performed on symptomatic ERMs to relieve the patient's visual complaints.3 Although imaging and operative techniques have improved over the years, about 10% of idiopathic ERMs recur and approximately 3% require a second surgery.4,5 The preoperative use of optical coherence tomography (OCT) imaging can allow physicians to better judge the full extent of a central macular disturbance and can help planning the surgery.6 However, the 6 mm × 6 mm field of view (FOV) available on most clinical spectral-domain OCT (SD-OCT) instruments may not be large enough to image the full extent of the ERM with a single scan. A wider FOV, like the FOV currently available on swept-source OCT (SS-OCT) instruments, may be useful to assess the full extent of ERMs and any associated vitreomacular adhesions or traction. The purpose of this study was to determine whether a 12 mm × 12 mm SS-OCT scan provided additional information about the full extent of large ERMs that could be clinically relevant in either explaining symptoms or planning surgery.
Patients and Methods
This case series is part of a prospective OCT imaging study at the Bascom Palmer Eye Institute. The institutional review board (IRB) of the University of Miami Miller School of Medicine approved the study. All patients signed an IRB-approved consent before OCT imaging was performed. The study complied with the tenets of the Declaration of Helsinki and with the Health Insurance Portability and Accountability Act of 1996.
The images were acquired using a SS-OCT instrument (PLEX Elite 9000; Carl Zeiss Meditec, Dublin, CA) operating at 100-kHz with a central wavelength of 1,050 nm. A cube scan pattern covering a 12 mm × 12 mm FOV (approximately 40°) centered on the fovea was used. This scan consists of 512 A-scans per each horizontal B-scan and 512 B-scans. The OCT fundus images (OFI) and retinal thickness maps (RTMs) were generated from the scans using the software available on the instrument.
Scans from eyes with the diagnosis of ERM were reviewed. En face structure OCT images were generated using a vitreoretinal interface (VRI) slab. This slab had a thickness of 210 μm with the upper boundary located 190 μm above the inner limiting membrane (ILM) and lower boundary located 20 μm below the ILM. Small adjustments involving the contrast and brightness settings were performed on each scan to optimize image quality.
Results
Six representative eyes with large ERMs that extended outside a 6 mm × 6 mm FOV (approximately 20°) are shown in Figures 1 to 6. In these figures, Panel A shows the OFI, Panel B shows the RTM, Panel C shows the en face image from the VRI slab, Panels D and E contain a B-scan located outside the central 6 mm × 6 mm FOV corresponding to the yellow line in Panel C, Panel F shows an overlay of the RTM with 60% transparency overlaid on the VRI en face image, and Panels G and H show foveal B-scans corresponding to the yellow line in Panel F. In the panels showing the OFI, RTM, and VRI slab images, the white square boxes centered on the fovea represent the area covered by a conventional 6 mm × 6 mm scan.
The full extent of the ERMs are clearly visualized on the VRI en face images (panel C). The ERMs appear as hyperreflective structures and their reflectivity depends on their thickness. The overlay of the RTM with 60% transparency on top of the VRI en face structure image highlights the areas with both traction and increased retinal thickness. Moreover, the VRI en face images can show evidence of traction and retinal striae even in the absence of retinal thickening. The composite images display the retinal thickening related to the traction forces associated with the ERMs. The horizontal lines on the en face images mark the position of the B-scans shown in the adjacent panels. The dashed yellow lines on the B-scans correspond to the boundary layers for the en face VRI slabs. The magnified B-scans emphasize the presence of the ERM. These six representative cases are intended to demonstrate how the 12 mm × 12 mm scan might be helpful to clinicians.
Discussion
In this report, we highlight cases in which the 12 mm × 12 mm SS-OCT scan can identify the full extent of ERMs outside the boundaries of a typical 6 mm × 6 mm SD-OCT scan. As shown in this paper, the SS-OCT can give us many different images using a single instrument and a single scan pattern. A single 12 mm × 12 mm scan provides an OCT fundus image that includes the macula, vessels, and optic nerve. Moreover, a RTM can also be generated and identifies any areas of abnormal thickening or thinning in the entire macula. In addition, the VRI slab image provides a widefield view of the ERM so that the clinician can appreciate the correlation between the ERMs and any thickening or distortions of the retina. Furthermore, traditional B-scans are also available from the same widefield raster scan so that any structural abnormality above, under, or in the retina can be appreciated. One concern might be the lower image quality due to the decreased density of A-scans and B-scans in the 12 mm × 12 mm scan pattern compared with the denser scans patterns used in the more traditional 6 mm × 6 mm scans, but the image quality of the 12 mm × 12 mm scan appears to be more than adequate to diagnose an ERM. The cases demonstrate how a single 12 mm × 12 mm scan can easily demonstrate the presence and the extent of the ERM, the traction component of the ERM that leads to retinal folds, the abnormal foveal contour associated with the traction, and the adherence and incomplete detachment of the posterior vitreous. This widefield imaging strategy might be an excellent screening tool in detecting asymptomatic or mildly symptomatic ERMs. In addition, it can identify vitreomacular adhesions and retinal traction, which should also help in explaining visual symptoms.
The en face VRI slab images clearly show the full extent of the ERMs. This slab image allows the clinician to correlate the visual complaints with the full extent of ERM. Moreover, VRI slab images can give a better perspective for the clinician when planning surgery. The en face VRI slab images can help clinicians fully appreciate the extent and three-dimensional (3-D) configuration of the ERM. In addition, this imaging strategy may assist in the training of residents and fellows, and 3-D printing of the ERM may aid in planning the surgical intervention.7 Currently, after an idiopathic ERM peeling, about 10% of them will recur and approximately 3% will require another surgery.4,5 In the future, pre- and post-surgical studies with widefield SS-OCT imaging might help us better understand these recurrences. Re-proliferation could start from unpeeled parts of the ERM located outside the central macula since the surgeon couldn't fully appreciated the extent of the ERM. Moreover, it might be useful to compare the widefield images with intraoperative OCT images during live surgeries, and as we enter an era where surgeries may be robotically assisted, the SS-OCT images could be used to assist with the surgeries.
In summary, the 12 mm × 12 mm SS-OCT en face VRI slab images can be useful in assessing the full extent and progression of large ERMs extending outside the standard 6 mm × 6 mm FOV. Moreover, the combined 12 mm × 12 mm RTM and VRI slab en face images can provide a better understanding of the traction forces on the retina, which may help explaining symptoms and aid in surgical planning.
References
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