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Macular OCT is useful for examining the RGCs and axons, the cells that are affected by glaucoma. The currently available devices differ in the areas sampled as well as the layers that they assess.
This article was reviewed by Jullia A. Rosdahl, MD, PhD
Imaging of the macula using optical coherence tomography (OCT) has become nearly indispensable in glaucoma clinical practice.
Jullia A. Rosdahl, MD, PhD, demonstrated the importance of the technology for her patients, describing the case of a 69-year-old Caucasian man who she last saw five years earlier, and who did not want treatment at that time. After another physician recommended treatment, he returned for a second opinion for his normal tension glaucoma.
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The patient had visual acuities of 20/25 in both eyes and intraocular pressures of 13 mm Hg in both eyes (untreated). He had a paracentral visual field (VF) defect that had not worsened measurably over the five years. Progressive retinal nerve fiber layer (RNFL) thinning was seen on the OCT maps and corresponded to the VF defect. The glaucoma appeared to have progressed. The patient was skeptical.
Dr. Rosdahl is associate professor of ophthalmology, Duke University, Department of Ophthalmology, Chapel Hill, NC.
Macular OCT showed inferior thinning and, she pointed out, progression analysis showed two characteristic arcuate-shaped areas of progression. With this information, the patient was convinced of his progressive disease and agreed to treatment.
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Preserving the RGCs
Most of the retinal ganglion cells (RCGs) are in the macula, in the inner three layers of the retina (retinal nerve fiber layer, ganglion cell layer, inner plexiform layer). These layers are collectively termed the ganglion cell complex and account for 40% of the macular thickness.
Once the RGCs are affected, blindness ensues. Measurement of the RCGs is useful to assess glaucoma because the RCGs numbers in the macula are relatively consistent amongst the population, Dr. Rosdahl explained.
More RCGs are present in the central retina than in the peripheral retina. Therefore, the numbers of RCGs sampled during automated VF testing varies considerably between the periphery and central tissues, with VF points in the periphery representing smaller numbers of RGCs than VF points centrally.
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Commercially available OCTs have the glaucoma protocol for image acquisition and analysis, all of which show high levels of structure/function correlation in patients with glaucoma.
Dr. Rosdahl noted that the devices appear to be more helpful in patients with a mean deviation better than -10 decibels on automated VF testing.
This is not a cut-off, though, as she showed an example where the macular OCT scans helped in a patient who could no longer perform automated VF testing.
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“The main difference among the machines is based on the differences in the areas of the retina sampled and the layers that are included in the analysis,” she said.
All machines sample areas of different sizes. The Cirrus 5000 (Carl Zeiss Meditec) uses data from an elliptical annulus 4 millimeters high and 4.8 millimeters wide that is centered on the fovea, the Spectralis (Heidelberg Engineering) uses data obtained from a 10 x 10-mm square, the 3D OCT-2000 (Topcon Medical Systems) from a 7 x 7-mm square, the Avanti widefield OCT (Optovue) a 7-mm circle centered 1 millimeter temporal to the fovea, and the RS-3000 Advance 2 (Nidek) a 9 x 9-mm square.
All machines have different macular analysis protocols. The Cirrus provides a thickness map of the ganglion cell and inner plexiform layers; the machine also has a deviation map and table that compare the obtained thickness to an age-matched control. Horizontal OCT scans of the fovea in each eye is another device feature.
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The Spectralis provides a total retinal thickness map and two plots that show the asymmetry between the patient’s eyes and compare the superior and inferior hemispheres in one eye. The Spectralis offers the Glaucoma Module Premium Edition, allowing segmentation of the individual retinal layers. The 3D OCT-2000 instrument produces thickness and deviation maps of the ganglion cell layer.
The Avanti reports on the ganglion cell complex thickness as a percentage of the age-matched control. The scans of the right and left eyes are shown with summary tables. The RS-3000, similar to the Avanti, shows the thicknesses of the three layers (nerve fiber layer, ganglion cell layer, and inner plexiform layer). The report includes thickness and deviation maps.
Pearls for using OCT in glaucoma
When using the glaucoma protocol, some noteworthy abnormalities may appear that are in fact not glaucoma. She described the case of a patient with diffuse macular thinning but an intact RNFL seen using the Cirrus. This would promote physicians to look for other non-glaucomatous disorders.
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In patients with age-related macular degeneration, the maps of the macula are irregular and should be used with caution after glaucoma. In patients with retinal vein occlusions, all layers of the retina will become thinner, not just layers containing ganglion cells.
In non-arteritic ischemic optic neuropathy, thinning of the retina occurs in affected eyes.
Dr. Rosdahl concluded that macular OCT is useful for examining the RGCs and axons, the cells that are affected by glaucoma.
“The currently available devices differ in the areas sampled as well as the layers that they assess,” she said. “However, all of the devices perform similarly with respect to structure and function correlations.”
Read more by Lynda Charters
Jullia A. Rosdahl, MD, PhD
Dr. Rosdahl has no financial interest in any aspect of this report.