Among the three most popular optical coherence tomography and angiography devices, there are important differences in hardware, resolution and the manner in which the data are analyzed and displayed.
“For clinicians interested in this parameter, this is an important difference to recognize among these machines,” he said.
When evaluating macular thickness for glaucoma diagnosis and monitoring, the Cirrus instrument reports the thickness of the ganglion cell layer and the inner plexiform layer (GCIPL).
The instrument (Avanti, Optovue) reports the ganglion cell complex (GCC), that is, everything from the internal limiting membrane to the inner plexiform layer. The Spectralis instrument can measure and report the entire macular thickness or the thickness of individual retinal layers in the macula—the retinal nerve fiber layer (RNFL), ganglion cell layer, or inner plexiform layer.
Cirrus offers a robust method for evaluating serial macular GCIPL or peripapillary RNFL scans for the detection of progression. The Avanti and Spectralis devices also offer methods of evaluating serial scans for progression detection; however, at present the clinician must determine whether an observed change is statistically meaningful.
The Cirrus optic nerve head and RNFL analysis printout includes an RNFL heatmap, the thickness deviation of the RNFL compared to the normative database, and tomograms that should be reviewed for segmentation errors or other artifacts.
The Ganglion Cell Analysis printout includes a heatmap of the GCIPL thickness, the deviation of the GCIPL thickness compared to the normative database, and one SD-OCT tomogram through the macula that should be reviewed for artifacts. This can sometimes disclose pathology such as macular edema or an epiretinal membrane that may have been missed clinically.
“The great power of the Cirrus instrument, in my opinion, is the methodology and analytics for looking at disease progression,” he said, and demonstrated the nerve fiber layer and the macular thicknesses obtained at different time points in a patient whose disease worsened over time.
“Most importantly, the instrument provides an analysis that facilitates complex comparisons between the current and previous scans,” Dr. Tanna explained. “The instrument also provides rates of change of various index averages and compares the thickness profiles along a 3.4-millimeter circular scan.”
The Spectralis OCT2 printout includes a confocal scanning laser ophthalmoscopy image of the posterior pole; a heatmap of the macular ganglion cell layer thickness (or a choice of other retinal layers); and the RNFL thickness profile along a 3.4-millimeter circular scan centered on the optic disc.
Angelo P. Tanna, MD
E: [email protected]
This article is based on Dr. Tanna’s presentation at the American Glaucoma Society 2019 annual meeting in San Francisco. Dr. Tanna is a consultant to Carl Zeiss Meditec