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Three devices for conducting structural analysis of glaucomatous eyes have pros and cons.
Dr. Parrish, a professor at Bascom Palmer Eye Institute, Miami, took a poll of the audience and found that optical coherence tomography ([OCT]; Stratus OCT or OCT III, Carl Zeiss Meditec Inc.) was the most widely used among the attendees. Scanning laser ophthalmoscopy ([SLO]; Heidelberg Retina Tomograph [HRT], Heidelberg Engineering Inc.) was next, followed by scanning laser polarimetry ([SLP]; GDx or GDx VCC, Carl Zeiss Meditec Inc.).
Dr. Parrish stressed that he was not representing any one manufacturer but acknowledged that the Bascom Palmer Eye Institute does use OCT heavily in the clinic and for research.
Against that backdrop, he said some of today's eye-care professionals wonder whether a place remains for stereo disc photography. Dr. Parrish said he believes one does. He works with a photographer who uses digital stereo disc photography, which makes the old split-frame 35-mm images seem out of date.
"It's valuable probably early in the course of the disease and again in moderate disc disease," Dr. Parrish said.
He added that retaining those photographs in patient files can help track in which patients ocular hypertension has progressed over time.
Dr. Parrish said images obtained from confocal SLO measure the intensity of reflected light from the retinal surface. The images generated by the device look like photographs but are really color-coded depictions of topographic measurements it has calculated.
SLO assesses optic disc topography, and its diagnostic accuracy is comparable with a glaucoma examination by an authority, he said. The technology offers instant storage and retrieval of images and has the longest track record of any quantitative instrument. It accurately predicts the development of future field loss and provides a measurement of the optic disc size.
Disadvantages of SLO, he said, include wide variations in IOP that may change disc topography and confound glaucoma progression analysis, and it cannot detect vessel shift, pallor, or disc hemorrhage.
The SLO printout provides probability values for parameters, with classification automatically being that p > 0.5 is within normal limits; p < 0.05 is borderline; and p < 0.001 is outside normal limits.
The latest iteration (HRT III) uses a different testing strategy that yields an image that is similar to OCT printouts. It uses an ethnic-specific database for all parameters, making it robust across many populations, as well as Moorfields regression analysis and retinal nerve fiber layer (NFL) profile graphs.
It generates a Glaucoma Probability Score (GPS) to determine whether change has occurred in a glaucomatous eye. The GPS analyzes three-dimensional modeling of the topographic image with a neural network classification technique.
"There is no contour line, there is no reference plane. It considers the shape of the doughnut hole rather just its two-dimensional configuration," Dr. Parrish said, adding that this view of the cup is based on its depth, width, and steepness of the layer walls.
The NFL is not viewed as a concave configuration but as a convex configuration. It looks at the vertical curvature along the superior/inferior line.
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SLP measures retardation or the change in polarization of incident light as it passes through the NFL and bounces back. Previous laser vision correction can lead to inaccurate readings due to the birefringent nature of the cornea, so a variable corneal compensator measures corneal birefringence and neutralizes the effect of the cornea.