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A scanning laser polarimetry device, which examines the retinal nerve fiber layer, is a beneficial diagnostic tool for glaucoma suspects because it targets the area where the earliest damage from glaucoma is likely to be seen. It also is valuable in monitoring stability or progression in patients with established disease.
Atlanta-A scanning laser polarimetry device (GDx-VCC, Carl Zeiss Meditec) is useful for identifying early damage in patients at risk for glaucoma, such as those with ocular hypertension. It also can help differentiate glaucomatous optic nerves from physiologic cupping, determine longitudinal development of retinal nerve fiber loss (RNFL) in at-risk individuals, and monitor progression of glaucomatous loss or stability over time, said Neil T. Choplin, MD, at the American Academy of Ophthalmology annual meeting.
"Of all the diagnostic instruments on the market, [this one] is the only one that was designed specifically to look at the RNFL. It does that very well, and it doesn't do anything else. It doesn't look at the retina, and it doesn't look at the optic nerve head, just strictly the RNFL," said Dr. Choplin, who is in private practice in San Diego. He said he has worked with this instrument since its inception more than a decade ago and has consulted with its developers on upgrades.
The clinical significance of the instrument's exclusive emphasis on the RNFL is that this area is where the earliest damage occurs in glaucoma, beginning with axonal dysfunction and the eventual death of the axon, Dr. Choplin said.
Printouts for each eye show an orientation image of the fundus represented in a color-coded retardation or thickness map in which warm colors represent areas of higher retardation and cooler colors represent lower values. The printouts also show areas of significant departure from an age-matched normative database, several summary parameters, and a plot graphing the values contained in a peripapillary circle.
The instrument has gone through several iterations over the years to address issues related to the signal that it acquires, said Dr. Choplin, who also is an adjunct clinical professor of surgery at the Uniformed Services University of Health Sciences in Bethesda, MD. The current model includes a built-in, fully automated variable corneal compensator (VCC) designed to remove a portion of the signal attributed to the anterior segment, which can confound the data, Dr. Choplin said. The addition of the VCC has improved the instrument's sensitivity and specificity, he added.
In his practice, Dr. Choplin said, he uses the laser at the initial visit for all patients referred for consultation, except those with advanced visual field loss or cloudy media. These referred patients typically have risk factors for glaucoma such as elevated IOP or suspicious-looking optic discs. The results of scanning will reveal whether any damage to the RNFL compatible with glaucoma is present.
The instrument also is useful for following patients with established glaucoma and assessing changes over time. The results can help the clinician determine whether the RNFL loss has occurred over time in a patient whose eye was normal at the initial evaluation, whether the loss has progressed in a patient with known loss, or whether the patient's status has remained stable over the follow-up period. Dr. Choplin said he repeats the scans once a year.
The device is less useful in a subset of patients who have "noisy" or atypical scans, he said. An atypical birefringence pattern (ABP) shows an irregular distribution of retardation values with alternating areas of warm and cool colors and relatively high nasal and temporal values. About 20% of eyes have this pattern, which is difficult to interpret.
A new software module (Extended Corneal Compensation, Carl Zeiss Meditec) has been developed to address these atypical scans and in testing has been effective at reducing ABP to 2%, Dr. Choplin said. A normative database has been collected, and the software package is awaiting FDA approval.