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New instrument strikes the right nerve in glaucoma diagnosis

by Bonnie Taher
Correspondent

Finally, a scanning-laser device for
early glaucoma detection that's available
to the practicing clinician
 

SAN DIEGO -- Scanning lasers that detect early glaucoma by measuring the thickness of the retinal nerve fiber layer have been around since the early '90s, but only research institutions had them.

 
(Image not available)
GDx scan: May reveal areas of future visual field loss.

Now, a scanning laser device is available for clinical use. Laser Diagnostic Technologies Inc., based here, recently released the GDx (for glaucoma diagnosis) glaucoma scanning system, the third generation of its Nerve Fiber Analyzer (NFA). Like the NFA, the GDx is a confocal laser scanning ophthalmoscope with an integrated polarimeter. Both instruments measure the thickness of the retinal nerve fiber layer.

And although both instruments use scanning laser technology, a key improvement that makes the GDx practical for clinical use is a database containing bilateral retinal-nerve-fiber measurements on hundreds of normal subjects, said Dennis J. Philpot, LDT's vice president of sales and marketing. The database automatically compares patient scans with normals matched for age, race and gender, then generates a color printout of the results, Philpot explained.

 

(Image not available)
Polarized light passing through the nerve fibers splits into two parallel rays. The change in velocity as the beams emerge from the nerve tissue correlates to fiber layer thickness.

The GDx and the NFA use polarized laser light to measure retinal-nerve-fiber-layer thickness. Both versions direct a laser beam through the nerve fibers, which split the light into two parallel rays traveling at different velocities. The change in velocity as the beams emerge from the nerve tissue correlates to fiber-layer thickness. The GDx images only the nerve fibers; an algorithm built into the software subtracts blood vessels from the evaluation.

In both instruments, the laser's sensitivity to minute changes in nerve-fiber thickness may reveal damage before it can be detected by conventional means. "People with glaucoma or ocular hypertension appear to have abnormal nerve-fiber-layer thicknesses. The GDx seems capable of identifying damage in patients in whom we have no other way of detecting it. My guess is we could identify patients before they ever have visual field loss," said Neil T. Choplin, MD, chairman of the Department of Ophthalmology at the San Diego Naval Medical Center. "We've seen abnormal nerve-fiber parameters in ocular hypertensive patients with normal visual fields, so we may be identifying damage much earlier."

The GDx may also prove useful for following disease progression. In a glaucoma patient Choplin re-scanned after a year, "the machine demonstrated continued loss of nerve fibers corresponding to where the visual field was worsening."

The GDx at work

E. Randy Craven, MD, assistant clinical professor of ophthalmology at the University of Colorado School of Medicine in Denver, uses the GDx in his private practice. He used the nerve fiber analyzer for 2 years and has had the GDx since October. Both Craven and Choplin contributed many of the normals to the GDx database. Craven's office was a software beta-test site.

 
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Change analysis of two nerve-fiber layer scans obtained 1 year apart in a patient with progressive glaucoma. Graph shows thinning of interior bundle (blue line) over time.

Craven explained that the laser acquires data through an undilated pupil in 0.7 seconds. Then the Windows-based software compares 65,000 measurement points to the normative database, based on the patient's age, race and gender.

Craven takes three images of each eye, and the software averages the results. New images can be acquired while the computer processes the information in the background. The patient is aware of a flickering red-light field in the examined eye, but there is no discomfort.

"This technology adds useful information to the decision-making process" Craven said. He noted that the one-page report shows both a digitized cartoon of the nerve fiber layer and the color-coded scan image. A graph compares the patient to the normal range and shows percentage deviation from normal for each quadrant. A numeric nerve-fiber-analysis chart codes normal values in green, abnormal in red and borderline in yellow.

The $65,000 instrument began shipping in January. More than 50 units were sold through March, most to clinical practices in the United States, Europe, South America and Australia. Previously, most nerve fiber analyzer units went to research institutions, Philpot said.

Upgrading nerve fiber analyzer units to incorporate the database, a fast-screening mode and other improvements runs about $5,000.

Neither Craven nor Choplin has a financial interest in the company. Choplin's opinions are his and not necessarily those of the U.S. Navy, which does not commercially endorse the instrument. .

Contact Information

Choplin: Phone: 619-532-6702
Craven: Phone: 303-797-1900
Philpot: Phone: 619-558-9144

 

Copyright 1997, ASCRS Ophthalmic Services Corp. All rights reserved.