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New instrument strikes the right nerve in glaucoma diagnosis
by Bonnie Taher
- 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
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
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.
(Image not available)
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. .
Choplin: Phone: 619-532-6702
Craven: Phone: 303-797-1900
Philpot: Phone: 619-558-9144
Copyright 1997, ASCRS Ophthalmic Services Corp. All rights reserved.