Retinal nerve fiber analyzer aids glaucoma detection

Issue: June 1998

NEW YORK - The GDx Nerve Fiber Analyzer (Laser Diagnostic Technologies Inc.) is a reliable instrument for determining questionable cases of glaucoma, according to a clinician who recently tested it on more than 300 patients.

The analyzer is most helpful in detecting glaucoma in patients with diffuse loss of axons. "In a normal young individual, the optic nerve consists of up to 1.4 million fibers," said Jerome Sherman, OD, a distinguished teaching professor at the State University of New York. However, "if you lose every other axon in a diffuse manner, we cannot pick that up clinically. Those patients still have normal visual fields," he said.

The GDx allows practitioners to detect a reduction in the retinal nerve fiber layer earlier than visual fields. "At this time, there is no other way we can glean such information," said Dr. Sherman, who evaluated the GDx as part of a university-approved research project.

Four groups tested

Dr. Sherman evaluated four groups of patients: normal, known glaucoma, known glaucoma suspects and those with other disorders (brain tumors, retina diseases) that indirectly affect the optic nerve.

"Patients who we thought were completely normal turned out to be completely normal in essentially every case," Dr. Sherman said. In most instances, patients with a previous diagnosis of glaucoma were confirmed as having abnormal nerve fiber layer readings.

The GDx also detected significant nerve fiber layer damage among some suspect cases. "This is something we could not have picked up clinically," Dr. Sherman said. Patients appreciate the test, too. "You don't have to dilate or touch the cornea," he said.

Furthermore, an image is obtained in 0.7 seconds. "The patient has to remain still, but when you push the button there is no light flashing," Dr. Sherman continued. "The patient is unaware of anything going on. The procedure is absolutely noninvasive."

It takes 1 to 2 minutes to complete data analysis. "Within 2 or 3 minutes from starting, you have all the information you need," Dr. Sherman noted. "It's information that sometimes we can't acquire in a full hour examination."

Nerve fiber analysis

Gary S. Gerber, OD, a practice building consultant in Westwood, N.J., has been using the GDx for a year. "The GDx has a scanning laser that's split into two beams: one bounces off the front surface of the nerve fiber layer and the other bounces off the back surface," he explained. "The thicker the nerve fiber layer, the longer the time delay between the two lasers. Thus, the longer the time delay, the less suspect the patient is for glaucoma."

Numerical values of various regions of the nerve fiber layer, including thicknesses, are provided as part of the printout analysis. "It gives you a statistical breakdown of all the images on the page," said Dr. Gerber, such as symmetry, superior ratio, inferior ratio and superior nasal ratio.

In addition, "it lists the probability of a patient having a specific problem with glaucoma in comparison to another patient of similar age and race," Dr. Gerber said. Color images and mapping (the T-SNIT curves) also help clinicians.

"But the instrument is not as reliable in picking up focal glaucomatous defects, where there is a loss in one small area," cautioned Dr. Sherman, who noted that the manufacturer is confident that this problem can be solved with the next generation of software.

Although primarily designed for diagnosis, the GDx can also benefit early glaucoma management. "For example, you may think a patient is not getting worse, but this instrument can show that the patient has lost another 200,000 nerve fibers," Dr. Sherman said. "In these cases, detection may indicate that you need to obtain to a lower pressure or to begin another treatment. In essence, the instrument tells you if there is further and further loss of the nerve fiber layer."

Likewise, a few of Dr. Sherman's patients thought to be glaucoma suspects had normal nerve fiber layer and remained suspects. The same held true for some patients thought to have glaucoma. "Based upon this test, the nerve fiber layer was so normal that they no longer required treatment. We can follow them carefully as glaucoma suspects without treatment," he said.

"The instrument is very fast, objective, easy to use and accurate," said Dennis J. Philpot, vice-president of sales and marketing for Laser Diagnostic Technologies. "Many more patients who have glaucoma but don't know it can become part of the health care system."

Further, the GDx, which has been in development since 1991 and is now in its third generation, has "all the capabilities of replacing other functional or structural tests or measurements currently performed," including perimetry, nerve fiber photography and optic nerve evaluation, said Mr. Philpot.

The high cost of the GDx, about $65,000, will likely deter widespread ownership. However, Dr. Sherman said, "Like an internist who desires to refer a patient for a CAT scan or MRI, I envision the GDx to be placed at all large referral centers."

Leasing is one way for cost-conscious practitioners to acquire the GDx. "Doctors need to use the instrument for only two or three patients a day to cover a lease payment," Mr. Philpot said. "And, in many states, Medicare and private insurance carriers are now paying for the GDx exam."

While visual fields remain the best single functional vision test for glaucoma, the GDx is a valuable adjunct, Dr. Sherman said. "It provides us with an extra piece of the puzzle," he said. "It allows for a diagnosis of glaucoma - or no glaucoma - to be made with much greater ability and confidence."

For Your Information:

Jerome Sherman, OD, can be reached at University Vision Associates, 315 Park Ave. South, New York, NY 10010; (212) 780-5004; fax: (212) 780-5207.

Gary S. Gerber, OD, can be reached at 372 Kinderkamack Rd., Westwood, NJ 07675; (201) 666-2021. Neither Dr. Sherman nor Dr. Gerber has a direct financial interest in the products mentioned in this article, nor is either a paid consultant for any companies mentioned.

Dennis J. Philpot is vice-president of sales and marketing for Laser Diagnostic Technologies Inc.; e-mail: jrennie@laserdiagnostic.com. Mr. Philpot has a direct financial interest in the GDx Nerve Fiber Analyzer.

GDx Nerve Fiber Analyzer is available from Laser Diagnostic Technologies Inc., 9550 Waples St., Suite 105, San Diego, CA 92121; (800) 722-6393; fax: (619) 558-9145.