Consider utility, marketability when choosing diagnostic tools

When selecting diagnostic instrumentation for the primary care optometry office, a practitioner must weigh a variety of factors, including the clinical utility of the device and the potential return on the investment.

“All of these diagnostic instruments provide useful information,” Ian Gaddie, OD, of Louisville, Ky., told Primary Care Optometry News. “But there is not one instrument that will be the panacea for diagnosing glaucoma. Each provides just a piece of the puzzle.”

The GDX: measuring nerve fiber layer

Laser Diagnostic Technology’s GDx is a confocal scanning laser polarimeter used to measure the linear polarization of light, or the retardation of light, by the retinal nerve fiber layer, according to Scot Morris, OD, of Centennial, Colo.

“It has a normative database and is able to compare not only the two eyes to each other, but also the current visit to previous visits,” Dr. Morris said.

According to the manufacturer, the patient’s results compare to a large, age- and ancestry-stratified database for diagnostic considerations on the first visit.

“It also provides a fairly simplistic printout that can be used to educate your patients,” Dr. Morris added.

Dr. Gaddie said he finds the GDx to be very useful in its ability to examine nerve fiber. “It is also useful for correlating optic nerve appearance and corresponding nerve fiber layer loss with functional vision loss,” he said.

Dr. Morris said that the GDx has a relatively quick capture time — 3 minutes for both eyes. “And it has been shown to be about 96% sensitive and 93% specific to nerve fiber layer changes,” he said. “It does not require dilation and has excellent reproducibility.”

“The GDx also is affected by lens and corneal birefringence, and thus postoperative refractive and cataract patients may have some minor degradation in sensitivity,” he said.

However, the manufacturer has recently introduced a variable corneal compensator that measures the birefringence of each cornea. There are now two options with the GDx: the GDx stand-alone unit, which has no upgrade available for the custom corneal compensator; and the GDx NFA portable unit, which replaces the Access and includes the upgrade. All existing Access units can be updated.

The main weakness of the GDx, Dr. Morris said, is the fact that it measures only nerve fiber layer. “Unfortunately, it does not allow us to evaluate the macula or retina,” he said. “It also does not allow us to obtain quantifiable data about optic nerve head depth and volume.”

HRT: ideal for multiple office use

The Heidelberg Retinal Tomograph (HRT) II is a confocal scanning laser ophthalmoscope that images and measures the surface topography of the optic nerve and retina, enabling their quantitative assessment.

“This unit is relatively inexpensive and is fairly portable,” Dr. Morris said. “This makes it ideal for multiple-office use. Also, it does not require dilation.”

Dr. Gaddie said he finds the HRT to be very helpful in measuring the size of the optic nerve head. “In my experience, the most useful clinical application for this instrument is in evaluating subtle changes in optic nerve head topography,” he said. “It gives an exact measure of the actual size and volume of the optic nerve head, which is useful in differentiating people who have large physiological optic nerves from those patients who have pathological damage to the neuroretinal rim.”

“While I agree with Dr. Gaddie in that it is a good tool for measuring optic nerve head size, the HRT uses a reference plane that is parallel to and 50 microns below the peripapillary retinal surface to analyze the depth and volume of the optic nerve,” Dr. Morris said. “Though this automation may sound beneficial, it does make reproducibility variable. Since this peripapillary nerve fiber layer is often altered during the course of glaucomatous optic neuropathy, loss of tissue in this area may cause a change in the positioning of this reference plane over time, which will ultimately affect the reproducibility of the results. Your staff must be carefully trained on how to properly analyze the margins of the optic disc to prevent errors.”

According to the manufacturer, neither this reference plane, nor the indices derived from it, is used to detect progression in glaucoma. Rather, a totally objective pixel analysis is used.

The HRT II has astigmatic lenses as well as a new software option: the macula edema module. The module quantifies diabetic retinopathy and expands the HRT II to both glaucoma and retinal applications.

OCT3: a recent upgrade

This device, manufactured by Zeiss Humphrey, is a relatively new upgrade of the older OCT2, Dr. Morris said.

“This update has increased the versatility of the device and now allows for a highly sensitive graphic representation of the thickness of the macula, retina and the nerve fiber layer,” he said. “It uses interferometry to objectively measure the volume and diameter of the cup, which makes it an excellent multipurpose diagnostic device.”

The OCT3 has software packages that allow for specific retinal nerve fiber layer thickness analysis, anatomical image comparisons and multiple bilateral comparison options.

“I have found that the OCT3 is excellent at determining small depth changes, which is especially helpful not only in glaucoma, but also in managing macular edema and retinal thickening,” Dr. Morris said. “Since this device does not assume a reference plane, but instead uses objective anatomical landmarks to measure cup volume and area, it has high reproducibility.”

One drawback to this instrument is its normative database, Dr. Morris said, “but it is reportedly in development,” he said.

The Optomap Exam

The Optos Optomap is made possible by the Panoramic200, a non-mydriatic ophthalmoscope. Because of its wide viewing area, no dilation is necessary.

“The instrument is able to view the inside of the eye in terms of retina and optic nerve 200° without dilating,” said William Jones, OD, of Albuquerque, N.M. “It allows you to examine most of the retina and all of the posterior pole.”

Dr. Jones said the instrument is basically used for screening purposes. “It allows you to see optic nerve problems, macular problems and peripheral retinal problems,” he said. “And then further investigation can be done with dilation and other techniques, for funduscopy.”

One weakness of this device, according to Dr. Jones, is its inability to get to the ora serrata on every patient. “It optically is not able to capture an image of the ora serrata,” he said. “But it is probably within 2 or 3 disc diameters of it.”

Talia’s RTA

Talia’s RTA (Retinal Thickness Analyzer) is used for diagnosing and monitoring glaucoma, retinal edema, macular degeneration and other pathologies. It combines a digital fundus camera with a scanning laser to analyze the retina. “It takes only 0.3 seconds to image 16 optical cross-sections that are approximately 190-µm apart, in a 3-mm by 3-mm area,” Dr. Morris said.

According to the manufacturer, data points from digitized slit images of the optic nerve head, peripapillary and macula are turned into meaningful 2-D and 3-D images and reports. Depending on the type of exam desired, between 4 and 13 of these 3-mm × 3-mm scans are taken and aggregated into a complete picture. Each of the scans is overlaid on the fundus image to give the viewer a 3-D representation of the chosen areas.

“The 3-D image allows this technology to be used as a very effective teaching tool,” Dr. Morris said. “The software packages allow for comparison analysis as well as normal deviation calculations. They allow for total retinal thickness measurements instead of simply nerve fiber layer thickness measurements.”

However, because of the large area scanned and the source of light, the device requires adequate dilation.

“This is often an inconvenience for those patients who do not want to be dilated,” Dr. Morris said. “And at the optic nerve head, it assumes a standardized reference plane, which makes reproducibility questionable.”

According to Talia, at the peripapillary and macula the RTA acquires objective measures of thickness without a reference plane, which allows for objective tracking of disease progression.

Making a choice

Among the factors to be considered when choosing diagnostic instrumentation are marketing, clinical utility, expandability and financial feasibility, Dr. Morris said.

An optometrist should consider how an instrument could potentially serve to promote his or her practice. “Find out if the manufacturer will help you market it,” Dr. Morris said. “Also, you want to consider how your patients will view the equipment.”

In terms of clinical value, a practitioner should question whether the equipment provides needed information, whether it is versatile or single-featured, whether it makes the OD’s job easier and whether staff will be able to operate the machine.

Other clinical considerations are whether the equipment has good sensitivity and specificity and whether the printouts are easy to interpret, Dr. Morris said.

Regarding expandability, the optometrist should consider whether the instrument can be linked with other machines or electronic medical fields in the future. Financially, consider not only affordability, but how to code or bill for the new instrumentation.

“Each practitioner’s needs are different. Ultimately, you need to decide what piece of equipment best suits those needs,” Dr. Morris said. “Regardless of which of these tools you choose, the key is that you use them and market your practice as having them.”

For Your Information:

• Ian Gaddie, OD, can be reached at fax: (502) 339-0571.
• Scot Morris, OD, can be reached at fax: (303) 346-2589.
• Neither Dr. Gaddie nor Dr. Morris has a direct financial interest in the products mentioned in this article, nor is either a paid consultant for any companies mentioned.
• William Jones, OD, is a Primary Care Optometry News Editorial Board member. He can be reached at 1828 Conestoga SE, Albuquerque, NM 87123; (505) 293-7347. Primary Care Optometry News was unable to determine if Dr. Jones has a direct financial interest in the products mentioned in this article or if he is a paid consultant for any companies mentioned.

• The GDx NFA is available from Laser Diagnostic Technologies, San Diego; (858) 673-7900; Web site: www.laserdiagnostic.com.
• The Heidelberg Retinal Tomograph is available from Heidelberg Engineering Inc., Vista, Calif.; (800) 931-2230; (760) 598-3770; Web site: www.heidelbergengineering.com.
• The OCT3 is available from Zeiss Humphrey Systems, Dublin, Calif.; (925) 557-4100; Web site: www.humphrey.com.
• The Panoramic200 is available from Optos, Marlborough, Mass.; (800) 854-3039; e-mail: info@optos.com; Web site: www.optos.com.
• The Retinal Thickness Analyzer is available from Talia Technology Inc., Tampa, Fla.; (800) 214-2030; Web site: www.talia.com.