Spotlight on Technology and Technique
Scanning for Glaucoma
New features help turn a familiar technology into a better tool for preventing vision loss.
BY CHRISTOPHER KENT, SENIOR ASSOCIATE EDITOR

Optical coherence tomography, the technology used in Zeiss Humphrey Systems' OCT optical scanner, has always held great promise as a means of tracking the retinal changes that indicate developing glaucoma. Now, the latest incarnation of this instrument -- the OCT2 -- has incorporated new features intended to make it even more effective for this purpose. The new model allows an ophthalmologist to use one instrument to monitor three different variables affected by developing glaucoma: the optic disc, the retinal nerve fiber layer (RNFL), and macular thickness and structure.

The OCT2 Optical Coherence Tomography Scanner from Zeiss Humphrey Systems.

Optical coherence tomography

OCT technology allows you to create an accurate representation of a two-dimensional cross-section through the retina. Essentially, this technology is a light-based equivalent of B-scan ultrasonography, with a resolution of 10 microns -- about 10 times better than ultrasound, according to Zeiss. (It also requires no direct contact with the eye or saline immersion.)

Perhaps the single most impressive aspect of this technology is that it shows the structural condition of the internal tissues, including photoreceptors, retinal pigment epithelium and choroid. In fact, according to Zeiss, the OCT2 is the only instrument that directly measures cross-sectional images of both the RNFL and macula. (Alternate technologies have to extrapolate macular thickness using other data.)

The OCT2 takes its measurements using a broad band of near-infrared light wavelengths, instead of a single wavelength, produced by a low coherence light source. This light is reflected off the structures in the retina; interferometry is used to analyze the reflected light and produce a two-dimensional cross-section of the area being scanned. The resulting data has a very high signal to noise ratio and very high resolution.

(Scientists at MIT are now using this technology to take scans through the tip of a needle using fiber-optics. This will make it possible to pierce organs and send back a scan of internal tissues, helping to locate cancers or abnormalities without the trauma of exploratory surgery.)

Going after glaucoma

The OCT2 can measure retinal changes directly using nerve fiber analysis or monitor glaucomatous progression by using volumetric analysis and checking cup to disk ratios.

In terms of managing glaucoma, the most important of the OCT's new features is a macular thickness analysis program, including an age-related normative database. This allows you to compare macular measurements to the database and/or previous measurements of the same patient. The comparative data can be presented in several formats, including side-by-side comparisons of color-coded, cross-sectional images and graphic or numerical representations of differences.

	In addition to providing quantitative analysis, the OCT2 generates images showing the structural detail of retinal sublayers.

Another feature that makes the OCT2 effective as a glaucoma management tool is its ability to take a measurement in either a straight line or a circle -- the latter being particularly useful for scanning the optic disc. (See illustration, right.) The OCT2 can also scan a series of concentric circles and create a donut-shaped retinal nerve fiber layer thickness map. Or, the OCT2 can make a series of straight line scans through a single point and use the information to analyze disc structure (or evaluate a macular hole).

Comments from the field

Joel S. Schuman, M.D., professor of ophthalmology at Tufts University School of Medicine and glaucoma service director at the New England Eye Center in Boston, helped develop the technology used in the OCT2. "This instrument has a lot going for it," he says. "In studies comparing the original OCT to other technologies as a glaucoma screening device, OCT has always done at least as well, and sometimes better, than the others. Now, the new OCT2 scans with a resolution of 10 microns and includes a normative database from multiple sites for comparison.

"Our technicians work with nearly every type of retinal scanner available, and they say the OCT2 is just as easy or easier to use than the others. It's certainly comfortable for the patient. And you can use it for multiple purposes, including diagnosing macular and retinal disease."

Rohit Varma, M.D., M.P.H., associate professor of ophthalmology at the University of Southern California in Los Angeles, has used the OCT2 since 1999. "Measuring the thickness of the nerve fiber layer is the best approach to early detection of glaucoma," says Dr. Varma, "and the OCT is the only instrument that gives a cross-sectional map of the NFL and provides an accurate estimate of its thickness. I think this technology has far and away the most potential for having an impact on detecting progression in glaucoma."

Dr. Varma admits that he still sees room for improvement in the OCT2. "I look forward to more automation of the scanning process, even higher scanning resolution and a broader bank of normative data. Nevertheless, I wouldn't be using this technology if I didn't think it was the wave of the future."

Practical improvements

Other new features incorporated into the OCT2 include:

• a Windows '98 platform, making data processing faster
• increased capacity for sorting and accessing images
• a smaller, more ergonomic design. (The original OCT had two monitors -- one for the black-and-white red-free image of the fundus showing the location of the scan, and one for the corresponding OCT cross-sectional image. The OCT2 has a single, flat-screen monitor that displays both images.)

The OCT2 is also about $10,000 less expensive than its predecessor. Current list price is $65,500.

(Note: OCT2 scans are currently billed using the 92135 procedure code. You can be reimbursed for testing Medicare patients diagnosed with glaucoma twice a year, glaucoma suspects once a year. In some areas, retinal edema is also allowed.)

Hope for the future

In the battle against glaucoma, every new development brings us closer to preventing -- and ultimately eliminating -- the disease. The OCT2 is clearly a step in the right direction.

For more information, contact Zeiss Humphrey Systems at (877) 486-7473, or via e-mail at info@humphrey.com.

Are you aware of new products or technology that have made (or are likely to make) a significant difference in practice? Contact Christopher Kent at kentcx@boucher1.com to find out about possible coverage in a future issue.