Spotlight: ON TECHNOLOGY AND TECHNIQUE
Through Thick and Thin
A new instrument maps retinal thickness, revealing macular holes, retinopathy, edema -- and providing a new way to catch early glaucoma.
BY CHRISTOPHER KENT, SENIOR ASSOCIATE EDITOR

Tiny changes in retinal thickness can herald the beginning of significant problems. Detecting those changes, however, requires highly sensitive technology, and in many cases the data is difficult to interpret. Now, the Retinal Thickness Analyzer (RTA) from Talia Technology is providing a new level of detailed, quantified information about the retina. Among other things, this makes it possible to detect glaucoma in its early stages by monitoring the posterior pole.

The Retinal Thickness Analyzer (RTA), from Talia Technology.

Scanning and mapping

The RTA combines a digital fundus camera with a computerized scanning laser biomicroscope. Using a helium-neon laser (wavelength 543 nm) it projects light onto the retina and measures the backscattering of the reflected light, calculating the distance between the nerve fiber layer and retinal pigment epithelium at each point in the scanned area. (During each scan the RTA measures 16 cross-sections, covering a 3 by 3 mm section of the retina in 0.3 seconds.)

The RTA can translate this information into a number of different color-coded 2-D or 3-D maps showing either retinal thickness or disc topography. The maps are overlaid on a fundus image to allow correlation of the thickness information with the retinal surface anatomy. (See sample maps, page 78.) The RTA can also produce a color-coded map showing deviation from a normative database to help you identify unusual thinning or thickening.

This technology can be used to scan the fovea, the peripapillary region, the posterior pole or the optic nerve head. When scanning the nerve head, the RTA can create a topographic map or generate quantitative stereometric measurements, if the operator draws the contour line along the edge of the nerve head. (The contour line can be stored for comparison in future exams.)

Detecting glaucoma at the posterior pole

A 20° diameter circle around the fovea, which falls within a standard RTA scan of the central macula, contains about half of the retina's ganglion cells, comprising up to one third of the retina's thickness in that area. Consequently, thinning in the region of the posterior pole may be one of the earliest clues that glaucomatous damage is occurring. (Because the cells are so concentrated, a large number of cells can die before a visual field test will detect any change.)

The RTA is ideally suited to detect this kind of change. In fact, the RTA can detect thinning in this area earlier than other technologies -- in many cases even before changes in the optic nerve head become evident. It's possible to recognize thinning in this region on a first scan because the number of ganglion cells located around the posterior pole is fairly consistent from individual to individual -- especially among patients with refractive errors between +5 D and -5 D.

Sanjay Asrani, M.D., of the Duke University Eye Center in Durham, N.C., helped develop the RTA. "We designed the RTA to diagnose diabetic macular edema, but I was amazed when I realized that it was able to detect retinal changes indicative of early glaucoma.

"The posterior pole is where the real action is taking place -- this is where the ganglion cells are dying. Thinning in this area appears to precede changes in the visual field, but until now we didn't have a way to see or measure this change."

Dr. Asrani uses the RTA to confirm suspicions aroused by an examination of the optic nerve head, and he finds this makes a big difference in his ability to determine whether glaucoma is present. "I believe this instrument has the potential to cause a paradigm shift in glaucoma diagnosis," he adds.

Diabetic macular edema . . . and beyond

Richard Rosen, M.D., is a retinal specialist at the New York Eye and Ear Infirmary. "I find the RTA especially helpful when diagnosing and monitoring diabetic maculopathy. It detects subtle changes while the patient still has good vision, enabling me to begin treatment before significant damage occurs. It also tells me whether the patient has responded to treatment.

"It's excellent for detecting other retinal problems as well -- central serous retinopathy, macular holes, retinal thickening secondary to epiretinal membranes, and so forth. It's much more sensitive than even a trained clinician." (In fact, a study conducted by Dr. Asrani showed that the RTA's findings were different from those of retinal specialists 42% of the time.)

Some researchers using the RTA have been surprised because the increased detail has revealed previously unknown variability in patients' response to therapy. For example, the RTA demonstrated -- for the first time -- that when treating macular edema with a laser, a thickness of 1.6 times normal has a 50% chance of reversal, but a thickness of 2.8 times normal is much less likely to reverse as a result of the treatment.

Practical advantages

Doctors agree that the RTA is easy to use and requires minimal training, but Dr. Rosen says the RTA's real strength is its maps. "The maps display a wide area for comparison, and because they're superimposed on a standard fundus image it's easy to tell exactly what I'm looking at." (Others report that the maps are also helpful as patient education tools, increasing compliance and willingness to undergo treatment.)

Dr. Rosen says the non-contact nature of the RTA is another big plus. "To get detailed data without the RTA, I need to put a contact lens on the patient. This can be a major drawback with a diabetic patient because the corneal epithelium can be scratched or even pulled off when I remove the lens."

The chief limitation of the RTA seems to be its inability to scan accurately through optical pathway inconsistencies such as cataracts. In spite of this, Dr. Asrani finds the instrument to be very useful. "I've been able to scan more than 500 patients in one year," he reports. "There's no question that it has tremendous potential application."

Making the most of it

A 5-year longitudinal study is currently underway to determine the predictive and diagnostic value of abnormalities picked up by the RTA. Meanwhile, a pilot program in Munich, Germany, is allowing doctors who can't afford to purchase this technology to send patients to a center equipped with an RTA and access the results over the Internet.

To learn more about the RTA, contact Talia Technology at (800) 214-2030, or visit their Web site at www.talia.com.

Left: An RTA 2-D map showing retinal edema. Center: An RTA 2-D map showing early glaucomatous thinning at the posterior pole. (The nerve head, omitted, is displayed on a separate map.) Right: An RTA posterior pole 3-D thickness map showing a macular hole. Note that all maps are overlaid on a fundus image to allow correlation of the thickness information with the retinal surface anatomy.

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.