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Imaging techniques aid diagnosis of glaucoma damage
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BOSTON - Advanced imaging techniques are giving glaucoma specialists a quantifiable, objective method of gathering information about disease progression, said Joel Schuman, MD.
Dr. Schuman, a glaucoma specialist at the New England Eye Center, said that nerve fiber layer analysis and ultrasound biomicroscopy could eventually change thinking about treating glaucoma.
"Clinicians have to rely on the tools at hand," he said. "The goal of these devices is to give objective, quantifiable data so that one visit to the next you can measure and determine whether or not there's been change."
Ultrasound biomicroscopy
Ultrasound biomicroscopy (UBM) is useful in studying difficult cases and has revealed that plateau iris is more common in narrow angles than previously thought, Dr. Schuman said.
UBM uses a high-resolution ultrasound to image details down to 50 µm. Because of the high frequency, UBM penetrates only 4 mm into the eye, so it is useful mostly for the anterior segment, he said.
"It's useful in terms of looking at the anatomy when you can't see particularly well by gonioscopy or when you want to see in greater detail," he said, in cases such as iris or ciliary body tumors, narrow or closed angles, pigment dispersion or malignant glaucoma.
In the case of plateau iris, he said, "We used to think it was a rare condition, but the plateau iris configuration is extremely common. We've learned why it occurs and what it looks like using the UBM."
Dr. Schuman said that scans of a patient conducted in both dark and light showed how iris movement affects angle closure. One patient with a narrow angle was scanned by UBM as physicians lowered the light. UBM revealed that as the iris dilated, the angle began to close and the iris became more convex. Within 5 minutes, the angle had closed entirely. When physicians raised the lights, the iris constricted and the angle opened.
"It demonstrates very nicely the pathophysiology of what's going on," Dr. Schuman said. "The aqueous continues to be produced in the ciliary body. There's relative pupillary block, and the fluid can't get through into the anterior chamber. It pushes the iris forward and it closes the angle. It's useful not only in terms of making a diagnosis, but also in terms of educating the patient that something needs to be treated even if you're not having any symptoms."
While the UBM is useful for imaging the anterior chamber, other devices have posterior chamber applications.
The GDx Nerve Fiber Analyzer (GDx/NFA), made by Laser Diagnostic Technologies Inc. (San Diego), uses polarized light to determine the thickness of the retinal nerve fiber layer, which shows the earliest effects of glaucoma, Dr. Schuman said.
"The nerve fiber layer is a highly reflective band," he said. "It looks like the striations that you see going toward the optic nerve head. It's just the cell bodies of the ganglion cells as they go from the retina to the lateral geniculate nucleus."
Nerve fiber layer evaluation
The GDx/NFA detects the shift in the axis of polarized light to determine the thickness of the nerve fiber layer, which is birefringent. The clinical reproducibility of the GDx/NFA is 5 to 8 µm.
Another technology, optical coherence tomography (OCT) produces cross-sectional images of the retina. Humphrey Instruments, in San Leandro, Calif., is now making one of these instruments, Dr. Schuman said.
The OCT is a non-contact, non-invasive tomographic imaging technique that uses short wavelength coherent light to achieve a resolution of about 10 µm. A superluminescent diode serves as the light source for an interferometer based system. The light beam scans transversely across the eye to make an image of the retina and optic nerve head.
"With confocal scanning lasers you are doing coronal sections, and with the nerve fiber analyzer you're looking at the shift in polarization," he said. "With OCT you're actually doing cross-sectional imaging of the tissue, and you see the foveal pit and the nerve fiber layer, as it comes closer to the optic nerve head, becomes thicker."
The OCT is useful for retinal and glaucoma work, and could have applications in the anterior segment. Dr. Schuman attaches his OCT to a Zeiss (Obezkochem, Germany) slit lamp, while the commercial system offered by Humphrey is used with a fundus camera.
Dr. Schuman is using his OCT to create circular scans around the optic nerve head.
"We've found these circular scans to be much more valuable in looking at glaucoma because it gives you a 360º view of the nerve fiber layer," he said. "You can think of it as a cylinder of information that is then unfolded and looked at in cross sections."
Quantitative imagers needed
Each of these devices could provide interesting information beneficial in patient management, Dr. Schuman said. He noted that these imaging technologies might not be immediately applicable in ophthalmic practice, but that is changing.
"The first question you must ask is, `Why do I need any of these? I can look in the eye, I can see the optic nerve head, why do I need something beyond that?' I'm not sure that you do right now."
But in the future, he added, "It will probably be important to have a device that gives quantitative objective information about the nerve head and the nerve fiber layer. We could see such instruments in the offices of general practitioners in the future."
For Your Information:
- Joel Schuman, MD, can be reached at the New England Eye Center, 750 Washington St., Box 450, Boston, MA 02111; (617) 636-7950; fax: (617) 636-4866; e-mail: joel.schuman@es.nemc.org; Web site: www.neec.com. Dr. Schuman has no direct financial interest in any of the products mentioned in this article, nor is he a paid consultant for any companies mentioned.