Glaucoma ranks as one of the most frequent causes for blindness in the
industrial world. With time, the disease damages nerve fibers leading to a loss
of visual field. A new technology, called laser scanning tomography, promises to
be the best method yet in the early detection of the malady.
Until now, the primary diagnostic tools for measuring glaucoma centered on the visual field exam and ophthalmic photography. While the visual field exam has proven to be an excellent indicator of visual impairment caused by glaucoma, it can detect the presence of glaucoma only after the optic nerve has become involved.
To detect the disease at an earlier stage, ophthalmologists must rely on the value of intraocular pressure. But results are mixed. Approximately two-thirds of all patients with high intraocular pressure do not develop glaucoma. Additionally, about one-half of glaucoma patients exhibit normal intraocular pressure.
It has been known for a long time, however, that during the early development of glaucoma, the optic nerve head becomes damaged due to the disappearance of nerve fibers. Laser scanning tomography lets doctors quantify these changes, in vivo, before there is a measurable visual field loss.
How it works. Developed in Germany by Heidelberg Engineering, the new system combines a laser scanning camera, a PC/AT, and specialized software. These components allow the acquisition of three-dimensional images at the posterior segment of the eye, a region normally inaccessible to direct in vivo examination. By manipulating these images on the screen, the doctor can document any topographical changes in the optic nerve head. It operates like this:
A visible red laser, focused to a single point, provides the illuminating light source. Scanning mirrors deflect the beam in two dimensions, constructing a planar 2-D field of the optic nerve head. Light reflected from the optic nerve head at the focal plane passes through a small diaphragm, focuses on a point source detector, and is registered by the computer.
Light reflected from those parts of the nerve head outside the focal plane are registered only partially, with the suppression of reflected light increasing rapidly as focal distance becomes greater. This technique yields a high resolution perpendicular and parallel to the optical axis.
Slicing the images. Such a scan comprises an optical "slice" through the object at the focal plane. A series of slices at different focal planes creates a volume, or 3-D image ready for digitizing and displaying on the computer screen. The "Heidelberg Retina Tomograph," reports Gerhard Zinser of Heidelberg Engineering, records the object volume as a series of 32 section images at 32 equally spaced focal planes. Total acquisition time: 1.6 seconds.
Topographical images store automatically in an image database, together with all relevant patient data. The operator analyzes these images in real-time at the screen, using various analysis, measurement, and data presentation menus.
For example, as the operator moves the cursor within the image, the computer displays the x, y, and z coordinates of each individual point. Additionally, horizontal and vertical cross sections representing the height variation along a horizontal or vertical line can be displayed to visualize the structure's shape.
These topographic maps prove useful in detecting changes over time, says Zinser. Aligning topographic images of the same patient, but recorded at different times, indicates if any deterioration of the optic nerve head has occurred, allowing early diagnosis and treatment. Computing time for aligning two images and subtracting one from the other: ten seconds.
In the U.S., specialists at the Emory Eye Center are among the first to use the laser-based system. "We know that the key to controlling glaucoma is to catch it early," says Mary Lynch, M.D., an associate professor of ophthalmology at the Emory University School of Medicine. "Emory glaucoma specialists are enthusiastic about applying this new laser technology to glaucoma research and screening."
Other medical applications for laser scanning tomography include: examination for macula degeneration; evaluation of retinal detachment; analysis and follow-up of tumors; and the evaluation of diabetic retinopathy.