In recent years there have been several advances in perimetry-related technologies, including a number that are still not widely used in clinical practice. Two of them—the new Glaucoma Progression Analysis software for the Humphrey Visual Field Analyzer, and the Frequency Doubling Technology perimeter—are well-proven and deserve much wider use than they're currently receiving. Other new developments aren't sufficiently validated to be a worthwhile investment for most clinical practices.
Here, I'd like to share some of my experience with these technologies, and offer reasons for incorporating them—or not incorporating them—into your office.
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| Optic disc photo of an elderly Hispanic POAG patient. (Perimetry on facing page.) |
Analysis of Visual Fields Progression
The development that I believe most deserves wider use is the Glaucoma Progression Analysis software for the Humphrey Visual Field Analyzer. The GPA software, which has been available for almost two years, helps determine whether test results are likely to indicate statistically significant progression, as defined in the Early Manifest Glaucoma Trial. The numbers used to differentiate nonsignificant change from true progression were taken from data collected at Bascom Palmer and other centers around the world during the development of the Swedish Interactive Threshold Algorithm (SITA) software for Humphrey. As part of that process, we tested several hundred stable glaucoma patients three or four times over a two-month period. That data allowed us to determine, to a statistically significant level, precisely what kinds of changing test results do not constitute true glaucoma progression, but could be mistaken for it.
The GPA software compares the patient's current visual field to two baseline fields and analyzes the difference. If the change in an individual point in the visual field has less than a 5-percent probability of falling within the range considered to be noise, that change is noted as progression. However, the program requires that three or more of the same points show change on three consecutive fields before the message "Likely Progression" is displayed.
Since many ophthalmologists already own the HFA-2 machine, this software is simply an upgrade. Most glaucoma specialists have already purchased the upgrade, but relatively few general ophthalmologists have. This is unfortunate, because the GPA software is a significant advance over subjective comparison of a series of visual fields on the table next to each other. The cost of an upgrade is relatively small, so I hope many more practices will invest in it.
Community Screening with FDT
The second technology that's proven its usefulness is Frequency Doubling Technology, or FDT, which has been around since the mid-1990s. Studies have shown that it's possible to lose 25 to 30 percent of your optic nerve fibers before standard white-on-white perimetry can pick up defects.1,2 FDT technology has been shown in several studies to have a high sensitivity and specificity for glaucoma,3,4 and there is some evidence that it may pick up glaucomatous damage earlier than white-on-white perimetry.5
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| Standard white-on-white perimetry vs. SITA SWAP of the left eye of an elderly Hispanic woman with POAG. | (Optic disc photograph on facing page.) |
Community screenings get little coverage in the literature, but they're very important if you want to prevent blindness; glaucoma doesn't cause symptoms until it's too late. (In my practice we routinely see patients who are blind in one eye and almost blind in the other because they haven't been in the medical system.) The FDT instrument is ideal for community screening because of its portability and level of accuracy; it weighs less than 20 pounds, and the test is very quick—one to two minutes when set on screening mode. In fact, used in combination with intraocular pressure measurement and visualization of the optic disc by an ophthalmologist, many consider it the gold standard for community glaucoma screening.
Of course, if you don't already have an FDT instrument, you'll need to buy or borrow one (or more) in order to do community screenings. We've solved this problem by getting help from Pfizer, which has loaned us multiple FDT instruments for this purpose. Whether you purchase or borrow the instrument(s), I encourage you to become involved in community screenings.
A quick tip for screening: A single FDT test will often produce a false positive result. If you find an abnormality at the 5-percent threshold level, run the test a second time. Two positives in a row probably indicate that the patient should be referred for a complete examination with a full-threshold Humphrey visual field.
Three Not Yet Time-Tested
Three other perimetry technologies have shown great promise, but aren't yet fully validated:
• The Matrix. This is the latest version of FDT technology, made by Welch Allyn in New York and sold by Humphrey Zeiss Meditec. It's intended to be an in-office instrument, and may allow earlier detection of glaucoma in the office setting. Instead of testing 17 locations in the visual field in one to two minutes using rather large stimuli, it tests 54 locations using smaller stimuli. (The Matrix isn't useful in community screenings because it's not portable.)
This instrument is so new that it hasn't really been put to the test; we're collecting data now. If it fulfills its promise, it could become commonplace in clinical practices. However, it won't replace standard white-on-white perimetry. It may detect glaucomatous damage sooner than white-on-white, but it will probably be unable to follow progression once the disease has become moderate to severe.
• SITA SWAP. Available as an option on several different perimeters, Short-Wavelength Automated Perimetry (SWAP) also has been shown to predict the development of glaucomatous loss earlier than white-on-white perimetry6 and may identify progression of visual field loss before white-on-white.7 The original technology wasn't adopted clinically because it produced a lot of false positives, partly due to interference caused by cataract and age-related macular degeneration. Also, the original test was very time-consuming—about 20 minutes per eye. However, by using the same SITA algorithm that shortened the white-on-white test, the time required to take the test has gone from 15 or 20 minutes to only five or 10 minutes. This test should be available soon.
Unfortunately, false positives are inherent in SWAP's blue-on-yellow technology. Cataracts filter out blue light, a potentially unavoidable hindrance. Also, the drusen and pigment changes in the macula caused by AMD affect SWAP results in ways not easy to identify. These factors may ultimately limit SWAP's usefulness in elderly glaucoma patients.
Nevertheless, some glaucoma specialists are using SWAP to test patients who have a suspicious optic disc but show nothing in a white-on-white visual field. If the SWAP test is normal, they feel safe in concluding that they're not missing early glaucoma development. However, if the test is positive, it's not conclusive; few doctors want to sentence a patient to lifelong treatment on the basis of a test known to have a lot of false positives.
Currently, SWAP is part of an ancillary study of the Ocular Hypertension Treatment Study; a subset of patients has been tested using SWAP throughout the study. We're waiting for the results to see whether SWAP, white-on-white visual fields or optic disc photos picked up glaucomatous progression first. If the data favors SWAP, that, plus the advent of SITA SWAP will probably lead to an increased use of this technology in the clinic.
• Objective perimetry. In recent years, two new psycho-physiologic tests—pattern ERG (electroretinography) and multifocal VEP (visual evoked potential)—have been discussed in the peer-reviewed literature. Unlike the tests we've talked about so far, these are objective measurements of the functional status of the visual field. Electrodes are applied to the patient's face or scalp and a stimulus is shown to the patient; the electrodes monitor brain activity to determine how effectively the retina is picking up the stimuli. So far, these tests are only being used at a few centers, but preliminary data suggest that they may be clinically valuable.
Pattern ERG is a global test of retinal ganglion cell function; it presents a reversing white and black checkerboard pattern. This test gives you a single number indicating the overall level of function, instead of a map of what the patient is seeing. Early studies suggest that the pattern ERG correlates well with the standard visual field in glaucoma patients and, as glaucoma progresses, the number changes.8,9 The really exciting thing is that the pattern ERG may turn out to be an earlier warning sign for glaucoma than other functional tests, although that has yet to be proven.
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| Standard white-on-white perimetry and Matrix visual field in an 83-year-old woman with early glaucoma. The white-on-white field shows little glaucomatous change in the pattern deviation plot, but the Matrix field shows an extensive superior arcuate defect. |
Multifocal VEP, developed by Stuart Graham, MD, in Sydney, Australia and Donald C. Hood, PhD, in New York, projects light stimuli onto different areas of the retina and then monitors brain-wave activity to determine whether the stimulus has been seen or not. It provides a map of the visual field, similar to standard perimetry, which correlates well with subjective measurements. It has shown excellent correspondence with standard perimetric defects,10,11 making it useful for patients who are not good visual field takers.
The Bottom Line?
I believe most practices that own a Humphrey white-on-white perimeter should add the GPA software upgrade; the cost is relatively small, and the gain is significant. If you're doing community screenings, an FDT instrument is a worthwhile investment. However, I believe it would be reasonable to wait for further developments before purchasing the other technologies described above.
Dr. Budenz is associate professor in the Departments of Ophthalmology, Epidemiology, and Public Health at the University of Miami Miller School of Medicine, and associate medical director of the Bascom Palmer Eye Institute, Miami.
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