spotlight on technology & technique
Decision-Free Perimetry
The first objective visual field test with a normative database works to prove its worth.
By Christopher Kent, Senior Associate Editor

Using a perimeter to test the visual field is a time-honored way to diagnose and monitor glaucoma. But subjective testing has always had limitations; many factors affect the results, and ganglion cell loss can be significant before evidence of it is detected.

Now, for the first time, an objective visual field test with a normative database designed for clinical use is available. The AccuMap Objective Perimetry System, created by ObjectiVision and distributed by Heidelberg Engineering, monitors electrical signals in the visual cortex as the subject watches a shifting set of patterns on a computer screen. The test is noninvasive and patient-friendly, and because it's objective, the patient doesn't have to make decisions. No learning curve is involved.

Studies have shown that the AccuMap instrument successfully detects and monitors visual field loss, and its ability to pick up on localized areas of dysfunction makes it particularly useful for monitoring glaucoma. It can even identify functional changes in pre-perimetric subjects.

The AccuMap system generates 58 simultaneous, unique flickering patterns onscreen and monitors the electrical signals being sent to the visual cortex for missing or diminished signal patterns.

How the Test Works

During the AccuMap test, the patient sits in front of a large computer monitor with chin slightly elevated so that eye level is just below the center of the screen. The patient's vision is corrected for maximum acuity at 16" (the distance to the screen). There's no need to dilate the pupils unless they're smaller than 2 mm.

The patient wears a headband/electrode holder which positions four sensors against the back of the head adjacent to the visual cortex. (See photo, below.) A fifth electrode is clipped to one ear. As the patient watches the computer screen, the flickering patterns generate multiple high-resolution visual evoked potentials (VEPs) -- electrical signals running from the eye to the visual cortex.

The design on the screen features 58 test zones corresponding to 58 segments of the visual field, covering the retina out to 24°. Each zone flickers from black to white rapidly (75 times a second) in its own unique sequence.

The patient is told to focus on a target at the center of the screen, which changes randomly during each 55-second run, and press a hand-held button when a specific target symbol (such as the number "3") is seen. This keeps the patient's eyes focused on the center of the pattern and also helps keep the patient alert. (Without this task the patient's concentration can wander, introducing alpha waves that can cause false positives.) Between runs, scenic images appear on the screen to help calm and relax the patient.

Mapping the Data

AccuMap's OPERA (Objective Perimetry Evoked Response Analysis) software is able to separate out and monitor the signals produced by each of the 58 patterns. It then performs statistical analysis of the data and determines whether one or more of the patterns is diminished or missing, indicating localized vision loss.

Because the system is measuring tiny signals -- only a few nanovolts -- it averages them over time to extract the signal from the background noise. For that reason, it's important to do at least six runs (lasting about 55 seconds each). With each run, the quality of the trace improves as more background noise is mathematically eliminated.

If contact between the scalp and electrodes becomes faulty during a run, it's obvious from the real-time feedback on your screen. This allows you to discard the faulty run without discarding data from other runs.

Once the feedback indicates that you've recorded a sufficient number of high-quality runs, you can stop testing. Doctors report that you usually achieve this within eight runs, which means 7 or 8 minutes of recording time.

Related Concerns

Issues worth noting:

Reproducibility. VEP recordings in the past have tended to show a large variation between normal subjects, mostly in terms of amplitude. This results from conductivity differences in the skull and tissues of the head. The designers of the AccuMap realized that the amplitude of the underlying brain signals, which are much larger, correlated with the amplitude of the VEPs. So, the AccuMap checks the strength of the underlying signals and uses it to scale the amplitude of the VEPs up or down. As a result, data from different patients can be meaningfully compared.

Normative database. So far, the database includes data from 200 subjects, far fewer than would be considered acceptable for subjective perimetry. However, researchers have noted that unlike subjective fields, the results of normal AccuMap tests don't vary significantly with age or sex, so the database won't need to be as large.

Potential limitations. This technology produces a lower signal to noise ratio in the periphery of the retina, which means that a larger reduction in signal will have to exist before the software will be able to detect it. So if you're monitoring changes in the periphery, other tests might be more helpful.

Feedback From the Field

Jeffrey M. Liebmann, M.D., is clinical professor of ophthalmology and director of glaucoma services at Manhattan Eye, Ear and Throat Hospital and New York University Medical Center. He's been investigating VEP for 4 years and has used the AccuMap system intermittently for 6 months.

"Our initial experience using the AccuMap system has been positive," he says. "The AccuMap often detects injury earlier than standard achromatic perimetry, and it's very useful when patients have unreliable standard visual fields. However, we don't yet have a mathematical model to help us use this technology to monitor disease progression.

"I think we'll always do regular perimetry in some form, but there will certainly be a role for objective perimetry; I'm just not yet sure what that role will be. This technology has the potential to change what we do. Right now, though, I believe it's still cutting edge, with limited clinical application."

For More Information

To find out more about the AccuMap system, call (800) 931-2230 or visit www. heidelbergengineering.com on the Web.