Blue-yellow perimetry brings promise of earlier glaucoma detection

Issue: July 1996

NEW YORK—A newly developed type of perimetry—short wave automated perimetry (SWAP), also known as blue-yellow perimetry—will give glaucoma practitioners an effective tool to enhance their ability to diagnose and track the progression of the disease, according to an optometrist here.

"Visual fields are a very nice way to monitor glaucoma," said Murray Fingeret, OD. "They're a good way to monitor a person's functional visual status." Using perimetry, he said, a practitioner can confirm the existence of glaucoma by the presence of a visual field defect and judge the defect to grade the glaucoma and monitor it over time.

Although there can be glaucomatous damage without visual field loss, Fingeret said, "We like to use visual fields as a helpful, confirmatory step in diagnosing glaucoma."

Moderate damage revealed

However, Fingeret said it was shown more than a decade ago that traditional automated perimetry will not reveal a scotoma until 25%-40% of the nerve fibers are damaged. "The fact is, a person needs to have a moderate amount of damage before you get a visual field defect, as least one that's reproducible," he said.

Because of this problem, researchers set out to discover a way to pick up the disease much earlier. "The idea was to find a more sensitive visual field test," Fingeret said.

The result of this research—particularly that done by two different groups headed by Chris Johnson, PhD; Pamela Sample, PhD; and Robert Weinreb, MD—was SWAP.

SWAP employs a bright yellow background that reduces the sensitivity of the retina's red-green receptors, said Johnson, and a large size five blue target color. The testing is then done in the same fashion as any automated perimetry test. Johnson is director of the Optics and Visual Assessment Laboratory at the University of California-Davis.

"You use the same instrument; all you're doing is changing the color of the background and the target as well as the target size," Fingeret said. "It's a cost-efficient way to test."

Sample and Johnson conducted independent studies involving ocular hypertensive patients. Johnson's original study, published 2 years ago in Archives of Ophthalmology followed 76 of these patients. They all had normal, regular visual fields, but on blue-yellow tests nine were found to have irregular fields. They were followed for 5 years, and five of those nine also developed scotomas on regular perimetry. Those who had normal blue-yellow tests were still normal 5 years later.

"The feeling is that you are picking up glaucomatous defects at an earlier point in time," Fingeret said.

Theories behind mechanism

It is still not clear how SWAP works, Johnson said, but two theories are proposed. One is that the nerve fibers that are most sensitive to blue light are also the ones first affected by early glaucoma. "The mechanisms that convey blue information to the brain may be more susceptible to various types of insult," Johnson said.

The second theory, and the more viable one, Johnson said, is that damage occurs in all the nerve fibers, but it is noticed more readily in the blue because "the blue system is very sparse," he said. "There's not a lot of redundancy and overlap. The other types of fibers can back each other up, almost masking the effects of minor nerve damage. But because the blue system is sparse, when you isolate it with blue-yellow perimetry there are not enough receptors to back up the missing ones, allowing for better detection of early glaucoma damage."

Johnson compared it to two workplaces: In a factory with 1,000 workers, 15 absentees would hardly be noticed. But in a business with 30 workers, if 15 were out the impact would be significant.

"Perhaps when a person starts to develop a problem you isolate it, and you don't have other receptors next door that can fill in, not allowing you to pick up the problem," Fingeret said. "It's probably a combination of both theories."

The test is not perfect. Johnson's latest study—they have 8 years worth of data—revealed that blue-yellow field defects appear about 3-4 years earlier in 80% or more of the cases, "but they have had a few cases where people develop scotomas on regular perimetry, yet never had a blue-yellow defect. Why? Nobody knows," Fingeret said.

"What this information conveys is that glaucoma is a very complex disease," said Johnson. "Not everybody shows the same kind of effects."

Database coming

Fingeret said the new Humphrey Instruments and Octopus perimeters come with blue-yellow perimetry. In the near future, Humphrey will release a database that will accompany the hardware, allowing practitioners to translate the raw blue-yellow scores into meaningful data that can be compared to age-matched norms.

Fingeret's office was one of four U.S. sites that helped build the database. Fingeret, Johnson and Sample ran three of the four U.S. sites that helped build a database.

"Where I see this having an impact," Fingeret said, "is when you have patients who are ocular hypertensive or have suspicious optic disks, and you're on the borderline of whether you should treat or not and their fields are full. This test will complement regular perimetry and give you more information.

"There may be times when you see there's a field defect on blue-yellow that may indicate that the patient has early glaucomatous damage occurring," Fingeret said. "I believe it will be a major advance in the diagnosis of early glaucoma."

He also said SWAP appears to be more effective at monitoring the progression of the disease. "For people who are unstable or are progressing, this picks up progression in a more sensitive mode," he said. "Progression in terms of automated perimetry is a very difficult area. We're just now understanding how variable the normal visual system is and how difficult it truly is to recognize progression." he said.

Dr. Fingeret has no financial interest in Humphrey Instruments or Octopus.