by Karla Gale

Last Updated: 2007-10-31 16:44:01 -0400 (Reuters Health)

NEW YORK (Reuters Health) – Ophthalmologists have found that a test of "twinkle after-effect (TwAE)," a type of illusory stimulus, can identify blind spots in people with age-related macular degeneration (AMD), even before they are aware of their visual loss.

"The TwAE has been known in the vision literature for over a decade, where it had been considered as something of a perceptual oddity," Dr. Peter J. Bex told Reuters Health.

Among normally sighted people, the after-effect occurs after looking for a few seconds at a ‘noisy pattern’ that contains a small blank patch, he explained. "When the noise is removed and replaced by a uniform grey field, most people see a noisy afterimage only in the region that was blank. The rest of the screen appears uniform."

"It occurred to us that a similar afterimage may be present in people with real blind spots," he added.

To test their theory, Dr. Bex at Harvard University in Cambridge, Massachusetts, and colleagues in London tested seven patients, aged 69 to 82 years, with retinal scotomas due to AMD, and one patient, aged 35, with traumatic maculopathy. Stimuli were projected from a computer screen.

According to their report in the October issue of PLoS ONE, six of the patients experienced an after-image that they could reliably trace around on a test screen. The boundaries they traced were closely correlated with lesions on microperimetry maps or angiography results.

"Early diagnosis of AMD and other retinal diseases is paramount because any loss of vision is irreversible," Dr. Bex said. The problem is that individuals who develop visual impairment may not be aware of it and may not seek clinical intervention until vision loss is catastrophic.

"This is why we are working hard to develop quick and simple tests, like the TwAE test, that can be self-administered regularly to monitor the boundaries of their sight loss, detect any worsening of vision, and to evaluate the effectiveness of any treatment over time."

The investigators believe that the TwAE test may also provide valuable information for other diseases that cause blind spots, particularly glaucoma and diabetic retinopathy.

"There is also potential for the test to help identify lesions from other retinal insults, including laser injury from military apparatus and weapons," Dr. Bex added.