Scientists at Schepens Eye Research Institute have discovered a simple and inexpensive way for patients with retinal and other eye disease to keep track of changes in their vision loss. In a study published in this week's PLoS One (October 24, 2007) they demonstrate that a compelling visual illusion known as the induced twinkle after-effect (TAE) can accurately identify the location and breadth of actual blind spots in people with retinal disease.
The twinkle after-effect is a "twinkling" that people can see in a blind spot when they stare at a blank screen after staring at a noisy visual target such as a detuned television screen.
"Our hope is that we can make this simple technique available online or on a DVD," says Dr. Peter Bex, associate scientist at Schepens Eye Research Institute and the principal investigator of the study. "This will be particularly helpful with patients who have glaucoma, diabetic retinopathy or macular degeneration where early detection of changes in vision can impact the effectiveness of treatments."
According to Bex, many people fail to seek help when they develop blind spots in their vision, because their brains automatically compensate or "fill in" the missing information in their visual field. Since everyone has a blind spot where the optic nerve meets the retina, this perceptual "fill in" process is useful for normally sighted people, allowing them a complete visual image. "But this innate process can mask the effects of serious disorders such as diabetic retinopathy and glaucoma and keep sufferers from seeking help until the vision loss is very serious or they bump into objects they can no longer see."
The traditional gold standard method for detecting blind spots (scotomas) is very expensive and time consuming and must be done in an ophthalmologist's office. The technique known as retinal specific microperimetry is a diagnostic tool that costs nearly 50 thousand dollars and requires specialized training to apply.
In 1992 scientists became aware of what they eventually named the "twinkle after effect." They discovered that when someone looks at a television screen filled with static noise while covering part of their visual field with a small patch, the formerly patched area is left with a twinkling sensation after the noise is turned off and the person looks at a blank screen. The rest of the visual field does not experience the twinkling effect, which was described by one patient as resembling a moving cumulous cloud. "While this discovery was intriguing, it wasn't clear how it could be used for patients," says Bex.
In the past several years, Bex and his team began to understand its potential. "We theorized that if people with blind spots stared at a noisy screen, the blind areas would "twinkle" when the screen was turned off and their eyes focused on a blank screen. These 'twinkling' blind spot areas could then easily be mapped," he says.
To test their theory, Bex and his team asked eight patients with macular degeneration to undergo the retinal specific microperimetry test and his "twinkling after-effect" test. The team provided a blank touch screen--after the noisy screen--so patients could outline the twinkling areas with their finger.
The team found that the results of the two tests matched in 75 percent of cases, and visual defects could be detected in areas that are not accessible to conventional microperimetry, confirming his belief that TAE could be used diagnostically. "This tool cannot replace the more sophisticated technique but we believe it is a powerful, simple tool that patients can use daily in the privacy of their home to detect any changes in their vision," he says. "If a patient detects a change, his or her physician can then study it more closely and offer therapy."
While the results of this small study are very encouraging, Bex says the next step is to do a larger clinical study.
Ultimately Bex sees this type of test being free to the public on the Internet or distributed through a public health entity. "We really believe this could have a great impact on the visual health of the community," says Bex.
Other members of the study team are Michael D. Crossland and Steven C. Dakin of the UCL Institute of Ophthalmology, London, UK.
Schepens Eye Research Institute is an affiliate of Harvard Medical School and the largest independent eye research institute in the world.