Sharper results for blindness test

Australian scientists have achieved a dramatic improvement in the speed and precision of tests for patchy blindness – by making their eye test stimuli blurrier.

Their advance is yielding a more reliable way of diagnosing and treating the onset of common blinding diseases like glaucoma and diabetic retinopathy, which can be hard to detect in the early stages. It may also improve the diagnosis of vision loss in cases ofmacular degeneration, multiple sclerosis and stroke.

Researchers at The ARC Vision Centre and Australian National University led by Professor Ted Maddess have pioneered a new approach to the science of perimetry – the mapping of patchy blindness across the visual field – and are currently developing a device that mayrevolutionise its diagnosis, and improve the effectiveness of treatment. 

“Currently, perimetry has a big problem: the results you get from one test are not easy to reproduce with the same patient in other tests. If the test can’t be relied on, it is hard for doctors to determine whether your vision is declining or not. You can get results that may look like your vision is getting better when it’s actually getting worse,” Prof. Maddess says.

“This may be partly due to the fact that the test requires the patient to concentrate on when to push a button exactly when they get a visual signal for up to 20 minutes, and they can make errors. But our research has established the poor results are also due to minute and very rapid movements of the eye and to distortions in the image received in the perimetry device, known as aliasing.

“Aliasing is the ‘jazzing’ effect seen when people wear clothing with thin stripes on TV, and is caused by fineness of the stripes being poorly read by the coarser array of sensors in the TV camera.

“Patches of blindness can vary rapidly across the visual field,” he explains. “The coarse sampling of perimetry then aliases the patchy blindness to create distorted maps, which are then made worse by very small eye movements,” Prof Maddess says. “The eye position and movements are different from one visit to the eye specialist to another, so differently distorted maps are produced each time, meaning you could even get a different diagnosis of vision loss each time.”

The team overcame the problem by using large blurry test stimuli, which give a more accurate overall match of where in the visual field vision has been lost. These have the added benefit that the patient does not need to be wearing perfect eye glasses during the test. 

Prof Maddess and his colleagues are now working on a new perimeter that uses the novel stimuli and which can complete the test far more rapidly and with less inconvenience to the patient than the current lengthy process.

Patchy blindness is a common condition across society, which becomes more frequent with ageing. It can be due either to loss of the light-sensing cells in the macula of the eye (as inmacular degeneration or diabetic retinopathy), or by damage to the optic nerve or part of the brain that interprets the visual signals (as in glaucoma, MS or stroke).

“Patchy blindness can become seriously disabling as people grow older,” Prof Maddess explains. “Since they have not learned to cope with vision loss, unlike people who go blind early in life, these older patients suffer loss of independence and need constant care. This makes it a very costly condition for society.”

Patchy blindness may mean loss of the ability to read, recognise faces or perform any taskrequiring fine vision, such as use a computer or watch TV. However research at The Vision Centre and elsewhere is coming up with new ways to restore damaged vision cells.”

The good news is that, with better perimetry, you can not only diagnose the condition better, you can also tell how well a treatment for patchy blindness is working,” he says. “That is an important step towards stabilising and restoring lost vision.”

(Source: The Vision Centre)