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Alzheimer’s screening with flickering light exam

Flicker test could provide simple screening tool for AD

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Pippa Wysong

Posted: Monday, December 5, 2016

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An example of retinal arterial (red) and venous (blue) diameter changes in response to flickering light (530-600nm, 12.5Hz, 20s) in a young healthy person. In the elderly and in diseases with vascular involvement, retinal vessel dilation is diminished and sometimes delayed. In AD it was also delayed but much more emphasised. Image from Konstantin Kotliar PhD

A flickering light exam of the retinal vessels is showing promise as a way to help screen for Alzheimer’s disease (AD), early research suggests.Because of so-called neurovascular coupling in the retina, the application of flickering light can evoke immediate dilation of both retinal arteries and veins in healthy people.
But in the elderly as well as in eye diseases with vascular involvement such as glaucoma or age-related macula degeneration, and in systemic diseases like arterial hypertension or diabetes mellitus, retinal vessel dilation is diminished and sometimes delayed, said Konstantin Kotliar PhD, a biomedical engineer at the Aachen University of Applied Sciences, Germany.
“In people with AD, retinal arteries and veins have a delayed reaction to a flickering light test too, but they dilate surprisingly more than those in people without the disease,” he told the 2016 Alzheimer’s Association International Conference in Toronto, Canada.
He presented a study measuring and comparing retinal vessel reactions to flickering light in groups of elderly people of the same age: 15 patients with mild-to-moderate dementia due to probable AD; 24 patients with mild cognitive impairment (MCI) due to AD; and 15 healthy controls without any cognitive impairment.

MAXIMAL ARTERIAL DILATION
Retinal arterial and venous reactions to 20-second flicker stimulation of 12.5Hz were measured with the Dynamic Vessel Analyzer (IMEDOS Systems). In AD patients, the maximal arterial dilation was on average 6.6%, whereas in the MCI group it amounted to 3.8%, and in healthy controls it was 2.7%.
Dilation was delayed in AD, with 30% of the maximal dilation being reached in seven seconds, whereas in MCI and in the control group this level occurred two seconds earlier, he said.
Maximal venous dilation to flicker was notably different between the groups too – being 3.7% in the control group, 4.7% in the MCI group, and 5.4% in the AD group. This reaction occurred in AD and MCI one second later than in the control group. Generally, the reactivity of retinal vessels was a surprise to the researchers.
“The reactivity of retinal vessels was a surprise, because in other diseases of vascular origin we see an impaired reaction. We believe this fascinating finding reflects a peculiar failure of the retinal autoregulation in AD,” said Dr Kotliar.
Being able to distinguish AD in the study also had a good level of sensitivity and specificity, making the use of retinal vessel reaction a promising tool for diagnostic purposes. As well as having potential to help screen for AD, the test may also help reveal the role of vascular factors in this disease. 
More research is needed to find the reason for such an enhanced vessel reaction in AD and also in its early stage, MCI, he stressed.