Dr Rosa Sancho from Alzheimer's Research UK said: "We know that diseases like Alzheimer's begin in the brain decades before symptoms like memory loss start".
For the study, scientists used Octa to compare the retinas of 39 Alzheimer's patients, 37 people with mild cognitive impairment, and 133 healthy individuals with normally functioning brains. After controlling for other factors, the researchers found this change to be statistically significant.
Because the retina is an extension of the brain and shares many similarities with the brain, researchers believe that the deterioration in the retina may mirror the changes going on in the blood vessels in the brain, thereby offering a window into the disease process.
They stressed that while they have proved that blood vessels become sparser in those with Alzheimer's, the next step is to show this happens before memory problems appear, which would give doctors a way to diagnose the condition years in advance.
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"It's possible that these changes in blood vessel density in the retina could mirror what's going on in the tiny blood vessels in the brain, perhaps before we are able to detect any changes in cognition", added Sharon Fekrat, ophthalmologist at the Duke University in the US.
"It's not possible for current techniques like a brain scan or lumbar puncture to screen the number of patients with this disease".
"But future studies need to focus on earlier stages of the disease", Isaacson said in an email.
"There is a strong effort in the research community to develop a less invasive test, such as a blood test that can yield information about Alzheimer's disease risk, " he said.
The study examined the blood flow at the back of the retina of over 200 participants using a non-invasive imaging technology known as optical coherence tomography angiography (OCTA).
In the USA alone, 5.8 million people are living with Alzheimer's dementia, according to 2019 data from the Alzheimer's Association.