Reduced blood flow in the retina may be a new way to identify early signs of Alzheimer’s disease.
One of the most common forms of dementia is Alzheimer’s disease affecting up to70% of people with dementia. We have known about it since the ancient Greeks but it was first classified in the early 1900s and it is mainly found in adults over the age of 65. The damage to the brain results in impaired behaviour, memory, language and the ability to think clearly. The disease can affect people as early as their forties but this is rare.
What causes Alzheimers?
The causes of the disease are multiple and poorly understood, although with newer technology we are getting a better understanding of the condition. Two different types of medications are used to manage the condition, although a 2012 review found that their benefit is small and, at best, they may temporarily delay the progression of the disease.
Why early detection is important.
Significant brain damage from Alzheimer’s-related plaques can occur up to 20 years before any symptoms appear. If we could detect the disease before it showed clinical signs we may be able to understand the mechanism by which it works and create better medications for its management.
Doctors currently use PET scans and lumbar punctures to accurately diagnose Alzheimer’s, but they are expensive and invasive. It has been known that there are changes that occur in the brain in the small blood vessels in people with Alzheimer’s disease, and because the retina is an extension of the brain, it seemed logical to investigate this further.
Eyes tell a story
In previous studies, researchers examining the eyes of people who had died from Alzheimer’s have reported that the eyes of such patients showed signs of thinning in the centre of the retina and degradation of the optic nerve. In the new studies, the researchers used a non-invasive technique, called optical coherence tomography angiography (OCT-A), to examine the retinas in eyes of participants with and without clinical symptoms of Alzheimer’s.
OCT-A first appeared in 2014 and is used for imaging the small blood vessels in the retina and choroid in conditions such as macula degeneration or macula oedema due to diabetes. It works by using a laser light which reflects of the surface of moving red blood cells and therefore eliminates the need to use dyes that are injected into the patient. The retina is scanned multiple times and allows the doctor to detect zones of low and high blood flow.
In the patients whose PET scans and cerebrospinal fluid showed preclinical Alzheimer’s, the area at the centre of the retina without blood vessels was significantly larger, suggesting less blood flow. Whereas, in the retinas of patients without the disease the blood flow was normal.
Hope for the future
Many of these instruments are currently found in ophthalmologists and are starting to appear in optometric practices. So it shouldn’t be too long before an eye test may help detect this debilitating condition much earlier and provide earlier treatment. In the future, it is hoped that this technique may assist is determining the cause of the disease.
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Stephen P. Yoon, Dilraj S. Grewal, Atalie C. Thompson, Bryce W. Polascik, Cynthia Dunn, James R. Burke, Sharon Fekrat. Retinal Microvascular and Neurodegenerative Changes in Alzheimer’s Disease and Mild Cognitive Impairment Compared with Control Participants. Ophthalmol Retina. 2019 Jun;3(6):489-499.
Yi Stephanie Zhang, Nina Zhou, Brianna Marie Knoll, Sahej Samra, Mallory R. Ward, Sandra Weintraub, Amani A. Fawzi. Parafoveal vessel loss and correlation between peripapillary vessel density and cognitive performance in amnestic mild cognitive impairment and early Alzheimer’s Disease on optical coherence tomography angiography. PLOS ONE, 2019; 14 (4)
By Arthur Stevens, Optometrist and Director of Kosmac & Clemens Optometrists