How the Eyes Might Be Windows to the Risk of Alzheimer’s Disease
Researchers say how quickly a person’s pupil dilates while taking cognitive tests
Scientists at the University of California, San Diego School of Medicine suggest that Alzheimer's disease could be detected years before the first symptoms appear by examining pupil dilation.
Measure pupils to identify Alzheimer's disease
This new study shows that measuring the rate of pupil dilation while a person is taking a cognitive test could help identify early and presymptomatic stages of Alzheimer's disease.
The eye is becoming a profound window into the health of a person's brain through a number of recent studies of eye biomarkers correlated with cognitive decline and neurodegeneration. A fascinating study conducted in 2018 revealed a distinct correlation between several degenerative eye diseases and the onset of Alzheimer's disease, suggesting that eye health can be an effective indicator of the health of many areas of the brain.
Other research has revealed fascinating links between thinning a layer of the retina, a decrease in small blood vessels from the retina to the back of the eye and the very early stages of Alzheimer's disease. Another study suggested that beta-amyloid proteins, toxic compounds associated with damaged brain cells, could be detected at the back of the eye.
None of these tests have yet been used in clinics, as they would all require complicated, or even expensive, imaging technologies to effectively screen patients. But a new study describes an innovative, low-cost, non-invasive method that could theoretically be implemented fairly quickly in clinics around the world.
Tau proteins develop in the cœruleus locus
This research is based on the observation that tau proteins, another protein associated with the cognitive decline of Alzheimer's disease, tend to develop initially and damage an area of the brain called locus coeruleus (LC).
Previous research had shown that LC largely induced responses to pupillary dilation. It was therefore intended to determine whether early accumulations of tau in the LC could modify pupil dilation to allow the identification of presymptomatic Alzheimer's disease.
A previously published study by the current research team showed that adults with mild cognitive impairment had greater pupil dilation in a cognitive test than a healthy adult. Importantly, these differences in pupil dilation appeared even when the two groups obtained the same results on cognitive tests.
In the research team's latest study, a large number of healthy middle-aged adults were tested. Their pupillary responses were then measured against the genetic risk scores for Alzheimer's disease to determine whether this test could identify individuals genetically at risk years before symptoms appeared.
The results confirmed an association between pupillary responses to cognitive tasks and genetic risk scores for Alzheimer's disease, despite the fact that all subjects still had normal cognition.
Proof that pupillary measurement is effective
"Given the evidence linking LC tau protein and the association between pupillary response and polygenic risk scores for AD, these results are compelling evidence that measuring pupillary response is effective. Cognitive tasks could be another screening tool to detect Alzheimer's disease before symptoms appear," says Willam Kremen, the first author of this new study.
As Kremen notes, this test is still at the proof of concept stage. More work is needed to verify these results in larger and more diverse cohorts.
