Alzheimer's disease often lacks distinctive warning signs early on, yet new, illuminating research shows how noninvasive ocular imaging could reveal what's unseen right in front of—and inside—our eyes.
Presented at this summer's Alzheimer's Association International Conference (AAIC) 2016 in Toronto, a study that used the eyes as a proxy for neural health was internationally lauded for its use of low-cost, in-vivo imaging to potentially detect early cognitive decline.
The ramifications of such findings could prove invaluable for families and caretakers as early detection and diagnosis regrettably can't stem Alzheimer's progression, but it can ensure another crucial component: time.
Alzheimer's, an irreversible and progressive brain disorder, currently affects 5.4 million Americans with estimates projecting 13.8 million by 2050. Onset is often gradual, making early diagnosis difficult; however, this early warning critically allows families time to plan ahead. Such complications make this new research a vitally intriguing part of the conversation during November's National Alzheimer's Disease Awareness Month, especially as it highlights optometry as a profession well-positioned to help.
Retinal nerves and cognition
Researchers at Moorfields Eye Hospital in London uncovered a link between cognitive ability and the thickness of subjects' retinal nerve fiber layer (RNFL) using a piece of technology quite familiar to optometry—Spectral Domain Optical Coherence Tomography (SD-OCT).
Examining more than 33,000 participants using SD-OCT, physical examinations, cognitive assessments and a self-questionnaire, researchers determined not only a significant correlation between thinner macular RNFL and poor cognition, but also specific gradations that pinpoint decline. For instance, in the prospective memory test, mean RNFL thickness measured 53.3 micrometers among those who correctly recalled on first attempt, while that fell to 52.5 micrometers for subjects needing two attempts and 51.9 micrometers for those who could not recall.
Thinner RNFL also correlated with poorer pairs matching, reasoning and reaction time, and appeared additive for each additional cognitive test failed, according to an Alzheimer's Association news release.
"Our findings show a clear association between thinner macular RNFL and poor cognition in the study population," said study author Fang Ko, M.D., of Moorfields Eye Hospital. "This demonstrates the potential utility of the eye as a noninvasive measure of neuronal loss, which is linked to cognitive performance, and provides a possible new biomarker for studies of neurodegeneration."
Amyloid biomarkers in the retina
Other findings published earlier this summer detail how in-vivo imaging of the retina can detect another calling card of Alzheimer's and cognitive decline, namely amyloid.
Although scientists don't fully understand what causes Alzheimer's, it is known that the buildup of amyloid plaques and neurofibrillary (tau) tangles progressively damage and kill neurons, causing the loss of brain tissue. Currently, positron emission tomography (PET) brain scans can measure the presence of amyloid buildup to determine Alzheimer's progression; however, this testing is expensive, and still it's only post-mortem that beta amyloid can effectively be measured for a definitive Alzheimer's diagnosis. Given these constraints, researchers at the University of Minnesota turned to the eyes.
Published in the journal Investigative Ophthalmology and Visual Science in June, the study used hyperspectral imaging to examine mice model retinas for changes associated with amyloid deposits. What researchers determined was a reduction in the light reflection off the retina correlated with Alzheimer's progression. The study's authors expect the technology to prove suitable for human trials, and such efforts are currently underway.
How optometry can make a difference
A window to the soul, the eye is a direct extension of the brain, as this research indicates. Doctors of optometry are afforded a rare glimpse into the brain directly without scanning or cutting into the body, and technology such as OCT—already employed in doctors' offices—paints a vivid picture that could be crucial to detection and diagnosis.
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