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Rates of neurodegenerative diseases such as Alzheimer’s and Parkinson’s are rising dramatically.
In fact, two out of every three people older than 80 years in North America will develop Alzheimer’s disease (AD) or Parkinson’s disease (PD), at tremendous cost to individuals, their families and health care systems (Michael J. Fox Foundation). Unfortunately, the onset of symptoms (memory loss, shaking and other motor issues) leading to diagnosis likely occurs 10 to 20 years after pathological changes begin to occur in the brain.
Earlier diagnosis of these conditions is essential for better outcomes, but it requires more sensitive and convenient screening tools than we currently have. The good news is that a tremendous amount of research over the past 5 years points to several ocular biomarkers for neurodegenerative disease — and puts optometry center stage in addressing these public health crises in the future.
Signposts in the retina
OCT holds great promise for the diagnosis of neurodegenerative disease.
OCT and OCT angiography studies have demonstrated that participants with neurodegenerative diseases have a number of associated retinal findings, including changes in retinal vasculature, degeneration of retinal ganglion cells and thinning of the retinal nerve fiber layer (RNFL) (Yoon et al., Hart et al.). Increasingly, optometrists are using OCT and evaluating RNFL to screen for or follow glaucoma. It turns out that RNFL thinning is also correlated with worsening visual acuity in PD and cognitive deterioration on the Montreal Cognitive Assessment (McCoskey et al.). This suggests that AD and PD share common neurodegenerative pathways with glaucoma and that future iterations of OCT software could also provide diagnostic clues to these diseases.
New retinal imaging techniques have also been able to identify amyloid-beta and tau protein levels in the retina. These proteins comprise the brain plaques and tangles that are characteristic of AD, but are typically only visible with an autopsy.
Changes in eye movements
More than any other health care provider, optometrists are the experts in eye movements. There is a growing consensus that eye tracking offers opportunities to identify neurodegenerative diseases earlier in their clinical course. Pupil response, for example, can distinguish between normal cognition and mild cognitive impairment (MCI) (Granholm et al.). The King-Devick test, a portable, easy-to-administer test that is often used to identify reading deficits and concussions, also may be able to differentiate between MCI and more advanced dementia and between AD and PD (Galetta et al).
Another interesting area of research is the effect of these diseases on antisaccade tasks, which require inhibition of attention and eye movement to a visual stimulus. The frequency of errors in antisaccade tasks is associated with AD severity. Among people with MCI, eye tracking and, specifically, performance on antisaccade tasks, can distinguish between those at lower and higher risk for progression to AD (Wilcockson).
Taken together, all of these research developments provide hope that earlier diagnosis of neurodegenerative diseases — and, therefore, earlier treatment interventions — is possible. They may also make optometry’s role in public health even more central than it is now. As primary eye care providers, we already talk to patients about the benefits of healthy diets and exercise in preventing glaucoma and age-related macular degeneration, and we already have insight into vascular problems and diabetes from the eye exam. And now, we may be on the front lines of identifying early neurodegenerative changes.