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The Strange Connection Between Vision Loss and Alzheimer’s

Alzheimer's disease, a severe form of dementia, affects an estimated 5.2 million Americans;1 200,000 of which are under the age of 65. According to recent data, Alzheimer's disease kills more than half a million Americans per year,2 making Alzheimer's the third leading killer in the U.S., right behind heart disease and cancer.

Since there's no conventional cure, it's really important to take prevention seriously. There are also few if any successful medical treatments available once Alzheimer's sets in. For example, memantine (sold under the brand name Namenda) is approved for moderate to severe cases of Alzheimer's, but doctors also prescribe it off-label for mild cases. Unfortunately, the drug has been found to be practically useless for mild to moderate Alzheimer's.3

Other go-to drugs for the treatment of Alzheimer's include cholinesterase inhibitor drugs such as Aricept, Exelon and Reminyl. These too may do more harm than good as they provoke slower heart rates, significantly increasing your chances of getting a permanent pacemaker. They also raise your risk of hip fracture.

Surprising Link Between Vision Loss and Alzheimer's

Interestingly, recent research4 shows loss of vision is associated with a higher risk for certain subtypes of Alzheimer's. As reported by Reuters:5

"Based on data from two large studies of older Americans … [t]he research team found that having distance vision worse than 20/40 and even the perception of having bothersome vision problems were associated with almost threefold higher odds of cognitive impairment.

Near-vision problems were less associated with higher odds of dementia or cognitive impairment … Regular vision screening of older Americans could help to catch people at greater risk of cognitive problems and dementia, the study team writes …"

This isn't the first time this link has been made. Two papers6,7 published last year also concluded that ocular changes could be used as biomarkers for Alzheimer's, suggesting noninvasive retinal imaging and visual testing could help with earlier diagnosis. In the first of these papers, the authors present a hypothesis for the link between declining vision and dementia:

"Alzheimer's disease … has multiple cognitive subtypes. These are usually broken down into memory, language, executive, attention and visuospatial functioning. The variant of AD in which visual symptoms are prominent due to the localized pathology in the parieto-occipital region is often referred to as visual variant Alzheimer's disease …

The interconnection between eye and brain suggests that it is reasonable to look for ocular manifestations of neurodegenerative disease and regard the eye as an extension of the [central nervous system] CNS. In embryological development, the eyes and brain have a similar origin. The eyes are formed from the anterior neural tube, an area that later gives rise to the forebrain.

Ocular development occurs through specification of the eye field post-neural induction. This process involves specific transcription factors that are also conserved in brain development. One such factor, a 'master regulator' gene of the development of the eye field, Pax6, plays an essential role in neural development. When expressed ectopically, Pax6 can induce ocular formation in other parts of the body, whereas its impairment or knockout disrupts neurogenesis in the cortex."

The Role of Amyloid Beta in Vision Loss and Dementia

Amyloid beta also plays an important role in both conditions. One of the hallmarks of Alzheimer's disease is the buildup of amyloid beta protein in the brain. The subsequent formation of brain plaque leads to progressive decline in cognitive and social functioning. Research has also linked amyloid beta deposition to neurodegeneration in the retina.8

For example, amyloid beta has been found in retinal drusen9 (yellow-colored fatty protein deposits beneath the retina) and is a hallmark of age-related macular degeneration (AMD), the most common cause of blindness among the elderly. As noted in a 2015 study:10"Multiple studies now link amyloid beta with key stages of AMD progression, which is both exciting and potentially insightful, as this identifies a well-established toxic agent that aggressively targets cells in degenerative brains."

Peripheral drusen has also been linked to a higher risk for Alzheimer's.11 In short, researchers suggest that by analyzing the presence of amyloid in the eye, one may be able to predict amyloid buildup in the brain with a fair degree of accuracy. As noted in the second paper published last year:12

"Kerbage et al. suggested using a florescent amyloid binding ligand in order to maximize the chances of detecting Aβ [amyloid beta] in the lens.

By compounding the ligand substance (aftobetin hydrochloride) into a sterile ophthalmic ointment suitable for topical application; in combination with an in vivo pulsed laser fluorescent spectroscopy in a group of 20 AD [Alzheimer's disease] and 20 healthy controls the authors were able to detect supranuclear amyloid in the lens of most AD patients with a sensitivity of 85 percent and specificity of 95 percent …

The authors found a correlation between fluorescence uptake values in the lens with amyloid burden in the brain detected quantified using PET."

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