Also known as vascular cognitive impairment, vascular dementia is characterized by impaired thinking skills caused by obstructions in the blood flow to various regions of the brain. This reduced blood flow deprives brain cells of much-needed oxygen and nutrients, which eventually leads to their death.

Many ongoing studies are exploring the relationship between poor vision and cognitive decline. Scientific interest in the topic was sparked by The Lancet Commission 2020 report on dementia prevention which identified hearing impairment, specifically hearing loss, as a modifiable risk factor for dementia. Since then, researchers have begun exploring the possibility of visual impairment also increasing the risk of cognitive impairment.

In a study published in the journal Human Brain Mapping, researchers looked at how visual impairment and retinal neurodegeneration are connected to cognitive impairment. They found that decreased visual acuity and the thinning of the ganglion cell layer were associated with global brain atrophy (decrease in size) and hippocampal atrophy. The ganglion cell layer is a layer in the retina that contains the neurons responsible for transmitting information to at least 50 different areas of the midbrain.

According to the study, neurodegeneration along the visual pathway, which leads to the disintegration of intermediate visual tracts, is behind the association between visual impairment and cognitive impairment. Although the entire process is still unclear, researchers believe that changes in the visual pathway caused by retinal neurodegeneration negatively affect cognitive function by altering the structure of brain regions that are crucial to cognition.

A similar study published in the journal Aging & Mental Health investigated whether cataracts, one of the most common causes of visual impairment in older adults, is associated with the risk of developing dementia or cognitive impairment. Meta-analysis of 11 studies involving a total of 489,211 participants revealed a significant association between cataracts and an increased risk of all-cause dementia. Subgroup analyses also showed that individuals who develop cataracts have an increased risk of Alzheimer's disease and vascular dementia.

Another study, this time by Chinese researchers, explored the relationship between cataracts and the incidence of cognitive impairment in older adults. Published in the journal Behavioral Brain Research, the meta-analysis looked at 13 studies that involved a total of 798,694 participants. The Chinese researchers found that compared to those without cataracts, participants that developed cataracts had a higher risk of all-cause dementia, Alzheimer's disease dementia, vascular dementia and mild cognitive impairment.

Meanwhile, an earlier study published in the journal JAMA Internal Medicine reported that undergoing cataract surgery could reduce the risk of dementia by 30 percent. This lowered risk, specifically of developing Alzheimer's disease dementia, persisted for over a decade after surgery, further cementing the link between cataracts and visual impairment and cognitive impairment.

However, much like the mechanisms underlying the link between visual impairment and cognitive decline, how cataract surgery lowers dementia risk remains unclear. Researchers, on the other hand, hypothesize that it may have to do with people receiving higher quality visual input and getting more blue light, which is specifically blocked by cataracts, after undergoing surgery. (Related: Extract from moringa trees found to prevent cataract formation.)

Cataracts are common among older adults. In fact, according to the National Eye Institute, more than half of American adults age 80 and above either have cataracts or have had surgery to remove them. Cataract refers to a cloudy area in the lens of the eye that obstructs vision, often caused by normal changes in your eyes as you age. Eye injury or damage from eye surgery, excessive sun exposure and health problems like diabetes can also raise your risk of developing cataracts.

In a new study published in the journal JAMA Network Open, American researchers sought to understand how vision and eye conditions are associated with an increased risk of Alzheimer's disease and related dementias (ADRDs). Specifically, they wished to clarify whether this association is causal and if vision is also a modifiable risk factor for dementia. (Related: Anti-inflammatory diet helps with cardiovascular and metabolic diseases, cuts dementia risk by 31%.)

Using data from the UK Biobank and genome-wide association studies on cataract, myopia (nearsightedness) and Alzheimer's, the researchers conducted an observational study to establish associations, followed by a 2-sample bidirectional mendelian randomization (MR) study which involved genetic analysis to determine causal relationships.

The analyzed data came from individuals aged 55 to 70 years who were dementia-free when they began to participate in the UK Biobank study. Of the 304,?953 selected participants, 14,295 had cataracts and 2,754 had worse than 20/40 vision. The researchers found that having cataracts and myopia was associated with a higher risk of ADRDs.

MR analyses further revealed that cataracts were associated with an increased risk of vascular dementia as well as lower total gray matter volume and white matter hyperintensities, which are believed to be predictive of stroke, dementia and death. MR analyses also estimated that cataracts are "associated with a 92 percent increase in the odds of vascular dementia risk." On the other hand, while poor visual acuity was linked to increased dementia risk, myopia was not associated with dementia in the MR analyses.

The researchers also conducted genetic analysis to determine potential reverse causality. They found that Alzheimer's disease is not associated with cataract formation. Based on their findings, the researchers concluded that "cataracts increase the risk of dementia through vascular and non-AD [Alzheimer's disease] mechanisms," meaning cataracts contribute to the development of Alzheimer's through pathways not commonly linked to the disease.

Because cataracts were also associated with reduced total brain and gray matter volumes, the researchers suspect that these pathways may have something to do with brain shrinkage or damage to the brain's small blood vessels. These suspicions are bolstered by the appearance of bright spots on the MRI scans of participants with cataracts, which suggests white matter lesions - clear markers of small vessel disease.

While the researchers were unable to explain how cataracts raise the risk of vascular dementia, their findings nevertheless support the hypothesis that treating or preventing cataracts could reduce a person's dementia risk. (Related: Prevent age-related cataracts by increasing your intake of vitamins and carotenoids.)

As with many health issues, adopting healthy lifestyle changes is the best strategy you can employ to reduce your risk of developing cataracts. Here are some tips for preventing cataract formation later in life: (h/to to VeryWellHealth.com)

Dementia can be prevented by ensuring good nutrition, living a healthy, active lifestyle and maintaining good eye health. Read more stories about how to keep your brain healthy at Brain.news.

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