How the Brain's Waste Clearance System Influences Dementia Risk

Dysfunction in the brain’s waste clearance system may play a central role in the development of dementia, offering new insights into why factors such as poor sleep and cardiovascular issues, including high blood pressure, are linked to an increased risk of cognitive decline (1^✔ ✔Trusted Source
MRI markers of cerebrospinal fluid dynamics predict dementia and mediate the impact of cardiovascular risk

Go to source). A recent study led by researchers at the University of Cambridge investigated the movement of cerebrospinal fluid (CSF)—the clear liquid responsible for cushioning the brain and removing metabolic waste—and found that impaired CSF flow strongly predicted the likelihood of developing dementia later in life.

Impaired Cerebrospinal Fluid (CSF) Flow Strongly Predicts Dementia Risk

The research analyzed data from 40,000 adults enrolled in the UK Biobank, providing one of the largest population-based assessments of CSF dynamics and dementia risk. The study’s findings are published in Alzheimer's & Dementia: The Journal of the Alzheimer's Association, highlighting the importance of brain fluid dynamics in long-term cognitive health.

In the healthy brain, the so-called glymphatic system serves to clear out toxins and waste materials, keeping the brain healthy. Only discovered as recently as 2012, this system functions by flushing CSF through the brain along tiny channels around blood vessels known as perivascular spaces. It collects waste then drains out of the brain, helping keep it clean and healthy.

The glymphatic system is thought to be important in protecting against many of the common forms of dementia, which are often characterized by the build-up of toxic substances in the brain – for example, Alzheimer's disease sees amyloid ‘plaques’ and tau ‘tangles accumulate in brain tissue.

One of the most common forms of dementia is vascular dementia, caused by reduced blood flow to the brain. The most common cause of this type of dementia is cerebral small vessel disease, which affects the small blood vessels in the brain. But the impact of cerebral small vessel disease is even greater because it also interacts with other dementias making them worse; for example, a study of nuns in the US found that among those nuns whose brains showed signs of Alzheimer's disease post mortem, only around a half exhibited symptoms of dementia – but this increased to around nine in 10 if they also had cerebral small vessel disease.

Cerebral Small Vessel Disease: A Critical Accelerator of Dementia and Alzheimer's

Professor Hugh Markus and colleagues at the University of Cambridge wanted to see whether cerebral small vessel disease and other cardiovascular risk factors damage the glymphatic system – and whether this in turn increases the risk of dementia.

Until recently, it has only been possible to study glymphatic function in mice, but recent advances in MRI scanning have made it possible to study it indirectly in humans. Even so, it was only possible to do this practically in relatively small numbers, but Yutong Chen, while a medical student at the University of Cambridge, developed machine learning algorithms capable of assessing glymphatic functions from MRI scans at scale.

The team applied the algorithm to MRI scans taken from around 40,000 adults in UK Biobank. They found three biomarkers – biological signatures – associated with impaired glymphatic function assessed at baseline, predicted the risk of dementia occurring over the subsequent decade. One of these was DTI-ALPS, a measure of the diffusion of water molecules along the perivascular spaces. Another was the size of the choroid plexus, where the CSF is produced. The third measure reflected the flow velocity of CSF into the brain.

Yutong Chen, from the Department of Clinical Neurosciences at Cambridge, said: “Although we have to be cautious about indirect markers, our work provides good evidence in a very large cohort that disruption of the glymphatic system plays a role in dementia. This is exciting because it allows to ask: how can we improve this?”

Further analysis showed that several cardiovascular risk factors impaired glymphatic function – and hence increased dementia risk, and that this was partly via causing cerebral small vessel disease, which is visible in the MRI scans.

First author Hui Hong, now a radiologist at the Second Affiliated Hospital of Zhejiang University, Hangzhou, China, said: “We already have evidence that small vessel disease in the brain accelerates diseases like Alzheimer's, and now we have a likely explanation why. Disruption to the glymphatic system is likely to impair our ability to clear the brain of the amyloid and tau that causes Alzheimer's disease.”

The research suggests possible approaches for reducing dementia risk. One is to look at strategies for improving glymphatic function. Sleep plays an important role in glymphatic function, and so disrupted sleep patterns are likely to impair its ability to clear toxins. Alternatively, there may be existing medicines that could be repurposed, or new ones that could be developed, to improve glymphatic function.

Therapeutic Avenues: Improving Sleep and Repurposing Drugs to Boost Glymphatic Function

Another possible approach is to treat vascular risk factors such as high blood pressure. This is supported by recent studies: the SPRINT MIND trial, for example, showed that intensive blood pressure control (maintaining a systolic blood pressure of less than 120 mm Hg) led to a 20% reduction in cognitive decline or dementia compared to participants in the standard treatment group.

Professor Markus, who leads the Stroke Research Group at the University of Cambridge and is a Fellow of Clare Hall, Cambridge, said: “We already know the importance of cardiovascular risk factors when it comes to dementia, and our findings further emphasize this link.

“At least a quarter of all dementia risk is accounted for by common risk factors like blood pressure and smoking. If these impair glymphatic function, then we can intervene. Treating high blood pressure or encouraging people to stop smoking would be an achievable way to helping the glymphatic system work better.”

Professor Bryan Williams, Chief Scientific and Medical Officer at the British Heart Foundation, said: “This study offers us a fascinating glimpse into how problems with the brain's waste clearance system could be quietly increasing the chances of developing dementia later in life. By improving our understanding of the glymphatic system, this study opens exciting new avenues for research to treat and prevent dementia. It also emphasizes the importance of managing known cardiovascular risk factors, such as high blood pressure, for reducing dementia risk.”

Reference:

1. MRI markers of cerebrospinal fluid dynamics predict dementia and mediate the impact of cardiovascular risk - (https://alz-journals.onlinelibrary.wiley.com/doi/10.1002/alz.70699)

Source-Eurekalert