Can restoring the brain's waste-clearance system improve brain function in Alzheimer’s and some autism?

 

One of the most exciting developments in modern neuroscience is the growing understanding of the glymphatic system—the brain's own waste-clearance system.

This network circulates cerebrospinal fluid (CSF) through the brain, removing metabolic waste products and helping maintain a healthy neuronal environment. Remarkably, the glymphatic system is most active during deep (slow-wave) sleep, suggesting that one of sleep's most important functions is to clean the brain.

Interest in this system has exploded over the past decade because impaired glymphatic function has now been implicated in Alzheimer's disease, Parkinson's disease, traumatic brain injury, stroke and several other neurological disorders.

Increasingly, researchers are also asking whether glymphatic dysfunction contributes to autism spectrum disorder (ASD).

 

A remarkably recent discovery

One of the most surprising aspects of this story is just how new it is.

The glymphatic system was only discovered in 2012 by Professor Maiken Nedergaard and colleagues. Until then, neuroscientists knew that the brain produced metabolic waste, but nobody really understood how it was removed. Unlike the rest of the body, the brain appeared to have no conventional lymphatic system.

The discovery of the glymphatic system transformed our understanding of brain biology. Researchers showed that cerebrospinal fluid flows alongside arteries into the brain, exchanges with the fluid surrounding brain cells, and then carries away waste products before leaving alongside veins.

Only three years later, in 2015, another major breakthrough followed when researchers discovered meningeal lymphatic vessels surrounding the brain. These vessels drain fluid from the glymphatic system into the body's lymphatic system.

Together, these discoveries revealed that the brain possesses its own sophisticated waste-disposal network.

Considering that this entire field is little more than a decade old, it is remarkable how quickly it has expanded. Today, impaired glymphatic function has been linked to Alzheimer's disease, Parkinson's disease, multiple sclerosis, traumatic brain injury, stroke, depression, schizophrenia and, increasingly, autism.

 

Autism and the glymphatic system

Until recently, the idea that glymphatic dysfunction might contribute to autism was based largely on indirect observations.

Researchers had reported:

• enlarged extra-axial cerebrospinal fluid in some infants who later develop autism,
• enlarged perivascular spaces,
• chronic neuroinflammation,
• and the very high prevalence of sleep disorders in autism.

Now, neuroimaging studies are beginning to provide more direct evidence.

A newly published MRI study examined 78 children with autism and 48 typically developing controls using the DTI-ALPS technique, a non-invasive MRI method that estimates glymphatic activity. The investigators found significantly reduced glymphatic function in the autism group. Even more interestingly, poorer glymphatic function correlated with more severe communication difficulties and poorer visual-motor integration. Statistical modelling suggested that impaired visual-motor integration partly explained the relationship between glymphatic dysfunction and communication deficits.

Glymphatic system dysfunction in children with autism spectrum disorder as evidenced by the diffusion tensor imaging along perivascular spaces index

The authors concluded that impaired glymphatic clearance may contribute to the pathophysiology of autism and represents a promising area for future mechanistic and interventional research.

These studies do not prove that impaired glymphatic clearance causes autism. However, they do suggest that it may contribute to the biology of at least a subgroup of autistic individuals.

 

Why deep sleep matters

Perhaps the most important aspect of the glymphatic system is that it works best during deep slow-wave sleep.

During deep sleep:

• cerebrospinal fluid flows more efficiently through the brain,
• the space between brain cells expands,
• metabolic waste products are removed,
• inflammatory molecules are cleared,
• and the brain performs what is essentially its nightly housekeeping.

Sleep disturbances affect around 50–80% of autistic children, often continuing into adulthood. If glymphatic clearance depends on deep sleep, this raises the possibility that chronic sleep disruption contributes to impaired waste clearance, neuroinflammation and altered brain function, creating a vicious cycle.

This may be one reason why improving sleep often has benefits that extend well beyond simply reducing daytime fatigue.

 

Aquaporin-4: the brain's plumbing protein

The glymphatic system depends on a protein called aquaporin-4 (AQP4), which forms tiny water channels in the end-feet of astrocytes—support cells that surround the brain's blood vessels.

These channels allow cerebrospinal fluid to move efficiently between blood vessels and brain tissue, helping wash away metabolic waste products. In animal experiments, deleting AQP4 dramatically reduces glymphatic clearance, highlighting its central role in the brain's waste-disposal system.

Interestingly, several studies have reported altered AQP4 expression or localisation in autism. Although it is still unclear whether these changes are a cause or a consequence of autism, they provide another possible explanation for why glymphatic function may be impaired in at least some autistic individuals.

 

Exercise may also support the brain's cleaning system

Exercise is another intervention that may enhance glymphatic function.

Experimental studies suggest that regular physical activity can:

• improve vascular health,
• improve sleep quality,
• enhance cerebrospinal fluid dynamics,
• reduce neuroinflammation,
• and potentially improve glymphatic clearance.

Even if future studies show only modest effects on glymphatic function, exercise already has well-established benefits for cardiovascular health, cognition, mood and healthy ageing.

 

 

 A fascinating new therapeutic idea

This brings us to one of the most intriguing studies published in the last 12 months.

The influence of intermittent hypercapnia on cerebrospinal fluid flow and clearance in Parkinson’s disease and healthy older adults

Researchers investigated whether they could artificially stimulate the glymphatic system by having volunteers breathe alternating short periods of air containing 5% carbon dioxide, followed by normal air.

Rather than continuously increasing carbon dioxide, the intermittent exposure generated rhythmic expansion and contraction of cerebral blood vessels.

Brain imaging demonstrated increased cerebrospinal fluid movement, while blood tests suggested increased movement of brain-derived proteins into the circulation, consistent with enhanced glymphatic clearance.

Importantly, this was not an autism or Alzheimer's treatment study. It was a proof-of-concept experiment involving healthy older adults and people with Parkinson's disease.

Nevertheless, it demonstrated something remarkable:

The human glymphatic system appears to be modifiable.

Mimicking what happens during deep sleep

Perhaps the most fascinating aspect of the study is the proposed mechanism.

The intermittent carbon dioxide exposure generated slow vascular oscillations remarkably similar to those naturally seen during deep sleep, when glymphatic clearance reaches its maximum.

In effect, the researchers may have temporarily reproduced one of the physiological mechanisms by which deep sleep naturally cleans the brain.

That opens an entirely new therapeutic concept.

Rather than trying to remove individual proteins such as amyloid with drugs, perhaps we can improve the brain's own housekeeping system.

 

How the intermittent CO₂ therapy was performed

The researchers did not expose participants to continuous carbon dioxide. Instead, they used a carefully controlled pattern of intermittent increase in the level of carbon dioxide (CO₂) in the blood (hypercapnia). The hypercapnia was designed to create rhythmic changes in cerebral blood flow.

Interestingly, several studies have reported abnormalities in cerebral blood flow regulation in autism, including reduced blood flow in specific brain regions and altered responses of cerebral blood vessels to changing carbon dioxide levels. Since the glymphatic system depends on rhythmic vascular pulsations to drive cerebrospinal fluid through the brain, impaired vascular regulation could potentially contribute to reduced glymphatic clearance.

Participants underwent three treatment sessions, each lasting approximately 10 minutes, for a total treatment time of about 30 minutes.

During each session they repeatedly alternated between:

• 35 seconds breathing a gas mixture containing 5% carbon dioxide
• 35 seconds breathing normal air

This cycle was repeated 24 times across the three sessions. The sessions were performed during a single visit, with short breaks between them.

The rationale was that the repeated dilation and relaxation of cerebral blood vessels would generate slow vascular oscillations similar to those that naturally occur during deep slow-wave sleep, when the brain's glymphatic system is most active.

MRI scans demonstrated increased cerebrospinal fluid movement through the brain, while blood tests showed increased concentrations of several brain-derived proteins—including amyloid-β, tau, GFAP and neurofilament light—in the circulation. The researchers interpreted these findings as evidence of enhanced glymphatic clearance rather than brain injury.

 

A new way of thinking

Much autism research has focused on inflammation, oxidative stress, mitochondrial dysfunction and altered neurotransmission.

But perhaps, in some autistic individuals, these abnormalities are partly downstream consequences of impaired brain waste clearance.

Instead of asking:

Which drug treats autism?

perhaps we should also ask:

Can we restore the brain's own housekeeping system?

That strategy might include:

• protecting deep sleep
• treating sleep disorders
• exercising regularly
• maintaining good cardiovascular health
• and perhaps one day therapies that directly stimulate glymphatic flow

 

Looking ahead

This remains a hypothesis.

Neither the imaging studies nor the intermittent hypercapnia study prove that improving glymphatic function will improve autism symptoms.

However, taken together they suggest something genuinely exciting.

For the first time we now have:

• evidence that glymphatic dysfunction is present in autism,
• evidence that poorer glymphatic function is associated with more severe communication difficulties,
• and evidence that the human glymphatic system may be therapeutically stimulated.

That does not yet constitute a treatment.

But it does provide the foundations for an entirely new direction of autism research—one aimed not at treating individual downstream abnormalities, but at restoring one of the brain's most fundamental maintenance systems.

If this hypothesis proves correct, the simplest interventions may remain among the most important: protect deep sleep, exercise regularly, maintain good cardiovascular health, and treat sleep disorders whenever possible. Future research will determine whether directly stimulating glymphatic flow can add to these fundamental approaches.

While a variety of therapies are marketed as improving lymphatic drainage, there is currently no convincing evidence that they enhance glymphatic clearance in humans.