This is a mechanism module, not a specific disease. Disorder entries reference individual nodes via conforms_to (e.g., "glymphatic_dysfunction#Impaired Perivascular CSF-ISF Exchange and Solute Clearance", the key conformance target and rate-limiting step). Candidate conformers span Alzheimer disease and the other neurodegenerative proteinopathies, traumatic brain injury, cerebral small vessel disease and stroke, idiopathic normal pressure hydrocephalus, and the sleep disorders that act as an upstream trigger. It is deliberately distinct from — and upstream of — the amyloidogenesis module, which models fibril nucleation and deposition once precursor concentration is permissive; this module models the *clearance* arm that sets that concentration. It is also distinct from loss_of_proteostasis, which covers intracellular chaperone/degradation capacity rather than extracellular perivascular bulk clearance.
IMPORTANT scope and epistemic caveats, which conforming entries must not quietly discard. (1) The microscopic mode of parenchymal solute transport is genuinely contested: whether movement through the parenchymal extracellular space is AQP4-dependent convective bulk flow or ordinary size-dependent diffusion is unresolved, and the two readings are curated here as competing mechanistic_hypotheses (convective_glymphatic_transport vs diffusive_parenchymal_transport) rather than being silently collapsed into the canonical model. What is not contested is that periarterial CSF influx is higher in wild-type than in Aqp4-null rodents and that interstitial solute clearance declines with age, injury, and dementia. (2) Most mechanistic evidence is rodent, frequently anesthetized, and acquired with invasive tracer delivery — all three of which are themselves determinants of measured glymphatic influx, so the human-model mismatch is recorded as an explicit HUMAN_MODEL_MISMATCH discussion. (3) The non-invasive human imaging surrogates (notably DTI-ALPS) are not validated against a gold-standard clearance measurement; a low ALPS-index should not be curated as a direct measurement of glymphatic dysfunction. Conforming disorder nodes that rest only on an imaging-surrogate association should say so.
AQP4-Dependent Convective (Bulk-Flow) Glymphatic Transport Model
convective_glymphatic_transport
CANONICAL
Evidence: 2
Evidence balance
2 support
The canonical reading: CSF is driven by arterial pulsation and vasomotion into periarterial spaces, moves convectively (bulk flow) through the parenchymal extracellular space in an AQP4-dependent manner, and exits along paravenous routes, carrying interstitial solutes with it. Under this model the astroglial end-foot AQP4 channel is a necessary conduit, so loss of perivascular AQP4 polarization is mechanistically — not merely correlatively — responsible for impaired clearance.
Note the scope boundary between the two hypotheses: the AQP4 dependence of periarterial CSF *influx* is the better-supported claim, whereas the convective nature of transport *within the parenchymal extracellular space* is what the alternative hypothesis disputes.
Diffusive, AQP4-Independent Parenchymal Transport Model
diffusive_parenchymal_transport
ALTERNATIVE
Evidence: 2
Evidence balance
1 support
1 refute
The principal alternative: convective flow is real in the subarachnoid and perivascular compartments, but movement of solutes through the parenchymal extracellular space itself is ordinary size-dependent diffusion rather than directed bulk flow, and does not require AQP4. Under this reading the clearance deficits observed in ageing, injury, and dementia are real, but are attributable to perivascular-space and extracellular-space geometry and to the rate of perivascular convection, not to an AQP4-gated parenchymal convective conduit. This matters for curation because it changes what an AQP4 finding in a conforming disorder entry may be taken to mean.
Retained as ALTERNATIVE rather than DEPRECATED. The rebuttal in PMID:30561329 addresses the AQP4-dependence experiments specifically; the separate size-dependence and post-arrest photobleaching results bearing on convection within the parenchyma have not been overturned in the same way, and the seed review (PMID:42419635) still lists bulk flow versus diffusion as an open controversy in 2026.
Is solute movement through the parenchymal extracellular space in the glymphatic pathway convective (AQP4-dependent bulk flow) or diffusive, and what fraction of measured interstitial clearance does each contribute?
CONTROVERSY
OPEN
controversy_glymphatic_bulk_flow_versus_diffusion
Attached to:
Perivascular AQP4 Depolarization and Reduced Periarterial CSF Influx
Impaired Perivascular CSF-ISF Exchange and Solute Clearance
This is the central unresolved mechanistic question about the pathway and it is not cosmetic. If parenchymal transport is predominantly diffusive, then an AQP4 mislocalization finding in a conforming disorder entry is a marker of astroglial reactivity rather than a lesion in the clearance conduit, and therapeutic strategies aimed at restoring AQP4 polarization lose their rationale. Curators should keep the two readings separated as mechanistic_hypotheses on the affected edge rather than asserting the canonical model as settled.
Proposed experiments:
Anaesthesia- and delivery-controlled discrimination of convective versus diffusive parenchymal transport
Which features of glymphatic transport measured in anaesthetized rodents with invasive tracer delivery hold in the awake human brain, given that anaesthetic regimen, age, and delivery route are themselves determinants of measured glymphatic influx?
HUMAN MODEL MISMATCH
OPEN
gap_glymphatic_human_model_translatability
Attached to:
Sleep Disruption and Loss of Sleep-Dependent Glymphatic Drive
Perivascular AQP4 Depolarization and Reduced Periarterial CSF Influx
Impaired Perivascular CSF-ISF Exchange and Solute Clearance
Evidence for this module exists in models, but its translational validity is the open question — the defining condition for HUMAN_MODEL_MISMATCH rather than KNOWLEDGE_GAP. Three specific mismatches apply. Anaesthesia is not a neutral substitute for sleep: measured glymphatic influx varies several-fold across anaesthetic regimens, so a result obtained under ketamine/xylazine may not describe natural sleep, let alone human sleep. Invasive tracer delivery itself suppresses glymphatic function, so intraparenchymal-injection studies may under-report transport. And the rodent brain differs from the human brain in scale, perivascular-space geometry, and transport distances, which is exactly the species-specific difference the seed review flags. Human confirmation to date rests on intrathecal-contrast MRI in small clinical cohorts and on non-invasive imaging surrogates.
Proposed experiments:
Awake, non-invasive cross-species comparison of glymphatic transport
Do the non-invasive human imaging surrogates for glymphatic function — principally DTI-ALPS, but also dynamic contrast-enhanced and diffusion MRI — measure glymphatic clearance, and against what gold standard have they been validated?
KNOWLEDGE GAP
OPEN
gap_glymphatic_imaging_biomarker_validation
Attached to:
Impaired Perivascular CSF-ISF Exchange and Solute Clearance
Nearly all human disease associations attributed to glymphatic dysfunction rest on imaging surrogates rather than on measured clearance, so the validity of those surrogates gates how much of the human literature this module may absorb. The ALPS-index in particular is an index of directional water diffusivity at the lateral ventricular body level; treating a low ALPS-index as a direct measurement of glymphatic dysfunction over-claims. Curators adding a conforming disorder node supported only by an imaging surrogate should record the surrogate as such in the evidence explanation rather than asserting measured clearance failure.
Proposed experiments:
Head-to-head validation of imaging surrogates against intrathecal tracer clearance
Sleep Disruption and Loss of Sleep-Dependent Glymphatic Drive
trigger
The shared initiating condition is loss of the sleep-associated brain state that normally drives perivascular fluid transport. During natural sleep and ketamine/xylazine anaesthesia the cortical interstitial space expands by roughly 60%, noradrenergic tone falls, and cortical slow (delta) activity rises; each of these covaries with CSF tracer influx. In humans, NREM slow waves are followed by hemodynamic oscillations that are in turn coupled to macroscopic CSF flow. Chronic sleep fragmentation, sleep deprivation, sleep apnoea, and the sleep-architecture changes of ageing remove this drive. Conforming disorder nodes substitute the disorder-specific route to lost glymphatic drive (primary sleep disorder, ageing, critical illness, disease-associated sleep fragmentation).
Downstream
-
Perivascular AQP4 Depolarization and Reduced Periarterial CSF Influx
Loss of the sleep-associated state contracts the interstitial space and raises noradrenergic tone, reducing periarterial CSF influx; ageing and injury additionally depolarize astrocytic AQP4 away from the perivascular end-foot.
Perivascular AQP4 Depolarization and Reduced Periarterial CSF Influx
amplifier
CSF enters the parenchyma along the perivascular spaces surrounding penetrating arteries, and this influx depends on the expression and perivascular localization of the astroglial water channel aquaporin-4 at astrocyte end-feet. Ageing, traumatic brain injury, and Alzheimer disease are each accompanied by loss of perivascular AQP4 polarization together with reduced arteriolar wall pulsatility, and Aqp4-null rodents show reduced CSF tracer influx. Conforming disorder nodes substitute the disorder-specific cause of reduced influx (AQP4 mislocalization, arteriosclerotic loss of pulsatility, perivascular space enlargement, meningeal lymphatic failure).
Downstream
-
Impaired Perivascular CSF-ISF Exchange and Solute Clearance
Reduced periarterial CSF influx and loss of perivascular AQP4 polarization lower the rate of exchange between subarachnoid CSF and brain interstitial fluid, and therefore the rate at which interstitial solutes are removed.
Impaired Perivascular CSF-ISF Exchange and Solute Clearance
central effector
The rate-limiting, disorder-agnostic step: exchange between subarachnoid CSF and brain interstitial fluid falls, and with it the clearance of interstitial solutes from the parenchyma. This is the key conformance target. It is reached from many upstream directions — ageing reduces CSF-parenchyma exchange and impairs intraparenchymal amyloid-beta clearance by about 40% in mice; traumatic brain injury reduces glymphatic pathway function by about 60% for at least a month; and in humans an intrathecally administered CSF tracer reaches all brain subregions but is cleared more slowly in a dementia cohort than in near-healthy references. Conforming disorder nodes preserve the impaired-clearance claim while substituting the disorder-specific insult and the solute measured.
Downstream
-
Accumulation of Aggregation-Prone Interstitial Proteins
When interstitial solute clearance falls below the rate of production, the steady-state interstitial concentration of aggregation-prone proteins rises.
Accumulation of Aggregation-Prone Interstitial Proteins
effector
Reduced clearance raises the interstitial concentration of the aggregation-prone proteins that the pathway normally removes — amyloid-beta, tau, and alpha-synuclein — pushing them toward the concentration-dependent nucleation regime modelled by the amyloidogenesis module. In humans, acute experimental slow-wave disruption raises CSF amyloid-beta40 and a single night of sleep deprivation raises PET amyloid burden in hippocampus and thalamus; in mice, Aqp4 deletion after traumatic brain injury promotes neurofibrillary tau pathology. Conforming disorder nodes substitute the disorder-specific accumulating species.
Downstream
-
Neuroinflammation and Progressive Neurodegeneration
Accumulated aggregation-prone protein and retained metabolic waste drive glial activation and neuroinflammation, and progressive neuronal loss.
Neuroinflammation and Progressive Neurodegeneration
consequence
Retained neurotoxic metabolites and accumulated protein aggregates provoke microglial and astroglial activation and chronic neuroinflammation, and the combination drives progressive neuronal dysfunction and loss. This is the clinical endpoint that makes glymphatic failure a proposed therapeutically targetable final common pathway to dementia, and it is where the module hands off to disease-specific downstream pathology in conforming entries.