The brain is one of the most metabolically active organs in the body. In awake young adults at rest, cerebral oxygen consumption is ≈50 mL O2/min or roughly 20% of the total utilized by the body.1 The brain’s high demand for energy in wakefulness persists during rapid eye movement sleep, and even during non–rapid eye movement sleep, the cerebral oxygen consumption rate is reduced by only 20%,2 although potentially more during stage 4 sleep. Importantly, this highly active bioenergetic state not only reflects oxidative metabolic fluxes, protein production, and substrate (re)cycling but also parallels metabolic waste production at a higher rate than in other body organs.
In every organ but the brain, lymphatic vasculature is pervasive and responsible for the recycling of undigested proteins and removal of excess fluid and metabolic waste from tissues.3,4 Given the high metabolic rate of the brain, the lack of lymphatic vasculature is a conundrum. A common assumption has been that with a tight blood–brain barrier restricting solute and fluid movements, a lymphatic system is superfluous in the central nervous system (CNS). As in other body organs, the CNS utilizes intracellular autophagy5 and the ubiquitin-proteasome pathway6,7 for clearance of damaged cytoplasmic organelles and misfolded or defect proteins. Amyloid-β and tau oligomers, products implicated in the pathogenesis of Alzheimer’s disease, are both degraded by these intracellular waste clearance systems.8 In addition, the blood–brain barrier also contributes to brain waste elimination9; for example, amyloid-β10 and α-synuclein (Parkinson’s disease)11 can cross from brain to blood via specialized blood–brain barrier transporters. However, dedicated transporters for other waste proteins, such as mutated huntingtin (Huntington’s disease), tau (Alzheimer’s disease and tauopathies), or the transactive response DNA-binding protein 43 (transactive response DNA binding protein-43, amyotrophic lateral sclerosis, and frontotemporal dementia) are lacking.
Cerebrospinal fluid (CSF) has long been thought to facilitate CNS tissue detoxification in place of lymphatics. Nonetheless, while CSF production and transport has been studied for decades, the exact processes involved in toxic waste clearance remain poorly understood.12–19 Over the past 5 years, emerging data have shed new light in the form of the glymphatic system, a novel brain-wide perivascular transit passageway dedicated to CSF transport and metabolic waste drainage from the brain.20,21 Here, we review the key anatomical components and operational drivers of the brain’s glymphatic system with a focus on its unique functional dependence on the state of arousal and anesthetic regimens. We also present evidence for the system’s existence in the human brain and discuss why clinical exploration of this new system may provide valuable insight into novel strategies for preventing delirium and cognitive dysfunction in perioperative and critical care settings.
THE GLYMPHATIC SYSTEM VERSUS THE LYMPHATIC SYSTEM
Except for the brain and eye balls, lymphatic vessels run parallel to venous blood vessels throughout the body and play a key role in tissue fluid homeostasis, lipid transport, and immune surveillance. Structurally, the “initial” lymphatic vessels are blind ending “stumps” imbedded as a network of small loops and larger interconnected circuits within the tissues that then converge into larger vessels connecting with the lymph nodes.3,4,22 These “collecting” lymphatics have 1-way valves and muscular walls enabling phasic contractive/relaxation capabilities, which involve endothelial nitric oxide synthase.23 Normal fluid movement out of the capillaries is important for tissue hydration, for nutrition, and to facilitate waste clearance. The lymphatics then collect fluid and solutes from the interstitial spaces that have both seeped out from the glycocalyx-covered intercellular clefts of the (leaky) capillaries24,25 and been generated within the tissue. Interstitial fluid and protein move into lymphatic vessels driven by a transmural hydrostatic pressure gradient, as well as external generated pressures from respiration and muscle contractions (eg, walking and peristalsis).4 There appears to be no restriction to the particle size that can enter from the interstitial space into the initial lymphatics, as large pores are present. Ultimately, the lymphatic system collects several liters of water daily while scavenging undigested proteins and other waste products that would otherwise be trapped in tissues.3,4,26 A considerable portion of lymph (fluid, proteins, and cells) is reabsorbed into blood vessels located within the lymph nodes. It has been reported that afferent lymphatics deliver 8 L/d of lymph to the lymph nodes, and 50% is reabsorbed at this level (Table). Thus, the postnodal collecting lymphatics carry the remaining 4 L to the thoracic duct and venous circulation.3
The Glymphatic System
The glymphatic system was introduced for the first time in 2012 by Iliff et al.20 To provide the reader with a better perspective of the glymphatic system, we present and discuss its anatomy and function in relation to the classical lymphatic system. In contrast to the lymphatic system, the glymphatic system operates beyond the blood–brain barrier and is not physically separated in form of a vasculature but instead uses the perivascular space as a conduit for fluid and solute transport (Figure 1; Table). Originally, the glymphatic system was described as a brain-wide perivascular transit passageway along arteries, arterioles, capillaries, venules, and veins that interconnects with interstitial fluid via aquaporin 4 water channels positioned on glial end-feet.20,21Figure 1 presents a model of the glymphatic system, demonstrating that the inner perimeter of the perivascular channels is the basement membrane, and the outer perimeter is made up of the glial end-feet encircling 99% of the cerebral vasculature.27 Although still debated, the perivascular spaces can be a physical space in some segments (eg, larger arteries) or a mesh-filled matrix comprising the basement membranes in smaller vessel segments, such as capillaries.28–31 Rapid perivascular CSF transport within the CNS was not in itself a novel discovery and was previously documented by other investigators.13 However, the dynamic connection between perivascular CSF and interstitial fluid via aquaporin 4 water channels on glia cells was a novel discovery that helped bridge the gap in knowledge as to how waste is drained from the brain.20
The glymphatic system in the rodent brain was discovered by tracking the movement of fluorescent dyes of different molecular weight administered into CSF via the cisterna magna or into brain parenchyma. The movement of dye-tagged CSF from periarterial channels to interstitial fluid and perivenous conduits was documented in real time using 2-photon optical microscopy or ex vivo techniques. Collectively, the first series of studies showed that (1) CSF and small molecular weight solutes moved readily and fast (5–10 minutes) from the periarterial space into interstitial fluid; (2) the large molecular weight dyes (eg, 2000 kDa fluorescein isothiocyanate dextran) were trapped in the perivascular space highlighting the restriction of solute transport due to the 20-nm gap between the glial end-feet; (3) ligation of the carotid artery impeded the periarterial influx of both small and large molecular weight solutes, suggesting that pulsatility (ie, bulk flow) drives CSF and solute transport; and (4) soluble amyloid-β1–40 administered into the parenchyma traversed the interstitial fluid toward the perivenous spaces, inferring that amyloid-β was cleared via the glymphatic system system.20 The documentation of amyloid-β clearance via the glymphatic system was an important finding given the implication of amyloid-β in Alzheimer’s disease.20
Next the investigators tested the role of aquaporin 4 on glymphatic system transport efficiency and demonstrated that glymphatic system transport and amyloid-β clearance were significantly reduced in aquaporin 4–deficient mice when compared to controls.20 Collectively, these experiments were important novel discoveries that pushed forward the field of CSF transport dynamics in relation to brain waste clearance and led to its designation as the glymphatic system, highlighting its dependency on glial cells. The unique role of the aquaporin 4 channels in CSF transport was recently challenged,32,33 encouraging further experimental validation of the original findings.34
“g-Lymph” of the Glymphatic System
Similar to the lymphatic system, which (re)absorbs fluid and solute waste from the tissues to the systemic circulation, the glymphatic system transports fluid and solutes within the CNS but drains the waste to the systemic circulation via authentic lymphatic vessels associated with the meninges.35–37 However, as the glymphatic system is operating within the closed skull and tight blood–brain barrier (as well as CSF-to-blood barriers), imposing physical and chemical constraints, the composition of the fluid transported by the glymphatic system is very different from that of the lymphatic system. “Lymph” is the major fluid source of the lymphatic system, with high content of protein and immune cells (Table). In contrast, the fluid transported in the glymphatic system is a mixture of CSF and interstitial fluid with waste solutes including amyloid-β and tau.38–40 The differences in protein content between lymph and CSF signify that their respective fluid densities and viscosities differ (Table).
CSF is mainly produced by the choroid plexus18,41,42 but also derived from secretion of fluids by the cerebral ventricular lining and by endothelial cells of the blood–brain barrier.43–45 CSF production in the choroid plexus is complex41 and involves the transport of osmotically active ions via the sodium-potassium adenosine triphosphatase, which forces passive movement of water across (primarily) aquaporin 1 water channels positioned at the luminal membranes of the choroid plexus.46 Very recently, the Na/K/2Cl cotransporter 1 also localized to the luminal membrane of the choroid plexus was identified as another important contributor to CSF formation.47 While several liters of lymph are produced and transported daily, the daily CSF production is 50048 and ≈1.7 mL in the human and rat brain, respectively.49 In the human brain, only 100–150 mL of CSF fills the fluid spaces of the CNS at any given time, inferring constant circulation and reabsorption.41 Studies show that CSF can “bypass” the brain parenchyma and directly exit via authentic lymphatics at the level of the dura sheaths associated with the exits of cranial and peripheral nerves.50 The percent volume fraction of the total CSF pool that actually exchanges with brain parenchymal interstitial fluid over a 24-hour period is unknown.
VISUALIZATION OF GLYMPHATIC SYSTEM TRANSPORT IN WHOLE BRAIN
Several techniques are used to visualize and quantify glymphatic system transport in the whole brain. Ideally, this should be done by tracking the fate of an endogenously produced waste substance dumped by cells into interstitial fluid (eg, amyloid-β or lactate) and with a technique that does not perturb the delicate glymphatic system, which operates under low pressure conditions. We first introduced dynamic contrast-enhanced magnetic resonance imaging in the anesthetized rat brain as an experimental platform for capturing glymphatic system transport, including periarterial influx and CSF interstitial fluid exchange.51 Typically, 20–30 μL of a small molecular weight paramagnetic contrast molecule (eg, gadoterate meglumine; Dotarem (Guerbet LLC, Carol Spring, IL; gadoterate meglumine), molecular weight 636 Da, Magnevist (Guerbet LLC, Carol Spring, IL), molecular weight 928 Da) diluted to best match the physical characteristics of CSF (baricity and density) is used as a surrogate waste solute and administered into rat brain CSF via the cisterna magna (smaller volumes are used in mice52,53). Administration of tracers via the lumbar intrathecal route is also possible.54Figure 2 shows the pattern of gadoterate meglumine transport from CSF into whole rodent brain using dynamic contrast-enhanced magnetic resonance imaging. The dynamic spatial progression of gadoterate meglumine uptake is displayed as color-coded, volume-rendered maps (Figure 2B–D) overlaid on the anatomical 3-dimensional magnetic resonance imaging template of the rat’s brain (Figure 2A). At 10 minutes after initiation of gadoterate meglumine infusion (Figure 2B), the cisterna magna and lateral reservoirs of the fourth ventricle are filled, and gadoterate meglumine is slowly progressing toward the spinal cord, along the large pial arteries at the base of the brain. At 20 minutes, gadoterate meglumine has reached the pineal recess, olfactory bulb, and rhinal fissure, and uptake into the cerebellum is evident (Figure 2C). At 30 minutes, contrast can be observed along the middle cerebral artery, and brain ventral parenchymal uptake is significant (Figure 2D). The magnitude of gadoterate meglumine uptake into the brain parenchyma varies dependent on the gadoterate meglumine concentration (in this particular experiment, gadoterate meglumine is delivered as an isobaric mixture). In previous experiments, we estimated that only 19% of an isobaric mixture of gadoterate meglumine administered into the cisterna magna was transported into the brain over a 3-hour period.55 The dynamic contrast-enhanced magnetic resonance imaging approach, which measures the uptake of gadoterate meglumine from CSF into brain parenchyma, can then be used to quantify glymphatic system transport using kinetic or other computational modeling approaches.51,55–58 This magnetic resonance contrast uptake pattern has been reproduced in several animal species including mouse, rat, canine, and nonhuman primate brain.52,56,59,60
Importantly, the same uptake pattern can be observed in the human brain after lumbar intrathecal administration of magnetic resonance contrast agents.61,62 Ringstad et al62 recently administered gadobutrol (0.5 mL of Gadovist (Guerbet LLC, Carol Spring, IL), 1.0 mmol/mL) as a bolus injection into CSF via the lumbar intrathecal route and performed dynamic contrast-enhanced magnetic resonance imaging at various time intervals over a 2-day period. In normal patients who were examined for dural tears and served as control subjects, contrast was observed along cerebral arteries and in parenchyma in a pattern similar to that observed in the rodent brain.51 However, CSF and parenchymal signal enhancement were slower in human brain compared to rodent brain likely due to differences in vascular pulsatility, cerebral metabolic rate, total brain mass, and expression pattern of aquaporin 4.63–65
The Glymphatic System of the Brain Drains to Authentic Lymphatic Vessels
Iliff et al20 reported that amyloid-β administered directly into brain parenchyma over time accumulated along capillaries and large central veins, including the caudal rhinal vein, highlighting the perivenous conduits as main clearance pathways that later would merge with lymphatics and then systemic circulation. However, several other investigators reported that interstitial fluid waste solutes drain along capillaries, arterioles, and arteries but not along veins29,45,66 (for more detail, see excellent review by Hladky and Barrand67). Regardless of these controversies, how do waste solutes transported via the glymphatic system actually exit the skull? In the lymphatic system, waste and undigested proteins are returned directly to the systemic circulation (Table). In contrast, in brain, metabolic waste drains first to authentic lymphatic vessels associated with the dura/epidural space connecting to cervical lymph nodes and from there to the systemic circulation.36,37,68,69 Indeed, authentic lymphatic vessels were confirmed at the level of the dural meninges70 and shown to be functionally capable of absorbing macromolecules from brain tissue and drain to deep cervical lymph nodes.36,37 The importance of the dural lymphatics was demonstrated in transgenic mouse models lacking these vessels because clearance of macromolecules from the brain to cervical lymph nodes was impeded.36,68 In a recent paper, Da Mesquita et al71 showed that destruction of meningeal lymphatics causes glymphatic system dysfunction without increasing the intracranial pressure (ICP) and also induces cognitive impairment in mice.
Interstitial Fluid Volume Fraction and Glymphatic System Transport: Role of Sleep States
The physical size of the interstitial fluid compartment and extracellular matrix is a major player in brain waste clearance via the glymphatic system.72 In most body organs, the extracellular matrix functions as a scaffold composed of cross-linked collagens (types I, III, and V), elastin, and glycosaminoglycans forming an entangled mesh surrounding the cells. The brain’s extracellular matrix is different because collagen is not pervasive but only present in the vascular basement membrane; instead, the major components include high amounts of brain-specific glycosaminoglycans (see reviews by Lei et al73 and Novak and Kaye74). The “stiffness” and hydraulic conductance of the extracellular matrix varies in sync with cell volume fluctuations (eg, neurons and astrocytes undergo volume changes during neuronal activity), as was shown recently in organotypic brain slices.75
The significance of the interstitial fluid volume on glymphatic system transport and waste clearance was elegantly demonstrated by Nedergaard’s team in unique experiments where these 2 parameters were measured in mouse brain during different states of arousal.72 They first trained the mice to sleep naturally while head-fixed for 2-photon optical imaging in the cortex. While the mice were sleeping naturally (confirmed by electrocorticography and electromyography), they infused small molecular weight solutes into CSF to measure glymphatic system transport and repeated these procedures on awakening the mice.72 While robust, rapid influx of CSF and solutes into cortex was observed during natural sleep, glymphatic system transport was dramatically reduced on awakening.72 Glymphatic system transport during ketamine/xylazine anesthesia was also shown to enhance glymphatic system transport similar to slow-wave sleep. To understand why sleep and ketamine/xylazine anesthesia accelerated glymphatic system transport and amyloid-β waste clearance compared to wakefulness, the interstitial fluid volume fraction was measured using real-time iontophoretic tetramethylammonium in head-fixed awake and anesthetized mice.72 The interstitial fluid volume fraction in wakefulness was 14% and increased to 23% during ketamine/xylazine anesthesia.72 It was noted that the increases in glymphatic system transport and expansion of the interstitial fluid volume fraction during sleep and/or ketamine/xylazine anesthesia were associated with increased power of slow-wave activity (delta oscillations) on the electrocorticography when compared to wakefulness.72 The authors concluded that the smaller interstitial fluid space during wakefulness compared to slow-wave sleep and anesthesia increases resistance to CSF influx and therefore decreases waste clearance via the glymphatic system.
Glymphatic System Transport and Body Position During Sleep
The enhanced effect of sleep and ketamine/xylazine anesthesia on glymphatic system transport and amyloid-β waste clearance compared to wakefulness was exciting new knowledge regarding the role of sleep. Indeed, it was hypothesized that the brain’s homeostatic need to continuously clear metabolic waste serves as an alternate biological driver for sleep.72 Regardless of the etiological reasons for sleep, there are prodigious implications of these findings for general brain health, for preventing neurodegenerative diseases, and for understanding adverse effects from both acute sleep deprivation and chronic partial sleep loss. We speculated that different sleep positions might also influence glymphatic system transport and amyloid-β waste clearance. We tested this hypothesis by measuring glymphatic system transport by dynamic contrast-enhanced magnetic resonance imaging and amyloid-β clearance in anesthetized (ketamine/xylazine) rodents positioned in the supine, prone, or lateral decubitus position.57 We discovered that the prone position with the head above the heart had the most significant negative effect on glymphatic system transport and waste clearance in comparison to supine and lateral decubitus positions.57 In fact, the magnetic resonance imaging experiments showed that the right lateral decubitus position was the best position from the point of view of glymphatic system transport.57 This result was intriguing, because studies have documented that humans favor the lateral body position during sleep.76 Humans and animals change body position several times during normal sleep cycles, and these rapid posture shifts may also affect glymphatic system function. Although still incompletely understood, the effect of body position during sleep on glymphatic system function may be explained by differences in breathing patterns and/or heart rate variability secondary to altered autonomic tone during the different sleep states. Alternatively, changes in ICP might be another explanatory factor.
Glymphatic System Function During Anesthesia
As discussed, the basis for sleep-induced enhancement of glymphatic system transport appears to be closely linked to expansion of the interstitial fluid space and slow-wave, delta oscillations.72 Interestingly, Xie et al72 also demonstrated that blockade of adrenergic signaling (using a mixture of α- and β-blockers infused directly into CSF) expanded interstitial fluid volume, accelerated glymphatic system transport, and was associated with slow-wave electrocorticography activity, although the mice were not naturally sleeping or anesthetized. Based on these experiments, we hypothesized that not all drug-induced sleep or deeper anesthetic states would enhance brain waste clearance to the same extent because of their varying effects on central adrenergic tone. In other words, some sleep states would be better than others from the point of view of brain waste clearance; and sleep states characterized by electrocorticography activity dominated by spindles and delta wave (0.5–4.0 Hz) oscillations would be superior for waste clearance via the glymphatic system. To address this hypothesis, we designed experiments to measure glymphatic system function in rodent brain using 2 different “sleep” regimens; one was composed of a hypnotic agent with specific effects on adrenergic tone and the other without it. Thus, one group of rats was exposed to dexmedetomidine and low-dose isoflurane (0.4%–0.8%) and the other to pure isoflurane (1.5%–2.2%). It is important to emphasize that we did not compare glymphatic system function with these 2-drug regimens from the point of view of anesthetic “equipotency.” Instead, our intent was to compare glymphatic system function in the 2 groups of rats while they remained “sleeping” and exposed to the same experimental conditions but to different anesthetic mixtures.
We showed that rats receiving dexmedetomidine, which blocks norepinephrine release from the locus coeruleus, in combination with low-dose isoflurane on average enhanced glymphatic system transport by 30% relative to rats receiving pure isoflurane.77Figure 3 shows glymphatic system transport measured by magnetic resonance imaging in a rat anesthetized with pure isoflurane (Figure 3A, B) versus one exposed to dexmedetomidine plus low-dose isoflurane (Figure 3C, D) 1 hour after administration of gadoterate meglumine into CSF. The color-coded map is a representative of gadoterate meglumine uptake in the whole rat brain. For example, in CSF, gadoterate meglumine levels are high (red colors), and in parenchyma, the levels are lower (green and blue colors). It is clear that there is more parenchymal uptake in rats exposed to dexmedetomidine plus low-dose isoflurane compared to isoflurane alone. We also confirmed that electrocorticography patterns with the 2 anesthetic states as expected were very different. In rats anesthetized with dexmedetomidine and low-dose isoflurane, the electrocorticography was dominated by spindles and delta waves, whereas rats anesthetized with pure isoflurane exhibited delta and theta oscillations in between frequent periods of burst suppression. In other words, the percentage of delta power and spindles were higher in the dexmedetomidine group when compared to pure isoflurane. How could the different effects on glymphatic system transport by the 2 anesthetic regimens be explained? The varying electrocorticography patterns recorded with the 2-drug regimens clearly indicated that anesthetic depths were different; and rodents exposed to dexmedetomidine were in a lighter, sleep state when compared to those exposed to pure isoflurane. We speculated that the addition of dexmedetomidine to the sleep mixture lowered adrenergic tone more than isoflurane alone and increased the interstitial fluid volume fraction to a greater extent, which would then accelerate glymphatic system transport. This hypothesis was indirectly corroborated because we observed a slight, 2% an increase in CSF volume in dexmedetomidine anesthetized rats compared to isoflurane alone.77 An increase in CSF volume indicates enhanced fluid transport through the glymphatic system. Alternatively, the differences in glymphatic system transport might also be caused by other factors, including changes in pulsatility, cerebral hemodynamics, or ICP.
Obvious questions arising in the context of glymphatic system function and different anesthetics are whether all IV anesthetics (eg, propofol and barbiturates) will enhance glymphatic system function to the same extent as dexmedetomidine when compared to inhalational anesthetics. or alternatively whether all inhalational anesthetics (independent of anesthetic depth) will adversely affect glymphatic system function when compared to sleep states induced with dexmedetomidine. Potential answers to these questions are probably best addressed from the point of view of electroencephalogram and/or interstitial volume fraction changes. There is currently no information available on the potential disparate effects of different hypnotics and inhalational anesthetics on the interstitial volume fraction changes when compared to the awake state. However, changes in electroencephalogram patterns with propofol and isoflurane are well described in humans.78–80 For example, in regards to propofol, the electroencephalogram signatures during light sedative states are characterized by alpha and beta oscillations, whereas in deeper states, slow-delta, alpha oscillation, and burst suppression dominate.80 For inhalational anesthetic such as isoflurane, a light anesthetic state (ie, sub–minimum alveolar concentrations [MAC]) are characterized by alpha and slow-delta oscillations and at ≥1 MAC levels theta oscillations dominate.80 Spindles intermingled with slow-wave delta rhythm as observed with dexmedetomidine are not described for propofol or inhalational anesthetics. Nevertheless, given that slow-wave delta oscillations can be observed with both propofol (deeper anesthetic states) and isoflurane (light sedation, sub-MAC concentrations), one could argue that both of these agents might accelerate glymphatic system function compared to wakefulness, if dose ranges for slow-wave delta oscillations were specifically targeted.
Within this framework, it is also important to briefly discuss studies comparing the sleep-restorative effects of various anesthetics after sleep deprivation with that of natural sleep, which might also shed light on the potential benefit of anesthetic agents on glymphatic system function.81–85 In other words, anesthetics that have the ability to decrease the “sleep pressure” that arises after sleep deprivation similar to natural sleep might also predict its benefit for potentiating glymphatic system function. Tung et al84 investigated the effect of propofol anesthesia on sleep homeostasis after sleep deprivation in a rat model to specifically address whether propofol anesthesia was comparable to ad libitum sleep in regards to sleep recovery. They measured increases in the intensity and amount of non–rapid eye movement and rapid eye movement sleep that are normally observed after a period of sleep deprivation (referred to as “rebound” non–rapid eye movement and rapid eye movement).84 They documented that recovery sleep behavior in rats allowed ad libitum sleep was no different from rats subjected to 6-hour propofol anesthesia and concluded that propofol anesthesia therefore appears to be a state that allows normal sleep homeostatic processes to occur.84 Mashour et al81 later documented that 4-hour isoflurane anesthesia (delivered at ≈1% to maintain unresponsiveness to toe pinch without causing continuous electroencephalogram burst suppression) did not appear to satisfy the homeostatic needs for rapid eye movement sleep.81 Interestingly, another study investigated the sleep restorative effects of short, 1-hour isoflurane and desflurane anesthesia administered at dose ranges set to specifically target an electroencephalogram pattern of almost continuous slow-delta waves.82 Indeed, at these dose ranges, it appeared that both inhalational anesthetics had sleep restorative power and were able to reduce the slow-wave rebound after sleep deprivation.82 However, they also noted that the slow-delta waves in the rats exposed to either isoflurane or desflurane were skewed toward frequencies below 1.5 Hz, which are typically observed during late sleep when sleep pressure is low and speculated that these particularly very slow-delta oscillations might represent a different component of the delta waves, observed in normal sleep.82 Clearly, more studies will be needed to further understand the impact of delta waves on glymphatic system function in the setting of sleep, anesthesia, and recovery sleep behavior after sleep deprivation.
GLYMPHATIC SYSTEM TRANSPORT, CSF, AND ICP
Normal CSF fluid production and transport into brain parenchyma are critical for glymphatic system function and waste clearance. For example, drainage of waste solutes from brain is impaired if CSF production is reduced pharmacologically86 by aging87 or if CSF is leaking (eg, dural tear or cisternostomy86). The latter also implies that glymphatic system transport is dependent on an “intact” skull and normal ICP. Researchers working in the glymphatic system field have noted that an open craniotomy for experimental purposes can be a confounder if not immediately resealed when studying glymphatic system transport, because an “intact skull” is a prerequisite for capturing the dynamic features of this delicate, low-pressure system. In human brain, it has been reported that if the skull is opened, the brain “collapses” when exposed to atmospheric pressure, which significantly dampens the pulse wave amplitude when compared to conditions where the skull is intact. The dampening of the pulse wave amplitude (as in open skull) will affect CSF transport and therefore glymphatic system transport. The role of ICP for glymphatic system transport is still not completely understood; however, as discussed, body position has been shown to affect glymphatic system transport in anesthetized rats.57 In humans, ICP has been measured to be 0.5 mm Hg in the supine (horizontal) position and increases to 3.7 mm Hg in the vertical position.88–90 Interestingly, in adult humans, the ICP when measured in the supine position is ≈1–2 mm Hg in daytime and it increases to ≈7 mm Hg at nighttime.91 In this context, it is important to highlight a recent study demonstrating that the total volume of “brain” inside the skull in humans varies by time a day and is greatest in the early morning.92 It could be speculated that the brain interstitial fluid is better “hydrated” after normal sleep on waking up in the morning due to the larger interstitial fluid space volume fraction changes, which occurred during slow-wave, non–rapid eye movement sleep cycles.
GLYMPHATIC SYSTEM TRANSPORT AND “PULSATILITY”: IN RELATION TO WAKEFULNESS AND SLEEP STATES
It is widely believed that the physical forces propelling CSF movement in the glymphatic system are derived from intracranial pulsatility.93,94 However, understanding of the processes contributing to pulsatility and how individual components drive waste clearance by the glymphatic system continues to evolve.95–97 Appreciation of the brain as a pulsating organ was initially based on direct observation via craniotomy of movement during the cardiac cycle. Subsequent measurement of ICP with the skull intact established the relationship between oscillations in blood pressure and ICP and eventually with bulk CSF flow.98 Measurement of cerebral blood flow velocity or volume flow later facilitated direct quantification of the temporal link among blood pressure oscillations, flow, and cyclic CSF movement, often described as a CSF “stroke volume.” Although intracranial pulsation coincident with the fundamental frequency of the heart rate has been prominently featured as a driving force for CSF and glymphatic system transport, the lower frequency events of respiration and spontaneous vasomotion also contribute and may play a particular role in the microlevel glymphatic system transport process.
In systemic vascular beds, arterial pressure and flow pulsations dissipate to become essentially continuous at the arteriolar and venous level. In contrast, within the rigid skull limited tissue compliance promotes propagation of arterial pressure pulsation throughout the brain leading to measurable pulsatile flow in the microvasculature and venous outflow.99 This preservation of pulsatility along the entire vascular bed is unique and may in part explain why the glymphatic system is effective as a waste drainage system despite operating within the tight skull and beyond the blood–brain barrier. At the microcirculatory level, Iliff et al100 demonstrated a decreased rate of glymphatic system transport following carotid ligation and an increased rate in response to the augmented blood pressure and flow produced by dobutamine, leading them to conclude that arterial pulsatility is a major driver for glymphatic system function. Overall, studies suggest that acute decreases in arterial pulsatility may impair glymphatic system function. Paradoxically, while it appears that the rate of CSF interstitial fluid exchange and amyloid-β clearance is highest during non–rapid eye movement sleep,72 this physiological state is actually associated with periods of significantly reduced blood pressure and cerebral blood flow, vasomotion,101 and ostensibly cerebral pulsatility.102–105 How or if altered arterial pulsatility during sleep affects glymphatic system function remains unknown.
For many years, it was thought that forces other than cardiac pulsation propelling CSF movement were minimal, in part due to the fact that common imaging techniques were cardiac gated. However, technological and computational advances in imaging techniques have now suggested that lower-frequency ICP oscillations produced by respiration compliment cardiac pulsation.106 The influence of respiration on CSF transport was recently visualized in normal human subjects from the lateral ventricles all the way down to the cervical subarachnoid space using ultrafast magnetic resonance imaging and demonstrated that forced inspiration was a main driver of CSF flow in the ventricles.106 The impact of spontaneous ventilation versus positive pressure ventilation on glymphatic system transport in brain is currently unknown.
POTENTIAL ROLE OF THE GLYMPHATIC SYSTEM IN METABOLIC AND NEUROTRANSMITTER HOMEOSTASIS
Almost all of the glutamate released by neurotransmission, and potentially most of the γ-aminobutyric acid (GABA) in the brain is taken up by the glial end-feet.107,108 Maintenance of extremely low glutamate and GABA concentrations in the interstitial fluid is critical for normal synaptic function, as well as avoiding neuronal damage from glutamate excitotoxicity. After being taken up by the astroglial, GABA and glutamate are converted to glutamine and released into the interstitial fluid, where it is actively transported into neurons for neurotransmitter resynthesis (ie, “glutamate/GABA/glutamine cycle107,108”). Maintenance of near-constant interstitial fluid glutamate and glutamine levels is critical for coordinating the glutamate/glutamine and GABA glutamine cycle. Functional neuroenergetics are coupled to the cycles and require an extraordinarily high amount of glucose metabolism that releases lactate into interstitial fluid. Lactate itself has been shown to have neuromodulatory and direct metabolic effects and therefore also needs to be maintained in the interstitial fluid at near-constant levels independent of brain activity.
There are several observations that suggest an important role for the glymphatic system in also maintaining local interstitial fluid metabolic and neurotransmitter homeostasis. It has been shown in multiple studies in human and rodent brains that there is a steady-state rate of aerobic glycolysis in the resting awake brain of ≈11% of the total glucose uptake flux. However, only small increases in the lactate concentration have been measured during activation, leaving the question of what happens to the excess lactate produced. Hertz et al109 have shown that lactate can rapidly diffuse between glial via gap junctions and proposed that this may provide lactate with preferred access to the glymphatic system and thus the ability to drain from the brain via this pathway, thereby explaining the fraction that cannot be detected via direct lactate or AV difference measurements. Coordinated action between glial processes and the glymphatic system has also been proposed as having a key role in maintaining metabolic homeostasis during sleep.110
EVIDENCE OF GLYMPHATIC SYSTEM IN THE HUMAN BRAIN
Though as of now no single study has been able to document unequivocally the existence of the glymphatic system in the human brain, the confluence of findings from CSF and brain imaging studies provides indirect evidence of its presence.
- CSF: Studies have reported increases in amyloid-β in CSF after 1 night of sleep deprivation111 and following the disruption of slow-wave sleep.112
- Magnetic resonance imaging: Studies on the transport of paramagnetic agents from perivascular and parenchymal CSF into the brain61,62,113 have reported similar findings to those in rodents.51,57 Diffusion magnetic resonance imaging has also been done to assess changes in apparent diffusion of water as a function of the state of sleep deprivation or arousal.114 These studies have documented differences in diffusivity between awake rested and awake sleep-deprived conditions and during sleep that, while not always consistent across studies, are indicative of dynamic changes in brain parenchymal volumes that appear to be influenced by arousal.92 A recent study has also demonstrated that meningeal lymphatics (which in rodents are considered the downstream drainage pathway of the glymphatic system) can be visualized in the live human by contrast-enhanced magnetic resonance imaging.35
- Positron emission tomography: Studies using positron emission tomography ligands to measure amyloid-β have shown increases in the concentration of amyloid-β in the brain of healthy controls after 1 night of sleep deprivation consistent with similar findings observed in rodents.115 Also, several independent studies have reported an association between restricted sleep hours and greater amyloid-β accumulation, both in healthy controls and individuals with mild cognitive impairment116 consistent with clearance of amyloid-β during sleep through the glymphatic system.
- Postmortem brain: Recently, human postmortem studies documented the presence of lymphatic vessels in the meninges,35 which are considered components of the glymphatic system in rodents.
CLINICAL IMPLICATIONS OF GLYMPHATIC SYSTEM IN RELATION TO PERIOPERATIVE AND CRITICAL CARE SETTINGS
Anesthesia and surgery in older patients (>65 years) are associated with a high incidence (≥40%) of delirium postoperatively and remain a major life-threatening problem.117 As the proportion of elderly patients continues to rise and more surgeries in this population are expected, postoperative delirium becomes a major socioeconomic burden.117 In a recent study, postoperative delirium was shown to be an independent risk factor associated with long-term mortality in patients undergoing elective operations with planned intensive care unit admission.118 While several risk factors for delirium have been identified, including age, preoperative morbidity,119 type of surgery,120 hypoxia, hypotension, length of intensive care unit stay,121 and “deep” sedation intraoperatively,122–124 the underlying mechanisms are still poorly understood. Preventive and treatment strategies for postoperative delirium are multidisciplinary and include optimizing pain control (multimodel and opioid sparing whenever possible), hydration, avoidance of hypoxia and hypotension, cognitive stimulation, and sleep “hygiene” in the intensive care unit setting.125 No specific medications are approved by the US Food and Drug Administration for the prevention of delirium.
Several clinical studies reported that the use of dexmedetomidine for sedation or “comfort sleep” is associated with lower incidence of delirium perioperatively, including in the intensive care unit setting, and improves outcomes when compared to benzodiazepines and/or opioid-based sedation strategies.126 Dexmedetomidine has also been associated with lower rates of agitation in delirious patients compared to conventional treatments.126 Given that brain waste clearance via the glymphatic system is more efficient during slow-wave sleep, which can be mimicked with drugs blocking central adrenergic transmission, it is tempting to suggest that dexmedetomidine delirium preventive effect is related to superior brain waste clearance during comfort sleep with dexmedetomidine compared to other hypnotic regimens in the intensive care unit setting. In other words, any hypnotic agent mimicking natural slow-wave sleep (including spindles) is likely to promote glymphatic system–enhanced waste drainage (“G-sleep”) like dexmedetomidine and would potentially curb or prevent delirium. Alternately, α-adrenergic blockers might also be advantageous if they cross the blood–brain barrier. New studies focused on developing noninvasive technologies to capture glymphatic system function in human brain would be required to test these hypotheses in the near future.
We would like to acknowledge 3-dimensional animation artist Elena Nikanorova for the medical illustration in Figure 1.
Name: Helene Benveniste, MD, PhD.
Contribution: This author helped write sections on the glymphatic system and Clinical Implications of Glymphatic System in Relation to Perioperative and Critical Care Settings, pose the scientific questions, and edit and review the paper.
Conflicts of Interest: None.
Name: Paul M. Heerdt, MD, PhD.
Contribution: This author helped contribute the section on Glymphatic System Transport and “Pulsatility”: in relation to Wakefulness and Sleep States, pose the scientific questions, and edit and review the paper.
Conflicts of Interest: P. M. Heerdt consults for Recro Pharma, Cheetah Medical, and Imperative LLC.
Name: Manuel Fontes, MD.
Contribution: This author helped write the section Clinical Implications of Glymphatic System in Relation to Perioperative and Critical Care Settings, pose the scientific questions, and edit and review the paper.
Conflicts of Interest: None.
Name: Douglas L. Rothman, PhD.
Contribution: This author helped contribute the section on Potential Role of the Glymphatic System in Metabolic and Neurotransmitter Homeostasis, pose the scientific questions, and edit and review the paper.
Conflicts of Interest: None.
Name: Nora D. Volkow, MD.
Contribution: This author helped contribute the section on Evidence of Glymphatic System in the Human Brain, pose the scientific questions, and edit and review the paper.
Conflicts of Interest: None.
This manuscript was handled by: Gregory J. Crosby, MD.
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