The lymphatic system is critical for the clearance of metabolites and the maintenance of fluid homeostasis, and previous studies have suggested that the lack of lymphatic circulation in the eye, and its metabolites are mainly excreted through the aqueous humor circulation concentrated in the anterior segment of the eye, but this does not explain how neurotoxic substances produced by retinal neurons with high metabolic activity are efficiently cleared [1]. Until 2020, Wang et al. [2] proposed the concept of glial lymphatic system, that is, after the aqueous humor forms a mixture with retinal interstitial fluid (ISF) behind the eye, it passes through the ethmoid plate through the axon to the periretinal vein space driven by glial cell terminal foot aquaporin 4 (AQP4), and finally is taken up by the dural lymphatic vessels and enters the lymphatic circulation. This article reviews the composition and function of the glial lymphatic system and its mechanism in glaucoma, papilledema, Terson syndrome (TS) and other diseases, so as to provide reference and ideas for in-depth understanding of the removal of metabolic waste in the eye and the pathological mechanism of related eye diseases.
Literature search strategy: "Glymphatic system, Lamina cribrosa, Glaucoma, Optic disc edema, Terson syndrome" as the English keyword search PubMed, Medline, Web of Science; "glial lymphatic system, lymphoid system, ethmoid plate, glaucoma, optic disc edema, Terson syndrome" were used as Chinese keywords to search CNKI, Wanfang Data Knowledge Service Platform, Weipu.com and China Biomedical Literature Service System. The search period was established until 2022-11-01. Inclusion criteria: published literature, with priority given to high-quality journal literature; Exclusion criteria: (1) literature with little data information, duplicate publications, or for which full text is not available; (2) Poor quality literature.
1 Glial lymphatic system
1.1 Glial lymphatic system The glial lymphatic system was first discovered in the brain and subsequently became the focus of attention in the field of neuroscience. The perivascular space (also known as the Virchow-Robin gap) [3], the glial lymphatic system [4], and the dural lymphatic vessels [5] have gradually confirmed the existence of the glial lymphatic system. Driven by respiratory, arterial pulsation and other factors, cerebrospinal fluid (CSF) is produced by the choroid plexus, flows through the ventricular system into the subarachnoid space, and enters the deep part of the brain along the peripheral spaces of arteries, arterioles and capillaries [6]. Under the mediation of AQP4 in astrocytes, CSF enters the brain parenchyma through the glial boundary membrane, forms a CSF-ISF mixture with ISF, flows out from the perivenous space that also relies on AQP4 in the form of "integral flow", and finally enters the peripheral lymphatic system by dural lymphatic vessels, which plays a role in removing metabolites and abnormal proteins in the brain, and can also transport nutrients such as glucose, lipids, apolipoproteins and neuroactive substances to brain tissue [7]. This process is regulated by circadian rhythms and anesthesia states, and glial lymphatic clearance affected by AQP4 polarization levels peaks during sleep in mice [8], which partly explains the causal relationship between sleep disorders and dementias such as Alzheimer's disease (AD) [9]. In addition, impaired glial lymphatic function has been observed in central system disorders such as ischemic stroke, traumatic brain injury, and idiopathic normal-pressure hydrocephalus [10-12].
1.2 Ocular glial lymphatic system
1.2.1 Retrograde pathway The optic nerve is a continuation of the central nervous system, both of which are surrounded by a large number of CSF in the subarachnoid space, and are tightly wrapped by the dura, arachnoid and pia mater, and the discovery of the glial lymphatic system has led to speculation that the retina and optic nerve may also have similar clearance pathways [13-14]. TAM et al. [15] and MATHIEU et al. [16] used in vivo hyperspectral imaging to inject quantum dot tracers into the anterior chamber and cerebellar medulla oblongata cisterns of mice, respectively, and detected the presence of tracer signals in their submandibular lymph nodes; AFTER WOSTYN ET AL. [17] INJECTED INDIAN INK STAINING SOLUTION INTO THE SUBARACHNOID SPACE OF CADAVERS, THE AGGREGATION OF INK WAS FOUND IN THE PERIVASCULAR SPACE OF THE OPTIC NERVE AND IN THE COLLAGEN FIBER BUNDLE. IN 2017, MATIEU ET AL. [18] FIRST FOUND EVIDENCE OF GLIAL LYMPHATIC PATHWAYS IN THE OPTIC NERVE: AFTER INJECTING 4 SIZES OF FLUORESCEIN ISOTHIOCYANATE (FITC) INTO MOUSE CSF, THE MASS FITC < 70 kDa enters the optic nerve from the perivascular space all the way to the glial layer of the mouse eyeball, the ethmoidal plate of the human eye. JACOBSEN ET AL. [19] VERIFIED THE EXISTENCE OF THIS PATHWAY IN VIVO, AND THE RESULTS SHOWED THAT THE SUBARACHNOID INJECTION OF CSF TRACER REACHED THE PERIVASCULAR SPACES OF THE OPTIC NERVE, OPTIC CHIASMATIC, OPTIC TRACT, AND PRIMARY VISUAL CORTEX. Since this retrograde pathway does not reach the optic disc, it may also be part of the glial lymphatic system [18].
1.2.2 Anterograde pathway As a known circulation mode in the eye, the aqueous humor cycle generally includes the trabecular reticulum pathway and the uveal scleral pathway, but these pathways are mainly concentrated in the anterior segment of the eye, which makes it difficult for metabolites on the retina to be effectively removed. Anatomical evidence further supports the existence of these pathways [20], but still does not explain how metabolic waste is excreted from the outside of the eye. IN 2020, WANG ET AL. [2] DISCOVERED A GLIAL LYMPHATIC PATHWAY THAT PROGRADE CLEARS AMYLOID β (Aβ) IN RODENT EYES: DRIVEN BY TRANSETHMOIDAL PRESSURE DIFFERENCE (TLPD) AND PUPIL CONTRACTION, THE AQUEOUS HUMOR GENERATED BY THE CILIARY BODY FORMS A MIXTURE WITH RETINAL ISF BACKWARDS. The mixture is transported along the retinal ganglion cell (RGCs) axon, through the ethmoid plate to the periretinal vein space, where it is taken up by dural lymphatic vessels and leaves the optic nerve, the whole process relies on AQP4 on the terminal foot of retinal müller cells and astrocytes, and then the mixture is prograde along the RGCs axon transport direction to remove neurotoxic substances such as Aβ in the eye, so as to maintain the fluid balance of the retina and optic nerve and protect the homeostasis in the retina [21]. The glial lymphatic system and the glial lymphatic system are transported in much the same way that aqueous humor/CSF enters from the periarterial space, mixes with ISF and is excreted from the perivenous space, and eventually the circulating fluid of the two pathways collects at the dural lymphatic vessels and enters the lymphatic circulation [22]. The sieve plate plays a key pivotal role in this system, and it is also a clear difference from the glial lymphatic system, and abnormal changes such as ethmoid plate deformation and defects may affect the clearance efficiency of the glial lymphatic system [23].
2 Ocular glial lymphatic system and ocular diseases
2.1 Glaucoma The exact pathological mechanism of glaucoma is currently inconclusive, and the damage of Aβ to the optic nerve is considered to be one of the important causes. Excess Aβ on the retina activates microglia-mediated neuroinflammation and induces apoptosis of RGCs [24], while α2-adrenergic agonists that inhibit the production of Aβ and amyloid precursor protein (APP) significantly reduce apoptosis of RGCs and play a neuroprotective role in vivo and in vitro [25]. Aβ in the central nervous system is mainly cleared through the ubiquitin-proteasome pathway, autophagy-lysosomal pathway, blood circulation and other ways [26], the damage to the clearance pathway is the root of the body's Aβ error accumulation, and the dysfunction of the glial lymphatic system has also been confirmed to be related to the excessive accumulation of Aβ on the retina and optic nerve [27-28].
WOSTYN ET AL. [17] BELIEVE THAT WHEN TLPD IS ENLARGED, THE BLOCKADE OF GLIAL LYMPHATIC SYSTEM TRANSPORT AT THE ETHMOID PLATE MAY BE THE MECHANISM OF PRIMARY OPEN-ANGLE GLAUCOMA (POAG), AND THE DEPOSITION OF Aβ near the ethmoid plate is the cause of optic nerve damage there. MATHIEU et al. [27] observed the tracer distribution of the optic nerve cross-section of mice by confocal microscopy and found that after injecting a 10 kDa dextrose tracer into the CSF of DBA/2J mice, compared with the control group, the concentration and distribution rate of tracer in the optic nerve of 10-month-old DBA/2J mice decreased significantly, and half of the mice showed a decrease in phosphorylated nerve filament (pNF) immunoreactivity associated with RGCs axonal lesions. This may be related to retrograde pathway blockade that reduces Aβ clearance, thereby damaging axons. The change of CSF fluid dynamics when the retrograde pathway is blocked may be another potential factor in the development of glaucoma, the difference between the CSF production rate and the outflow rate determines the size of intracranial pressure (ICP), and too low ICP is positively correlated with the incidence of glaucoma [29], such as normal intraocular pressure glaucoma (NTG) patients with 3~7 mm behind the eye The area of the subarachnoid space around the optic nerve is smaller (that is, the CSF pressure here is lower) [30].
RANGROO et al. [28] believe that the optic nerve damage of glaucoma is not due to the reduction of Aβ clearance, but rather the excessive excretion of Aβ induces this process, and proposes that the defect of the sieve plate in the anterograde pathway is the root cause. During glial lymphatic transport in normal mice, the sieve plate is able to transfer the extracellular fluid in front of it into the axon of RGCs for directional anterograde transport. The extracellular fluid of DBA/2J mice would flow directly from the notch in the sieve plate, and this erroneous transport mode would reduce the normal glial lymphatic transport of liquid. In addition, changes in properties such as pressure gradient (TLPG) of the sieve plate may also be involved in the glaucoma pathological process. TLPG is the ratio of TLPD to sieve thickness, and elevated TLPD has been observed in many types of glaucoma, which not only causes mechanical damage to the optic disc and nerves in front of the sieve plate [31], but also causes the backward displacement of the front surface of the sieve plate [32]; Ethmoid plates in NTG patients also tend to be thinner and have a higher curvature than those in healthy people and patients with high intraocular pressure [33].
2.2 Optic disc edema The current research on glial lymphatic system and optic disc edema mainly focuses on space-related neuroophthalmic syndrome (SANS), SANS refers to long-term space flight caused by optic disc edema, choroidal folds, and flat eyeballs, and the probability of astronauts being diagnosed with optic disc edema during or after space is > 15％［34］。 The mechanism of SANS is not well understood, but it is thought to be related to the expansion of bilateral optic nerve sheaths and changes in intracranial fluid in microgravity [35]. With the discovery of the glial lymphatic system, the transport mode of fluid and solute in the optic nerve has changed completely new, so some scholars have proposed that the edema of the optic disc of astronauts may be related to the dysfunction of the glial lymphatic system [36-37].
WOSTYN ET AL. [34] PROPOSED THAT PAPILLEDEMA IN SANS MAY BE CAUSED BY CSF RETROGRADE ENTRY FROM THE GLIAL LYMPHATIC SYSTEM INTO THE EYE. Microgravity causes fluid (including CSF) to redistribute intracranial fluids (including CSF) and changes the anatomy and compliance of the optic nerve sheath, causing CSF to enter the optic nerve from the periretinal periartery space, which in turn infiltrates the optic disc in front of the ethmoid plate. This idea is supported by an experiment investigating the effect of microgravity on ICP, that the space environment prevents the subject's ICP from decreasing when upright, and that the measurement of ICP for 24 hours is consistently higher than that measured in the terrestrial environment [38], and that elevated ICP accelerates the retrograde movement of CSF in the optic nerve and induces papilledema, given the effect of TLPD on the fluid in the glial lymphatic system. It has also been suggested that this disc edema may also occur because intraocular fluid is blocked during the anterograde outflow of the optic nerve [34]. In microgravity, elevated ICP or CSF retention in the pararetinal space leads to increased pressure in the optic nerve sheath, resulting in reversal of positive TLPD, and obstruction of intraocular fluid drainage induces papilledema. The study on the relationship between long-term space flight and ocular structure found that the thickness of the choroid around the optic disc of the subject needs to return to the initial value after more than 30~90 days caused by microgravity [39]. Given that the clearing efficiency of the glial lymphatic system at the back of the eye is much lower than that of the anterior aqueous humor circulation [2], this may explain the slow improvement of optic disc edema after spaceflight. Finally, anatomical evidence also validates the relationship between papilledema and the glial lymphatic system, with magnetic resonance imaging (MRI) finding that the volume of perivascular spaces in the white matter and basal ganglia of the brain increases after space flight [40-41]. In short, the imbalance of fluid outflow and inflow in the glial lymphatic pathway may be a new insight into this optic disc edema, and the future monitoring of astronauts' optic disk structure by optical coherence tomography (OCT) and the analysis of their optic nerve and optic nerve sheath through MRI are expected to further reveal the specific mechanism of the disease, so as to provide targeted treatment plans and strategies to reduce unnecessary eye damage for astronauts in long-term space life.
2.3 TS The disease is usually manifested as subarachnoid hemorrhage with vitreous hemorrhage caused by acute elevation of ICP, and the bleeding site in the eye can also be subinferential, intraretinal, subretinal space, etc. The exact mechanism of TS is not well understood, but the well-established hypothesis is that acutely elevated ICP pushes CSF into and dilates the optic nerve sheath, thereby mechanically compressing the central retinal vein, resulting in retinal vascular rupture and hemorrhage. However, TS also tends to occur in one eye, and ICP acting on both optic nerves can cause retinal hemorrhage in both eyes, so this theory is still insufficient. The discovery of the glial lymphatic system creates the only extravascular anatomical channel between the subarachnoid space and the retina, and it has been suggested that blood entering the eye along this pathway may be the mechanism of TS [42-43].
This view was first confirmed by a 2007 case report in which preoperative CT in patients with TS showed subarachnoid hemorrhage, bilateral optic nerve sheath hemorrhage, and bilateral intraocular hemorrhage [44], suggesting that intracranial hemorrhage may have been transmitted into the eye via the optic nerve; In order to explore the micropathology of TS, KO et al. [45] studied ocular histology sections of 109 patients with TS, and found that TS intracranial hemorrhage continued in the optic nerve, subdural and subarachnoid space in the optic nerve sheath, and the preretinal hemorrhage was mostly diffuse, while the subinferential hemorrhage was clearly defined. Subsequent T2-weighted magnetic resonance (MR) images of a patient with TS showed that subarachnoid hemorrhage entered the optic nerve of both eyes, and the perivascular space of the central retina showed high signal intensity due to hyperemia and dilation, and the authors speculated that intracranial hemorrhage entered the subendomembrane from the perivascular space in the optic nerve sheath [46]; This hypothesis is further confirmed by a 2014 case report that concluded that the spread of blood within the retina is limited, so that blood entering the vitreous mostly comes from the retinal vasculature or perivascular space [47]. With the proposal of the glial lymphatic system, the above research on the pathological mechanism of TS has been confirmed, when ICP is acutely elevated, subarachnoid hemorrhage may enter the optic nerve from the periarterial space and the pericapillary space, and enter the retina and vitreous along the optic nerve. KUMARIA ET AL. [48] PROPOSED ANOTHER CASE OF VENTRICULAR HEMORRHAGE CAUSED BY PRIMARY VITREOUS HEMORRHAGE AND REFERRED TO IT AS "REVERSE TS", IN WHICH CT ANGIOGRAPHY SHOWED THAT OCULAR BLOOD ENTERED THE SYSTEMIC CIRCULATION FROM THE PERIVASCULAR SPACE, WHICH WAS CONSISTENT WITH THE GLIAL LYMPHATIC SYSTEM. In addition, it has been reported that the leakage site of fluorescein angiography after vitrectomy is near the basal layer of retinal astrocytes [49-50], and whether this is related to glial cells in the glial lymphatic system of the ocular deserves further investigation.
2.4 Other diseases The course of age-related macular degeneration (AMD) may also involve disorders of the glial lymphatic system. On the one hand, AMD and glaucoma belong to retinal degenerative diseases, and they and AD jointly exhibit Aβ and hyperphosphorylated Tau (p-Tau) deposition, chronic inflammation and iron homeostasis imbalance [51]; On the other hand, increasing age affects the clearance efficiency of the glial lymphatic system, which is a major risk factor for AMD [52].
3 Summary and outlook
In summary, the discovery of glial lymphatic system puts forward new possibilities for the study of the pathogenesis of glaucoma, optic disc edema, TS and other eye diseases. However, the research of this system is still in its infancy, the evidence of relevant case studies and animal experiments is still insufficient, there are many questions to be answered urgently, and whether the dysfunction of the glial lymphatic system and glaucoma Aβ deposition-like changes are two links of an event rather than a causal relationship, and the specific role of astrocytes, AQP4, perivascular space and other links also needs to be clarified. Considering similar anatomical structures, physiological functions and other factors, the current research of the glial lymphatic system is mainly carried out in rodents, so in order to evaluate this system in more depth, further non-invasive imaging studies of primates and even living organisms can be carried out. In the future, the specific regulatory links of the glial lymphatic system can be determined, and the function of the system can be improved for specific links, such as using the advantages of multi-target and multi-pathway of traditional Chinese medicine to promote the removal of metabolic wastes in the eye, which will have the opportunity to bring hope for the treatment of many eye diseases such as glaucoma and some central nervous system diseases.
There is no conflict of interest in this article.
Bibliography
Source: ZHU Guangyu,CHENG Yuxin,LU Xuejing. Research progress on glial lymphatic system and related eye diseases[J]. Chinese Journal of General Practice,2023,26(26):3330-3334. DOI：10.12114/j.issn.1007-9572.2022.0901.