Lymphatic drainage from the eye: A new target for therapy

Aqueous humor as eye lymph: A crossroad between venous and lymphatic system

Aqueous humor (AQH) is a transparent fluid with characteristics similar to those of the interstitial fluid, which fills the eyeball posterior and anterior chambers and circulates in them from the sites of production to those of drainage. The AQH volume and pressure homeostasis is essential for the trophism of the ocular avascular tissues and their normal structure and function. Different AQH outflow pathways exist, including a main pathway, quite well defined anatomically and referred to as the conventional pathway, and some accessory pathways, more recently described and still not fully morphofunctionally understood, generically referred to as unconventional pathways. The conventional pathway is based on the existence of a series of conduits starting with the trabecular meshwork and Schlemm's Canal and continuing with a system of intrascleral and episcleral venules, which are tributaries to veins of the anterior segment of the eyeball. The unconventional pathways are mainly represented by the uveoscleral pathway, in which AQH flows through clefts, interstitial conduits located in the ciliary body and sclera, and then merges into the aforementioned intrascleral and episcleral venules. A further unconventional pathway, the lymphatic pathway, has been supported by the demonstration of lymphatic microvessels in the limbal sclera and, possibly, in the uvea (ciliary body, choroid) as well as by the ocular glymphatic channels, present in the neural retina and optic nerve. It follows that AQH may be drained from the eyeball through blood vessels (TM-SC pathway, US pathway) or lymphatic vessels (lymphatic pathway), and the different pathways may integrate or compensate for each other, optimizing the AQH drainage. The present review aims to define the state-of-the-art concerning the structural organization and the functional anatomy of all the AQH outflow pathways. Particular attention is paid to examining the regulatory mechanisms active in each of them. The new data on the anatomy and physiology of AQH outflow pathways is the key to understanding the pathophysiology of AQH outflow disorders and could open the way for novel approaches to their treatment.

Technological Advances in a Therapy of Primary Open-Angle Glaucoma: Insights into Current Nanotechnologies

Glaucoma is a leading cause of irreversible blindness and is characterized by increased intraocular pressure (IOP) and progressive optic nerve damage. The current therapeutic approaches for glaucoma management, such as eye drops and oral medications, face challenges including poor bioavailability, low patient compliance, and limited efficacy. In recent years, nanotechnology has emerged as a promising approach to overcome these limitations and revolutionize glaucoma treatment. In this narrative review, we present an overview of the novel nanotechnologies employed in the treatment of primary open-angle glaucoma. Various nanosystems, including liposomes, niosomes, nanoparticles, and other nanostructured carriers, have been developed to enhance the delivery and bioavailability of antiglaucoma drugs. They offer advantages such as a high drug loading capacity, sustained release, improved corneal permeability, and targeted drug delivery to the ocular tissues. The application of nanotechnologies in glaucoma treatment represents a transformative approach that addresses the limitations of conventional therapies. However, further research is needed to optimize the formulations, evaluate long-term safety, and implement these nanotechnologies into clinical practice. With continued advancements in nanotechnology, the future holds great potential for improving the management and outcomes of glaucoma, ultimately preserving vision and improving the lives of millions affected by this debilitating disease.

Pierre CL, Bimschleger H, Gao J, Cheng R, Mohammadi P, Chen H, Palmer AA, Polesskaya O, Jablonski MM. Genome-wide association study finds multiple loci associated with intraocular pressure in HS rats

Elevated intraocular pressure (IOP) is influenced by environmental and genetic factors. Increased IOP is a major risk factor for most types of glaucoma, including primary open angle glaucoma (POAG). Investigating the genetic basis of IOP may lead to a better understanding of the molecular mechanisms of POAG. The goal of this study was to identify genetic loci involved in regulating IOP using outbred heterogeneous stock (HS) rats. HS rats are a multigenerational outbred population derived from eight inbred strains that have been fully sequenced. This population is ideal for a genome-wide association study (GWAS) owing to the accumulated recombinations among well-defined haplotypes, the relatively high allele frequencies, the accessibility to a large collection of tissue samples, and the large allelic effect size compared to human studies. Both male and female HS rats (N = 1,812) were used in the study. Genotyping-by-sequencing was used to obtain ∼3.5 million single nucleotide polymorphisms (SNP) from each individual. SNP heritability for IOP in HS rats was 0.32, which agrees with other studies. We performed a GWAS for the IOP phenotype using a linear mixed model and used permutation to determine a genome-wide significance threshold. We identified three genome-wide significant loci for IOP on chromosomes 1, 5, and 16. Next, we sequenced the mRNA of 51 whole eye samples to find cis-eQTLs to aid in identification of candidate genes. We report 5 candidate genes within those loci: Tyr, Ctsc, Plekhf2, Ndufaf6 and Angpt2. Tyr, Ndufaf6 and Angpt2 genes have been previously implicated by human GWAS of IOP-related conditions. Ctsc and Plekhf2 genes represent novel findings that may provide new insight into the molecular basis of IOP. This study highlights the efficacy of HS rats for investigating the genetics of elevated IOP and identifying potential candidate genes for future functional testing.

Normal-Tension Glaucoma: A Glymphopathy?

Glaucoma is one of the main causes of irreversible blindness in the world. The most common form, primary open-angle glaucoma, is an optic neuropathy that is characterized by a progressive loss of retinal ganglion cells and their axons, leading to structural changes in the optic nerve head and associated visual field defects. Elevated intraocular pressure remains the most important modifiable risk factor for primary open-angle glaucoma. However, a significant proportion of patients develop glaucomatous damage in the absence of increased intraocular pressure, a condition known as normal-tension glaucoma (NTG). The pathophysiology underlying NTG remains unclear. Several studies have revealed that vascular and cerebrospinal fluid (CSF) factors may play significant roles in the development of NTG. Vascular failure caused by functional or structural abnormalities, and compartmentation of the optic nerve subarachnoid space with disturbed CSF dynamics have been shown to be associated with NTG. In the present article, based on the concept of the glymphatic system and observations in patients with NTG, we hypothesize that failure of fluid transport via the glymphatic pathway in the optic nerve may be involved in the pathogenesis of some if not many cases of NTG. According to this hypothesis, vascular and CSF factors may share reduced glymphatic transport and perivascular waste clearance in the optic nerve as a final common pathway leading to the development of NTG. In addition, we speculate that some cases of NTG may reflect glymphatic dysfunction in natural brain aging and central nervous system diseases, such as Alzheimer's disease. Clearly, further studies are needed to gain additional insight into the relative contribution of these factors and conditions to reduced glymphatic transport in the optic nerve.

Progress on Brain and Ocular Lymphatic System

In recent years, 2 major discoveries have modified the traditional understanding of the brain. First, meningeal lymphatic vessels (MLV) were found in the dural sinus, which may absorb and drain cerebrospinal fluid (CSF). Second, the glymphatic system was discovered, composed of para-arterial CSF influx channel, paravenous interstitial fluid (ISF) efflux channel, and the water channel aquaporin-4 (AQP4) in astrocytes connecting the 2 channels. Accumulating evidence demonstrates that the lymphatic system of the brain plays a vital role within the circulation of CSF and, therefore, in the removal of metabolites. Therefore, it is involved in the incidence and development of some central nervous system (CNS) diseases. The optic nerve and retina are the extension of the CNS in the orbit. Whether they have a lymphatic system and how they clear the metabolites of the optic nerve and retina are still unclear. Recent studies have found that the ocular lymphatic system has a crucial impact on bounding eye diseases, like disorders of the optic nerve and retina. Therefore, here we review the recent research progress concerning the structure and function of MLV and glymphatic system. We also discuss the biomarkers for identification of lymphatic vessels, the composition of ocular lymphatic systems, and the possible association with diseases.

Ocular Lymphatic and Glymphatic Systems: Implications for Retinal Health and Disease

Clearance of ocular fluid and metabolic waste is a critical function of the eye in health and disease. The eye has distinct fluid outflow pathways in both the anterior and posterior segments. Although the anterior outflow pathway is well characterized, little is known about posterior outflow routes. Recent studies suggest that lymphatic and glymphatic systems play an important role in the clearance of fluid and waste products from the posterior segment of the eye. The lymphatic system is a vascular network that runs parallel to the blood circulatory system. It plays an essential role in maintenance of fluid homeostasis and immune surveillance in the body. Recent studies have reported lymphatics in the cornea (under pathological conditions), ciliary body, choroid, and optic nerve meninges. The evidence of lymphatics in optic nerve meninges is, however, limited. An alternative lymphatic system termed the glymphatic system was recently discovered in the rodent eye and brain. This system is a glial cell-based perivascular network responsible for the clearance of interstitial fluid and metabolic waste. In this review, we will discuss our current knowledge of ocular lymphatic and glymphatic systems and their role in retinal degenerative diseases.

Bleb-Independent Glaucoma Surgery to Activate the Uveolymphatic Route of Non-Trabecular Aqueous Humor Outflow: Short-Term Clinical and OCT Results

The deep sclerectomy technique was modified to enhance aqueous humor (AH) outflow via the non-trabecular pathway. A pilot study was carried out to assess its safety and effectiveness. Thirty-eight patients were under observation. After superficial scleral flap (4 × 4 mm), deep scleral layers were divided into three parts by three parallel-to-limbus incisions. Deep sclerectomy without creating a window in the Descemetes' membrane was carried out in the distal part. A collagen implant was placed under the sclera of the remaining two parts with one end in the intrascleral pool. The third proximal part was excised to expose the uvea and implant. A Nd:YAG laser trabeculotomy at the surgery site was made on postoperative days 7-10. Outcome measures were IOP change, use of hypotensive medication(s), complications, and the need for a second surgery. At six months, the mean IOP decreased from 29.1 ± 9.2 mm Hg to 14.0 ± 4.3 mm Hg (p = 1.4 × 10-9); hypotensive medication use reduced from 2.9 ± 0.9 to 0.6 ± 1.0 (p = 1.3 × 10-10); complete success was achieved in 68.4% of cases and partial success was achieved in 31.6% of cases. Intraoperative and postoperative complications were rare and manageable. The OCT of the surgery site revealed the absence of bleb in all cases. Lymphatic vessels with characteristic bicuspid valves in their lumen were detected in conjunctiva near the operation site and over it in 32 patients. IOP decrease in the proposed technique was achieved by activation of the uveolymphatic route of AH outflow.

Glymphatic imaging and modulation of the optic nerve

Optic nerve health is essential for proper function of the visual system. However, the pathophysiology of certain neurodegenerative disease processes affecting the optic nerve, such as glaucoma, is not fully understood. Recently, it was hypothesized that a lack of proper clearance of neurotoxins contributes to neurodegenerative diseases. The ability to clear metabolic waste is essential for tissue homeostasis in mammals, including humans. While the brain lacks the traditional lymphatic drainage system identified in other anatomical regions, there is growing evidence of a glymphatic system in the central nervous system, which structurally includes the optic nerve. Named to acknowledge the supportive role of astroglial cells, this perivascular fluid drainage system is essential to remove toxic metabolites from the central nervous system. Herein, we review existing literature describing the physiology and dysfunction of the glymphatic system specifically as it relates to the optic nerve. We summarize key imaging studies demonstrating the existence of a glymphatic system in the optic nerves of wild-type rodents, aquaporin 4-null rodents, and humans; glymphatic imaging studies in diseases where the optic nerve is impaired; and current evidence regarding pharmacological and lifestyle interventions that may help promote glymphatic function to improve optic nerve health. We conclude by highlighting future research directions that could be applied to improve imaging detection and guide therapeutic interventions for diseases affecting the optic nerve.

Systemic exposure following intravitreal administration of therapeutic agents: an integrated pharmacokinetic approach. 2. THR-687

Intravitreal (IVT) injection remains the preferred administration route of pharmacological agents intended for the treatment of back of the eye diseases such as diabetic macular edema (DME) and neovascular age-related macular degeneration (nvAMD). The procedure enables drugs to be delivered locally at high concentrations whilst limiting whole body exposure and associated risk of systemic adverse events. Nevertheless, intravitreally-delivered drugs do enter the general circulation and achieving an accurate understanding of systemic exposure is pivotal for the evaluation and development of drugs administered in the eye. We report here the full pharmacokinetic properties of THR-687, a pan RGD integrin antagonist currently in clinical development for the treatment of DME, in both rabbit and minipig. Pharmacokinetic characterization included description of vitreal elimination, of systemic pharmacokinetics, and of systemic exposure following IVT administration. For the latter, we present a novel pharmacokinetic model that assumes clear partition between the vitreous humor compartment itself where the drug is administered and the central systemic compartment. We also propose an analytical solution to the system of differential equations that represent the pharmacokinetic model, thereby allowing data analysis with standard nonlinear regression analysis. The model accurately describes circulating levels of THR-687 following IVT administration in relevant animal models, and we suggest that this approach is relevant to a range of drugs and analysis of subsequent systemic exposure.

Immunology and Pathology in Ocular Drug Development

Clear vision is dependent on features that protect the anatomical integrity of the eye (cornea and sclera) and those that contribute to internal ocular homeostasis by conferring hemangiogenic (avascular tissues and antiangiogenic factors), lymphangiogenic (lack of draining lymphatics), and immunologic (tight junctions that form blood-ocular barriers, immunosuppressive cells, and modulators) privileges. The later examples are necessary components that enable the eye to maintain an immunosuppressive environment that responds to foreign invaders in a deviated manner, minimizing destructive inflammation that would impair vision. These conditions allowed for the observations made by Medawar, in 1948, of delayed rejection of allogenic tissue grafts in the anterior chamber of mouse eye and permit the sequestration of foreign invaders (eg, Toxoplasma gondii) within the retina of healthy individuals. Yet successful development of intraocular drugs (biologics and delivery devices) has been stymied by adverse ocular pathology, much of which is driven by immune pathways. The eye can be intolerant of foreign protein irrespective of delivery route, and endogenous ocular cells have remarkable plasticity when recruited to preserve visual function. This article provides a review of current understanding of ocular immunology and the potential role of immune mechanisms in pathology observed with intraocular drug delivery.

Eye lymphatic defects induced by bone morphogenetic protein 9 deficiency have no functional consequences on intraocular pressure

Aqueous humor drainage is essential for the regulation of intraocular pressure (IOP), a major risk factor for glaucoma. The Schlemm's canal and the non-conventional uveoscleral pathway are known to drain aqueous humor from the eye anterior chamber. It has recently been reported that lymphatic vessels are involved in this process, and that the Schlemm's canal responds to some lymphatic regulators. We have previously shown a critical role for bone morphogenetic protein 9 (BMP9) in lymphatic vessel maturation and valve formation, with repercussions in drainage efficiency. Here, we imaged eye lymphatic vessels and analyzed the consequences of Bmp9 (Gdf2) gene invalidation. A network of lymphatic vessel hyaluronan receptor 1 (LYVE-1)-positive lymphatic vessels was observed in the corneolimbus and the conjunctiva. In contrast, LYVE-1-positive cells present in the ciliary bodies were belonging to the macrophage lineage. Although enlarged conjunctival lymphatic trunks and a reduced valve number were observed in Bmp9-KO mice, there were no morphological differences in the Schlemm's canal compared to wild type animals. Moreover, there were no functional consequences on IOP in both basal control conditions and after laser-induced ocular hypertonia. Thus, the BMP9-activated signaling pathway does not constitute a wise target for new glaucoma therapeutic strategies.

Unraveling the Possible Routes of SARS-COV-2 Invasion into the Central Nervous System

PURPOSE OF REVIEW

To describe the possible neuroinvasion pathways of Severe Acute Respiratory Syndrome-related Coronavirus-2 (SARS-CoV-2), the virus responsible for the Coronavirus disease-19 (Covid-19) pandemic.

RECENT FINDINGS

We present data regarding the family of Coronaviruses (CoVs) and the central nervous system (CNS), and describe parallels between SARS-CoV-2 and other members of the family, which have been investigated in more depth and combine these findings with the recent advancements regarding SARS-CoV-2.

SUMMARY

SARS-CoV-2 like other CoVs is neuroinvasive, neurotropic and neurovirulent. Two main pathways of CNS penetration seem to be the strongest candidates, the hematogenous and the neuronal. Τhe olfactory route in particular appears to play a significant role in neuroinvasion of coronaviruses and SARS-CoV-2, as well. However, existing data suggest that other routes, involving the nasal epithelium in general, lymphatic tissue and the CSF may also play roles in SARS-CoV-2 invasion into the CNS.

TBL1XR1 Mutations in Primary Marginal Zone Lymphomas of Ocular Adnexa are Associated with Unique Morphometric Phenotypes

PURPOSE

Extranodal marginal zone B-cell lymphoma (EMZL) of mucosa-associated lymphoid tissue (MALT) that affects the ocular adnexa, also known as ocular adnexal MALT lymphomas (OAML), are low-grade lymphomas that mostly affect elderly individuals. This study was conducted to explore the genetic and microbial drivers of OMAL, and unique morphometric phenotypes associated with these mutations and infections.

MATERIALS AND METHODS

In this study, we performed targeted deep sequencing of 8 OAML cases to identify its potential genetic and microbial drivers. We additionally performed computational digital image analysis of cases to determine if morphologic features corresponded to genetic mutations and disease biology.

RESULTS

We identified TBL1XR1 as recurrently mutated in OAML (4/8), and mutations in several other oncogenes, tumor suppressors, transcription regulators, and chromatin remodeling genes. Morphologically, OAML cases with mutations in TBL1XR1 showed lymphoma cells with significantly lower circularity and solidity by computational digital image analysis (p-value <0.0001). Additionally, cases of OAML with mutations in TBL1XR1 showed equivalent or increased vascular density compared to cases without mutations in TBL1XR1. Finally, we did not find any infectious microbial organisms associated with OAML.

CONCLUSIONS

Our study showed recurrent mutations in TBL1XR1 are associated with unique morphometric phenotypes in OMAL cases. Additionally, mutations in genes associated with the methylation status of histone 3, nuclear factor (NF)-κB pathway, and NOTCH pathway were enriched in OMAL cases. Our findings have biologic and clinical implications as mutations in TBL1XR1 and other genes have the potential to be used as markers for the diagnosis of OAML, and also demonstrate a specific biologic phenotypic manifestation of TBL1XR1 mutations.

Morphological analysis of aqueous humor drainage using QD nanoparticles and indocyanine green

This study represents the first morphological description of the lymphatic drainage of the ciliary body in vivo by comparative hyperspectral fluorescence imaging techniques of Quantum Qdot655 (QD) nanoparticles and indocyanine green (ICG). A volume of 1.25 μl of QD was injected into the left anterior camera of all rats. Similarly,1.25 μl of ICG diluted at a ratio of one-fourth with physiological saline solution was injected into the right anterior camera of all rats. The thickness of the skin in the mandibular area, connective tissue, and the depth of the mandibular lymph node (MLN) made image retrieval difficult. For QD, 302 nm UV excitation and 605 nm fluorescence peak emission were applied. The detection of QD and ICG used in this study in the MLNs is definitive evidence that aqueous humor (AH) follows a uveolymphatic pathway. Scanning electron microscope and the energy dispersive X-ray analyzer spectrum were used to examine both the Schlemm's canal and the MLN. For the first time, the QD was detected in the cortex of MLN. The QCM analysis of both QD-AH and ICG-AH was used to determine whether there was any interaction between them. This comparative study shows the importance of experimental animal modeling in pharmacological studies regarding eye research and drugs. In a female rat, the signal was taken from the parotid lymph node with QD injections.

Neurological manifestations of patients with COVID-19: potential routes of SARS-CoV-2 neuroinvasion from the periphery to the brain

Coronavirus disease 2019 (COVID-19), caused by severe acute respiratory syndrome Coronavirus 2 (SARS-CoV-2), has caused a global pandemic in only 3 months. In addition to major respiratory distress, characteristic neurological manifestations are also described, indicating that SARS-CoV-2 may be an underestimated opportunistic pathogen of the brain. Based on previous studies of neuroinvasive human respiratory coronaviruses, it is proposed that after physical contact with the nasal mucosa, laryngopharynx, trachea, lower respiratory tract, alveoli epithelium, or gastrointestinal mucosa, SARS-CoV-2 can induce intrinsic and innate immune responses in the host involving increased cytokine release, tissue damage, and high neurosusceptibility to COVID-19, especially in the hypoxic conditions caused by lung injury. In some immune-compromised individuals, the virus may invade the brain through multiple routes, such as the vasculature and peripheral nerves. Therefore, in addition to drug treatments, such as pharmaceuticals and traditional Chinese medicine, non-pharmaceutical precautions, including facemasks and hand hygiene, are critically important.

Nanotechnology for Medical and Surgical Glaucoma Therapy-A Review

Glaucoma is the second leading cause of blindness worldwide. Even though significant advances have been made in its management, currently available antiglaucoma therapies suffer from considerable drawbacks. Typically, the success and efficacy of glaucoma medications are undermined by their limited bioavailability to target tissues and the inadequate adherence demonstrated by patients with glaucoma. The latter is due to a gradual decrease in tolerability of lifelong topical therapies and the significant burden to patients of prescribed stepwise antiglaucoma regimens with frequent dosing which impact quality of life. On the other hand, glaucoma surgery is restricted by the inability of antifibrotic agents to efficiently control the wound healing process without causing severe collateral damage and long-term complications. Evolution of the treatment paradigm for patients with glaucoma will ideally include prevention of retinal ganglion cell degeneration by the successful delivery of neurotrophic factors, anti-inflammatory drugs, and gene therapies. Nanotechnology-based treatments may surpass the limitations of currently available glaucoma therapies through optimized targeted drug delivery, increased bioavailability, and controlled release. This review addresses the recent advances in glaucoma treatment strategies employing nanotechnology, including medical and surgical management, neuroregeneration, and neuroprotection.

Reverse meridional cyclodialysis ab interno in management of open angle glaucoma — a preliminary report

The uveoscleral outflow as an alternate route of aqueous drainage is of great interest in glaucoma surgical treatment. A cyclodialysis cleft allows one to create a direct connection between the anterior chamber (AC) and the suprachoroidal space (SCS) which is the key element of uveoscleral outflow. The purpose of the study was to evaluate the safety and effectiveness of reverse meridional cyclodialysis ab interno (RMCai) in decreasing intraocular pressure (IOP) in patients with primary open-angle glaucoma (POAG) and refractory glaucoma (RG). Fourteen patients who exhibited POAG and RG (11 men and 3 women, age 77.3 ± 7.8 years) were included in the study. All patients underwent RMCai with the help of custom-designed spatula. The spatula, inserted through a clear corneal incision, was used to detach the ciliary body from the scleral spur to create a 2.0–2.5 mm wide and 6.0–6.5 mm deep cleft. Outcome measures were IOP change, use of hypotensive medication(s), complications, and need for a second surgery. Decrease in IOP by more than 20% and IOP between 6 and 21 mmHg without hypotensive medication constituted complete success. Similar changes in IOP with medication constituted partial success. Need for second surgery constituted failure. The follow-up period was >3 months. Baseline IOP and hypotensive medication use were 22.0 ± 8.5 mmHg (95% confidence interval (CI), 17.6–26.4) and 2.6 ± 0.9 (95% CI, 2.2–3.1). At 3, 6, 12, 18, and 24 months, complete success was achieved in 64.3%, 77.8%, 55.6%, 37.5%, and 40% of patients respectively; partial success — in 14.3%, 22.2%, 44.4%, 50.0%, and 60.0%. Four patients required a second surgery. Failure occurred because of cleft closure by fibrosis. It was concluded that RMCai is safe and effective in decreasing IOP in POAG and RG patients.

Immunological considerations and concerns as pertinent to whole eye transplantation

PURPOSE OF REVIEW

The advent of clinical vascularized composite allotransplantation (VCA), offers hope for whole eye transplantation (WET) in patients with devastating vison loss that fails or defies current treatment options. Optic nerve regeneration and reintegration remain the overarching hurdles to WET. However, the realization of WET may indeed be limited by our lack of understanding of the singular immunological features of the eye as pertinent to graft survival and functional vision restoration in the setting of transplantation.

RECENT FINDINGS

Like other VCA, such as the hand or face, the eye includes multiple tissues with distinct embryonic lineage and differential antigenicity. The ultimate goal of vision restoration through WET requires optimal immune modulation of the graft for successful optic nerve regeneration. Our team is exploring barriers to our understanding of the immunology of the eye in the context of WET including the role of immune privilege and lymphatic drainage on rejection, as well as the effects ischemia, reperfusion injury and rejection on optic nerve regeneration.

SUMMARY

Elucidation of the unique immunological responses in the eye and adnexa after WET will provide foundational clues that will help inform therapies that prevent immune rejection without hindering optic nerve regeneration or reintegration.

Cyclodialysis ab externo with implantation of a collagen implant in surgical management of glaucoma

Glaucoma is one of the main causes of irreversible blindness in the Russian Federation and it is the leading cause of visual impairments among working age population. The primary goal of glaucoma therapy is to preserve the visual function, which is mainly achieved through persistent normalization of IOP by instillation of hypotensive drugs, laser therapy and/or surgery/ In this clinical study safety and efficacy of a glaucoma surgical technique implying valve cyclodialysis ab externo with implantation of a non-absorbable collagen implant (NACI) (Xenoplast, Dubna-Biofarm, Russia) in the supraciliary space were evaluated. All patients exhibited moderate and severe primary open-angle glaucoma (POAG). The efficacy assessment criterias were intraocular pressure (IOP) dynamics, use of hypotensive medications, need for repeat surgical intervention and complications. A total of 26 patients (26 eyes) were operated upon and under observation. Twelve months after surgery, 34% IOP decrease from the baseline level was observed: from 29.5 ± 6.8 to 18.8 ± 4.3 mmHg. The amount of hypotensive medications used reduced from 2.8 ± 0.9 to 0.6 ± 0.9. Applying the criteria recommended by the World Glaucoma Association, complete success was registered in 73.1% of patients and partial success — in 26.9% patients. No surgery ended in a failure through the follow-up period. Post-operatively, one patient developed hyphema, 2 patients had some blood elements in aqueous humor and 1 patient had shallow anterior chamber (AC). The suggested surgical technique proved to be an efficient and safe way to decrease IOP and reduce the number of hypotensive medications and had a minimal number of complications associated with the surgery, therefore it can be recommended as a method of choice in patients with advanced stage POAG.

How many aqueous humor outflow pathways are there?

The aqueous humor (AH) outflow pathways definition is still matter of intense debate. To date, the differentiation between conventional (trabecular meshwork) and unconventional (uveoscleral) pathways is widely accepted, distinguishing the different impact of the intraocular pressure on the AH outflow rate. Although the conventional route is recognized to host the main sites for intraocular pressure regulation, the unconventional pathway, with its great potential for AH resorption, seems to act as a sort of relief valve, especially when the trabecular resistance rises. Recent evidence demonstrates the presence of lymphatic channels in the eye and proposes that they may participate in the overall AH drainage and intraocular pressure regulation, in a presumably adaptive fashion. For this reason, the uveolymphatic route is increasingly thought to play an important role in the ocular hydrodynamic system physiology. As a result of the unconventional pathway characteristics, hydrodynamic disorders do not develop until the adaptive routes cannot successfully counterbalance the increased AH outflow resistance. When their adaptive mechanisms fail, glaucoma occurs. Our review deals with the standard and newly discovered AH outflow routes, with particular attention to the importance they may have in opening new therapeutic strategies in the treatment of ocular hypertension and glaucoma.

Lymphatic Vascular Structures: A New Aspect in Proliferative Diabetic Retinopathy

Diabetic retinopathy (DR) is the most common diabetic microvascular complication and major cause of blindness in working-age adults. According to the level of microvascular degeneration and ischemic damage, DR is classified into non-proliferative DR (NPDR), and end-stage, proliferative DR (PDR). Despite advances in the disease etiology and pathogenesis, molecular understanding of end-stage PDR, characterized by ischemia- and inflammation-associated neovascularization and fibrosis, remains incomplete due to the limited availability of ideal clinical samples and experimental research models. Since a great portion of patients do not benefit from current treatments, improved therapies are essential. DR is known to be a complex and multifactorial disease featuring the interplay of microvascular, neurodegenerative, metabolic, genetic/epigenetic, immunological, and inflammation-related factors. Particularly, deeper knowledge on the mechanisms and pathophysiology of most advanced PDR is critical. Lymphatic-like vessel formation coupled with abnormal endothelial differentiation and progenitor cell involvement in the neovascularization associated with PDR are novel recent findings which hold potential for improved DR treatment. Understanding the underlying mechanisms of PDR pathogenesis is therefore crucial. To this goal, multidisciplinary approaches and new ex vivo models have been developed for a more comprehensive molecular, cellular and tissue-level understanding of the disease. This is the first step to gain the needed information on how PDR can be better evaluated, stratified, and treated.

[Lymphatic system of the eye]

Existence of lymphatic outflow of intraocular fluid is still an open question. Identification of the lymphatic capillaries and vessels in various human organs in normal and pathological conditions became possible with discovery of endotheliocyte markers for lymphatic vessels. However, the available information on the presence of lymphatic structures in the human eye is inconsistent and uncertain. The data on lymphatic drainage of the eye is of interest, particularly because it may help understand glaucoma pathogenesis, mechanisms of development of eye inflammatory diseases, and develop pathogenetic therapies. The article reviews literature and presents the authors' own views on lymphatic drainage of the human eye.

Glymphatic stasis at the site of the lamina cribrosa as a potential mechanism underlying open-angle glaucoma

The underlying pathophysiology of primary open-angle glaucoma remains unclear, but the lamina cribrosa seems to be the primary site of injury, and raised intraocular pressure is a major risk factor. In recent years, a decreased intracranial pressure, leading to an abnormally high trans-lamina cribrosa pressure difference, has gained interest as a new risk factor for glaucoma. New research now lends support to the hypothesis that a paravascular transport system is present in the eye analogous to the recently discovered 'glymphatic system' in the brain, which is a functional waste clearance pathway that promotes elimination of interstitial solutes, including β-amyloid, from the brain along paravascular channels. Given that β-amyloid has been reported to increase by chronic elevation of intraocular pressure in glaucomatous animal models and to cause retinal ganglion cell death, the discovery of a paravascular clearance system in the eye may provide powerful new insights into the pathophysiology of primary open-angle glaucoma. In this review, we provide a new conceptual framework for understanding the pathogenesis of primary open-angle glaucoma, present supporting preliminary data from our own post-mortem study and hypothesize that the disease may result from restriction of normal glymphatic flow at the level of the lamina cribrosa owing to a low intracranial pressure and/or a high trans-lamina cribrosa pressure gradient. If confirmed, this viewpoint could offer new perspectives for the development of novel diagnostic and therapeutic strategies for this devastating disorder.

Intraocular Pressure-Lowering Effect of Latanoprost Is Hampered by Defective Cervical Lymphatic Drainage

Purpose

To evaluate whether defects in cervical lymphatic drainage influence the intraocular pressure (IOP)-lowering effect of latanoprost in patients with primary open-angle glaucoma (POAG) who have undergone unilateral radical neck dissection (uRND).

Methods

We enrolled (1) bilateral POAG patients who had started (bilateral) latanoprost (0.005%) monotherapy prior to their uRND and (2) treatment-naïve, bilateral glaucoma suspects (GSs) who had undergone the same surgery. We compared the eyes ipsilateral to the uRND with their fellow eyes in terms of the changes in IOP between the baseline (prior to the uRND) and the follow-up visits (1, 3, and 6 months after the uRND).

Results

The study involved 22 eyes of 11 POAG patients and 14 eyes of 7 GSs. In the POAG patients, IOP had increased significantly after surgery in the eyes ipsilateral to the uRND (from 14.7±1.4mmHg to 17.1±2.2mmHg; P = 0.007). Interestingly, in the eyes contralateral to the uRND, IOP had not changed significantly after surgery (from 14.2±1.8mmHg to 14.4±2.0mmHg; P = 0.826). In GSs, the eyes ipsilateral to the uRND did not differ significantly from their fellow eyes in terms of post-operative IOP change (ipsilateral value: 0.3±0.5mmHg, fellow eyes: -0.1±0.7mmHg; P = 0.242).

Conclusion

In the POAG patients, IOP had increased significantly in the eyes ipsilateral to the uRND. However, it had not changed significantly in the eyes contralateral to the surgery or in the eyes of the GSs. These findings suggest that, latanoprost works, at least in part, by enhancing outflow from the aqueous humor via the uveolymphatic pathway.