Intracranial pressure and glaucoma: Is there a new therapeutic perspective on the horizon?

Age- and glaucoma-induced changes to the ocular glymphatic system

The ocular glymphatic system supports bidirectional fluid transport along the optic nerve, thereby removes metabolic wastes including amyloid-β. To better understand this biological process, we examined the distributions of intravitreally and intracisternally infused tracers in full-length optic nerves from different age groups of mice. Aging was linked to globally impaired ocular glymphatic fluid transport, similar to what has seen previously in the brain. Aging also reduced the pupillary responsiveness to light stimulation and abolished light-induced facilitation in anterograde ocular glymphatic flow. In contrast to normal aging, in the DBA/2 J model of glaucoma, we found a pathological increase of glymphatic fluid transport to the anterior optic nerve that was associated with dilation of the perivascular spaces. Thus, aging and glaucoma have fundamentally different effects on ocular glymphatic fluid transport. Manipulation of glymphatic fluid transport might therefore present a new target for the treatment of glaucoma.

Could Young Cerebrospinal Fluid Combat Glaucoma? Comment on Lee et al. Association between Optic Nerve Sheath Diameter and Lamina Cribrosa Morphology in Normal-Tension Glaucoma. J. Clin. Med. 2023, 12, 360

I enjoyed reading the article by Lee et al. [...].

An ocular glymphatic clearance system removes β-amyloid from the rodent eye

Despite high metabolic activity, the retina and optic nerve head lack traditional lymphatic drainage. We here identified an ocular glymphatic clearance route for fluid and wastes via the proximal optic nerve in rodents. β-amyloid (Aβ) was cleared from the retina and vitreous via a pathway dependent on glial water channel aquaporin-4 (AQP4) and driven by the ocular-cranial pressure difference. After traversing the lamina barrier, intra-axonal Aβ was cleared via the perivenous space and subsequently drained to lymphatic vessels. Light-induced pupil constriction enhanced efflux, whereas atropine or raising intracranial pressure blocked efflux. In two distinct murine models of glaucoma, Aβ leaked from the eye via defects in the lamina barrier instead of directional axonal efflux. The results suggest that, in rodents, the removal of fluid and metabolites from the intraocular space occurs through a glymphatic pathway that might be impaired in glaucoma.

Can the Treatment of Normal-Pressure Hydrocephalus Induce Normal-Tension Glaucoma? A Narrative Review of a Current Knowledge

Ventriculoperitoneal shunt placement is the most commonly used treatment of normal-pressure hydrocephalus (NPH). It has been hypothesized that normal-tension glaucoma (NTG) is caused by the treatment of NPH by using the shunt to reduce intracranial pressure (ICP). The aim of this study is to review the literature published regarding this hypothesis and to emphasize the need for neuro-ophthalmic follow-up for the concerned patients. The source literature was selected from the results of an online PubMed search, using the keywords "hydrocephalus glaucoma" and "normal-tension glaucoma shunt". One prospective study on adults, one prospective study on children, two retrospective studies on adults and children, two case reports, three review papers including medical hypotheses, and one prospective study on monkeys were identified. Hypothesis about the association between the treatment of NPH using the shunt to reduce ICP and the development of NTG were supported in all reviewed papers. This suggests that a safe lower limit of ICP for neurological patients, especially shunt-treated NPH patients, should be kept. Thus, we proposed to modify the paradigm of safe upper ICP threshold recommended in neurosurgery and neurology into the paradigm of safe ICP corridor applicable in neurology and ophthalmology, especially for shunt-treated hydrocephalic and glaucoma patients.

[Dependency of intraocular pressure on body posture in glaucoma patients : New approaches to pathogenesis and treatment]

BACKROUND

Human intraocular pressure (IOP) depends on the position of the head in relation to the body in space. Physiologically, the IOP increases in a lying position compared to an upright posture. Microgravity in space also appears to cause an increase in intraocular pressure, accompanied by other ophthalmological changes, which are summarized under the term spaceflight associated neuro-ocular syndrome (SANS). Bed rest studies are being carried out to investigate the effects of weightlessness on the human body. So here there is an intersection between research into SANS and glaucoma. Increased intraocular pressure remains the most important risk factor for glaucoma development and progression that can be influenced by treatment. The influence of position-dependent IOP fluctuations on glaucoma is still not sufficiently understood.

MATERIALS AND METHODS

A literature search was carried in PubMed on the subject of IOP fluctuations related to posture. Analysis and evaluation of the published study results and a summary of available clinical data.

RESULTS

The increase in IOP when changing from a seated to a lying body position is greater in glaucoma patients with an increase of up to 8.6 mm Hg compared to healthy subjects with an increase up to 5 mm Hg. In small pilot studies the increase in lying IOP in some glaucoma patients and healthy volunteers could be attenuated by elevation of the head by 30%. A lower compartmental pressure in the subarachnoid space has been associated with glaucoma and may represent a risk factor for glaucoma development. Not only the level of IOP but also IOP fluctuations were associated with an increased risk of disease progression.

CONCLUSION

The clinical significance of IOP peaks during sleep on glaucoma is still not sufficiently understood. New methods for continuous IOP measurement offer promising opportunities for further research into the importance of IOP fluctuations related to changes of body and head posture.

Cholinergic nervous system and glaucoma: From basic science to clinical applications

The cholinergic system has a crucial role to play in visual function. Although cholinergic drugs have been a focus of attention as glaucoma medications for reducing eye pressure, little is known about the potential modality for neuronal survival and/or enhancement in visual impairments. Citicoline, a naturally occurring compound and FDA approved dietary supplement, is a nootropic agent that is recently demonstrated to be effective in ameliorating ischemic stroke, traumatic brain injury, Parkinson's disease, Alzheimer's disease, cerebrovascular diseases, memory disorders and attention-deficit/hyperactivity disorder in both humans and animal models. The mechanisms of its action appear to be multifarious including (i) preservation of cardiolipin, sphingomyelin, and arachidonic acid contents of phosphatidylcholine and phosphatidylethanolamine, (ii) restoration of phosphatidylcholine, (iii) stimulation of glutathione synthesis, (iv) lowering glutamate concentrations and preventing glutamate excitotoxicity, (v) rescuing mitochondrial function thereby preventing oxidative damage and onset of neuronal apoptosis, (vi) synthesis of myelin leading to improvement in neuronal membrane integrity, (vii) improving acetylcholine synthesis and thereby reducing the effects of mental stress and (viii) preventing endothelial dysfunction. Such effects have vouched for citicoline as a neuroprotective, neurorestorative and neuroregenerative agent. Retinal ganglion cells are neurons with long myelinated axons which provide a strong rationale for citicoline use in visual pathway disorders. Since glaucoma is a form of neurodegeneration involving retinal ganglion cells, citicoline may help ameliorate glaucomatous damages in multiple facets. Additionally, trans-synaptic degeneration has been identified in humans and experimental models of glaucoma suggesting the cholinergic system as a new brain target for glaucoma management and therapy.

Glaucoma as a dangerous interplay between ocular fluid and cerebrospinal fluid

Glaucoma is a leading cause of irreversible blindness worldwide. Primary open-angle glaucoma, the most common type, is characterized by progressive degeneration of retinal ganglion cells and their axons in the optic nerve, resulting in progressive deterioration of visual fields. The optic nerve head is generally considered to be the primary site of axonal injury in glaucoma. Although the pathophysiology of glaucomatous optic neuropathy is not well understood, elevated intraocular pressure is considered the most important modifiable risk factor. However, in normal-tension glaucoma, intraocular pressure is not elevated and thus other risk factors must also be involved in the optic neuropathy of primary open-angle glaucoma. Several studies reported significantly lower intracranial pressure in patients with glaucoma compared with healthy subjects, suggesting that low intracranial pressure may result in a high pressure difference across the lamina cribrosa, influencing the physiology and pathophysiology of the optic nerve head by the effect of a mechanical force. Moreover, a rapidly evolving literature suggests the existence of an 'ocular glymphatic system'. This opens up new ways to understand the mechanisms underlying a range of ocular diseases such as glaucoma. In the present paper, I hypothesize that an imbalance between intraocular pressure and intracranial pressure, apart from generating mechanical forces at the lamina cribrosa, may lead to a dangerous interplay between ocular fluid and cerebrospinal fluid resulting in impaired cerebrospinal fluid entry into the optic nerve subarachnoid space and optic nerve perivascular spaces, and the perivascular space surrounding the central retinal artery in particular, thereby inhibiting glymphatic clearance of waste products from the retrobulbar or retrolaminar portion of the optic nerve. Should further research demonstrate that the proposed viewpoint is largely correct, it would hold great potential for our understanding of glaucomatous optic nerve damage and would have important implications for diagnosis and therapy of this devastating disorder.

The brain’sglymphatic system:physiological anatomy and clinical perspectives

The recently discovered glymphatic system (GS) ensures the efficient clearance of interstitial fluid and soluble compounds from the central nervous system into cerebrospinal fluid (CSF), which compensates for the lack of conventional lymphatic vessels in the brain parenchyma. This unique anatomical and physiological phenomenon had been unknown until 2012. GS lacks inherent proper vessels Р the current of CSF and interstitial fluid is carried out directly inside the arterial walls (the perivascular pathway) or near the walls of the cerebral arteries and veins (the paravascular pathway). Current biorheological technologies could establish a special role of aquaporin-4 in the filtration of CSF and interstitial fluid. The close link between GS and the CSF circulatory system allows the established views on fluid dynamics within the brain to be reconsidered. The discovery of GS can contribute to our understanding of the pathogenesis of increased intracranial pressure and neurodegenerative diseases, as well as to the elaboration of new therapeutic approaches to their treatment.