Role of cerebrospinal fluid in glaucoma: Pressure and beyond

Assessment of Cerebral Vasomotor Reactivity in Patients With Primary Open-angle Glaucoma and Ocular Hypertension Using the Breath-Holding Index

PRCIS

Patients with ocular hypertension (OHT) do not show impaired cerebral vasodilation responses to hypercapnia but patients with primary open-angle glaucoma (POAG) do. Impaired vasoreactivity in patients with POAG may have neuronal or vascular origins and increase stroke risk.

PURPOSE

To investigate changes in cerebral blood flow and cerebral vasomotor reactivity using the breath-holding index in patients with POAG and OHT, to examine whether these parameters contribute to the risk of ischemic stroke.

METHODS

Thirty patients with POAG, 30 patients with OHT, and 30 age- and sex-matched healthy control subjects were included in this university hospital-based, cross-sectional, and observational study. Eyes with a greater degree of visual field loss and/or more severe optic disc damage were selected for the study in patients with POAG, whereas in patients with OHT and controls, the study eye was chosen randomly. The mean blood flow velocity and breath-holding index were measured in the middle cerebral artery ipsilaterally in patient and control groups, by using transcranial Doppler ultrasonography.

RESULTS

The mean blood flow velocity and breath-holding indexes were significantly lower in patients with POAG than in the control group (all P<0.05). In the OHT group, the mean blood flow velocity and breath-holding indexes were not different from those in the control group.

CONCLUSIONS

Patients with POAG have impaired vasodilation response to hypercapnia. Presumably, the neuronal changes and deterioration of the endothelium-mediated vasodilatation in patients with glaucoma may disrupt the regulation of arteries and potentially present functional insufficiency on vasoreactivity. Moreover, impaired cerebral vascular regulation may contribute to the increased risk of stroke in patients with POAG.

Resveratrol Protects Optic Nerve Head Astrocytes from Oxidative Stress-Induced Cell Death by Preventing Caspase-3 Activation, Tau Dephosphorylation at Ser422 and Formation of Misfolded Protein Aggregates

Optic nerve head astrocytes (ONHAs) are the major cell type within the optic nerve head, providing both structural and nutrient support to the optic nerve. Astrocytes are necessary for the survival of neurons with controlled activation of astrocytes being beneficial to neurons. However, overactive astrocytes can be harmful and the loss of normal astrocyte function can be a primary contributor to neurodegeneration. The neuroprotective properties of reactive astrocytes can be lost or they might gain neurotoxic properties in neurodegenerative diseases. The activated astrocytes are crucial in the development of glaucoma, where they serve as a source for cytotoxic substances that participate in ganglion apoptosis. There is increasing evidence indicating that neuroinflammation is an important process in glaucoma. Under pathological conditions, astrocytes can induce an inflammatory response. Extensive evidence shows that inflammatory responses mediated by astrocytes can also influence pathology development, synapse health, and neurodegeneration. The elimination of activated astrocytes by apoptosis is also expected in unfavorable conditions. In neurodegenerative diseases, a common feature is the presence of aggregates found in astrocytes, which can disrupt astrocyte function in such a way as to be detrimental to the viability of neurons. The biological processes involved in vision loss in glaucoma are not well understood. Despite the rapid advances in our understanding of optic nerve head (ONH) structure and function, numerous potential contributions of the ONHAs to optic nerve damage remain unanswered. The present study investigated the role of ONHAs during oxidative stress in order to determine novel cell biological processes underlying glaucoma pathogenesis. ONHAs were exposed to chemically induced oxidative stress using tert-butyl hydroperoxide (tBHP) in order to model extracellular oxidative stress as it occurs in the glaucomatous retina and ONH. In order to determine the impact of an intervention approach employing potential glioprotective treatments for central nervous system tissue we pretreated cells with the polyphenolic phytostilbene and antioxidant trans-resveratrol (3,5,4'-trihydroxy-trans-stilbene). ONHAs exposed to tBHP-mediated oxidative stress displayed decreased viability and underwent apoptosis. In addition, increased levels of activated caspases, dephosphorylation of Tau protein at Ser422, an important site adjacent to the caspase cleavage site controlling Tau cleavage, caspase-mediated Tau cleavage, and cytoskeletal changes, specifically formation of neurofibrillary tangles (NFTs) were detected in ONHAs undergoing oxidative stress. When cells were pretreated with resveratrol cell viability increased along with a significant decrease in activated caspases, cleaved Tau, and NFT formation. Taken together, ONHAs appear to act similar to neurons when undergoing oxidative stress, where proteolytic cleavage of Tau by caspases leads to NFT formation. In addition, resveratrol appears to have promise as a potential protective treatment preventing ONHA dysfunction and degeneration. There is currently no cure for glaucoma or a neuro- and glioprotective treatment that directly targets the pathogenic mechanisms in the glaucomatous retina and optic nerve. The present study identified a potential mechanism underlying degeneration of astrocytes that is susceptible to pharmaco-therapeutic intervention in the eye and potentially elsewhere in the central nervous system. Identification of such mechanisms involved in glaucoma and other disorders of the eye and brain is critical to determine novel targets for effective therapies.

Association between glaucoma and the risk of Alzheimer's disease: A systematic review of observational studies

To address inconsistency as well as investigate the relationship between glaucoma and the risk of Alzheimer's disease (AD). We systematically conducted this meta-analysis based on observational studies published up to 15 January 2018, identified from PubMed and Web of Science. Two team members independently extracted the data and assessed the quality of each included study. Summary relative risk (RR) and 95% confidence intervals (CIs) were calculated using a random-effects model. Eight observational studies with 6870 AD cases were included. The majority of these studies (n = 6) were graded as low risk according to the Newcastle-Ottawa Quality Assessment Scale. Individuals diagnosed with glaucoma, compared to those who were not, had an increased risk of AD (RR = 1.52; 95% CI: 1.41-1.63; I²  = 97%, p < 0.001). A significant finding was also observed for primary open-angle glaucoma (RR = 1.52; 95% CI: 1.41-1.63; I²  = 97%, p < 0.001). However, when stratified by study design, only the case-control studies (RR = 1.08; 95% CI: 0.89-1.31; I²  = 37.3%, p = 0.207) yielded significant results, while the cohort studies did not (RR = 1.08; 95% CI: 0.89-1.31; I²  = 97.7%, p < 0.001). Of note, our meta-regression analysis suggested that study design might be a source of heterogeneity (p = 0.009). Additionally, a significantly positive association was observed when the analyses were restricted to Asia (RR = 2.03; 95% CI: 1.02-4.07). There was no significant publication bias in these analyses. Recent evidence suggests that glaucoma may increase the risk of AD. Additional cohort studies are needed to confirm these findings and to have improved knowledge on the true nature of this association.

Elevated Plasma Levels of Drebrin in Glaucoma Patients With Neurodegeneration

Glaucoma is an optic neuropathy characterized by progressive degeneration of retinal ganglion cells (RGCs). Aberrations in several cytoskeletal proteins, such as tau have been implicated in the pathogenesis of neurodegenerative diseases, could be initiating factors in glaucoma progression and occurring prior to axon degeneration. Developmentally regulated brain protein (Drebrin or DBN1) is an evolutionarily conserved actin-binding protein playing a prominent role in neurons and is implicated in neurodegenerative diseases. However, the relationship between circulating DBN1 levels and RGC degeneration in glaucoma patients remains unclear. In our preliminary study, we detected drebrin protein in the plasma of glaucoma patients using proteomic analysis. Subsequently, we recruited a total of 232 patients including primary angle-closure glaucoma (PACG), primary open-angle glaucoma (POAG) and Posner-Schlossman syndrome (PS) and measured its DBN1 plasma levels. We observed elevated DBN1 plasma levels in patients with primary glaucoma but not in patients with PS compared to nonaxonopathic controls. Interestingly, in contrast to tau plasma levels increased in all groups of patients, elevated drebrin plasma levels correlated with retinal nerve fiber layer defect (RNFLD) in glaucoma patients. To further explore the expression of DBN1 in neurodegeneration, we conducted experiment of optic nerve crush (ONC) models, and observed increased expression of DBN1 in the serum as well as in the retina and then decreased after ONC. This result reinforces the potentiality of circulating DBN1 levels are increased in glaucoma patients with neurodegeneration. Taken together, our findings suggest that circulating DBN1 levels correlated with RNFLD and may reflect the severity of RGCs injury in glaucoma patients. Combining measurement of circulating drebrin and tau levels may be a useful indicator for monitoring progression of neurodegenerative diseases.

The potential impact of recent insights into proteomic changes associated with glaucoma

INTRODUCTION

Glaucoma, a major ocular neuropathy, is still far from being understood on a molecular scale. Proteomic workflows revealed glaucoma associated alterations in different eye components. By using state-of-the-art mass spectrometric (MS) based discovery approaches large proteome datasets providing important information about glaucoma related proteins and pathways could be generated. Corresponding proteomic information could be retrieved from various ocular sample species derived from glaucoma experimental models or from original human material (e.g. optic nerve head or aqueous humor). However, particular eye tissues with the potential for understanding the disease's molecular pathomechanism remains underrepresented. Areas covered: The present review provides an overview of the analysis depth achieved for the glaucomatous eye proteome. With respect to different eye regions and biofluids, proteomics related literature was found using PubMed, Scholar and UniProtKB. Thereby, the review explores the potential of clinical proteomics for glaucoma research. Expert commentary: Proteomics will provide important contributions to understanding the molecular processes associated with glaucoma. Sensitive discovery and targeted MS approaches will assist understanding of the molecular interplay of different eye components and biofluids in glaucoma. Proteomic results will drive the comprehension of glaucoma, allowing a more stringent disease hypothesis within the coming years.

Estimated Cerebrospina Fluid Pressure and the 5-Year Incidence of Primary Open-Angle Glaucoma in a Chinese Population

Purpose

We aim to assess the longitudinal association between baseline estimated cerebrospinal fluid pressure (CSFP) and 5-year incident primary open angle glaucoma (POAG) in a population-based sample of Bai Chinese living in rural China.

Methods

Among the 2133 Bai Chinese aged 50 years or older who had participated in the baseline examination of the Yunnan Minority Eye Study, 1520 (71.3%) attended the follow-up examination after five years and 1485 were at risk of developing POAG. Participants underwent comprehensive ophthalmic examinations at both baseline and follow-up surveys. CSFP in mmHg was estimated as 0.55 × body mass index (kg/m2) + 0.16 × diastolic blood pressure (mmHg)-0.18 × age (years)-1.91. Glaucoma was defined using the International Society of Geographical and Epidemiological Ophthalmology Classification criteria. Multivariate logistic regression models were established to determine the association between baseline CSFP and incident POAG.

Results

After a mean follow-up time of 5 years, 19 new cases of POAG were detected, with an incidence rate of 1.3% (95% confidence interval, 0.7–1.9%). In multivariate logistic regression analysis, after adjusting for age, gender, education, intraocular pressure, central corneal thickness, hypertension and diabetes, no significant associations, nor any trends, were evident between baseline estimated CSFP and incident POAG. The association between estimated CSFP per mmHg increase in baseline and 5-year incidence of POAG was also non-significant, with adjusted relative risk of 0.96 (P = 0.11) in multivariate analysis.

Conclusions

This longitudinal cohort study does not support previously observed cross-sectional association between estimated CSFP and POAG in population-based studies.

The interaction between intracranial pressure, intraocular pressure and lamina cribrosal compression in glaucoma

This review examines some of the biomechanical consequences associated with the opposing intraocular and intracranial forces. These forces compress the lamina cribrosa and are a potential source of glaucomatous pathology. A difference between them creates a displacement force on the lamina cribrosa. Increasing intraocular pressure and/or decreasing intracranial pressure will increase the trans-lamina cribrosa pressure difference and the risk of its posterior displacement, canal expansion and the formation of pathological cupping. Both intraocular pressure and intracranial pressure can be elevated during a Valsalva manoeuvre with associated increases in both anterior and posterior lamina cribrosa loading as well as its compression. Any resulting thinning of or damage to the lamina cribrosa and/or retinal ganglion cell axons and/or astrocyte and glial cells attached to the matrix of the lamina cribrosa and/or reduction in blood flow to the lamina cribrosa may contribute to glaucomatous neuropathy. Thinning of the lamina cribrosa reduces its stiffness and increases the risk of its posterior displacement. Optic nerve head posterior displacement warrants medical or surgical lowering of intraocular pressure; however, compared to intraocular pressure, the trans-lamina cribrosa pressure difference may be more important in pressure-related pathology of the optic nerve head region. Similarly important could be increased compression loading of the lamina cribrosa. Reducing participation in activities which elevate intraocular and intracranial pressure will decrease lamina cribrosa compression exposure and may contribute to glaucoma management and may have prognostic significance for glaucoma suspects.

Fast circulation of cerebrospinal fluid: an alternative perspective on the protective role of high intracranial pressure in ocular hypertension

As ocular hypertension refers to a condition in which the intraocular pressure is consistently elevated but without development of glaucoma, study of it may provide important clues to factors that may play a protective role in glaucoma. β-amyloid, one of the key histopathological findings in Alzheimer's disease, has been reported to increase by chronic elevation of intraocular pressure in animals with experimentally induced ocular hypertension and to cause retinal ganglion cell death, pointing to similarities in molecular cell death mechanisms between glaucoma and Alzheimer's disease. On the other hand, recent studies have reported that intracranial pressure is higher in patients with ocular hypertension compared with controls, giving rise to the idea that elevated intracranial pressure may provide a protective effect for the optic nerve by decreasing the trans-lamina cribrosa pressure difference. The speculation that the higher intracranial pressure reported in ocular hypertension patients may protect against glaucoma mainly through a lower trans-lamina cribrosa pressure difference remains at least questionable. Here, we present an alternative viewpoint, according to which the protective effect of higher intracranial pressure could be due, at least in part, to a pressure-independent mechanism, namely faster cerebrospinal fluid production leading to increased cerebrospinal fluid turnover with enhanced removal of potentially neurotoxic waste products that accumulate in the optic nerve. This suggests a new hypothesis for glaucoma, which, just like Alzheimer's disease, may be considered then as an imbalance between production and clearance of neurotoxins, including β-amyloid. If confirmed, then strategies to improve cerebrospinal fluid flow are reasonable and could provide a new therapeutic approach for stopping the neurotoxic β-amyloid pathway in glaucoma.

Proof-of-Concept Prototype for Noninvasive Intracranial Pressure Monitoring Using Ocular Hemodynamics Under Applied Force

Studies have suggested that elevated cerebrospinal fluid (CSF) pressure can have a damaging effect on the optic nerve and visual acuity. There is need for a noninvasive CSF pressure measurement technique. A portable device for noninvasive intracranial pressure (ICP) monitoring would have a significant impact on clinical care. A proof-of-concept prototype is used to test the feasibility of a technique for monitoring ICP changes. The proposed methodology utilizes transcranial Doppler ultrasonography to monitor blood flow through the ophthalmic and central retinal arteries while forces are applied to the cornea by a controlled actuator. Control algorithms for the device were developed and tested using an integrated experimental platform. Preliminary results using tissue-mimicking materials show the ability to differentiate between materials of differing stiffness that simulates different levels of ICP. These experiments are an initial step toward a handheld noninvasive ICP monitoring device.

A novel rat model to study the role of intracranial pressure modulation on optic neuropathies

Reduced intracranial pressure is considered a risk factor for glaucomatous optic neuropathies. All current data supporting intracranial pressure as a glaucoma risk factor comes from retrospective and prospective studies. Unfortunately, there are no relevant animal models for investigating this link experimentally. Here we report a novel rat model that can be used to study the role of intracranial pressure modulation on optic neuropathies. Stainless steel cannulae were inserted into the cisterna magna or the lateral ventricle of Sprague-Dawley and Brown Norway rats. The cannula was attached to a pressure transducer connected to a computer that recorded intracranial pressure in real-time. Intracranial pressure was modulated manually by adjusting the height of a column filled with artificial cerebrospinal fluid in relation to the animal’s head. After data collection the morphological appearance of the brain tissue was analyzed. Based on ease of surgery and ability to retain the cannula, Brown Norway rats with the cannula implanted in the lateral ventricle were selected for further studies. Baseline intracranial pressure for rats was 5.5±1.5 cm water (n=5). Lowering of the artificial cerebrospinal fluid column by 2 cm and 4 cm below head level reduced ICP to 3.7±1.0 cm water (n=5) and 1.5±0.6 cm water (n=4), a reduction of 33.0% and 72.7% below baseline. Raising the cerebrospinal fluid column by 4 cm increased ICP to 7.5±1.4 cm water (n=2) corresponding to a 38.3% increase in intracranial pressure. Histological studies confirmed correct cannula placement and indicated minimal invasive damage to brain tissues. Our data suggests that the intraventricular cannula model is a unique and viable model that can be used to study the effect of altered intracranial pressure on glaucomatous optic neuropathies.

Is there a link between open-angle glaucoma and dementia? The Three-City-Alienor cohort

OBJECTIVE

Previous research has suggested an association between dementia and glaucoma through common risk factors or mechanisms. Our aim was to evaluate the longitudinal relationship between open-angle glaucoma (OAG) and incident dementia.

METHODS

The Three-City-Bordeaux-Alienor study is a population-based cohort of 812 participants with a 3-year follow-up period. All participants were aged 72 years or older. An eye examination was performed on all subjects. An OAG was determined based on optic nerve damage and visual field loss. Incident dementia was actively screened for and confirmed by a neurologist.

RESULTS

A total of 41 participants developed dementia over the 3-year follow-up period. Future incident dementia cases had an increased prevalence of OAG (17.5% vs 4.5% for nondemented participants, p = 0.003). After adjustment for age, gender, education, family history of glaucoma, vascular comorbidities, and apolipoprotein ε4, our results showed that participants with an OAG were four times more likely to develop dementia during the 3-year follow-up period (odds ratio = 3.9, 95% confidence interval = 1.5-10.4, p = 0.0054). An increased risk of dementia was also associated with 2 markers of optic nerve degeneration (vertical cup:disk ratio and minimal rim:disk ratio). However, no association was found between a high intraocular pressure and/or the use of intraocular pressure-lowering medications and incident dementia.

INTERPRETATION

If the association between OAG and dementia is confirmed, direct and noninvasive quantification of the amount of retinal ganglion cell axonal loss may be a useful biomarker of cerebral axonal loss in the future. It may also offer new breakthroughs in understanding the underlying pathophysiological mechanisms of both diseases.

Hemodynamic interactions in the eye: a review

The ocular circulation provides readily visible information about the state of the systemic circulation, as well as being potentially of relevance to the pathogenesis of ocular disorders such as glaucoma. The interaction between intraocular pressure, retinal vessels and cerebrospinal fluid pressure located at the retrolaminar portion of the eye has been of great interest for both ophthalmic and neurological clinicians and researchers. Understanding the relationship between these physiological parameters can explain phenomena such as spontaneous retinal venous pulsatility, and characterize the effects of the translaminar pressure gradient. It may be feasible to use measurable changes in venous pulsatility to enhance clinical assessment in different diseases. In this article we review recent findings on ocular hemodynamics and the relevance of these parameters in the diagnosis of ophthalmic and neurological diseases.

Systemic adeno-associated virus-mediated gene therapy preserves retinal ganglion cells and visual function in DBA/2J glaucomatous mice

A slow progressive death of neurons is the hallmark of neurodegenerative diseases, such as glaucoma. A therapeutic candidate, erythropoietin (EPO), has shown promise in many models of these diseases; however, it also causes polycythemia, a potentially lethal side effect. We have developed a novel mutant form of EPO that is neuroprotective but no longer erythropoietic by altering a single amino acid (arginine to glutamate at position 76; R76E). We hypothesized that a single intramuscular injection of recombinant adeno-associated virus carrying EpoR76E (rAAV2/5.CMV.EpoR76E) would protect retinal ganglion cells in a mouse model of glaucoma without inducing polycythemia. This systemic treatment not only protected the retinal ganglion cell somata located within the retina; it also preserved axonal projections within the optic nerve, while maintaining the hematocrit within normal limits. The rescued retinal ganglion cells retained their visual function demonstrated by flash visual evoked potentials. To our knowledge, this is the first demonstration of a therapy that protects neurons from death and prevents loss of visual function from the slow neurodegenerative effects of glaucoma. Because of its broad range of cellular targets, EpoR76E is likely to be successful in treating other neurodegenerative diseases as well.

Are intracranial pressure fluctuations important in glaucoma?

Glaucoma is one of the leading causes of irreversible blindness. Primary open-angle glaucoma (POAG), the most common type, is a progressive optic neuropathy with characteristic structural changes in the optic nerve head and functional changes in the visual field. Mechanical and vascular theories for the pathogenesis of glaucomatous optic neuropathy have been proposed. Elevated intraocular pressure (IOP) is a strong risk factor, although a subset of POAG patients has normal IOP and is designated normal tension glaucoma (NTG). Clearly, factors other than IOP are likely to be involved in retinal ganglion cell death in glaucoma. An intriguing finding of recent studies is that intracranial pressure (ICP) is lower in patients with POAG and NTG when compared with nonglaucomatous control subjects. It has been suggested that the relationship between IOP and ICP may play a fundamental role in the development of glaucoma. A decreased ICP could result in an increased trans-lamina cribrosa pressure difference (IOP minus ICP) and lead to glaucomatous damage. In the present paper, we raise the question of whether ICP fluctuations also may be important in glaucoma. The effect of ICP fluctuation might be comparable to that of IOP fluctuation, which has been recognized as an independent risk factor for glaucoma progression.