Introduction

Glaucoma has traditionally been regarded as a disease primarily caused by elevated intraocular pressure (IOP), and for decades clinical management has focused mainly on lowering IOP as the principal strategy for preventing optic nerve damage. However, accumulating clinical observations and experimental evidence have challenged this traditional concept. Progressive optic neuropathy may develop in patients with normal-tension glaucoma (NTG) despite IOP remaining within the normal range, whereas many individuals with persistently elevated IOP never develop glaucomatous optic nerve damage (1–3). These observations strongly suggest that elevated IOP is an important risk factor rather than the sole pathogenic mechanism responsible for glaucoma. Consequently, glaucoma is now increasingly recognized as a multifactorial neurodegenerative disease characterized by the progressive degeneration of retinal ganglion cells (RGCs) and their axons, which together form the optic nerve (4–6).

In several respects, the pattern of neuronal loss resembles that observed in other neurodegenerative disorders such as Alzheimer’s disease and Parkinson’s disease, although glaucoma possesses distinct pathological characteristics specific to the visual system (13–15).

Over the past two decades, substantial advances in molecular biology and neuroscience have revealed multiple cellular and molecular pathways contributing to glaucomatous neurodegeneration. These include glutamate-mediated excitotoxicity, oxidative stress, mitochondrial dysfunction, vascular insufficiency, impaired axonal transport, deprivation of neurotrophic factors, neuroinflammation, glial cell activation, as well as genetic and environmental influences (4–12,16–20).

Understanding these mechanisms not only provides a more comprehensive explanation for optic nerve degeneration but also offers promising opportunities for the development of novel therapeutic strategies aimed at protecting retinal ganglion cells and preserving visual function. Neuroprotection has therefore emerged as one of the most important directions in contemporary glaucoma research (21–24).

 

Anatomy of the Optic Nerve and the Lamina Cribrosa

The optic nerve is composed of more than one million axons originating from retinal ganglion cells (RGCs), which transmit visual information from the retina to the visual centers of the brain. These axons converge at the optic disc to form the optic nerve before exiting the globe. As they leave the eye, all axons pass through a specialized connective tissue structure known as the lamina cribrosa, a multilayered collagenous meshwork located within the optic nerve head. This structure provides essential mechanical support for both the optic nerve axons and the blood vessels traversing the scleral canal (1,4,5).

The lamina cribrosa is widely recognized as one of the most vulnerable sites to glaucomatous injury. Mechanical strain induced by elevated intraocular pressure, together with age-related connective tissue remodeling and vascular insufficiency, may result in deformation of the lamina cribrosa, compression of axonal bundles, and

disruption of axoplasmic transport. These changes ultimately contribute to retinal ganglion cell degeneration and progressive optic neuropathy (4–7).

Recent advances in optical coherence tomography (OCT) and enhanced-depth imaging have enabled detailed visualization of the lamina cribrosa in vivo, demonstrating that structural alterations of this region may occur during the early stages of glaucoma, even before significant visual field loss becomes clinically detectable (8–10).

 

Early Clinical Signs of Optic Nerve Damage

Structural alterations of the optic nerve head represent some of the earliest clinically detectable manifestations of glaucoma and frequently precede measurable functional defects on standard automated perimetry (1,4).

 

Optic Disc Cupping

Progressive loss of retinal ganglion cell axons results in enlargement and deepening of the optic cup, leading to an increased cup-to-disc (C/D) ratio. Although physiological variations exist, progressive optic disc cupping remains one of the hallmark features of glaucomatous optic neuropathy (4,5).

 

Neuroretinal Rim Thinning

The neuroretinal rim contains the axons of viable retinal ganglion cells. As glaucoma progresses, gradual axonal loss causes thinning of the neuroretinal rim, particularly in the inferior and superior regions, consistent with the well-established ISNT rule observed in healthy optic discs. Progressive violation of this pattern is considered an important indicator of glaucomatous damage (5,9).

 

Optic Disc Hemorrhage

Optic disc hemorrhage is an important clinical marker associated with active glaucomatous progression. Numerous longitudinal studies have demonstrated that disc hemorrhages are associated with an increased risk of retinal nerve fiber layer (RNFL) thinning and subsequent visual field deterioration, particularly in patients with normal-tension glaucoma (11–13).

 

Retinal Nerve Fiber Layer Thinning

Thinning of the retinal nerve fiber layer (RNFL) can be accurately quantified using optical coherence tomography (OCT) and is considered one of the most sensitive biomarkers for the early detection of glaucomatous damage. In many patients, structural RNFL loss precedes detectable visual field defects by several years, emphasizing the importance of OCT in the diagnosis and monitoring of glaucoma (8–10,14).

Collectively, these structural changes reflect the progressive degeneration of retinal ganglion cells and their axons and represent the earliest clinical manifestations of the neurodegenerative cascade underlying glaucomatous optic neuropathy.

 

Glutamate and Excitotoxicity in Glaucoma

Glutamate is the principal excitatory neurotransmitter in both the retina and the central nervous system. Under physiological conditions, it plays a crucial role in visual signal transmission from photoreceptors to bipolar cells and subsequently to retinal ganglion cells (RGCs). Extracellular glutamate concentrations are tightly regulated to maintain normal synaptic function and prevent neuronal toxicity (16–18).

Under conditions of chronic cellular stress, ischemia, mitochondrial dysfunction, or glaucomatous injury, glutamate homeostasis may become disrupted, leading to excessive extracellular accumulation of glutamate. This, in turn, causes prolonged activation of N-methyl-D-aspartate (NMDA) receptors expressed on retinal ganglion cells (16–19).

Excessive NMDA receptor stimulation results in an abnormal influx of calcium ions into neurons, initiating a cascade of pathological events that includes:

• Activation of proteolytic and lipolytic enzymes. Mitochondrial dysfunction.
• Excessive production of reactive oxygen species (ROS). DNA damage.
• Lipid peroxidation and disruption of cellular membranes.
• Activation of intrinsic apoptotic pathways leading to programmed cell death.

This process, known as glutamate-mediated excitotoxicity, is considered one of the principal mechanisms responsible for retinal ganglion cell degeneration in glaucoma (16–20).

 

Factors Contributing to Glutamate Excitotoxicity

Several pathological processes enhance glutamate-mediated neurotoxicity and accelerate retinal ganglion cell loss.

 

Vascular Insufficiency

Reduced ocular blood flow and chronic ischemia impair oxygen and glucose delivery, resulting in decreased ATP production. Consequently, ATP-dependent ion pumps fail to maintain ionic homeostasis, reducing the ability of both neurons and glial cells to remove extracellular glutamate efficiently. The resulting glutamate accumulation further amplifies excitotoxic neuronal injury (18–21).

 

Neuronal Injury

As retinal ganglion cells undergo degeneration, damaged neurons release additional glutamate into the extracellular space. This creates a vicious cycle in which injured neurons promote further excitotoxic damage to neighboring healthy cells, thereby accelerating disease progression (17–20).

 

Mitochondrial Dysfunction

Mitochondria are the primary source of cellular energy within retinal ganglion cells.

Mitochondrial impairment reduces ATP production, compromises glutamate recycling, and increases oxidative stress, thereby enhancing neuronal susceptibility to excitotoxic injury (20–23).

 

Glial Cell Dysfunction

Glial cells play indispensable roles in maintaining the physiological environment of the retina and optic nerve. Their supportive functions include regulation of neurotransmitter concentrations, maintenance of ionic balance, metabolic support, and protection of neuronal tissue.

 

Müller Cells

Müller cells span nearly the entire thickness of the retina and perform numerous essential functions, including:

• Structural and metabolic support of retinal neurons. Regulation of water and electrolyte homeostasis.
• Uptake of excess extracellular glutamate.
• Maintenance of retinal metabolic balance.

These cells express specialized glutamate transporters, particularly EAAT1 (GLAST), which efficiently remove extracellular glutamate and thereby protect retinal ganglion cells from excitotoxic damage (18–21).

 

Astrocytes

Astrocytes are predominantly located within the retinal nerve fiber layer, optic nerve head, and lamina cribrosa. They contribute to:

Structural support of optic nerve axons. Maintenance of the blood–optic nerve barrier. Regulation of local blood flow.

Preservation of extracellular homeostasis. Protection of retinal ganglion cell axons.

During the early stages of glaucoma, astrocytes exert protective effects by limiting neuronal injury. However, persistent mechanical stress and chronic inflammation eventually induce reactive gliosis, characterized by astrocyte hypertrophy, proliferation, and altered gene expression. Although initially beneficial, prolonged reactive gliosis contributes to extracellular matrix remodeling, chronic inflammation, and progressive optic nerve damage (21–24).

 

 

Neuroinflammation and Microglial Activation

Neuroinflammation is now recognized as a central component of glaucomatous neurodegeneration. Among the various inflammatory cells involved, microglia play a pivotal role in initiating and regulating the immune response within the retina and optic nerve (22–25).

Under physiological conditions, microglia remain in a resting surveillance state, continuously monitoring the neural microenvironment. Elevated intraocular pressure, ischemia, oxidative stress, and neuronal injury trigger microglial activation, transforming these cells into a pro-inflammatory phenotype (22–25). Activated microglia release numerous inflammatory mediators, including:

Tumor Necrosis Factor-alpha (TNF-α)

Interleukin-1 beta (IL-1β) Interleukin-6 (IL-6)

Nitric oxide (NO)

Reactive oxygen species (ROS)

Although this inflammatory response may initially serve a protective function by removing cellular debris and promoting tissue repair, persistent activation results in chronic neuroinflammation. Sustained production of inflammatory cytokines and oxidative mediators accelerates retinal ganglion cell apoptosis, disrupts axonal integrity, and contributes significantly to progressive optic nerve degeneration (22–27). Experimental studies have further demonstrated that microglial activation may occur during the earliest stages of glaucoma, preceding detectable structural damage to the optic nerve head. Consequently, modulation of microglial activity has emerged as a promising therapeutic target for future neuroprotective strategies (24–27).

 

Oxidative Stress and Mitochondrial Dysfunction

Oxidative stress is widely recognized as one of the principal mechanisms contributing to the development and progression of glaucoma. It occurs when the production of reactive oxygen species (ROS) exceeds the capacity of endogenous antioxidant defense systems, resulting in cumulative cellular damage (20,23,28).

Retinal ganglion cells (RGCs) have exceptionally high metabolic demands and rely heavily on mitochondrial oxidative phosphorylation for ATP production. Consequently, they are particularly vulnerable to mitochondrial dysfunction induced by aging, ischemia, genetic susceptibility, or chronic mechanical stress. Impaired mitochondrial function not only reduces cellular energy production but also promotes excessive ROS generation, thereby creating a self-perpetuating cycle of oxidative injury (20,23,28–30). Excessive ROS production leads to multiple pathological alterations, including: Oxidative damage to nuclear and mitochondrial DNA.

Lipid peroxidation of cellular membranes. Oxidative modification of intracellular proteins. Impairment of mitochondrial respiratory enzymes. Activation of intrinsic apoptotic pathways.

Increased susceptibility of retinal ganglion cells to neurodegeneration.

Growing experimental evidence suggests that mitochondrial dysfunction is not merely a consequence of retinal ganglion cell death but may represent one of the earliest pathogenic events in glaucoma. Furthermore, inherited mitochondrial abnormalities and age-related mitochondrial decline may increase individual susceptibility to glaucomatous optic neuropathy (28–31).

These findings have identified mitochondrial preservation and enhancement of cellular bioenergetics as promising therapeutic targets for future neuroprotective interventions.

 

The Trabecular Meshwork and Elevated Intraocular Pressure

The trabecular meshwork (TM) is the principal conventional outflow pathway responsible for maintaining physiological intraocular pressure through drainage of aqueous humor.

Recent studies have demonstrated that oxidative stress and chronic low-grade inflammation affect not only retinal neurons but also trabecular meshwork cells. Persistent oxidative injury results in progressive loss of trabecular endothelial cells, accumulation of extracellular matrix (ECM), increased expression of transforming growth factor-beta (TGF-β), and increased resistance to aqueous humor outflow (4,29,32).

These pathological alterations ultimately impair aqueous humor drainage, leading to elevated intraocular pressure, which further exacerbates optic nerve injury through mechanical deformation of the lamina cribrosa and disruption of axonal transport.

Accordingly, elevated intraocular pressure should no longer be viewed solely as the primary cause of glaucoma. Rather, in many patients, it represents one component of a complex pathological cascade involving oxidative stress, chronic inflammation, extracellular matrix remodeling, vascular dysregulation, and neurodegeneration (4,29,32).

 

Neurotrophic Factors and Neurodegeneration

The survival and functional integrity of retinal ganglion cells depend on a continuous supply of neurotrophic factors. Among these, brain-derived neurotrophic factor (BDNF) plays a central role in promoting neuronal survival, maintaining synaptic function, and protecting axons from degeneration (24,30,33).

Under physiological conditions, BDNF is transported retrogradely from central visual targets in the brain to retinal ganglion cells through axonal transport. However, glaucomatous deformation of the lamina cribrosa disrupts this retrograde transport, resulting in progressive deprivation of neurotrophic support (24,30,33).

Deficiency of BDNF renders retinal ganglion cells increasingly susceptible to oxidative stress, glutamate-mediated excitotoxicity, mitochondrial dysfunction, and apoptosis. Experimental studies have demonstrated that restoration of BDNF signaling or enhancement of its receptor activity may significantly improve neuronal survival and delay glaucomatous neurodegeneration (30,33,34).

These observations have positioned neurotrophic factor supplementation as one of the most promising strategies for future neuroprotective therapy.

 

Biomarkers of Neurodegeneration in Glaucoma

The identification of reliable biomarkers has become a major focus of contemporary glaucoma research, with the aim of improving early diagnosis, monitoring disease progression, and evaluating treatment response (31–35). Several candidate biomarkers have shown considerable promise.

 

Neurofilament Light Chain (NfL)

Neurofilament light chain is a highly sensitive biomarker of axonal injury. Elevated NfL concentrations have been reported in several neurodegenerative disorders, including glaucoma, reflecting ongoing degeneration of retinal ganglion cell axons (31–34).

 

Glial Fibrillary Acidic Protein (GFAP)

GFAP is a structural protein expressed predominantly by astrocytes and serves as an established marker of glial activation and neuroinflammation. Increased GFAP expression reflects reactive gliosis and has been associated with glaucomatous optic nerve injury (22,24,35).

 

Brain-Derived Neurotrophic Factor (BDNF)

Reduced levels of BDNF may indicate impaired neurotrophic support and progressive retinal ganglion cell degeneration, making it a potential biomarker of disease severity and therapeutic response (30,33,34).

 

Oxidative Stress Biomarkers

Several biochemical markers of oxidative damage—including products of lipid peroxidation, protein oxidation, and oxidative DNA injury—have been investigated as indicators of glaucomatous neurodegeneration. Although promising, further validation is required before these biomarkers can be incorporated into routine clinical practice (28,31,35).

Despite substantial advances, no single biomarker currently possesses sufficient sensitivity and specificity for routine clinical use. Future diagnostic strategies will likely combine structural imaging, functional testing, and molecular biomarkers to enable personalized glaucoma management and earlier detection of neurodegenerative changes.

 

The Pathophysiological Cascade of Glaucomatous Neurodegeneration

Current evidence indicates that glaucoma develops through a complex and interconnected cascade of pathological events rather than through a single causative mechanism. Mechanical stress, vascular insufficiency, oxidative stress, mitochondrial dysfunction, glutamate-mediated excitotoxicity, chronic neuroinflammation, impaired neurotrophic support, and genetic susceptibility interact to promote progressive retinal ganglion cell (RGC) degeneration and optic nerve damage (4–35).

These pathogenic processes reinforce one another through multiple positive feedback loops. For example, mitochondrial dysfunction increases reactive oxygen species (ROS) production, which further amplifies oxidative stress, activates microglia, enhances glutamate excitotoxicity, and accelerates apoptotic cell death. Simultaneously, disruption of axonal transport at the level of the lamina cribrosa deprives retinal ganglion cells of essential neurotrophic support, rendering them increasingly vulnerable to degeneration (20–35).

This integrated model provides a more comprehensive explanation for disease progression than the traditional pressure-dependent theory and supports the contemporary concept of glaucoma as a multifactorial neurodegenerative disorder.

 

Future Neuroprotective Therapies in Glaucoma

Although lowering intraocular pressure remains the cornerstone of glaucoma management, many patients continue to experience progressive visual field loss despite achieving target IOP levels. Consequently, considerable research efforts are now directed toward the development of neuroprotective therapies aimed at preserving retinal ganglion cell survival and function (21,24,30).

 

Citicoline

Citicoline is among the most extensively investigated neuroprotective agents in glaucoma. Experimental and clinical studies suggest that it may exert beneficial effects through several mechanisms, including:

• Enhancement of neuronal membrane phospholipid synthesis.
• Improvement of mitochondrial function.
• Facilitation of neurotransmission.
• Reduction of oxidative stress.
• Promotion of neuronal survival.

Several clinical studies have reported improvements in visual function and electrophysiological parameters when citicoline is used as an adjunctive therapy in patients with glaucoma, although larger randomized trials are still needed to establish its long-term efficacy (21,30,36).

 

Coenzyme Q10

Coenzyme Q10 (CoQ10) is a potent endogenous antioxidant and an essential component of the mitochondrial electron transport chain.

Experimental evidence suggests that CoQ10 may:

Reduce reactive oxygen species production.

• Improve mitochondrial bioenergetics.
• Protect retinal ganglion cells from apoptosis.
• Enhance neuronal resistance to oxidative stress.

These findings support its potential role as an adjunctive neuroprotective therapy in glaucoma (28,30,36).

 

Memantine

Memantine is a non-competitive NMDA receptor antagonist originally developed for the treatment of Alzheimer's disease.

Its proposed mechanism in glaucoma involves attenuation of glutamate-mediated excitotoxicity by limiting excessive calcium influx into retinal ganglion cells.

Although large multicenter clinical trials did not demonstrate definitive clinical benefit, targeting excitotoxic pathways continues to represent an important area of ongoing glaucoma research (17,18,36).

 

Brain-Derived Neurotrophic Factor (BDNF)

Because BDNF plays a fundamental role in retinal ganglion cell survival, several experimental approaches have attempted to enhance BDNF signaling through:

• Direct BDNF supplementation.

• Activation of TrkB receptors.
• Gene-mediated delivery of neurotrophic factors.
• Improvement of retrograde axonal transport.

Preclinical studies have demonstrated significant neuroprotective effects, although effective long-term delivery methods remain a major challenge (30,33,34).

 

Stem Cell Therapy

Stem cell-based therapies represent one of the most promising emerging fields in regenerative ophthalmology.

Potential therapeutic mechanisms include:

• Replacement of damaged retinal neurons.
• Secretion of neurotrophic factors.
• Modulation of neuroinflammation.
• Enhancement of retinal repair.
• Improvement of the retinal microenvironment.

Although these approaches remain largely experimental, ongoing research continues to demonstrate encouraging results (34–37).

 

Gene Therapy

Recent advances in gene-editing technologies have opened new avenues for glaucoma treatment.

Potential applications include:

• Increasing neuronal resistance to oxidative stress.
• Modulating genes involved in apoptosis.
• Enhancing mitochondrial function.
• Reducing chronic neuroinflammation.
• Improving aqueous humor outflow.

Gene therapy may eventually provide individualized treatment strategies for patients with genetically susceptible forms of glaucoma (34–37).

 

Mitochondria-Targeted Therapies

Given the central role of mitochondrial dysfunction in glaucomatous

neurodegeneration, several investigational therapies specifically target mitochondrial preservation by:

• Enhancing ATP production.