Causes and Risk Factors for Glaucoma
Glaucoma is a multifactorial optic neuropathy caused by a complex interplay of elevated intraocular pressure (IOP), vascular insufficiency, genetic predisposition, and structural vulnerabilities of the optic nerve, with the primary modifiable risk factor being elevated IOP. 1
Primary Etiologic Mechanisms
Elevated Intraocular Pressure (IOP)
IOP elevation is the most important modifiable risk factor and the primary target for treatment. Population-based studies demonstrate that glaucoma prevalence increases dramatically with rising IOP levels—at 30 mmHg, nearly 7% of Caucasians and 25% of African Americans develop primary open-angle glaucoma (POAG). 1 However, IOP above 21 mmHg has poor predictive value as a screening cutoff, since 13-71% of patients with elevated IOP (depending on the population studied) do not have glaucomatous damage, highlighting significant interindividual variation in optic nerve susceptibility. 1
Vascular and Perfusion Abnormalities
Low ocular perfusion pressure represents a critical vascular mechanism, particularly in normal-tension glaucoma. The relationship between blood pressure and glaucoma is complex:
- Systemic hypertension shows conflicting evidence, but after adjusting for confounders, patients with hypertension alone have 17% increased risk of POAG, rising to 48% when combined with diabetes 1
- Low diastolic perfusion pressure increases glaucoma risk specifically in patients receiving antihypertensive treatment 1
- Nocturnal hypotension and exaggerated nighttime blood pressure dips create unstable ocular blood flow, particularly affecting central visual fields in normal-tension glaucoma 2
Migraine headaches and peripheral vasospasm (Raynaud's syndrome) are established risk factors that decrease autoregulation of optic disc blood flow, making the optic nerve more vulnerable to damage. 1
Major Demographic and Genetic Risk Factors
Non-Modifiable Risk Factors
Age and race are the strongest demographic predictors:
- African Americans have threefold higher prevalence than non-Hispanic whites and it is the leading cause of blindness in this population 1
- Hispanics/Latinos have comparable prevalence rates to African Americans 1
- Asian populations have highest rates of angle-closure glaucoma, with PACG accounting for nearly as many cases as open-angle glaucoma in some Asian populations 3
- Older age consistently increases risk across all populations 1
Family history of glaucoma is one of the most important risk factors, with genetic mutations in MYOC, OPTN, and TBK1 implicated in disease pathogenesis. 4 Polygenic risk scores are emerging as tools for personalized risk assessment. 4
Male sex is associated with higher glaucoma risk, possibly due to protective effects of female hormones on ganglion cell loss, though women carry larger population burden due to longer survival. 1
Structural and Ocular Risk Factors
Corneal and Anterior Segment Factors
- Thin central cornea is a well-established risk factor for POAG 1
- Shallow anterior chamber depth (both central and peripheral) increases angle-closure risk 3
- Hyperopia and short axial length predispose to angle-closure disease 3
Optic Nerve Characteristics
- Large cup-to-disc ratio at baseline 1
- Disc hemorrhages are significant markers of progressive disease 1
- High pattern standard deviation on visual field testing 1
Myopia
Axial myopia independently increases POAG risk across all ethnic groups studied (Afro-Caribbeans, Hispanics, non-Hispanic whites, Chinese, Asian Indians, Japanese). 1 The mechanism involves weaker scleral support at the optic nerve, increasing susceptibility to glaucomatous damage. 1
Systemic Disease Associations
Type 2 Diabetes Mellitus
Diabetes increases glaucoma risk by 40-100% depending on ethnicity, with longer disease duration conferring higher risk. 1 A meta-analysis of 47 studies confirmed this association. 1 Mechanisms include microvascular changes in the optic nerve and potentially elevated IOP from altered corneal biomechanics. 1
Hypothyroidism
Hypothyroidism may increase glaucoma risk through decreased cellular metabolism increasing ganglion cell susceptibility and alterations in trabecular meshwork mucopolysaccharides that elevate IOP. 1
Emerging Associations
Lower intracranial pressure creates an increased translaminar pressure gradient (difference between IOP and intracranial pressure), with studies showing glaucoma patients have lower intracranial pressure than controls, particularly pronounced in normal-tension glaucoma. 1 However, additional research is needed to confirm this as a definitive risk factor. 1
Infectious Etiologies (Secondary Glaucoma)
Viral infections can cause secondary or uveitic glaucoma through anterior segment inflammation. Implicated viruses include herpes simplex virus, varicella-zoster virus, cytomegalovirus, rubella virus, dengue virus, chikungunya virus, Ebola virus, Zika virus, and SARS-CoV-2. 5 These are relatively uncommon causes, often underreported due to lack of long-term follow-up. 5
Clinical Pitfalls
Avoid using IOP >21 mmHg as a rigid diagnostic cutoff—this has poor predictive value and misses many patients with normal-tension glaucoma while falsely flagging many with ocular hypertension who will never develop glaucoma. 1
Do not overlook vascular risk factors in patients with normal IOP—these patients may have normal-tension glaucoma driven primarily by perfusion abnormalities rather than pressure. 2
Screen family members of glaucoma patients aggressively—family history is one of the strongest risk factors, and genetic predisposition may not always be apparent from patient history. 6
Consider the cumulative effect of multiple risk factors—patients with combined hypertension and diabetes have 48% increased risk versus 17% for hypertension alone. 1