PanOph

Primary open-angle glaucoma

Glaucoma

Key Points

  • •POAG is the most common form of glaucoma and leading cause of irreversible blindness worldwide — approximately 57.5 million affected
  • •Diagnosis requires open angles on gonioscopy with characteristic optic disc cupping and visual field loss — gonioscopy is mandatory
  • •IOP is the only modifiable risk factor — target 25–30% reduction from baseline (AGIS, EMGT, CIGTS)
  • •OCT RNFL thinning can precede visual field loss by up to 5 years, enabling earlier detection
  • •SLT is a valid first-line alternative to topical medications — 74% drop-free at 3 years (LiGHT trial, Lancet 2019)
1. Definition

Primary open-angle glaucoma (POAG) is a chronic, progressive optic neuropathy characterized by open anterior chamber angles on gonioscopy, characteristic optic disc cupping with corresponding visual field loss, and no identifiable secondary cause. It is the most common form of glaucoma worldwide and the leading cause of irreversible blindness globally (AAO PPP Open-Angle Glaucoma 2020).

2. Epidemiology

POAG is the leading cause of irreversible blindness globally, affecting approximately 57.5 million people worldwide. Prevalence increases with age: ~1.5% at age 40–49, rising to ~5% at age 70–79. African descent populations have 3–4x higher prevalence (up to 8–10% over age 70) compared to European descent (~2–3%). Hispanic/Latino populations have intermediate risk. Male and female prevalence is roughly equal. The Baltimore Eye Survey and Barbados Eye Study provided key epidemiological data. Approximately 50% of POAG cases remain undiagnosed in developed countries; this figure exceeds 90% in developing nations.

3. Aetiology and causes

The exact etiology remains multifactorial and incompletely understood. Key mechanisms include:

  1. Increased resistance to aqueous outflow through the trabecular meshwork (TM) — particularly the juxtacanalicular tissue and inner wall of Schlemm's canal
  2. Extracellular matrix (ECM) changes in the TM: increased cross-linked actin networks (CLANs), accumulation of sheath-derived plaques, and reduced cellularity
  3. Genetic factors: over 120 loci identified in GWAS; MYOC (myocilin) mutations account for 3–5% of POAG (most common: Gln368Stop); OPTN, TBK1, WDR36, and CAV1/CAV2 also implicated
  4. Vascular dysregulation and reduced ocular blood flow contributing to optic nerve head (ONH) ischemia
  5. Oxidative stress and mitochondrial dysfunction in TM cells and retinal ganglion cells (RGCs)
4. Pathogenesis

Two main theories explain RGC death in POAG:

Mechanical theory: Elevated IOP causes posterior bowing of the lamina cribrosa, compressing RGC axon bundles as they pass through the laminar pores. This disrupts axoplasmic flow (both anterograde and retrograde), depriving RGCs of neurotrophic factors (BDNF, CNTF) from the lateral geniculate nucleus.

Vascular theory: Reduced ocular perfusion pressure (OPP = 2/3 MAP − IOP) leads to ischemia at the ONH. Impaired autoregulation of ONH blood flow, endothelin-1-mediated vasoconstriction, and nocturnal blood pressure dips exacerbate ischemic damage.

Both mechanisms converge on a final common pathway: RGC apoptosis mediated by excitotoxicity (glutamate), oxidative stress, TNF-α from reactive astrocytes, and mitochondrial dysfunction. Loss of RGCs leads to thinning of the retinal nerve fiber layer (RNFL) and characteristic optic disc cupping.

Aqueous outflow resistance: The trabecular (conventional) pathway handles ~80–95% of outflow. In POAG, accumulation of ECM material, reduced TM cellularity, collapse of Schlemm's canal, and herniation of inner wall tissue into collector channel ostia increase outflow resistance.

5. Classification

IOP-based classification:

  • High-tension POAG (HTG): IOP consistently >21 mmHg (majority of POAG cases)
  • Normal-tension glaucoma (NTG): IOP consistently ≤21 mmHg with glaucomatous damage (considered a subtype or separate entity)

Staging by visual field loss (Hodapp-Parrish-Anderson criteria):

  • Early: MD better than −6 dB, <25% of points depressed below 5% on pattern deviation, <10% below 1%, no point in central 5° worse than 15 dB
  • Moderate: MD −6 to −12 dB, 25–50% of points depressed below 5%, <50% below 1%, no point in central 5° worse than 25 dB, only one hemifield with a point <15 dB in central 5°
  • Severe/Advanced: MD worse than −12 dB, >50% of points depressed below 5%, >20% below 1%, any point in central 5° at 0 dB, points within central 5° <15 dB in both hemifields

AAO Preferred Practice Pattern also uses AGIS and GSS scoring systems.

6. Risk factors and associations

Established risk factors:

  • Elevated IOP: Most important modifiable risk factor; each 1 mmHg increase raises risk by ~10% (OHTS)
  • Age: Risk doubles every decade after age 40
  • African descent: 3–4x higher risk, earlier onset, more aggressive course
  • Family history: 4–9x increased risk with affected first-degree relative
  • Central corneal thickness (CCT): Thin CCT (=<555 μm==) is an independent risk factor (OHTS, EMGT)
  • Myopia: Moderate to high myopia increases risk 2–3x

Probable risk factors:

  • Low ocular perfusion pressure / systemic hypotension
  • Diabetes mellitus (controversial, but Rotterdam and Blue Mountains studies suggest association)
  • Disc hemorrhages: Indicate disease progression
  • Pattern standard deviation (PSD) on baseline visual field

Protective factors: Higher CCT may be protective; exercise may lower IOP transiently.

7. Clinical features

Symptoms:

  • Typically asymptomatic until advanced stages — 'the silent thief of sight'
  • Peripheral visual field loss is not noticed until significant damage has occurred
  • Late: difficulty with mobility, reading, driving; tunnel vision in end-stage disease
  • No pain, redness, or acute symptoms (unlike angle-closure)

Signs:

  • IOP: May be elevated (>21 mmHg) or within statistical normal range
  • Gonioscopy: Open angles (Shaffer grade III–IV), no synechiae
  • Optic disc changes (in order of progression):

— Asymmetric cupping (>0.2 difference between eyes) — earliest sign

— Increased vertical cup-to-disc ratio (>0.6 or progressive increase)

— Focal notching of the neuroretinal rim (inferior > superior, following ISNT rule violation)

— Bayoneting of vessels at the disc margin

— Laminar dot sign (visibility of lamina cribrosa pores)

— Disc hemorrhages (Drance hemorrhages) — usually at 6 or 7 o'clock

— Peripapillary atrophy (beta-zone: atrophic RPE, associated with glaucoma)

— Nasal shifting of vessels

  • RNFL defects: Wedge-shaped defects visible on red-free photography
  • Visual field defects (in order of appearance): nasal step of Rönne → arcuate (Bjerrum) scotoma → paracentral scotoma → ring scotoma → temporal island with central island (end-stage)
8. Investigations

IOP measurement:

  • Goldmann applanation tonometry (GAT): Gold standard; measures force to flatten 3.06 mm diameter cornea; affected by CCT
  • Pachymetry: CCT measurement; thin corneas underestimate IOP, thick overestimate

Gonioscopy:

  • Shaffer/Spaeth grading; must confirm open angles to diagnose POAG
  • Rule out secondary causes (pigment, pseudoexfoliation, neovascularization)

Optic disc assessment:

  • Stereoscopic disc photography (baseline documentation)
  • OCT RNFL and ganglion cell analysis: RNFL thinning precedes VF loss by up to 5 years; inferior and superior sectors affected earliest
  • HRT (Heidelberg Retinal Tomograph): Confocal scanning laser ophthalmoscopy for disc topography

Visual field testing:

  • Standard automated perimetry (SAP): Humphrey 24-2 or 30-2 SITA Standard/Fast
  • Look for: nasal step, arcuate defects, paracentral scotomas
  • Reliability indices: fixation losses =<20%, false positives =<15%, false negatives =<33%==
  • Progression: GPA (Guided Progression Analysis), VFI trend analysis
  • 10-2 testing for central field assessment in advanced disease

Additional:

  • Diurnal IOP curve (phasing): Measure IOP at multiple times; peak IOP and fluctuation important
  • Disc photos for serial comparison
  • Anterior segment OCT or UBM if angle anatomy in question
9. Differential diagnosis
  1. Normal-tension glaucoma (NTG): IOP consistently ≤21 mmHg; may represent spectrum of POAG; disc hemorrhages more common; consider overlap
  2. Ocular hypertension (OHT): IOP >21 mmHg without disc or field damage; OHTS showed ~10% convert to POAG over 5 years untreated
  3. Physiological large cup: Large cup-to-disc ratio but proportional to disc size; stable over time; normal RNFL; no field defects; symmetric
  4. Secondary open-angle glaucomas: Pigmentary glaucoma (Krukenberg spindle, iris transillumination, heavy TM pigment), pseudoexfoliation, steroid-induced, lens-related
  5. Chronic angle-closure glaucoma: May mimic POAG if PAS not detected; mandatory gonioscopy in all new glaucoma patients
  6. Compressive optic neuropathy: Pituitary adenoma, meningioma; visual field defects may mimic glaucoma; look for pallor > cupping, respect vertical midline; MRI indicated if field loss is atypical
10. Complications

Ocular complications of disease:

  • Progressive visual field loss leading to legal blindness (VA ≤6/60 or VF <20°)
  • Disc hemorrhages indicating ongoing damage
  • Central visual acuity loss in advanced/end-stage disease
  • Complete blindness if untreated (lifetime risk ~15% for bilateral blindness)

Complications of medical treatment:

  • Beta-blockers (timolol): Bronchospasm, bradycardia, hypotension, depression, impotence
  • Prostaglandin analogues (latanoprost): Iris pigmentation, periorbital fat atrophy (prostaglandin-associated periorbitopathy — PAP), eyelash growth, CME in pseudophakes, HSV reactivation
  • Alpha-agonists (brimonidine): Allergic conjunctivitis (12–15%), granulomatous anterior uveitis (rare)
  • CAIs (dorzolamide): Corneal decompensation in low endothelial cell counts, metallic taste, aplastic anemia (oral)

Complications of surgical treatment:

  • Trabeculectomy: Bleb leak, bleb-related infection/endophthalmitis (0.2–1.5%/year), hypotony maculopathy, choroidal effusion, cataract progression, malignant glaucoma
  • Tube shunts: Tube erosion, corneal decompensation, diplopia, hypertensive phase
  • MIGS: Generally lower complication rates; transient hyphema, IOP spikes
11. Management

Target IOP: Reduce IOP by at least 25–30% from baseline (AGIS, CIGTS, EMGT); adjust target based on disease severity, rate of progression, life expectancy.

First-line medical therapy:

  • Prostaglandin analogues (PGA): Latanoprost 0.005% once nightly, travoprost 0.004% once nightly, bimatoprost 0.01–0.03% once nightly, tafluprost 0.0015% (preservative-free). Reduce IOP 25–33%. Most effective single agents.

Second-line / adjunctive:

  • Beta-blockers: Timolol 0.5% BD (or gel-forming QD), betaxolol 0.25–0.5% BD (beta-1 selective). Reduce IOP 20–25%.
  • Alpha-2 agonists: Brimonidine 0.1–0.2% TDS. Reduce IOP 18–25%. Neuroprotective properties (Low-Pressure Glaucoma Treatment Study).
  • Topical CAIs: Dorzolamide 2% TDS, brinzolamide 1% TDS. Reduce IOP 15–20%.
  • Rho-kinase inhibitors: Netarsudil 0.02% once nightly. Reduce IOP 15–20%. Works on TM/Schlemm's canal.

Fixed combinations: Timolol-dorzolamide (Cosopt), timolol-brimonidine (Combigan), timolol-latanoprost (Xalacom), netarsudil-latanoprost (Rocklatan).

Laser therapy:

  • Selective laser trabeculoplasty (SLT): Can be used as first-line (LiGHT trial, Lancet 2019); 532 nm Q-switched Nd:YAG; 360 degrees; repeatable. Reduces IOP 20–25%. ~80% success at 1 year. 74% of patients drop-free at 3 years (LiGHT).

Surgical options (when medical/laser fail):

  • Trabeculectomy with MMC (0.2–0.4 mg/mL for 1–3 minutes): Gold standard incisional surgery. IOP reduction 30–50%.
  • Tube shunt surgery (Ahmed, Baerveldt): TVT study showed comparable outcomes to trabeculectomy at 5 years; Ahmed has flow restrictor (fewer early hypotony); Baerveldt 350 has better long-term IOP control.
  • MIGS (minimally invasive glaucoma surgery): iStent, Hydrus microstent, goniotomy/GATT, Xen gel stent, endocyclophotocoagulation. Best for mild–moderate disease combined with cataract surgery.
  • Cyclodestructive procedures: Transscleral CPC, micropulse CPC — reserved for refractory or end-stage glaucoma.

Follow-up: IOP check 1 month after starting treatment, then every 3–4 months; VF and OCT every 6–12 months; gonioscopy annually.

12. Prognosis

POAG is a chronic progressive disease requiring lifelong monitoring and treatment. With appropriate therapy, the majority of patients retain functional vision throughout life.

Key prognostic data:

  • OHTS: Treatment reduced conversion from OHT to POAG by ~60% over 5 years (4.4% vs 9.5% in observation → 2.0% vs 4.4% in treatment at 5 yrs)
  • EMGT: Each 1 mmHg of IOP reduction reduced progression risk by ~10%; 45% of treated patients progressed at 6 years vs 62% untreated
  • AGIS: Eyes maintaining IOP =<18 mmHg at all visits showed no mean VF progression== over 7 years
  • CIGTS: Medical and surgical treatment equally effective at 5 years, though surgery achieved lower IOPs

Prognostic factors for progression:

  • Higher baseline IOP and greater IOP fluctuation
  • Disc hemorrhages (strongest predictor of progression)
  • Thinner CCT
  • Older age at diagnosis
  • Advanced disease at presentation
  • African descent
  • Poor medication compliance (estimated at only 50–70%)

Lifetime risk of bilateral blindness: ~5% with treatment, ~15% without treatment.

Clinical Pearls

1
POAG is the most common cause of irreversible blindness worldwide — always check IOP, disc, and fields in patients over 40, especially with risk factors.
2
The ISNT rule (Inferior rim thickest > Superior > Nasal > Temporal) is violated early in POAG — look for inferior notching first as the earliest structural sign.
3
Disc hemorrhages (Drance hemorrhages) are the strongest predictor of glaucoma progression — document and act on them by intensifying treatment.
4
Central corneal thickness must be measured in all glaucoma suspects: OHTS showed thin CCT (=<555 μm==) is an independent risk factor, and GAT underestimates true IOP in thin corneas.
5
The LiGHT trial (Lancet 2019) established SLT as a valid first-line treatment for POAG/OHT, with 74% of patients drop-free at 3 years — discuss with patients as an alternative to lifelong drops.
6
AGIS showed that maintaining IOP =<18 mmHg at all visits resulted in no mean VF progression== over 7 years — a landmark finding that established IOP target concepts.
7
Medication compliance in POAG is only 50–70% — SLT-first strategy (LiGHT) or sustained-release implants may improve real-world outcomes.

Oral-exam questions

  • What is the most important modifiable risk factor for POAG? — Elevated IOP. Each 1 mmHg increase raises risk by ~10% (OHTS). IOP is the only modifiable risk factor currently known.
  • What IOP target prevents progression in advanced POAG? — The AGIS study showed that maintaining IOP =<18 mmHg at all visits resulted in no mean VF progression== over 7 years.
  • What trial supports SLT as first-line treatment? — The LiGHT trial (Lancet 2019) showed SLT was at least as effective as topical medications, with 74% of patients drop-free at 3 years. SLT was also more cost-effective.
  • What is the ISNT rule and how is it used? — The neuroretinal rim is normally thickest Inferiorly, then Superior, Nasal, Temporal. Violation of this order (especially inferior thinning/notching) is an early sign of glaucomatous damage.
  • Why is CCT important in POAG assessment? — OHTS showed thin CCT (=<555 μm) is an independent risk factor for POAG. Additionally, thin corneas cause GAT to underestimate true IOP==, potentially leading to under-treatment.
  • What is the strongest predictor of glaucoma progression? — Disc hemorrhages (Drance hemorrhages). When found, treatment should be intensified regardless of current IOP level.
  • How does OCT complement visual field testing? — OCT RNFL thinning can precede detectable VF loss by up to 5 years, allowing earlier detection of progression and pre-perimetric glaucoma.

Mnemonics

POAG Big 5 Trials

O — OHTS (OHT → POAG prevention) E — EMGT (early treatment halves progression) A — AGIS (IOP <18 = no progression) C — CIGTS (meds = surgery at 5 yrs) L — LiGHT (SLT as first-line)

ISNT Rule

I — Inferior (thickest) S — Superior N — Nasal T — Temporal (thinnest) Violation of this order = suspect glaucoma

Silent Thief

S — Silent (asymptomatic until late) T — Trabecular meshwork outflow block H — High IOP (>21 mmHg in HTG) I — ISNT rule violation E — Every decade doubles risk F — Field loss (nasal step → arcuate → tunnel)

Comparison Tables

Landmark Glaucoma Trials: Key Results
OHTS
Year
2002
Population
OHT (IOP 24–32)
Key Finding
Treatment reduced POAG from 4.4% to 2.0% at 5 yrs
Clinical Impact
Established IOP-lowering prevents glaucoma in OHT
EMGT
Year
2002
Population
Early manifest OAG
Key Finding
Treatment halved progression (45% vs 62%); 10% risk reduction per mmHg
Clinical Impact
Proved benefit of early aggressive treatment
AGIS
Year
2000
Population
Advanced OAG
Key Finding
IOP <18 mmHg → no mean VF progression over 7 yrs
Clinical Impact
Established IOP target concept
CIGTS
Year
2001
Population
Newly diagnosed OAG
Key Finding
Medical and surgical Rx equally effective at 5 yrs
Clinical Impact
Supports medical-first approach
CNTGS
Year
1998
Population
NTG
Key Finding
30% IOP reduction → progression 12% vs 35%
Clinical Impact
Proved IOP reduction benefits NTG
LiGHT
Year
2019
Population
Treatment-naive OAG/OHT
Key Finding
SLT ≥ drops; 74% drop-free at 3 yrs
Clinical Impact
SLT validated as first-line treatment
IOP-Lowering Drug Classes: Comparison
PGA
Example
Latanoprost 0.005%
IOP Reduction
25–33%
Frequency
Once nightly
Key Side Effect
Iris pigmentation, PAP
Beta-blocker
Example
Timolol 0.5%
IOP Reduction
20–25%
Frequency
BD
Key Side Effect
Bronchospasm, bradycardia
Alpha-2 agonist
Example
Brimonidine 0.2%
IOP Reduction
18–25%
Frequency
TDS
Key Side Effect
Allergic conjunctivitis (12–15%)
Topical CAI
Example
Dorzolamide 2%
IOP Reduction
15–20%
Frequency
TDS
Key Side Effect
Metallic taste, corneal decompensation
ROCK inhibitor
Example
Netarsudil 0.02%
IOP Reduction
15–20%
Frequency
Once nightly
Key Side Effect
Conjunctival hyperemia
Osmotic
Example
Mannitol 20%
IOP Reduction
Acute (large)
Frequency
Single IV dose
Key Side Effect
Fluid overload, CHF risk
Hodapp-Parrish-Anderson (HPA) Visual Field Staging
Mean Deviation (MD)
Early
Better than −6 dB
Moderate
−6 to −12 dB
Severe
Worse than −12 dB
Points depressed <5% (PD)
Early
<25%
Moderate
25–50%
Severe
>50%
Points depressed <1% (PD)
Early
<10%
Moderate
<50%
Severe
>20%
Central 5° (worst point)
Early
Better than 15 dB
Moderate
Better than 25 dB (one hemifield <15)
Severe
0 dB or <15 dB in both hemifields
Clinical significance
Early
Early detection opportunity
Moderate
Escalate treatment
Severe
Consider surgery; high blindness risk

Self-Assessment (5)

MCQ

A 55-year-old African American man has an IOP of 28 mmHg, CDR 0.7 with inferior notching, and a superior arcuate scotoma on Humphrey 24-2. Gonioscopy shows wide open angles. What is the most likely diagnosis?

MCQ

According to the AGIS study, at what IOP level did eyes show no mean visual field progression over 7 years?

MCQ

The LiGHT trial (2019) established that SLT as first-line treatment for POAG achieved what proportion of patients drop-free at 3 years?

MCQ

A glaucoma patient on maximum medical therapy with latanoprost, timolol, and dorzolamide still has IOP 22 mmHg with progressive VF loss. What is the most appropriate next step?

MCQ

Which clinical sign on optic disc examination is the strongest predictor of glaucoma progression?

References

  1. AAO Preferred Practice Pattern: Primary Open-Angle Glaucoma (2020)
  2. Kass MA, Heuer DK, Higginbotham EJ, et al. The Ocular Hypertension Treatment Study (OHTS). Arch Ophthalmol. 2002;120(6):701-713.
  3. Heijl A, Leske MC, Bengtsson B, et al. Reduction of intraocular pressure and glaucoma progression: results from the Early Manifest Glaucoma Trial. Arch Ophthalmol. 2002;120(10):1268-1279.
  4. The AGIS Investigators. The Advanced Glaucoma Intervention Study (AGIS): 7. Am J Ophthalmol. 2000;130(4):429-440.
  5. Gazzard G, Konstantakopoulou E, Garway-Heath D, et al. Selective laser trabeculoplasty versus eye drops for first-line treatment of ocular hypertension and glaucoma (LiGHT). Lancet. 2019;393(10180):1505-1516.
  6. Shields MB. Textbook of Glaucoma, 6th Edition. Lippincott Williams & Wilkins.
  7. Stamper RL, Lieberman MF, Drake MV. Becker-Shaffer's Diagnosis and Therapy of the Glaucomas, 11th Edition.
  8. Ehlers JP, Shah CP. The Wills Eye Manual: Office and Emergency Room Diagnosis and Treatment of Eye Disease, 8th Edition.

Related Content

OHTS — Ocular Hypertension Treatment StudyEMGT — Early Manifest Glaucoma TrialAGIS — Advanced Glaucoma Intervention StudyCIGTS — Collaborative Initial Glaucoma Treatment StudyLiGHT — Laser in Glaucoma and Ocular HypertensionNormal-Tension GlaucomaPseudoexfoliation GlaucomaPrimary Angle-Closure GlaucomaCongenital Glaucoma
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