Open Angle Glaucoma
Pathophysiology · Classification · Management
Examination question · ~800 words
What is open angle glaucoma? Discuss its pathophysiology, classification, clinical features, investigations, and management.
Try to outline your answer mentally before expanding sections below.
Open angle glaucoma (OAG) is a chronic, progressive optic neuropathy with characteristic structural changes at the ONH and corresponding VF loss, occurring in the presence of an open, normal-appearing iridocorneal angle on gonioscopy. POAG accounts for ~74% of all glaucoma globally, with worldwide prevalence ~80 million and responsibility for 12.3% of global blindness. Understanding aqueous dynamics, trabecular meshwork pathology, optic nerve damage mechanisms, landmark trials (OHTS, EMGT, CNTGS, AGIS, CIGTS, LiGHT, TVT, ABC), medical and surgical management, and evolving concepts such as neuroprotection and MIGS is essential for the MD examination.
Imagine the eye as a sink. Aqueous humour (water) is constantly produced by the tap (ciliary processes). It drains through the trabecular meshwork (the plughole) into Schlemm's canal and the collector channels (the pipes). In open angle glaucoma, the angle is open — the plughole is visible — but the drain mesh is clogged. Water backs up silently, pressure rises, and the optic nerve (the wall behind the sink) sustains damage under constant hydraulic pressure. Unlike the sink, the damage to the nerve is silent and irreversible — the patient feels nothing until >40% of ganglion cells are lost.
Open angle glaucoma (OAG) is a chronic, progressive optic neuropathy with characteristic structural changes at the ONH and corresponding VF loss, occurring in the presence of an open, normal-appearing iridocorneal angle on gonioscopy.
Key Epidemiology
- Worldwide prevalence: ~80 million; leading cause of irreversible blindness (12.3% of global blindness).
- POAG accounts for ~74% of all glaucoma globally.
- Black populations: 4× higher prevalence; earlier onset and more aggressive progression.
- Bilateral but often asymmetric.
- 50% of patients undiagnosed at time of presentation (silent disease).
Aqueous Dynamics and IOP
- Normal IOP: 10–21 mmHg (Gaussian distribution; 95th percentile)
- Aqueous produced at ~2–2.5 μL/min by ciliary processes (active secretion via Na/K ATPase + carbonic anhydrase; passive ultrafiltration and diffusion.
- Conventional outflow (80%): Trabecular meshwork (TM) → Schlemm's canal → collector channels → episcleral veins. Flow-dependent, pressure-sensitive.
- Uveoscleral outflow (20%): Through ciliary muscle → supraciliary / suprachoroidal space. Pressure-independent. Enhanced by PGAs.
TM Pathology in POAG
- Loss of TM cellularity (cell dropout, loss of phagocytic function → glycosaminoglycan accumulation).
- Cross-linking of ECM (fibronectin, laminin) → increased outflow resistance.
- MYOC gene mutations (GLC1A) → misfolded myocilin accumulates in ER of TM cells → apoptosis.
- Oxidative stress, mechanical stretch (IOP-dependent), and abnormal transforming growth factor-β2 (TGF-β2) signalling drive TM dysfunction.
Optic Nerve Damage — Mechanical vs Vascular Theories
| Theory | Mechanism | Evidence |
|---|---|---|
| Mechanical | Elevated IOP compresses lamina cribrosa → axoplasmic flow blockade → RGC death | Lamina cribrosa deflection on OCT; beam-like collagen disruption |
| Vascular | Reduced ONH perfusion pressure → ischaemia → RGC loss (NTG model) | Disc haemorrhages, low OPP, NTG association with vascular risk |
| Combined | Both act synergistically; vascular dysregulation amplifies mechanical stress | Explains progression at low IOP in NTG |
| Risk Factor | Effect / Evidence | Landmark Study |
|---|---|---|
| Elevated IOP | Strongest modifiable risk; linear dose-response | OHTS, EGPS |
| Age >40 years | Prevalence doubles every decade after 40 | Barbados Eye Study |
| African/Caribbean descent | 4× higher risk; younger onset | OHTS subgroup |
| Family history | FDR: OR 3.2 (Blue Mountains); RR ~4× across populations | Blue Mountains / Rotterdam / Baltimore Eye Surveys |
| Thin CCT (<555 μm) | Structural susceptibility + IOP underestimation | OHTS |
| High myopia | Thin sclera, optic disc vulnerability | Multiple cohort studies |
| Disc haemorrhage | Strongest independent predictor of progression (NTG) | CNTGS |
| Low OPP / nocturnal hypotension | Vascular mechanism; NTG link | Drance, Hayreh |
| Type | IOP | Gonioscopy | Key Features |
|---|---|---|---|
| POAG | Usually >21 | Open; no identifiable cause | Commonest; bilateral asymmetric; painless |
| NTG | ≤21 mmHg | Open | Vascular mechanism; disc haemorrhages; paracentral scotomas near fixation |
| OHT | ≥22 mmHg | Open | No structural/functional damage; conversion risk 10% at 5 years (OHTS) |
| PXF Glaucoma | Often very high | Open; PXF on TM | Pigment on TM, lens; worse prognosis |
| Pigmentary Glaucoma | Elevated; exercise spikes | Dense TM pigment; Sampaolesi line | Young myopic males; Krukenberg spindle; Scheie line |
| Steroid-induced | Elevated (responder) | Open | Steroid use history; usually reversible |
Symptoms
- Usually asymptomatic until late stage — "silent thief of sight".
- Peripheral VF loss first → tunnel vision → central field lost last (paracentral preservation hallmark).
- May present with: difficulty driving, bumping into objects, reduced contrast sensitivity.
Signs — Optic Disc
| Sign | Description | Significance |
|---|---|---|
| Increased CDR | >0.5 CDR or vertical > horizontal (asymmetry >0.2) | Pathological cupping |
| ISNT rule violation | Inferior > Superior > Nasal > Temporal (normal); violation = glaucoma | Thinnest rim in inferior pole = first loss |
| Notching | Focal NRR loss (inf > sup); bayonet sign of vessels | High specificity for glaucoma |
| RNFL defect | Arcuate wedge defect visible in red-free fundus photo | Pre-perimetric marker |
| Disc haemorrhage | Splinter haemorrhage at disc margin; transient | Strongest predictor of progression (NTG) |
| Bayonet sign | Vessel angulation at disc margin due to NRR loss | Advanced cupping |
| Baring of circumlinear vessel | Vessel loses its background rim | Progressive cupping |
| Laminar dot sign | Visible lamina cribrosa pores | Deep cupping |
| Nasalisation of vessels | Central retinal vessels emerge nasal to centre | Advanced glaucoma |
1. Tonometry
- Goldmann applanation tonometry (GAT) — gold standard. Measures IOP by applanating a 3.06 mm corneal disc (Imbert-Fick law). Error: CCT, corneal curvature, astigmatism.
- Pascal DCT (Dynamic Contour Tonometry): CCT-independent; measures actual IOP + OPA (ocular pulse amplitude).
- Ocular Response Analyser (ORA): Measures corneal hysteresis (CH) — viscoelastic property. Low CH predicts faster progression independent of IOP.
- Non-contact tonometry (air-puff): Screening only; multiple readings needed.
2. Gonioscopy
- Mandatory in all glaucoma suspects. Goldmann 3-mirror or Zeiss 4-mirror lens.
- Identifies open angle; rules out secondary causes (PXF, pigment, synechiae).
- Shaffer grading: Grade 4 (wide open, 35–45°) to Grade 0 (closed). Grades 3–4 = low risk of closure.
- Sampaolesi line (anterior to Schwalbe) = PXF/PDG marker.
3. Perimetry (Visual Field Testing)
| Test | Strategy | Use |
|---|---|---|
| SAP — SITA Standard/Fast | Bayesian; 24-2 or 30-2 programme | Baseline and follow-up |
| SAP — 10-2 | Central 10° grid | When MD < −12 dB; monitor fixation |
| FDT (Frequency Doubling Technology) | Magnocellular pathway (My cells) | Early glaucoma screening |
| SWAP (Short Wavelength AP) | Blue-on-yellow; koniocellular | Pre-perimetric; less used now |
| mfERG | Retinal function mapping | Research; malingering |
Key VF Defects in OAG
- Bjerrum (arcuate) scotoma: Arcuate defect from blind spot to nasal horizontal raphe — most classic.
- Nasal step: Asymmetric VF defect straddling horizontal meridian (Rönne step).
- Paracentral scotoma: More common in NTG (near fixation; superior hemifield predominance).
- Temporal wedge: Early peripheral defect.
- Late: Constricted tubular field → central island (temporal crescent last to go).
4. OCT (Optical Coherence Tomography)
- RNFL OCT: Measures peripapillary RNFL thickness. Inferior > Superior loss first (ISNT pattern). Floor effect below ~50 μm.
- ONH OCT: BMO-MRW (Bruch membrane opening — minimum rim width) is superior to classic NRR area. More sensitive in early glaucoma.
- GCC (Ganglion Cell Complex): Macular OCT. GCL + IPL + RNFL. Detects early macular ganglion cell loss. Useful in myopes and OHT.
- Lamina cribrosa OCT: Depth, curvature, focal defects — research tool.
- Swept-source OCT: Deeper penetration; better lamina cribrosa and choroidal imaging.
5. Corneal Pachymetry
- Thin CCT (<555 μm): Independent risk factor (OHTS) + IOP underestimated by GAT (Goldmann).
- Thick CCT (>580 μm): IOP overestimated — benign OHT may be misdiagnosed.
6. Optic Disc Photography
- Stereoscopic disc photos: Gold standard for NRR documentation.
- HRT3 (Heidelberg Retina Tomograph): Confocal scanning laser; Moorfields Regression Analysis.
- GDx (Nerve Fibre Analyser): Scanning laser polarimetry — RNFL birefringence mapping.
| Trial | Design | Key Finding |
|---|---|---|
| OHTS (2002) | OHT → treated vs untreated; 5 years | Treating OHT reduces 5-year conversion from 9.5% to 4.4%. CCT, CDR, age, IOP predict conversion |
| EMGT (2002) | Early POAG → laser+meds vs untreated | Treatment reduced progression from 62% to 45% at 6 years. Every 1 mmHg reduction = ~10% lower progression risk |
| AGIS (2000) | Advanced OAG — ALT/trab sequence | White patients: trab first better. Black patients: ALT first better. IOP <18 all visits = VF preservation |
| CIGTS (2000) | Newly dx POAG → meds vs trabeculectomy | Surgery achieved lower IOP; better VF at 5 years in advanced disease. Similar QoL |
| CNTGS (1998) | NTG → 30% IOP reduction vs untreated | 30% IOP reduction reduces progression from 35% to 12%. Disc haemorrhage = strongest progression predictor |
| LiGHT (2019) | SLT vs meds as first-line; RCT | SLT achieved target IOP without drugs in 74% at 3 years. Cost-effective. No inferiority to drops |
| TVT (2012) | Tube vs trab (Baerveldt 350 vs MMC-trab) | Tube shunts had higher success than trabeculectomy with MMC through 1, 3, and 5 years. 5-year cumulative failure: 29.8% tube vs 46.9% trab. IOP similar after 3 months |
| PTVT (2018) | Primary tube vs trabeculectomy (treatment-naive) | At 3 years: similar success. Tubes needed fewer re-ops. Trab had more hypotony |
| ABC Trial (2013) | Ahmed vs Baerveldt GDD | Baerveldt: lower IOP (13.6 vs 16.6 mmHg) at 5 years. Ahmed: earlier stable IOP; fewer early complications |
| OHTS + EGPS (2007) | Combined cohort meta-analysis | Validated 5 predictive factors: IOP, VCD (CDR), CCT, age, VF PSD |
| Drug Class | Mechanism | IOP Reduction | Side Effects |
|---|---|---|---|
| Prostaglandin analogues (Latanoprost, Bimatoprost, Travoprost, Tafluprost) | ↑ Uveoscleral outflow via FP receptor / prostamide | 25–35% | Iris hyperpigmentation, hypertrichosis, periorbital fat atrophy (bimatoprost>), CME |
| Beta-blockers (Timolol 0.5%, Betaxolol 0.5%) | ↓ Aqueous production (β2 receptor on NPE) | 20–25% | Bradycardia, bronchospasm, depression, masking hypoglycaemia. Betaxolol: selective β1, safer in COPD |
| CAIs — Topical (Dorzolamide, Brinzolamide) | ↓ Aqueous production (carbonic anhydrase II in NPE) | 15–20% | Stinging, metallic taste, superficial punctate keratitis |
| CAIs — Systemic (Acetazolamide) | Same; stronger effect | 25–30% | Paraesthesia, renal stones, Stevens-Johnson, aplastic anaemia, metabolic acidosis |
| Alpha-2 agonists (Brimonidine 0.1–0.2%) | ↓ production + ↑ uveoscleral outflow; neuroprotection (BDNF) | 20–25% | Allergy (up to 25%), fatigue, dry mouth; CNS depression in infants — CONTRAINDICATED |
| ROCK inhibitors (Netarsudil) | ↑ TM/conventional outflow; ↓ episcleral venous pressure | 20–25% | Conjunctival hyperaemia, cornea verticillata |
| Fixed combinations | Dual mechanism; reduce instillations | 30–35% | Combined side effects; improved compliance |
PGAs cause more uveoscleral outflow enhancement with evening dosing — this is the preferred timing.
Systemic CAIs should be avoided in sickle cell disease — metabolic acidosis precipitates sickling.
Brimonidine is CONTRAINDICATED in infants (apnoea, bradycardia, CNS depression).
Preservative-free formulations reduce OSD (ocular surface disease) in long-term users.
Selective Laser Trabeculoplasty (SLT)
- Nd:YAG laser, Q-switched, 532 nm (frequency-doubled). Spot size 400 μm, typically 0.2–1.7 mJ, 100 spots over 180°–360°.
- MOA: Selective photothermolysis of melanin-containing TM cells (Type I and II) → macrophage recruitment → cytokine cascade → ECM remodelling → improved aqueous outflow. No thermal coagulation of TM. Repeatable.
- LiGHT Trial: SLT as primary therapy — 74% achieve target IOP without medication at 3 years. More cost-effective than drops.
- IOP reduction: 20–30%. Duration: 1–5 years. Can be repeated (unlike ALT).
- Indications in late OAG: adjunct to maximum medical therapy; reduce drop burden; post-filtration surgery adjunct.
Trabeculectomy — Operative Anatomy and Technique
- Site: Superior limbus, under upper eyelid (best bleb protection).
- Fornix-based vs limbus-based conjunctival flap: Fornix-based — more anterior bleb, easier suture access; Limbus-based — more posterior bleb.
- Scleral flap: Partial thickness (50%), triangular or rectangular, 4×4 mm.
- Sclerostomy: Kelly punch (1 mm) or Vannas scissors — removes full-thickness sclera + TM block including internal ostium.
- Peripheral iridectomy: Prevents iris plugging of sclerostomy; also prevents pupil block.
- MMC application: 0.2–0.4 mg/mL on sponges, 2–3 minutes, under conjunctival flap away from wound. Irrigate thoroughly. Prevents sub-Tenon fibrosis by inhibiting fibroblast proliferation.
- Scleral flap sutures: Releasable or laser-lysable sutures allow post-op IOP titration.
Post-operative Management
- Steroids (prednisolone 1% hourly tapering) + antibiotic drops.
- Slit lamp: Monitor bleb morphology, IOP, anterior chamber depth, corneal clarity.
- Laser suture lysis (Hoskins/Ritch lens): Day 7–14; increases filtration if IOP elevated.
- Bleb needling (with 5-FU injection): For failing/encapsulated bleb. 5-FU 0.1 mL × 5 injections, 5 mm from bleb edge.
Complications of Trabeculectomy
| Complication | Mechanism | Management |
|---|---|---|
| Hypotony | Excessive filtration; wound leak; bleb over-drainage | Pressure patch; aqueous suppressants; bleb compression; autologous blood injection |
| Hypotony maculopathy | IOP <6 → choroidal expansion → macular folds | Urgent IOP elevation; bleb revision; transconjunctival sutures |
| Bleb leak / Seidel+ | Wound dehiscence; thin avascular bleb | Aqueous suppressants; bandage CL; bleb revision |
| Blebitis / Endophthalmitis | Late infection; Strep. pneumoniae, H. influenzae | Urgent intravitreal antibiotics; bleb excision if needed |
| Accelerated cataract | MMC, hypotony, surgical trauma | Planned phaco; combined surgery if needed |
| Flat AC | Wound leak, overfiltration, suprachoroidal haemorrhage | Identify cause; AC reformation if needed |
| Encapsulated bleb (Tenon cyst) | Fibroblast proliferation; dome-shaped, vascularised bleb | Needling + 5-FU; MMC injection |
Glaucoma Drainage Devices (GDD)
- Ahmed FP-7: Plate area 184 mm²; silicone; Venturi-shaped valve with two silicone elastomer membranes (opens at IOP ~8–12 mmHg). Provides immediate IOP control. Hypertensive phase common at 4–6 weeks.
- Baerveldt 350: Silicone; plate 350 mm²; non-valved. Intraluminal suture/Vicryl ligature used initially; opens at 4–6 weeks. Lower long-term IOP.
- Tube placement: Pars plana (vitrectomised eye) or anterior chamber (phakic/pseudophakic).
- Tube erosion risk: Cover with donor cornea or scleral patch graft.
| Device | Mechanism | IOP Reduction | Best Indication |
|---|---|---|---|
| iStent inject W (3rd gen) | Bypasses TM into Schlemm's canal; 2 stents | 15–25% | Mild-moderate OAG with cataract |
| Hydrus Microstent | Scaffold in Schlemm's canal; 8 mm; 3 clock hours | 20–25% | Mild-moderate OAG with cataract; HORIZON trial |
| XEN 45 gel stent | Ab interno bleb-forming device; gelatin; subconjunctival drainage | 25–35% | Moderate-advanced OAG; failed meds |
| PRESERFLO MicroShunt | SIBS polymer; subconjunctival drainage; ab externo | 25–35% | Moderate OAG; MicroShunt vs trab RCT ongoing |
| Kahook Dual Blade (KDB) | Ab interno trabeculotomy; excises TM strip | 15–25% | Mild-moderate OAG with cataract |
| GATT (Gonioscopy-assisted transluminal trabeculotomy) | 360° TM incision via microcatheter; opens Schlemm's | 30–40% | POAG, steroid-induced, juvenile OAG |
| CPC / Micropulse CPC | Cyclodestruction; reduces AH production | Variable | End-stage; refractory; poor VA potential |
MIGS Pearl: MIGS are best for mild-moderate OAG in conjunction with phacoemulsification. XEN and PRESERFLO are the only MIGS appropriate for moderate-to-advanced disease as standalone procedures due to their bleb-forming mechanism. GATT is particularly useful in younger patients and steroid-induced OAG due to high outflow restoration.
Structure-Function Correlation
- OCT RNFL detects structural loss before VF defects (pre-perimetric stage; ~50% ganglion cell loss before VF threshold).
- Once RNFL reaches floor (~50 μm), further structural loss not measurable — only VF (functional) monitoring valid.
- BMO-MRW: Better reflects true neuroretinal rim area; accounts for disc size variability. Superior to classical CDR assessment.
Progression Analysis Tools
- Event-based analysis (GPA): Detects when VF points worsen beyond test variability threshold from baseline. Fast/conservative.
- Trend-based analysis (VFI slope, MD slope): Linear regression of MD over time. Rate >1 dB/year = clinically significant (rapid progressor). Predicts years to blindness.
- Pointwise linear regression (PLR): Identifies which specific test locations are progressing.
- Structure-function index (SFI): Combines OCT and VF data for a composite progression score.
Follow-up Protocol
| Stage | VF Frequency | OCT Frequency | IOP Target |
|---|---|---|---|
| OHT / Pre-perimetric | Annual 24-2 | 6–12 monthly | <21 or individualised |
| Early OAG | 6-monthly × 2 yrs; then annually | 6–12 monthly | ≤18 mmHg |
| Moderate OAG | 6-monthly | 4–6 monthly | ≤15 mmHg |
| Late / Advanced OAG | 4–6 monthly; add 10-2 | 4–6 monthly | ≤12 mmHg |
- Brimonidine: Upregulates BDNF and CNTF in RGCs; reduces glutamate excitotoxicity. Low Pressure Glaucoma Treatment Study (LoGTS) — brimonidine showed less VF progression than timolol independent of IOP.
- Memantine: NMDA receptor antagonist; blocks glutamate excitotoxicity. Low Pressure Glaucoma phase 3 trial — failed primary endpoint.
- Citicoline: Neuroprotective; phase 2 RCTs show reduced VF progression. Oral and intramuscular formulations.
- Nicotinamide (Vitamin B3): Restores mitochondrial function in RGCs. Phase 2 trial (Williams et al. 2021) — significant neuroprotection at 3g/day.
- Rho kinase (ROCK) inhibitors: Netarsudil — also provides neuroprotection via axonal cytoskeletal stability.
- Gene therapy: AAV2-CNTF intravitreal injection; CRISPR targeting MYOC mutation in TM; eotaxin pathway modulation.
- Stem cell therapy: RGC replacement; TM cell transplantation to restore outflow.
- Sustained drug delivery: Bimatoprost SR implant (Durysta) — FDA approved; 6-month intracameral sustained release. iDose travoprost implant — Phase 3.
Q: Which glaucoma drug is CONTRAINDICATED in infants?
A: Brimonidine — causes CNS depression, apnoea, bradycardia.
Q: Why does pigmentary glaucoma worsen after exercise?
A: Vigorous exercise → iris pigment epithelium rubbing on zonules (reverse pupillary block in concave iris) → pigment shedding → TM pigment loading → acute IOP spike.
Q: OCT shows RNFL thickness <50 μm. Can you monitor with OCT?
A: No — floor effect. Structural OCT loses sensitivity below ~50 μm. Switch to functional monitoring (VF 24-2 and 10-2).
Q: AGIS found worse outcomes in which group with trabeculectomy first?
A: Black patients. In AGIS, Black patients had better outcomes with ALT first; White patients fared better with trabeculectomy first.
Q: What is the significance of Bruch membrane opening (BMO-MRW)?
A: BMO-MRW is the minimum distance from BMO to inner limiting membrane surface. It is the most accurate ONH structural parameter as it corrects for disc size and BMO area — superior to classic CDR.
Q: NTG patient on target IOP still progressing — next step?
A: Assess diurnal curve for nocturnal hypotension (24-hour ABPM), look for disc haemorrhages, sleep apnoea, migraine, hyperviscosity. Consider calcium channel blocker if vasospastic profile.
Q: A patient on brimonidine develops conjunctival follicles — diagnosis?
A: Brimonidine hypersensitivity reaction (up to 25%). Presents with follicular conjunctivitis, lid oedema, periocular dermatitis. Stop drug; substitute with dorzolamide.
Q: Why is low corneal hysteresis (CH) significant?
A: Low CH on ORA predicts faster VF progression independent of IOP — the viscoelastic dampening of the cornea reflects scleral/ONH biomechanical vulnerability. Target lower IOP in these patients.
A 48-year-old myopic (-8D) patient presents with IOP 24 mmHg OU, CDR 0.7 OU, mild inferior RNFL thinning on OCT, and a normal 24-2 VF. CCT is 490 μm. His father had glaucoma and lost vision. Gonioscopy shows grade 4 open angles with moderate TM pigmentation. How would you calculate his 5-year conversion risk and what treatment threshold would you apply?
Model Answer
This patient has multiple converging risk factors for POAG conversion from OHT/pre-perimetric glaucoma: (1) IOP 24 mmHg — elevated, (2) thin CCT 490 μm — independent risk factor (OHTS validated) and IOP underestimated by GAT, (3) vertical CDR 0.7 (VCD) — borderline suspicious, (4) high myopia — structural ONH and RNFL vulnerability, (5) family history (FDR) — RR 3.7×, (6) young age — more years of disease.
OHTS-EGPS Risk Calculator
Inputs — IOP, VCD, CCT, age, VF PSD. With CCT 490 μm, IOP 24, VCD 0.7, and FH — 5-year conversion risk is likely >15% (high risk threshold per OHTS: >10% at 5 years).
Treatment threshold
OHTS showed treating high-risk OHT (CCT <555 μm, IOP >24, CDR >0.5) reduces 5-year conversion from 9.5% to 4.4%. This patient exceeds all three criteria. Additionally, his OCT already shows inferior RNFL thinning — suggesting pre-perimetric glaucoma, not mere OHT. I would classify him as pre-perimetric POAG and initiate treatment (first-line PGA), with target IOP of ≤17 mmHg (25% reduction from corrected IOP). Baseline 10-2 perimetry, stereoscopic disc photography, and 4–6 monthly OCT monitoring are essential. SLT may be offered as primary therapy per LiGHT trial data.
References
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