Primary Open Angle Glaucoma (POAG)
Summary
Primary Open Angle Glaucoma (POAG) is a chronic, progressive optic neuropathy characterized by degeneration of retinal ganglion cells, leading to characteristic Optic Disc Cupping / atrophy and corresponding Visual Field defects. It is traditionally associated with raised Intraocular Pressure (IOP >21mmHg), although Normal Tension Glaucoma exists. It is the leading cause of irreversible blindness worldwide. The condition is termed "The Thief of Sight" because it is typically asymptomatic, destroying peripheral vision before the patient notices. The angle of the Anterior Chamber (between Iris and Cornea) is open, distinguishing it from Angle Closure Glaucoma. [1,2]
Key Facts
- Mechanism: Increased resistance to outflow of aqueous humour through the Trabecular Meshwork (unlike Angle Closure where the meshwork is physically blocked by the iris).
- The Turning Point (LiGHT Trial): Historically, eye drops were first line. The landmark LiGHT trial (Lancet 2019) demonstrated that Selective Laser Trabeculoplasty (SLT) provides better clinical outcomes and quality of life than drops. NICE guidelines (2022 update) now recommend SLT as first-line treatment.
- Corneal Thickness: Central Corneal Thickness (CCT) affects IOP readings. Thick corneas give falsely high readings (Ocular Hypertension artifact). Thin corneas give falsely low readings (Risk of missing Glaucoma).
Clinical Pearls
The "Cup": The optic cup is the pale centre of the disc. The neuroretinal rim is the pink tissue. In glaucoma, the rim thins (ISNT rule: Inferior -> Superior -> Nasal -> Temporal) and the cup expands. Vertical elongation of the cup is highly specific.
Don't wait for symptoms: If a patient complains of "tunnel vision", they have already lost 90% of their optic nerve fibres. Early diagnosis relys entirely on screening (Optometrist visits).
Systemic Beta-Blockers: Be wary of prescribing Timolol eye drops in Asthmatics or patients with Heart Block. Drops are absorbed systemically (via nasolacrimal duct) and can cause disastrous bronchospasm/bradycardia.
Incidence
- Affects 2% of people >40 years, rising to 10% in >80s.
- Ethnicity: 4-6x more common and more severe in Black African/Caribbean populations (onset often earlier).
- Genetics: Strong familial link (10x risk if sibling affected).
Aqueous Dynamics
- Production: Ciliary Body (Active secretion).
- Flow: Posterior Chamber -> Pupil -> Anterior Chamber.
- Outflow:
- Trabecular Meshwork (90%): Through Schlemm's canal to episcleral veins. (Blocked in POAG).
- Uveoscleral (10%): Through ciliary muscle face. (Enhanced by Prostaglandins).
Optic Neuropathy
Raised IOP causes mechanical compression of the lamina cribrosa, disrupting axoplasmic flow in the optic nerve axons, leading to apoptosis.
Symptoms
Signs
- Tonometry: Goldmann Applanation Tonometer (GAT) is the gold standard. Air-puff is for screening only.
- Gonioscopy: Use a mirrored lens to inspect the angle. Critical to rule out Angle Closure. In POAG, the angle is open (structures visible).
- Slit Lamp: Deep anterior chamber.
- Fundoscopy: Assess C:D ratio.
Diagnostics
- Visual Field Test (Perimetry): Humphrey Fields. Look for reproducing defects on Black-on-White grid.
- OCT (Optical Coherence Tomography): Measures Retinal Nerve Fibre Layer (RNFL) thickness. Detects damage years before visual field loss occurs.
- Pachymetry: Measures Corneal Thickness (to correct IOP).
Management Algorithm
DIAGNOSED POAG / OHT
↓
NICE GUIDELINE
↓
┌──────────┴──────────┐
FIRST LINE UNAVAILABLE /
(New Diagnosis) UNSUITABLE for SLT
↓ ↓
SLT EYE DROPS (1st Line)
(Select Laser (Prostaglandin Analogue)
Trabeculoplasty) ↓
↓ NOT CONTROLLED
IOP Reduced? ↓
│ ADD 2nd AGENT
┌───┴───┐ (Beta Blocker etc)
YES NO ↓
│ │ TRABECULECTOMY
Monitor Drops (Surgery)
1. Laser (First Line)
- Selective Laser Trabeculoplasty (SLT): Application of laser to the meshwork stimulates remodeling and improves outflow.
- Efficacy: Works in 75% of patients. Action lasts 3-5 years. Can be repeated.
2. Medical (Drops)
- Prostaglandin Analogues (Latanoprost, Bimatoprost):
- Mechanism: Increases Uveoscleral outflow.
- Dose: Once at Night (ON).
- SEs: Long eyelashes, brown pigmentation of iris (heterochromia), red eye.
- Beta Blockers (Timolol, Levobunolol):
- Mechanism: Reduces aqueous production.
- Dose: BD.
- SEs: Bronchospasm (Avoid in Asthma/COPD), Bradycardia, Fatigue.
- Carbonic Anhydrase Inhibitors (Dorzolamide): Reduces production.
- Alpha-2 Agonists (Brimonidine): Reduces production + Increases uveoscleral outflow.
3. Surgical
- Trabeculectomy: Creating a fistula (trapdoor) from the anterior chamber to the sub-conjunctival space (forming a "bleb"). Mitomycin C is used to prevent scarring.
- MIGS (Minimally Invasive Glaucoma Surgery): Stents (e.g. iStent) placed in meshwork during cataract surgery.
- Blindness: Irreversible.
- Driving: DVLA must be notified if affects both eyes or significant field loss.
- Blebitis: Infection of the surgical bleb (Endophthalmitis risk).
- Without treatment: Progression to blindness is certain but slow (15-20 years).
- With treatment: Most retain useful vision for life.
Key Guidelines
| Guideline | Organisation | Key Recommendations |
|---|---|---|
| NG81 | NICE (2017, Upd 2022) | SLT is now First Line treatment for newly diagnosed OHT/POAG. 24mmHg is threshold for treating Ocular Hypertension (OHT). |
| Driving | DVLA | Group 1: 120 degree horizontal field required. |
Landmark Trials
1. LiGHT Trial (2019)
- Comparison: SLT Laser vs Eye Drops (Latanoprost).
- Findings: SLT had better IOP control, fewer surgeries required, lower cost, and better symptom profile (no daily drops).
- Impact: Changed NICE guidelines to make Laser first line.
2. OHTS (Ocular Hypertension Treatment Study)
- Findings: Treating high IOP prevents conversion to Glaucoma.
What is Glaucoma?
Your eye is like a sink with a tap (fluid in) and a drain (fluid out). In Glaucoma, the drain (meshwork) gets clogged up with sludge. The tap keeps running, so pressure builds up. This pressure squashes the delicate optic nerve at the back of the eye, killing the wires that send pictures to your brain.
Can you fix my vision?
No. Vision lost from glaucoma is gone forever. The treatment is purely to save what you have left.
What is the Laser?
It is a gentle "clean out" of the drain using light. It takes 15 minutes, is painless, and means you might not need to put drops in every night.
Primary Sources
- NICE Guideline NG81. Glaucoma: diagnosis and management. 2017 (Updated 2022).
- Gazzard G, et al. Selective laser trabeculoplasty versus eye drops for first-line treatment of ocular hypertension and glaucoma (LiGHT). Lancet. 2019;393:1505-1516. PMID: 30862377.
- Quigley HA. Glaucoma. Lancet. 2011;377:1367-1377.
Common Exam Questions
- Pharmacology: "Side effect of Latanoprost?"
- Answer: Eyelash growth (Hypertrichosis) and Iris darkening.
- Ophthalmology: "Triad of Glaucoma?"
- Answer: Raised IOP + Disc Cupping + Field Loss.
- Triage: "Asthmatic patient with glaucoma. Which drop to avoid?"
- Answer: Timolol (Beta blocker).
- Pathology: "Loss of rim obeying ISNT rule. Order of thinning?"
- Answer: Inferior and Superior rim thin first (hence vertical elongation of cup).
Viva Points
- Normal Tension Glaucoma: Classic cupping and field loss but IOP less than 21. Often associated with migraine/Raynaud's (vascular theory).
- Ocular Hypertension: High IOP (>21) but NORMAL disc/fields. We treat if Risk is high or IOP >24 (NICE).
Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists.