Primary Open-Angle Glaucoma

POAG Defined - The Sneaky Thief

• A chronic, progressive optic neuropathy, often asymptomatic in early stages.
• Key diagnostic features:
  • Characteristic optic nerve head (ONH) cupping (e.g., ↑ cup-to-disc ratio).
  • Specific patterns of visual field (VF) loss.
  • Open, normal-appearing anterior chamber angle (ACA) on gonioscopy.
  • Often linked to ↑ Intraocular Pressure (IOP > 21 mmHg), though Normal Tension Glaucoma exists.
• Epidemiology:
  • Most common type of glaucoma worldwide.
  • Prevalence significantly ↑ with age (typically affecting those > 40 years).
  • Major risk factors: positive family history, African descent, myopia, diabetes mellitus.
  • Its insidious, asymptomatic nature earns it the name "sneaky thief of sight".

⭐ Elevated IOP is the most significant modifiable risk factor for the development and progression of POAG.

Risk Factors & Pathophysiology - Pressure Cooker Eye

• Risk Factors: 📌 (Mnemonic: Age, Family Hx, Race, IOP, CCT, Associated diseases, Nearsightedness)
  • ↑IOP (most significant modifiable)
  • Age > 40 yrs
  • Family Hx (1st degree)
  • Race (African, Hispanic)
  • Thin CCT (< 555 microns)
  • High Myopia
  • DM, HTN
• Pathophysiology:
  • ↓Aqueous outflow via TM → ↑IOP.
  • ONH Damage:
    • Mechanical: Lamina distortion.
    • Vascular: Ischemia, ↓axonal flow.

  ⭐ A thin Central Corneal Thickness (CCT < 555 microns) is an independent risk factor for progression from ocular hypertension to POAG.

Clinical Picture - Silent Signs

• Largely asymptomatic initially ("silent thief of sight"); insidious onset.
• Late: Gradual, painless peripheral vision loss (classic tunnel vision).
• Signs:
  • ↑IOP (typically > 21 mmHg, but variable).
  • Open angle on Gonioscopy (Shaffer grade III-IV).
  • Optic Disc Damage (Glaucomatous Optic Neuropathy):
    • ↑CDR (> 0.5 or asymmetry > 0.2).
    • Neuroretinal rim notching (often inferotemporal/superotemporal).
    • RNFL defects (e.g., slit or wedge-shaped).

  ⭐ The ISNT rule (normal rim: Inferior > Superior > Nasal > Temporal) is often violated in glaucomatous optic neuropathy. oka

Diagnostic Toolkit - Unmasking POAG

• Tonometry: Measures Intraocular Pressure (IOP); Goldmann applanation tonometry is gold standard. Normal IOP: 10-21 mmHg.
• Gonioscopy: Visualizes iridocorneal angle; differentiates open vs. closed angle.
• Ophthalmoscopy/Stereo-biomicroscopy: Optic disc assessment (cupping, CDR >0.5, notching, NRR thinning).
• Perimetry (Humphrey Visual Field - HVF): Detects functional loss.
  • Early: Nasal step, paracentral scotoma.
  • Late: Arcuate scotoma (Bjerrum's), temporal wedge, tunnel vision.
• Optical Coherence Tomography (OCT): Quantifies structural damage.
  • Retinal Nerve Fiber Layer (RNFL) thinning.
  • Ganglion Cell Complex (GCC) analysis.

⭐ Vertical elongation of the cup (vertical C:D ratio > horizontal) is an early sign of glaucomatous optic neuropathy, often preceding RNFL changes detectable by OCT or visual field defects.

Management Matrix - Lowering Pressure

Goal: Achieve target IOP (↓ 20-30% from baseline or individualized).

• Medical Therapy (Stepwise):
  • 1st Line: Prostaglandin analogs (PGAs) (e.g., Latanoprost) - ↑ Uveoscleral outflow.
  • Adjunctive: Beta-blockers (e.g., Timolol), Carbonic Anhydrase Inhibitors (CAIs) (e.g., Dorzolamide) - ↓ aqueous production; α2-Agonists (e.g., Brimonidine) - dual mechanism (↓ aqueous production, ↑ uveoscleral outflow).
• Laser Therapy:
  • Selective Laser Trabeculoplasty (SLT) or Argon Laser Trabeculoplasty (ALT) - ↑ Trabecular outflow.
• Surgical Therapy (if medical/laser fails or advanced disease):
  • Trabeculectomy (gold standard).
  • Glaucoma Drainage Devices (GDDs).
  • Minimally Invasive Glaucoma Surgery (MIGS).

⭐ Latanoprost (a PGA) is often the first-line choice due to potent IOP lowering, once-daily dosing, and a favorable systemic side-effect profile.

High‑Yield Points - ⚡ Biggest Takeaways

• POAG: Most common glaucoma; painless, progressive optic neuropathy.
• Key signs: ↑ cup-disc ratio, characteristic visual field defects (e.g., arcuate scotoma).
• IOP is a major risk factor; gonioscopy confirms open angle.
• Prostaglandin analogues are often first-line medical treatment to ↓ IOP.
• Early diagnosis and treatment are crucial to prevent irreversible vision loss.
• Often asymptomatic until advanced stages, highlighting screening importance.