Primary Open-Angle Glaucoma

Primary Open-Angle Glaucoma

Primary Open-Angle Glaucoma

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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". Healthy vs Glaucomatous Optic Nerve

⭐ 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.

Optic nerve changes in glaucoma

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). Optic disc cupping progression
  • 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.

Practice Questions: Primary Open-Angle Glaucoma

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What is the best drug for open-angle glaucoma?

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_____ cornea is a risk factor for POAG development

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_____ cornea is a risk factor for POAG development

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