Refractive Surgery

Refractive Errors & Candidacy - Setting the Stage

• Refractive Errors:
  • Myopia: Focus before retina; concave (-) lens.
  • Hyperopia: Focus behind retina; convex (+) lens.
  • Astigmatism: Multiple foci; cylindrical lens.
  • Presbyopia: Age-related ↓ accommodation.
• Candidacy Essentials: (📌 S.A.F.E.R.)
  • Stable Refraction: <0.5D change/ ≥1 yr.
  • Age: ≥18 yrs (ideally >21).
  • Favourable Corneal Thickness (CT): Adequate (LASIK: pre-op ~500µm, RSB ≥250-300µm).
  • Expectations: Realistic.
  • Robust Health: No active ocular/uncontrolled systemic disease.
  • (Not pregnant/lactating).

⭐ Absolute Contraindications: Keratoconus, active autoimmune disease, unrealistic expectations, severe dry eye.

Laser Vision Correction - Zapping to Clarity

Laser vision correction reshapes the cornea. Common types: LASIK, PRK, SMILE.

• Mechanism: Excimer laser (LASIK, PRK) ablates cornea; Femtosecond laser creates SMILE lenticule.
• Munnerlyn Formula: $t = S^2 \times D / 3$ ($t$=ablation depth, $S$=optical zone, $D$=diopters).
• Key Thresholds: Residual Stromal Bed (RSB) > 250-300 µm. LASIK flap: 100-120 µm.

⭐ Exam Favourite: Most common LASIK complication: dry eye syndrome.

Lens-Based & Incisional Options - Alternative Routes

• Phakic IOLs (PIOLs): Implanted lenses, natural lens retained.

  • Indications: High myopia/hyperopia, astigmatism; laser unsuitable.
  • Key Req: Anterior Chamber Depth (ACD) >2.8 mm (posterior), >3.0-3.2 mm (anterior); adequate Endothelial Cell Count (ECC).
  • Types:

    Type	Location	Fixation	Examples
    Anterior	Ant. Chamber	Angle/Iris-claw	Cachet, Artisan
    Posterior	Post. Chamber	Sulcus	ICL (Visian/EVO)

  • Complications: Endothelial loss, cataract, glaucoma, pigment dispersion.
  • 

  ⭐ Phakic IOLs preserve accommodation & are reversible; key for high myopia, avoiding excessive corneal ablation.

• Refractive Lens Exchange (RLE/CLE): Clear lens replaced with IOL.

  • Indications: High errors, presbyopia, early cataract.
  • Pros: Definitive, premium IOLs. Cons: Loses accommodation (monofocal), surgical risks.

• Incisional Keratotomy (AK/LRIs): Corneal incisions for astigmatism.

  • Uses: Native or post-op astigmatism.
  • Note: Less predictable than laser methods.

Post-Op & Complications - The Aftermath

• Post-Op Regimen:
  • Topical steroids (e.g., Prednisolone) & antibiotics (e.g., Moxifloxacin) tapered.
  • Frequent preservative-free artificial tears.
  • Protective eyewear.
• Complications:
  • Early: Dry eye, flap striae, Diffuse Lamellar Keratitis (DLK), infection.
  • Late: Regression, haze (PRK > LASIK), corneal ectasia, epithelial ingrowth.

Start["👁️ Flap Striae
• Post-LASIK finding• Corneal wrinkles"]

Decision["📋 Assessment
• Visual impact?• Check symptoms"]

Monitor["👁️ Monitoring
• Observe progress• Routine follow-up"]

Treatment["💊 Flap Repair
• Lift and reposition• Stretch the flap"]

Outcome["✅ Assess Result
• Check VA levels• Resolution status"]

Start --> Decision Decision -->|No| Monitor Decision -->|Yes| Treatment Treatment --> Outcome

style Start fill:#F7F5FD, stroke:#F0EDFA, stroke-width:1.5px, rx:12, ry:12, color:#6B21A8 style Decision fill:#FEF8EC, stroke:#FBECCA, stroke-width:1.5px, rx:12, ry:12, color:#854D0E style Monitor fill:#EEFAFF, stroke:#DAF3FF, stroke-width:1.5px, rx:12, ry:12, color:#0369A1 style Treatment fill:#F1FCF5, stroke:#BEF4D8, stroke-width:1.5px, rx:12, ry:12, color:#166534 style Outcome fill:#F6F5F5, stroke:#E7E6E6, stroke-width:1.5px, rx:12, ry:12, color:#525252

> ⭐ Corneal ectasia: most feared long-term complication. Risk factors: thin residual stromal bed (<**300**µm), high myopia, abnormal pre-op topography. 

##  High‑Yield Points - ⚡ Biggest Takeaways

> * **LASIK**: **Corneal flap** for excimer ablation. **PRK**: **Epithelium removed**, then **stromal surface ablation**.
> * **SMILE**: **Flapless**; femtosecond laser creates intrastromal **lenticule**, then extracted.
> * **Wavefront/Topography-guided** ablations reduce **higher-order aberrations** (HOAs) for better vision quality.
> * Commonest side effect: **Dry eye**. Most serious risk: **Corneal ectasia** (maintain **Residual Stromal Bed**).
> * Ideal candidates: Age >**18 years**, **stable refraction** (≥1 year), adequate **corneal thickness** & topography.
> * Contraindications: **Keratoconus**, **thin corneas**, **unstable refraction**, active ocular/systemic inflammation.