Anesthetics in Ophthalmology

Topical Anesthetics - Surface Spell

• Mechanism: Reversibly block Na+ channels in nerve endings → inhibit nerve impulse conduction.
• Onset & Duration: Rapid onset (20-60 sec); short duration (10-30 min).
• Common Agents:
  • Esters:
    • Proparacaine (0.5%): Least irritating, preferred for routine procedures. Onset ~20s, duration ~15 min.
    • Tetracaine (0.5%-1%): More stinging, longer duration than proparacaine. Onset ~30s, duration ~20-30 min.
    • Cocaine (historical): Vasoconstrictor, mydriatic; diagnostic for Horner's.
  • Amides:
    • Lidocaine (gel 2%-4%): Longer duration, good for minor OR procedures, e.g., before intravitreal injections.
• Clinical Uses:
  • Tonometry, gonioscopy.
  • Foreign body/suture removal.
  • Pre-operative for cataract surgery (part of topical regimen).
  • Schirmer's test (after initial dry eye assessment).
• Adverse Effects:
  • Transient stinging, burning upon instillation.
  • Allergic reactions (more common with esters).
  • ⚠️ Corneal epithelial toxicity: Punctate keratitis, delayed healing, ring infiltrates, stromal edema with prolonged/frequent use (anesthetic abuse keratopathy).

  ⭐ Proparacaine (0.5%) is generally the least irritating topical anesthetic and is widely used for routine ophthalmic procedures like applanation tonometry.

• 📌 Mnemonic: "Poor Tired Lions" (Proparacaine, Tetracaine, Lidocaine) for common topical agents.

Injectable Anesthetics - Precision Blocks

• Purpose: Profound ocular akinesia & anesthesia for intraocular surgery.

• Common Agents:

  • Lidocaine (1-2%): Onset 5-10 min, duration 1-2 hrs. Max: 4.5 mg/kg (plain), 7 mg/kg (+ adrenaline).
  • Bupivacaine (0.5-0.75%): Onset 5-15 min, duration 4-8 hrs. Max: 2 mg/kg.
  • Hyaluronidase: 5-15 IU/mL added to ↑ spread, ↓ onset.
  • 📌 Mnemonic: "Lido is fast, Bupi lasts."

• Types of Blocks:

  • Retrobulbar Block (Intraconal)
    • Injection: 3-5 mL into muscle cone.
    • Needle: 23-27G, 31-38mm.
    • Effect: Excellent akinesia/anesthesia.
    • Risks: Globe perforation, retrobulbar hemorrhage (RBH), optic nerve injury, brainstem anesthesia.
  • Peribulbar Block (Extraconal)
    • Injection: 6-12 mL into extraconal space (1-2 sites).
    • Needle: 23-27G, 25mm.
    • Effect: Good akinesia/anesthesia.
    • Advantages: Safer than retrobulbar; ↓ globe/nerve injury risk.
    • Disadvantages: Slower onset, more chemosis.
  • Sub-Tenon's Block (Episcleral)
    • Technique: Blunt cannula into posterior sub-Tenon's space.
    • Volume: 2-4 mL.
    • Advantages: Very safe, effective. Good for high myopes, anticoagulated pts.
  • Subconjunctival Injection
    • Use: Localized anesthesia for minor procedures (e.g., pterygium).
    • Volume: 0.1-0.5 mL.

⭐ Retrobulbar hemorrhage is a critical complication of retrobulbar blocks, potentially causing vision loss. Immediate management involves lateral canthotomy and cantholysis to decompress the orbit.

General Anesthesia & Systemic Aspects - Eyes Wide Shut Safely

• Indications: Pediatrics, patient anxiety, long/complex surgery (retinal, orbital), open globe (prevents LA-induced ↑IOP), LA allergy.
• Anesthetic Goals: Akinesia, analgesia, amnesia, stable IOP, Oculocardiac Reflex (OCR) control, smooth induction/emergence (no cough/strain → ↑IOP).
• Agent Effects on IOP:
  • ↓IOP: IV agents (propofol), volatiles (sevo), NDMRs.
  • ↑IOP: Ketamine (caution), succinylcholine (transient ↑ $5-10 \text{ mmHg}$ for $5-10 \text{ min}$).
    • 📌 Succinylcholine "SUX" IOP up briefly.
• Oculocardiac Reflex (OCR):
  • Triggers: Globe pressure, EOM traction (esp. medial rectus).
  • Pathway: CN V (afferent) → CN X (efferent).
  • Signs: Bradycardia, arrhythmia, asystole.
  • Management: Stop stimulus, O₂, Atropine (0.01-0.02 mg/kg IV).
• Key Systemic Concerns: PONV (can ↑IOP), corneal abrasion (protect eyes), malignant hyperthermia.

⭐ Succinylcholine is relatively contraindicated in open globe injuries unless pre-treated with a defasciculating dose of a non-depolarizing muscle relaxant, due to its risk of increasing IOP and potential for extrusion of intraocular contents.

High‑Yield Points - ⚡ Biggest Takeaways

• Topical anesthetics like Proparacaine offer rapid surface anesthesia for tonometry and foreign body removal.
• Cocaine is unique: anesthetic, vasoconstrictor, mydriatic; key for Horner's syndrome diagnosis.
• Lidocaine is a common injectable; Bupivacaine provides longer analgesia and akinesia.
• Hyaluronidase is often added to injectables to improve spread and speed onset.
• Monitor for systemic toxicity (CNS, CVS effects) with all injectable anesthetics.
• Use preservative-free formulations for intraocular use to prevent corneal toxicity.