Autonomic Drugs in Ophthalmology

Ocular Autonomic Innervation - Eye's Command System

• Parasympathetic (PNS): "Rest & Digest"
  • Pathway: CN III (E-W nucleus) → Ciliary ganglion.
  • NT: ACh; Receptor: M3.
  • Actions:
    • Sphincter pupillae: Contraction → Miosis.
    • Ciliary muscle: Contraction → Accommodation; ↑Aqueous outflow (trabecular).
• Sympathetic (SNS): "Fight or Flight"
  • Pathway: Hypothalamus → Ciliospinal Ctr (C8-T2) → Sup. Cervical Ganglion.
  • NT: NE.
  • Receptors & Actions:
    • Dilator pupillae ($\alpha_1$): Contraction → Mydriasis.
    • Ciliary epithelium: $\beta_2$ → ↑Aqueous production; $\alpha_2$ → ↓Aqueous production.

⭐ M3 agonism: Miosis & ciliary muscle contraction → ↑trabecular aqueous outflow.

Pupil Dilators - Wide‑Eyed Wonders

• Anticholinergics (Mydriasis + Cycloplegia): Block muscarinic receptors in iris sphincter & ciliary muscle.
  • Atropine:
    • Potent, long duration (7-10 days).
    • Uses: Refraction (children <6 yrs), uveitis.
    • ⚠️ Risk: Angle-closure glaucoma, systemic toxicity.
    • 📌 Atropine toxicity: "Blind as a bat, mad as a hatter, red as a beet, hot as a hare, dry as a bone."
  • Homatropine: Duration 1-3 days. Less potent.
  • Cyclopentolate: Duration ~24 hrs. Preferred for cycloplegic refraction.
  • Tropicamide: Shortest acting (4-6 hrs). Mydriasis > Cycloplegia.

    ⭐ Tropicamide: Fastest onset (20-30 min) & shortest duration; preferred for routine fundoscopy.

• Sympathomimetics (Mydriasis only, NO cycloplegia): Stimulate α1 receptors on iris dilator muscle.
  • Phenylephrine (2.5%, 5%, 10%):
    • Duration ~3-5 hrs.
    • Uses: Fundoscopy, decongestion, breaking posterior synechiae, pre-op mydriasis.
    • ⚠️ Risk: Systemic (hypertension, especially with 10%), rebound miosis.

Pupil Constrictors & Glaucoma Go‑Tos - Pressure Point Players

• Miotics (Cholinomimetics): E.g., Pilocarpine (direct M3 agonist).
  • Mechanism:
    • Ciliary muscle contraction → ↑trabecular outflow → ↓IOP.
    • Sphincter pupillae contraction → miosis (pupil constriction).
  • Primary Uses:
    • Acute Angle-Closure Glaucoma (AACG): Emergency ↓IOP.
    • Open-Angle Glaucoma (OAG): Adjunct, rarely primary.
    • Counteract mydriasis; Sjogren's syndrome (oral for xerostomia).
  • Side Effects: Brow ache, dim vision, accommodative spasm, induced myopia.
• 📌 Mnemonic: "Pilocarpine: Pupils In, Liquid Out."
• Context: While miotics are key for AACG, other classes (e.g., prostaglandin analogs, beta-blockers) are first-line for OAG, acting by ↓aqueous production or ↑uveoscleral outflow.

⭐ In acute angle-closure glaucoma, pilocarpine's miosis is crucial: it pulls the peripheral iris away from the trabecular meshwork, unblocking aqueous outflow and rapidly lowering IOP.

Glaucoma Drug Arsenal - IOP Warriors

Primary goal: ↓ Intraocular Pressure (IOP). Achieved by:

• ↓ Aqueous Humor Production
• ↑ Aqueous Humor Outflow

Key Drug Classes:

• Prostaglandin Analogs (PGAs): Latanoprost, Travoprost
  • Mechanism: ↑ Uveoscleral outflow.
  • 📌 "-prost" suffix. Often first-line.
• Beta-Blockers: Timolol (non-selective), Betaxolol (β1-selective)
  • Mechanism: ↓ Aqueous production by ciliary body.
  • ⚠️ Caution: Systemic side effects (bronchospasm, bradycardia).
• Alpha-Adrenergic Agonists: Brimonidine, Apraclonidine
  • Mechanism: Dual - ↓ Aqueous production & ↑ Uveoscleral outflow.
  • 📌 Alpha: ↓ Aqueous prod, ↑ outflow.
• Carbonic Anhydrase Inhibitors (CAIs): Dorzolamide, Brinzolamide (topical); Acetazolamide (systemic)
  • Mechanism: ↓ Aqueous production (↓ $HCO_3^-$ formation).
  • 📌 "-zolamide" suffix.

⭐ Latanoprost can cause irreversible iris hyperpigmentation and eyelash growth.

High‑Yield Points - ⚡ Biggest Takeaways

• Pilocarpine: Direct muscarinic agonist for glaucoma (↑trabecular outflow); causes miosis, ciliary spasm.
• Timolol: Non-selective β-blocker for glaucoma (↓aqueous production).
• Latanoprost: Prostaglandin F2α analog for glaucoma (↑uveoscleral outflow); causes iris pigmentation.
• Atropine: Muscarinic antagonist for mydriasis and cycloplegia (long-acting).
• Phenylephrine: α1-agonist for mydriasis without cycloplegia.
• Brimonidine: α2-agonist for glaucoma (↓aqueous production, ↑outflow).
• Dorzolamide: Carbonic anhydrase inhibitor for glaucoma (↓aqueous secretion).