Ocular Pharmacology & Glaucoma

Ocular Pharmacology & Glaucoma

1. Functional anatomy relevant to ocular drug delivery

• The eye is a specialized sensory organ relatively secluded from systemic access by the blood–retinal, blood–aqueous, and blood–vitreous barriers, giving it unusual pharmacodynamic and pharmacokinetic properties (G&G 14e Ch.74, p.1453).
• Extraocular structures — eyelids, orbit (bony cavity with fissures/foramina conducting nerves, muscles, vessels), Tenon's capsule, adipose tissue, and six extraocular muscles; the retrobulbar region lies immediately behind the globe. Knowledge of this anatomy underlies safe periocular drug delivery (subconjunctival, sub-Tenon, peribulbar, retrobulbar injections) (G&G 14e Ch.74, p.1453).
• Conjunctiva — vascularized mucous membrane; the inferior fornix (inferior cul-de-sac) is where topical medications are usually placed (G&G 14e Ch.74, p.1453).
• Lacrimal system — secretory (main lacrimal gland + accessory conjunctival glands) and excretory (puncta → canaliculi → lacrimal sac → nasolacrimal duct → nasal cavity) elements; lacrimal gland is autonomically innervated (parasympathetic secretomotor) (G&G 14e Ch.74, p.1453).
  • Anticholinergic drugs (tricyclic antidepressants, antihistamines, antiparkinsonian drugs) can produce dry-eye symptoms by blocking parasympathetic lacrimal secretion (G&G 14e Ch.74, p.1453).
• Tear film is trilaminar: anterior lipid layer (meibomian glands); middle aqueous layer (~98% of tear film; main + accessory lacrimal glands); posterior mucin layer (conjunctival goblet cells, adherent to corneal epithelium) (G&G 14e Ch.74, p.1453).
• The nasal cavity is lined by highly vascular mucosa; topical ophthalmic drugs reaching it via the nasolacrimal system gain direct systemic access, bypassing hepatic first-pass metabolism — basis of systemic side effects from eye drops (G&G 14e Ch.74, p.1453).