Ocular Pharmacology & Glaucoma

Introduction & ocular barriers

• Secluded organ — The eye is relatively isolated from systemic access by the blood–retinal, blood–aqueous and blood–vitreous barriers, giving it distinctive ocular pharmacokinetics; topical drops placed in the inferior fornix (cul-de-sac) are the commonest route.
• Cornea = trilaminar barrier — A "fat–water–fat" barrier — lipophilic epithelium → hydrophilic stroma (~90% of thickness) → lipophilic endothelium; an amphipathic drug (both lipid- and water-soluble) penetrates best.
• Autonomic receptor map of the eye — Iris dilator (radial) = α₁ → mydriasis; iris sphincter (circular) = M₃ → miosis; ciliary muscle = M₃ → accommodation; ciliary epithelium = β₂ stimulates / α₂ suppresses aqueous production; carbonic anhydrase isoenzyme II in ciliary epithelium drives bicarbonate-dependent secretion.
• Systemic access via tears — Drug draining through the nasolacrimal duct reaches highly vascular nasal mucosa and enters the circulation directly, bypassing hepatic first-pass metabolism — the basis of systemic side effects from eye drops.