Ocular Pharmacology and Toxicology: Drug Classes, Routes, and Toxicants
What to know cold
Understand drug delivery and barriers: corneal epithelium is lipophilic, stroma hydrophilic, so topical drugs need balanced solubility; the blood-aqueous and blood-retinal barriers limit systemic drug penetration. Know the classes: mydriatics/cycloplegics (atropine, tropicamide, phenylephrine - sympathomimetic vs parasympatholytic), miotics (pilocarpine), topical antibiotics and their spectrum, antifungals (natamycin, itraconazole), antivirals (for feline herpesvirus), corticosteroids vs NSAIDs (and the risk of corticosteroid use with corneal ulceration), and glaucoma drugs (prostaglandin analogs, carbonic anhydrase inhibitors, beta-blockers, osmotic agents).
Systemic toxicology is high yield: know that cats are uniquely sensitive to permethrin (tremors, hypersalivation, mydriasis) and that topical ivermectin/permethrin products can cause ocular and neurologic signs. Know ethylene glycol (oxalate crystals, acute blindness), lead, organophosphates/carbamates (miosis, SLUD), and that many topical agents (atropine, corticosteroids) have systemic absorption and endocrine or GI effects.
Traps
Questions test contraindications: corticosteroids worsen fungal and herpes infections and delay corneal healing; atropine raises IOP in some patients and causes ileus; prostaglandins cause miosis and conjunctival hyperemia and are avoided in uveitis. Another trap: confusing the mechanism of phenylephrine (alpha-agonist, mydriasis without cycloplegia) with atropine (anticholinergic, mydriasis plus cycloplegia).
Rule of thumb
Match the drug to the compartment and the barrier. Worked example: a corneal ulcer with a melting appearance - avoid corticosteroids, use topical antibiotic plus serum or anticollagenase therapy, and consider systemic NSAID for pain; do not use a prostaglandin analog.