Ophthalmic Drug Preparations
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Direct answer
Ophthalmic preparations deliver drugs topically as drops (about 25-50 microlitres per drop, of which under 5 percent penetrates the cornea), as gels and ointments for longer contact, and by injection for sight-threatening disease — intravitreal anti-VEGF agents (ranibizumab, aflibercept, off-label bevacizumab) for diabetic macular oedema and neovascular age-related degeneration. Glaucoma therapy illustrates the class logic: once-nightly latanoprost (prostaglandin F2 analogue increasing uveoscleral outflow, iris and lash pigmentation as signature effects), timolol (systemically absorbed beta-blocker — caution in asthma and bradycardia), brimonidine (avoid in infants), and carbonic anhydrase inhibitors or mannitol for acute pressure. Preservatives, notably benzalkonium chloride, and nasolacrimal occlusion after instillation are the practical pharmacology that separates an ophthalmic prescriber from a prescription writer.
What you must remember
- Drop pharmacokinetics: a drop exceeds tear-film capacity, so most drains through the punctum — closing the medial canthus for 1-2 minutes after instillation doubles contact time and cuts systemic absorption; waiting 5 minutes between two drops.
- Prostaglandin analogues: latanoprost 0.005% once at night (bimatoprost, travoprost similar) — increase uveoscleral outflow; adverse effects: gradual iris and periocular skin pigmentation, longer thicker lashes, conjunctival hyperaemia; refrigerate latanoprost.
- Timolol 0.25-0.5% and brimonidine: timolol's beta-blockade reaches the circulation through the nasal mucosa — bronchospasm, bradycardia, masking hypoglycaemia in diabetics, contraindicated in asthma, heart block and decompensated heart failure; brimonidine, an alpha-2 agonist, causes somnolence and apnoea in infants — avoid under about 2 years.
- Acute angle-closure attack: pilocarpine 2% after pressure falls (ischaemic sphincter is unresponsive initially), plus intravenous acetazolamide 500 mg and mannitol 20% 1-1.5 g/kg, topical timolol and apraclonidine — an emergency dose list examiners expect.
- Intravitreal therapy: ranibizumab 0.3-0.5 mg and aflibercept 2 mg injections every 4-8 weeks for wet ARMD and diabetic macular oedema; dexamethasone implant for macular oedema; endophthalmitis is the feared complication.
- Mydriatics and cycloplegics: tropicamide 0.5-1% (6 hours, no cycloplegia for refraction), homatropine (1-2 days), atropine 1% (7-14 days, refraction in children); phenylephrine 2.5% for dilation — the 10 percent strength raises blood pressure and is used with caution.
- Preservative pharmacology: benzalkonium chloride causes punctate keratitis with chronic use and discolours soft contact lenses — preservative-free unit-dose drops for dry eye and long-term glaucoma therapy in compromised ocular surfaces.
- Steroid warning: topical corticosteroids raise intraocular pressure, accelerate cataract, and worsen herpetic and fungal keratitis — never prescribe without slit-lamp follow-up.
A glaucoma clinic morning walked through
A 58-year-old office worker is referred with bilateral ocular pressures of 28 mmHg and early visual field loss: primary open-angle glaucoma. Step one is latanoprost once nightly in the right strength for adherence — one drop, one minute of canthal closure, the bottle kept in the fridge at home. Six weeks later pressure is 21 mmHg but not at target, so a fixed-combination latanoprost-timolol drop is introduced — but only after his asthma history is interrogated, because topical timolol has hospitalised asthmatics. The second patient is an emergency: a 64-year-old with a painful red right eye, hazy cornea and fixed mid-dilated pupil — acute angle closure. She receives intravenous acetazolamide 500 mg, topical timolol and apraclonidine, mannitol if pressure resists, and pilocarpine 2% an hour later once pressure has begun to fall — the sequencing question (why pilocarpine fails at 60 mmHg: an ischaemic sphincter cannot constrict) is a final-professional classic. The third is a diabetic with falling vision: optical coherence tomography confirms macular oedema, and she begins monthly intravitreal aflibercept — with counselling that these injections, not oral tablets, are now the sight-saving therapy.
Where students slip
Two slips recur in practicals. First, topical does not mean local: timolol's systemic absorption through nasolacrimal mucosa produces real bronchospasm and bradycardia, and the examiner's question — "which glaucoma drop is dangerous in asthma?" — has only one answer. Second, sequencing in angle closure: giving pilocarpine first, when the pressure-ischaemic iris sphincter cannot respond, wastes the window; pressure must fall before the moiotic works. Lesser slips include confusing mydriatic duration (tropicamide hours versus atropine days), forgetting latanoprost's once-at-night schedule (prostaglandins work better nocturnally and daytime dosing is less effective), and prescribing topical steroids for a red eye without excluding keratitis — the classic path to an ulcer that perforates.
Frequently asked questions
Why should the lacrimal punctum be occluded after eyedrop instillation?
It prolongs ocular contact and prevents drainage to the nasal mucosa, increasing efficacy and reducing systemic absorption and side effects.
Which glaucoma medication is contraindicated in asthma?
Timolol and other topical beta-blockers — systemic absorption causes bronchospasm, bradycardia and masked hypoglycaemia.
What are the signature adverse effects of latanoprost?
Increased brown iris pigmentation (irreversible), longer and darker eyelashes, conjunctival hyperaemia, and the once-nightly dosing schedule.
Why is pilocarpine delayed in acute angle-closure treatment?
The ischaemic sphincter at very high pressure cannot constrict; acetazolamide, timolol and mannitol lower pressure first, then pilocarpine opens the angle.
Which drugs are injected into the vitreous and for what?
Anti-VEGF agents ranibizumab, aflibercept and off-label bevacizumab for wet ARMD and diabetic macular oedema; steroid implants for persistent macular oedema.