23.1 Routes of Drug Delivery, Prescription Components & Instillation Technique

Routes of ocular drug delivery

The blood-ocular barriers — the blood-aqueous and blood-retinal barriers — are why systemic drugs often fail to reach therapeutic intraocular levels and why intravitreal injection exists. Inflammation breaks those barriers down, which is why systemic antibiotics penetrate an inflamed eye better than a quiet one.

The arithmetic of a drop

The conjunctival sac holds about 7 to 10 microlitres. A commercial eye drop is typically 30 to 50 microlitres. Most of a drop therefore overflows onto the cheek or drains down the nasolacrimal duct within seconds.

Three consequences follow directly and all are examinable:

1. A second drop of the same medication immediately after the first is wasted, because there is no space for it.
2. When two different drops are prescribed, wait at least five minutes between them, or the second washes out the first.
3. Drug that drains down the nasolacrimal duct is absorbed across the highly vascular nasal mucosa directly into the systemic circulation, bypassing first-pass hepatic metabolism. This is why a topical beta-blocker can cause bronchospasm and bradycardia.

Punctal occlusion — pressing gently over the medial canthus for one to two minutes after instillation — or simply gentle lid closure for the same period, both increases ocular absorption (by prolonging contact) and reduces systemic absorption (by preventing nasolacrimal drainage). It is a free intervention that every patient on a chronic drop should be taught.

Correct instillation technique

Drops:

1. Wash hands.
2. Check the drug, concentration, expiry date and the eye — the same five-rights discipline as any medication.
3. Ask the patient to tilt the head back or lie back, and to look up.
4. Gently pull the lower lid down to form a pocket in the inferior fornix.
5. Hold the bottle above the eye without touching the lashes, lid or globe — contact contaminates the bottle and can abrade the cornea.
6. Instil one drop into the fornix, not onto the cornea (a drop landing on the cornea is uncomfortable and triggers reflex tearing and blinking that expels it).
7. Ask the patient to close the eye gently — not squeeze — and apply punctal occlusion for one to two minutes.
8. Blot excess from the lid with a tissue.
9. Record the drug, concentration, eye, time and who instilled it.

Ointments: discard the first small bead, then lay a ribbon of about 1 cm along the inferior fornix from the nasal to the temporal side, ask the patient to close the eye and roll it around, and warn about blurred vision. Drops first, ointment last — the ointment forms a barrier that impedes anything applied after it.

Common errors: touching the bottle tip to the eye or lashes, instilling multiple drops "to be sure," letting the drop land on the cornea, the patient squeezing the lids so the drop is expelled, failing to wait between different medications, and failing to teach punctal occlusion.

Patients who cannot instil their own drops — tremor, arthritis, poor vision, cognitive impairment — are a major hidden cause of apparent treatment failure. Ask the patient to demonstrate rather than describe their technique, and consider drop aids, a carer, or a simplified regimen.

Components of a medication prescription

The blueprint names "describe the components of a medical prescription" as an explicit task. A complete ophthalmic prescription contains:

1. Patient name, date of birth and address
2. Date of issue
3. Drug name — generic preferred, to avoid brand confusion
4. Concentration / strength — for example 0.5%, 2 mg/mL
5. Dosage form — solution, suspension (must be shaken), ointment, gel
6. Route and eye — right eye, left eye, both eyes; the blueprint's five-domain history and the prescription both depend on unambiguous laterality
7. Frequency — written out ("four times daily"), not QD or QID abbreviations that are on the do-not-use list
8. Duration or total quantity, and the number of refills
9. Specific instructions — shake well, refrigerate, taper schedule, use with punctal occlusion
10. Prescriber name, signature, licence or registration number and contact details

Safety conventions: always a leading zero (0.5%), never a trailing zero (write 1%, not 1.0%), write out unit rather than U, and write the frequency in words. Suspensions such as many steroid preparations must be shaken before every use or the delivered dose varies enormously.

Cap colour conventions are a practical aid, standardised in the United States and widely used elsewhere, though they should never substitute for reading the label: