Introduction to the Eyes

Core Concepts

The eye's structure

Think of the eye as a fluid-filled sphere with three layers. The outer layer is the protective shell: the tough white sclera and the transparent cornea in front, through which light enters and topical drugs pass. The middle layer (uvea) is vascular: the iris (the colored diaphragm controlling the pupil), the ciliary body (makes aqueous humor, focuses the lens), and the choroid (blood supply behind the retina). The inner layer is the retina, which converts light into nerve signals. The lens sits behind the iris; the space between cornea and iris is the anterior chamber, filled with aqueous humor — whose pressure is the key issue in glaucoma. The conjunctiva is the thin membrane over the inner eyelids and the white of the eye.

Tear film and drainage

A thin tear film — oil, water, and mucus layers — keeps the cornea moist and is the first barrier a drop meets: a new drop mixes with, dilutes, and is washed away by existing tears. Tears drain through tiny openings (puncta) at the inner corners of the eyelids into the nasolacrimal duct to the nose and throat — why an overflowing drop tastes bitter and systemic absorption occurs via the nasal mucosa. Blinking pumps tears and drug toward the puncta, so blinking right after instillation pushes drug out rather than helping it absorb.

Why topical delivery is inefficient

Only a small fraction of an instilled drop is absorbed. The conjunctival sac holds far less than a typical drop, so most overflows; tears dilute it; blinking pumps it down the nasolacrimal duct; the corneal epithelium has tight junctions that resist penetration; and the blood–ocular barriers (blood–aqueous and blood–retinal) restrict movement between the bloodstream and the eye's interior. These barriers explain why posterior-segment disease (retina, vitreous) usually needs intravitreal or systemic therapy, while anterior-segment disease is treatable topically.

Routes of ocular drug delivery

• Topical (drops, ointments, gels): default for anterior-segment problems — convenient, noninvasive, but low bioavailability.
• Periocular injections (around the eye, e.g., subconjunctival): higher drug levels than drops for severe disease.
• Intravitreal injection (into the vitreous cavity): delivers drug directly to the retina and vitreous — the route for many retinal therapies.
• Systemic (oral, IV): for eye disease that is part of a systemic infection or when no topical option exists; limited by blood–ocular barriers.

Nursing technique: making drops work

Instillation follows specific steps: wash hands; tilt the head back; pull the lower lid down to make a small pocket (the lower conjunctival sac); place the drop in the pocket — never on the cornea; avoid touching the bottle tip to the eye or lid (contamination); release the lid. Two techniques improve retention and cut systemic absorption: punctal occlusion (gentle pressure over the inner-corner punctum, about a minute per facility guidance) and gentle lid closure after the drop. When multiple drops are ordered, wait long enough between them (about five minutes per facility policy) so the first isn't washed away. If both drops and ointment are ordered, the drop goes first, the ointment last — ointment coats the eye and blocks later drops.

Administration safety and patient teaching

Teach patients: don't share bottles or droppers; don't let the tip touch anything; store products per label (some need refrigeration or shaking); check expiration dates; remove contact lenses for most products; expect ointments to blur vision temporarily. Stinging or burning is common and brief — but persistent pain, worsening redness, or vision change must be reported. Educational draft only — no doses or administration schedules; verify products, concentrations, storage, and technique against references, formulary, and prescriber orders.

Common Confusions