38.1 Introduction to the Eyes

Anatomy and Physiology of the Eye

The eye converts light into electrical signals that the brain interprets as vision. For pharmacology, understanding eye anatomy matters because drug targets, delivery methods, and side effects all depend on which structures are involved.

Pupil, Iris, and Retina

The cornea is the clear, dome-shaped outer layer at the front of the eye. It protects the inner structures and does most of the work of bending (refracting) light so it can be focused.

Behind the cornea sits the iris, the colored part of the eye. The iris contains two sets of smooth muscle that control the size of the pupil, the central opening where light enters. In bright light, the iris constricts the pupil to limit light entry; in dim light, it dilates the pupil to let more in. Many ophthalmic drugs work by targeting these iris muscles, so you'll see this concept again with miotics and mydriatics.

The lens is a transparent, flexible structure just behind the iris. It fine-tunes focus by changing shape, a process called accommodation, directing light precisely onto the retina.

The retina lines the back of the eye and contains photoreceptor cells:

• Rods handle low-light and peripheral vision
• Cones handle color vision and sharp central (visual acuity) detail

These photoreceptors convert light into electrical signals, which travel along the optic nerve to the brain for processing.

One more structure to know: the space between the cornea and the lens is filled with aqueous humor, a clear fluid that nourishes the eye and maintains intraocular pressure (IOP). Aqueous humor drains out through a structure called the trabecular meshwork. This drainage system is directly relevant to glaucoma and the drugs used to treat it.

Common Eye Disorders

Glaucoma

Glaucoma is a group of conditions where increased intraocular pressure (IOP) damages the optic nerve over time, leading to vision loss. The underlying problem is usually impaired drainage of aqueous humor through the trabecular meshwork. Fluid builds up, pressure rises, and the optic nerve gradually deteriorates.

Risk factors include older age, family history, African American or Hispanic descent, and conditions like diabetes or severe myopia.

There are two main types to know:

• Open-angle glaucoma (most common): Drainage is slow but not fully blocked. Symptoms develop gradually, starting with loss of peripheral vision and progressing to tunnel vision. Patients often don't notice until significant damage has occurred, which is why it's called the "silent thief of sight."
• Acute angle-closure glaucoma: The drainage angle is suddenly blocked. This is a medical emergency with rapid onset of severe eye pain, redness, headache, nausea, and blurred vision. It requires immediate treatment to prevent permanent vision loss.

Conjunctivitis

Conjunctivitis (pink eye) is inflammation of the conjunctiva, the thin membrane covering the white of the eye and lining the inner eyelids. Causes include:

• Viral infection (most common): Highly contagious, usually produces watery discharge
• Bacterial infection: Produces thicker discharge (mucus or pus), also contagious
• Allergies or irritants: Not contagious, often accompanied by intense itching

Common symptoms across types include redness and swelling of the conjunctiva, itching or burning, a gritty sensation, and discharge. Identifying the cause matters because treatment differs: bacterial conjunctivitis may require antibiotic drops, while viral conjunctivitis is typically self-limiting, and allergic conjunctivitis responds to antihistamine or mast cell stabilizer drops.

Medication Administration Techniques

Proper technique for ophthalmic medications directly affects how well the drug works and whether it causes unwanted systemic effects. Even small errors in administration can reduce drug delivery to the eye or increase absorption into the bloodstream.

Eye Drops

1. Wash hands thoroughly.
2. Tilt the patient's head back and have them look up toward the ceiling.
3. Gently pull down the lower eyelid to create a small pocket (the conjunctival sac).
4. Hold the dropper close to the eye but do not touch the dropper tip to the eye, eyelid, or eyelashes. Contact contaminates the bottle.
5. Instill the prescribed number of drops into the conjunctival pocket.
6. Have the patient gently close the eye (not squeeze it shut) and apply light pressure to the inner corner of the eye (the nasolacrimal duct area) for 1–2 minutes. This technique, called punctal occlusion, prevents the medication from draining into the nasal passage and being absorbed systemically.

Eye Ointments

1. Wash hands thoroughly.
2. Tilt the patient's head back and have them look up.
3. Gently pull down the lower eyelid to create a pocket.
4. Apply a thin ribbon of ointment (approximately 1 cm) along the inside of the lower eyelid, moving from the inner corner to the outer corner. Avoid touching the tube tip to the eye.
5. Have the patient close the eye gently and roll the eyeball in all directions to distribute the ointment evenly. Warn the patient that vision will be temporarily blurry.

General Tips

• Use a separate dropper or tube for each eye to prevent cross-contamination, especially with infections.
• When administering more than one eye medication, wait at least 5 minutes between drops. If using both drops and ointment, administer drops first, then ointment. Ointment creates a barrier that can block absorption of subsequent drops.
• Store medications as directed on the label. Many ophthalmic drugs are light-sensitive or require refrigeration.
• Teach patients not to share eye medications and to check expiration dates, since contaminated or expired solutions increase infection risk.