The eye is a specialized sensory organ that is relatively secluded from systemic access by the blood-retinal, blood-aqueous, and blood-vitreous barriers. As a consequence, the eye exhibits some unusual pharmacodynamic and pharmacokinetic properties.
AMD: age-related macular degeneration
CAI: carbonic anhydrase inhibitor
FDA: U.S. Food and Drug Administration
ICAM-1: intercellular adhesion molecule 1
IOP: intraocular pressure
LFA-1: lymphocyte function–associated antigen 1
NSAID: non-steroidal anti-inflammatory drug
tPA: tissue plasminogen activator
VEGF: vascular endothelial growth factor
WHO: World Health Organization
The eye is protected by the eyelids and by the orbit, a bony cavity of the skull that has multiple fissures and foramina that conduct nerves, muscles, and vessels (Figure 74–1). In the orbit, connective (i.e., Tenon’s capsule) and adipose tissues and six extraocular muscles support and align the eyes for vision. The retrobulbar region lies immediately behind the eye (or globe). Understanding ocular and orbital anatomy is important for safe periocular drug delivery, including subconjunctival, sub-Tenon, and peribulbar or retrobulbar injections.
Anatomy of the globe in relation to the orbit and eyelids. Routes of administration of anesthesia are represented by the blue needles.
The external surface of the eyelids is covered by a thin layer of skin; the internal surface is lined with the palpebral portion of the conjunctiva, which is a vascularized mucous membrane continuous with the bulbar conjunctiva. At the reflection of the palpebral and bulbar conjunctivae is a space called the fornix, located superiorly and inferiorly behind the upper and lower eyelids, respectively. Topical medications usually are placed in the inferior fornix, also known as the inferior cul-de-sac.
The lacrimal system consists of secretory glandular and excretory ductal elements (Figure 74–2). The secretory system is composed of the main lacrimal gland, which is located in the temporal outer portion of the orbit, and accessory glands located in the conjunctiva. The lacrimal gland is innervated by the autonomic nervous system (Table 74–1 and Chapter 10). The parasympathetic innervation is clinically relevant because a patient may complain of dry eye symptoms while taking medications with anticholinergic side effects, such as tricyclic antidepressants (see Chapter 18), antihistamines (see Chapter 43), and drugs used in the management of Parkinson’s disease (see Chapter 21). Muscarinic cholinergic and α adrenergic receptors that mediate responses of several pupillary muscles from autonomic nerves also provide means of dilating the pupil for examination of posterior structures.
Anatomy of the lacrimal system.
TABLE 74–1AUTONOMIC PHARMACOLOGY OF THE EYE AND RELATED STRUCTURES