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.
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 64-1). In the orbit, connective (i.e., Tenon's capsule) and adipose tissues and 6 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's, and retrobulbar injections.
Anatomy of the globe in relationship to the orbit and eyelids. Various routes of administration of anesthesia are demonstrated by the blue needle pathways.
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 lids, 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 64-2). 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 64-1 and Chapter 8). 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 15), antihistamines (see Chapter 32), and drugs used in the management of Parkinson disease (see Chapter 22).
Table 64–1Autonomic Pharmacology of the Eye and Related Structures |Favorite Table|Download (.pdf) Table 64–1 Autonomic Pharmacology of the Eye and Related Structures
| ||ADRENERGIC RECEPTORS ||CHOLINERGIC RECEPTORS |
|TISSUE ||SUBTYPE ||RESPONSE ||SUBTYPE ||RESPONSE |
|Corneal epithelium ||β2 ||Unknown ||Ma ||Unknown |
|Corneal endothelium ||β2 ||Unknown ||Undefined ||Unknown |
|Iris radial muscle ||α1 ||Mydriasis || || |
|Iris sphincter muscle || || ||M3 ||Miosis |
|Trabecular meshwork ||β2 ||Unknown || || |
|Ciliary epitheliumb ||α2/β2 ||Aqueous production || || |
|Ciliary muscle ||β2 ||Relaxationc ||M3 ||Accommodation |
|Lacrimal gland ||α1 ||Secretion ||M2, M3 ||Secretion |
|Retinal pigment epithelium ||α1/β2 ||H2O transport/unknown || || |