OVERVIEW OF OCULAR ANATOMY, PHYSIOLOGY, AND BIOCHEMISTRY
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. Because of its anatomical isolation, the eye offers a unique, organ-specific pharmacological laboratory in which to study the autonomic nervous system and the effects of inflammation and infectious diseases. No other organ in the body is so readily accessible or as visible for observation; however, the eye also presents some unique challenges as well as opportunities for drug delivery (Robinson, 1993).
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 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'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 eyelids serve several functions. Foremost, their dense sensory innervation and eyelashes protect the eye from mechanical and chemical injuries. Blinking, a coordinated movement of the orbicularis oculi, levator palpebrae, and Müller's muscles, serves to distribute tears over the cornea and conjunctiva. In humans, the average blink rate is 15-20 times/minute. 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, also known as the glands of Krause and Wolfring, 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 (Chapter 15), antihistamines (Chapter 32), and drugs used in the management of Parkinson disease (Chapter 22). Located just ...