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INTRODUCTION

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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.

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ABBREVIATIONS

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Abbreviations

ACh: acetylcholine

ADME: absorption, distribution, metabolism, excretion

ARMD: age-related macular degeneration

CA: carbonic anhydrase

CAI: carbonic anhydrase inhibitor

CMV: cytomegalovirus

CNS: central nervous system

COMT: catechol-O-methyltransferase

DME: diabetic macular edema

FDA: Food and Drug Administration

5FU: 5-fluorouracil

GSH: reduced glutathione

HIV: human immunodeficiency virus

IOP: intraocular pressure

IP3: inositol triphosphate

MAO: monoamine oxidase

NMDA: N-methyl-D-aspartate

PDE: phosphodiesterase

PG: prostaglandin

PK: phosphokinase

PLC: phospholipase C

RAR: retinoic acid receptor

RXR: retinoid X receptor

tPA: tissue plasminogen activator

VEGF: vascular endothelial growth factor

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EXTRAOCULAR STRUCTURES

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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 69–1). 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 retrobulbar injections.

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Figure 69–1

Anatomy of the globe in relation to the orbit and eyelids. Routes of administration of anesthesia are represented by the blue needles.

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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.

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The lacrimal system consists of secretory glandular and excretory ductal elements (Figure 69–2). 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 69–1 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 39), and drugs used in the management of Parkinson disease (see Chapter 18). 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.

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Figure 69–2

Anatomy of ...

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