Ophthalmic Drugs



Ophthalmic Drugs


Objectives


When you reach the end of this chapter, you will be able to do the following:



Drug Profiles



Key Terms


Accommodation The adjustment of the lens of the eye for variations in distance. (p. 919)


Angle-closure glaucoma Glaucoma that occurs as a result of a narrowed anatomic angle between the lens and cornea. Also called closed-angle glaucoma, narrow-angle glaucoma, congestive glaucoma, and pupillary closure glaucoma. (p. 920)


Anterior chamber The bubble-like portion of the front of the eye between the iris and the cornea. (p. 919)


Aqueous humor The clear, watery fluid circulating in the anterior and posterior chambers of the eye. (p. 919)


Canal of Schlemm A tiny circular vein at the angle of the anterior chamber of the eye through which the aqueous humor is drained and ultimately funneled into the bloodstream. Also called Schlemm canal. (p. 919)


Cataract An abnormal progressive condition of the lens of the eye, characterized by loss of transparency with resultant blurred vision. (p. 919)


Ciliary muscle The circular muscle between the anterior and posterior chambers of the eye behind the iris. It is connected to the suspensory ligaments that control the curvature of the lens. (p. 919)


Cones Photoreceptive (light-receiving) cells in the retina of the eye that enable a person to perceive colors and play a large role in central (straight-ahead) vision. (p. 920)


Cornea The convex, transparent anterior part of the eye. (p. 919)


Cycloplegia Paralysis of the ciliary muscles, which prevents the accommodation of the lens for variations in distance. (p. 919)


Cycloplegics Drugs that paralyze the ciliary muscles of the eye. (p. 919)


Dilator muscle A muscle that constricts the iris of the eye but dilates the pupil. Also called dilator pupillae. (p. 919)


Glaucoma An abnormal condition of elevated pressure within an eye because of obstruction of the outflow of aqueous humor. (p. 920)


Intraocular pressure The pressure of the fluids of the eye against the tunics (retina, choroid, and sclera). (p. 919)


Iris The round, muscular portion of the eye that gives the eye its color and serves as an aperture controlling the amount of light passing through the pupil. (p. 919)


Lacrimal ducts Small tubes that drain tears from the lacrimal glands into the nasal cavity. (p. 919)


Lacrimal glands Glands located at the medial corners of the eyelids that produce tears. (p. 919)


Lens The transparent, curved structure of the eye that is located directly behind the iris and the pupil and is attached to the ciliary body by ligaments. (p. 919)


Lysozyme An enzyme with antiseptic actions that destroys some foreign organisms. It is normally present in tears, saliva, sweat, and breast milk. (p. 919)


Miotics Drugs that constrict the pupil. (p. 919)


Mydriatics Drugs that dilate the pupil. (p. 919)


Open-angle glaucoma A type of glaucoma that is often bilateral, develops slowly, is genetically determined, and does not involve a narrowing of the angle between the iris and the cornea. (Also called chronic glaucoma, wide-angle glaucoma, and simple glaucoma.) (p. 920)


Optic nerve A major nerve that connects the posterior end of each eye to the brain, to which it transmits visual signals. (p. 920)


Pupil A circular opening in the iris of the eye, located slightly to the nasal side of the center of the iris. The pupil lies behind the anterior chamber of the eye and the cornea and in front of the lens. (p. 919)


Retina The innermost layer of the eye, containing both rods and cones that receive visual stimuli and transmit them to the optic nerve. (p. 920)


Rods The photoreceptive elements arranged perpendicularly to the surface of the retina. Rods are especially sensitive to low-intensity light and are responsible for black-and-white and peripheral (“off-to-the-side”) vision. (p. 920)


Sphincter pupillae A muscle that expands the iris while constricting or narrowing the diameter of the pupil. (p. 919)


Tears Watery saline or alkaline fluid secreted by the lacrimal glands to moisten the conjunctiva (see Figure 57-1). (p. 919)


Uvea The fibrous tunic beneath the sclera that includes the iris, the ciliary body, and the choroid of the eye (see Figure 57-1). Also called tunica vasculosa bulbi or uveal tract. (p. 919)


Vitreous body A transparent, semigelatinous substance contained in a thin membrane filling the cavity behind the lens. Also called the corpus vitreum. (p. 919)


Vitreous humor The fluid component of the vitreous body. (p. 919)


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http://evolve.elsevier.com/Lilley



Anatomy, Physiology, and Pathophysiology Overview


The eye is the organ responsible for the sense of sight. The structures of the eye are illustrated in Figure 57-1. Each eyeball is nearly spherical and approximately 1 inch in diameter. Each eye is recessed into a small frontal skull cavity known as an orbit. The exposed anterior (front) portion of the eye is covered by three layers: the protective external layer (cornea and sclera), a vascular middle layer known as the uvea (includes the choroid, iris, and ciliary body), and the internal layer, known as the retina. All of these layers are protected by the eyelid, which serves as an external protective tissue.



Each eye is held in place and moved by six muscles that are controlled by cranial nerves. These muscles include the rectus and oblique muscles. There are four types of rectus muscles: inferior, superior, medial, and lateral. There are two types of oblique muscles: inferior and superior. These muscles are shown in Figure 57-2. (The medial rectus muscle is hidden from view in this figure but is directly across from the lateral rectus muscle.) The levator palpebrae superioris muscle opens the eyelid (see later in the chapter). This muscle rests on top of the superior rectus muscle. There are several other important structures that are either part of or adjacent to the eye. The structures and the purpose of each are as follows:



• Eyebrow: Rows of short hair above (superior to) the upper eyelids. The eyebrow protects the eye from direct light, falling dust or other small particles, and perspiration coming from the forehead.


• Eyelid: The layer of muscle and skin lined interiorly by the conjunctiva. The conjunctiva also covers the outer anterior surface of the eye, which includes the cornea. The eyelid is moveable and can open or close. It protects the eye when closed and allows vision when open. The eyelid is raised by contraction of the levator palpebrae superioris muscle and is lowered by relaxation of this muscle (see Figure 57-2).


• Cornea: The convex (outward-projecting; opposite of concave), transparent, anterior portion of the eye. It can be thought of as a window that sits in front of the lens and allows the passage of light.


• Eyelashes: Two or three rows of hairs that are located on the edge (margin) of the eyelids. They help prevent small particles from falling into the eye when it is open.


• Palpebral fissure: The space between the upper and lower eyelids when the eyelids are open but relaxed.


• Sclera: A tough, white coat of fibrous tissue that surrounds the entire eyeball except for the cornea. It helps maintain the shape of the eye. Commonly called the white of the eye, the sclera is nonvascular and allows light to pass through it to the lens.


• Choroid: One of the middle-layer structures of the eyeball that contains blood vessels that supply the eye; it also absorbs light.


• Ciliary body: The structure that supports the ciliary muscles that control the curvature of the lens via attached suspensory ligaments.


• Conjunctiva: The mucous membrane that lines the eyelids and also covers the exposed anterior surface of the eyeball.


• Iris: The colored (pigmented) muscular apparatus behind the cornea.


• Pupil: The variable-sized opening in the center of the iris that allows light to enter into the eyeball when the eyelids are open. The pupil is the rear portion of the window of the eye through which light passes to the lens and the retina (the cornea is the front part of this window).


• Medial canthus: The site of union of the upper and lower eyelids near the nose.


• Lacrimal caruncle: A small, red, rounded elevation covered by modified skin at the medial angle of the eye; the site of the lacrimal glands (see later in the chapter).


• Lateral canthus: The site of union of the upper and lower eyelids away from the nose.



Lacrimal Glands


The eye is kept moist and healthy by an intricate network of connected canals, ducts, and sacs that work together. The lacrimal glands produce tears that bathe and cleanse the exposed anterior portion of the eye. Tears are composed of an isotonic, aqueous solution that contains an enzyme called lysozyme, which acts as an antibacterial to help prevent eye infections. Tears drain into the nasal cavity through the lacrimal ducts.


Layers of the Eye


Overall, the eye can be thought of as having three separate anatomic layers. The fibrous outer layer of the eye has two parts that are continuous with each other: the sclera and the cornea. The sclera is a tough, fibrous layer that protects and maintains the shape of the eye. The cornea is a nonvascular transparent portion of the outer layer that allows light to enter the eye. It is located at the very front of the eye and is continuous with the sclera. It is pain-sensitive (a protective function) and obtains nutrition from the aqueous humor, the clear watery fluid that circulates in the anterior and posterior chambers of the eye.


The vascular middle layer of the eye is composed of the iris (to the anterior), ciliary body, and choroid (to the posterior). These three structures are collectively called the uvea. The iris gives color to the eye and has an adjustable opening in the center called the pupil. The main function of the iris is to regulate the amount of light that enters the eye by causing the size of the pupil to vary. Pupil size is controlled by two smooth muscles. The sphincter pupillae muscle is controlled by the parasympathetic nervous system and constricts the diameter of the pupil (called miosis) (Figure 57-3). In contrast, the pupil is opened (called mydriasis) by a radial smooth muscle called the dilator muscle. It is composed of radiating fibers, like spokes of a wheel, which converge from the circumference of the iris toward its center. Sympathetic nervous system impulses control this muscle (see Figure 57-3).



The anterior portions of both the retina and choroid merge to become the ciliary body, which produces aqueous humor. This is the clear, watery fluid that circulates in both the anterior and posterior chambers, not to be confused with tears. Aqueous humor contributes, along with vitreous humor to the intraocular pressure of the eye. This is the internal pressure of all fluids against the tunics (retina, choroid, sclera) of the eye. Given the small space of the eye, any change in the volume of aqueous humor present can lead to increased or reduced intraocular pressure. Normally, the aqueous humor is removed from the anterior chamber via the canal of Schlemm at a rate that balances out its production by the ciliary body. The ciliary body also provides a support for the suspensory ligaments to which the lens is attached. The lens is the transparent crystalline structure of the eye, located directly behind the iris and the pupil. It has a biconvex (oval-spherical) shape and is held in place by suspensory ligaments that are attached to the ciliary muscle. Contraction of the ciliary muscle changes the shape of the lens. This function is important for visual accommodation as well as the focusing of light (and visual images) onto the retina. The ciliary muscle is controlled by the parasympathetic nervous system through the oculomotor cranial nerve (cranial nerve III). The lens divides the interior of the eyeball into posterior (rear) and anterior (forward) chambers. The larger chamber behind the lens is filled with a jellylike fluid called the vitreous body. The lens is transparent to allow light to pass through easily. A loss of lens transparency results in a visual condition called a cataract. A cataract is a gray-white opacity that can be seen within the lens. If cataracts are untreated, sight may eventually be completely lost. At the onset of a cataract, vision is blurred and may be further worsened by the glare of bright lights. Diplopia or double vision may also develop.


Before light rays reach the retina, they are focused into a sharp image by the lens of the eye. The elasticity of the lens enables it to change its shape and focusing power. This process is called accommodation and is facilitated by the ciliary body. Paralysis of accommodation is called cycloplegia. Mydriatics are drugs that dilate the pupil (e.g., apraclonidine). Drugs that constrict the pupil are called miotics (e.g., acetylcholine, pilocarpine). Drugs that paralyze the ciliary body are called cycloplegics, but they also have mydriatic properties (e.g., atropine, cyclopentolate) (Figure 57-4). All of these medications are used to facilitate visualization of the inner eye during ophthalmic examinations.



The third and inner layer of the eye is a thin delicate layer known as the retina. It contains light-sensitive photoreceptors called rods and cones. The basic function of the retina is to receive the light image formed by the lens and to convert it via the rods and cones into the neural signals that support vision. Rods produce black-and-white vision, including shades of gray, and are especially sensitive in low light; cones are responsible for color vision (Figure 57-5). In addition, rods are more active in providing peripheral (to-the-side) vision, whereas cones are more active in central (straight-ahead) vision. In the posterior central part of the retina, the nerve fibers of retinal cells join to form the optic nerve. The function of this nerve is to connect the retina with the visual center of the brain, located within the occipital lobe that extends above and behind the cerebellum. It is this portion of the brain that interprets incoming visual stimuli.



Pathophysiology of Glaucoma


Glaucoma is a group of eye disorders that damages the optic nerve. In most cases, this is due to increased intraocular pressure that is caused by abnormally elevated levels of aqueous humor. Glaucoma occurs when the aqueous humor is not drained through the canal of Schlemm as quickly as it is formed by the ciliary body. The accumulated aqueous humor creates a backward pressure that pushes the vitreous humor against the retina. Continued pressure on the retina destroys its neurons, which leads to impaired vision and eventual blindness (Figure 57-6). Unfortunately, glaucoma is often without early symptoms, and many patients are not diagnosed until some permanent sight loss has occurred.



Two major types of glaucoma are discussed in this chapter: angle-closure glaucoma and open-angle glaucoma. Figure 57-7 shows the pathophysiology of each and provides an enlarged view of the involved eye structures. Table 57-1 lists additional characteristic features of each type. Glaucoma can be a primary illness (occurring on its own), or it can be secondary to another eye condition or injury (e.g., posttraumatic glaucoma). Congenital glaucoma can also occur in infants. The visual and optic nerve changes typical of glaucoma can also occur in the absence of increased intraocular pressure (normotensive glaucoma). There are a few other less common forms of glaucoma (e.g., pigmentary glaucoma, pseudoexfoliative glaucoma) that are beyond the scope of this chapter.




Pharmacology Overview


Medications used to treat disorders of the eye can be divided into several major drug groups: antiglaucoma drugs, antimicrobials, antiinflammatory drugs, topical anesthetics, diagnostic drugs, antiallergic drugs, and lubricants and moisturizers. There are also a variety of combination drug products that include two or more medications from different subclasses. The reader can assume the same therapeutic indications and drug effects for these combination products as for the single-ingredient drug products corresponding to their individual components. The focus of this chapter is on commonly used therapeutic medications.


A multitude of various products are also available for use in the care of contact lenses, including contact lens–cleaning enzymes, irrigating solutions, and eye washes. Their use is fairly straightforward, and they carry limited risk. More complicated surgical drugs are beyond the scope of this chapter. The reader is advised to refer to the manufacturer’s packaging information for details about any unfamiliar product encountered in clinical practice.


Antiglaucoma Drugs


Treatment of glaucoma involves reducing intraocular pressure by either increasing the drainage of aqueous humor or decreasing its production. Some drugs may do both. Drug therapy can delay and possibly even prevent the development of glaucoma. Glaucoma eyedrops are color-coded according to medication class to aid the patient in identification, and they are listed in Table 57-2. Drug classes used to reduce intraocular pressure include the following:




See Table 57-2 for a comparison of the effects of these drugs on aqueous humor.


Cholinergic Drugs (Miotics)


There are two categories of ocular parasympathetic drugs, more concisely referred to as cholinergic drugs: direct acting and indirect acting. Directing-acting cholinergics include acetylcholine, carbachol, and pilocarpine. Indirect-acting drugs, which are also called cholinesterase inhibitors, include echothiophate, currently the only available drug in this class. Because the primary drug effect of these drugs is pupillary constriction, or miosis (see later), they are also commonly called miotics.


Mechanism of Action and Drug Effects


Acetylcholine is the neurochemical mediator of nerve impulses in the parasympathetic nervous system. It stimulates parasympathetic or cholinergic receptors located in the brain and throughout the body along parasympathetic nerve branches. This results in several effects on the eye: miosis (pupillary constriction), vasodilation of blood vessels in and around the eye, contraction of ciliary muscles, drainage of aqueous humor, and reduced intraocular pressure. Ciliary muscle contraction promotes aqueous humor drainage by widening the space where the drainage occurs. Miosis promotes aqueous humor drainage by causing the iris to stretch, which also serves to widen this space.


Both direct- and indirect-acting miotics have effects similar to those of acetylcholine, but their actions are more prolonged (Figure 57-8). The direct-acting miotics are able to directly stimulate ocular cholinergic receptors and mimic acetylcholine. Indirect-acting miotics work by binding to and inactivating the cholinesterases acetylcholinesterase and pseudocholinesterase, the enzymes that break down acetylcholine. As a result, acetylcholine accumulates and acts longer at the cholinergic receptor sites. This leads to drug effects that include miosis, ciliary muscle contraction, enhanced aqueous humor drainage, and reduced intraocular pressure by an average of 20% to 30% (Figure 57-9). Drug-induced miotic effects may be less pronounced in individuals with dark eyes (e.g., brown or hazel) than in those with lighter eyes (e.g., blue). This is because the pigment of the iris also absorbs the drug (which reduces its therapeutic effects), and dark eyes have more pigment.




Indications


The direct- and indirect-acting miotics are used for treatment of open-angle glaucoma, angle-closure glaucoma, and convergent strabismus (a condition in which one eye points toward the other, or “cross-eye”) and in ocular surgery. They are also used to reverse the effect of mydriatic (pupil-dilating) drugs after ophthalmic examination. Specific indications may vary for different drugs, as shown in Table 57-3.



Contraindications


Contraindications to the use of miotics include known drug allergy and any serious active eye disorder in which induction of miosis might be harmful.


Adverse Effects


Most of the adverse effects from the use of cholinergic and cholinergic inhibitors (miotics) are local and limited to the eye. Adverse effects are more likely to occur with indirect-acting miotics because they have longer-lasting effects. Effects include blurred vision, drug-induced myopia (nearsightedness), and accommodative spasms. Such effects are secondary to contraction of the ciliary muscle. Miotic drugs also cause vasodilation of blood vessels supplying the conjunctiva, iris, and ciliary body, which may lead to vascular congestion and ocular inflammation. Other undesirable effects include temporary stinging upon drug instillation, reduced nighttime or low-light vision, conjunctivitis, lacrimation (tearing), twitching of the eyelids (blepharospasm), and eye or brow pain. Prolonged use can result in iris cysts, lens opacities, and, rarely, retinal detachment. Systemic effects are uncommon but are more likely to occur with cholinesterase inhibitors (indirect-acting miotics). See Chapters 20 and 21 for more information on the systemic effects of these drugs.


Interactions


Drug interactions are unlikely because of the local actions of these drugs. When miotic drugs are given with topical



Dosages


For dosage information on selected miotic drugs, see the table on this page.


Drug Profiles


Direct-acting ocular cholinergics include acetylcholine (Miochol-E), carbachol (Carboptic), and pilocarpine (Pilocar). Indirect-acting drugs, which are also called cholinesterase inhibitors, include echothiophate (Phospholine Iodide). These drugs are used for management of glaucoma, as adjuncts for ocular surgery, and for treatment of various other ophthalmic conditions.


Direct-Acting Miotics


acetylcholine


Acetylcholine (Miochol-E) is a direct-acting cholinergic drug that is used to produce miosis during ophthalmic surgery. It is a pharmaceutical form of the naturally occurring neurotransmitter in the body. It has very quick onset and may begin to work almost immediately. It is administered directly into the anterior chamber of the eye before and after securing one or more sutures.



♦ pilocarpine


Pilocarpine (Pilocar) is a direct-acting cholinergic drug that is used as a miotic in the treatment of glaucoma. Pilocarpine is available in different strengths as an ocular gel and solution. One special formulation is the pilocarpine ocular insert system (Ocusert Pilo-20), which is applied once weekly by the patient.


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May 9, 2017 | Posted by in NURSING | Comments Off on Ophthalmic Drugs

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