Chapter 57: Ophthalmic Drugs
Chapter Summary
OCULAR ANATOMY AND PHYSIOLOGY
The eye is the organ responsible for the sense of sight.
The exposed anterior portion of the eye is covered by three layers: (1) the protective external layer (cornea and sclera), (2) a vascular middle layer known as the uvea (includes the choroid, iris, and ciliary body), and (3) 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: (1) inferior, (2) superior, (3) medial, and (4) lateral. The two types of oblique muscles are inferior and superior.
Lacrimal Glands
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
The fibrous outer layer of the eye has two parts that are continuous with each other: (1) the sclera and (2) 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 obtains nutrition from the aqueous humour, 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), collectively called the uvea. The iris gives colour to the eye and has an adjustable opening in the centre 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 the sphincter pupillae muscle and by a radial smooth muscle called the dilator muscle.
Aqueous humour contributes, along with vitreous humour, to the intraocular pressure of the eye.
The lens is the transparent crystalline structure of the eye, located behind the iris and the pupil. It has a biconvex shape and is held in place by ligaments attached to the ciliary muscle.
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.
The elasticity of the lens enables it to change its shape and focusing power (accommodation), facilitated by the ciliary body. Paralysis of accommodation is called cycloplegia.
Mydriatics (e.g., apraclonidine) are drugs that dilate the pupil. Drugs that constrict the pupil (e.g., acetylcholine and pilocarpine) are called miotics. Drugs that paralyze the ciliary body (e.g., atropine and cyclopentolate) are called cycloplegics, but they also have mydriatic properties.
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 into the neural signals that support vision.
Glaucoma
Glaucoma is a disorder of the eye caused by inhibition of the normal flow and drainage of aqueous humour, and treatment helps to reduce intraocular pressure either by increasing the drainage of aqueous humour or by decreasing its production.
Drugs that increase aqueous humour drainage are direct cholinergics, indirect cholinergics, and β-blockers.
Two major types of glaucoma are angle-closure glaucoma and open-angle glaucoma.
Glaucoma can be a primary illness, or it can be secondary to another eye condition or injury (e.g., post-traumatic glaucoma). Congenital glaucoma can also occur in infants.
TREATMENT OF EYE DISORDERS
Medications used to treat disorders of the eye can be divided into several major drug groups: antiglaucoma drugs, antimicrobials, anti-inflammatory drugs, topical anaesthetics, diagnostic drugs, antiallergic drugs, and lubricants and moisturizers.
Antiglaucoma Drugs
Treatment of glaucoma involves reducing the intraocular pressure either by increasing the drainage of aqueous humour or by decreasing its production. Some drugs may do both.
Drug classes used to reduce intraocular pressure include direct-acting cholinergics (also called miotics and parasympathomimetic drugs), indirect-acting cholinergics (also called miotics, parasympathomimetic drugs, and cholinesterase inhibitors), adrenergics (also called mydriatics and sympathomimetic drugs), antiadrenergics (β-blockers, also called sympatholytic drugs), carbonic anhydrase inhibitors, osmotic diuretics, and prostaglandin agonists.
Cholinergic Drugs (Miotics)
Direct-acting cholinergics include acetylcholine, carbachol, and pilocarpine.
Since their primary effect is pupillary constriction, or miosis, indirect-acting drugs are commonly called miotics. Indirect-acting drugs (cholinesterase inhibitors) include echothiophate, the only available drug.
Acetylcholine is the neurochemical mediator of nerve impulses in the parasympathetic nervous system, stimulating parasympathetic or cholinergic receptors resulting in several effects on the eye: miosis, vasodilation of blood vessels in and around the eye, contraction of ciliary muscles, drainage of aqueous humour, and reduced intraocular pressure.
Ciliary muscle contraction promotes aqueous humour drainage by widening the space where the drainage occurs. Miosis promotes aqueous humour drainage by causing the iris to stretch, which also serves to widen this space.
Direct- and indirect-acting miotics have effects similar to those of acetylcholine, but their actions are more prolonged.
The direct- and indirect-acting miotics are used for the treatment of open-angle glaucoma, angle-closure glaucoma, and convergent strabismus (“cross-eye”) and in ocular surgery.
Most adverse effects from the use of cholinergics and cholinergic inhibitors are local and limited. Adverse effects are more likely with indirect-acting miotics because medication is longer lasting. The adverse effects include blurred vision, drug-induced myopia (nearsightedness), and accommodative spasms.
Sympathomimetics (Mydriatics)
Sympathomimetic drugs are used for the treatment of glaucoma and ocular hypertension. These medications include α-receptor agonists brimonidine (Αlphagan P®, Combigan®, Onrel Tea®) and apraclonidine (Iopidine®), and the α-β–receptor agonist dipivefrin.
Sympathomimetic drugs mimic the neurotransmitters norepinephrine and epinephrine, stimulating dilator muscle contraction by α- or β-receptor interaction, resulting in mydriasis.
Both epinephrine and dipivefrin are used to reduce elevated intraocular pressure in the treatment of chronic open-angle glaucoma, either as initial or long-term therapy. Increases in intraocular pressure during surgery are usually mediated via increased catecholamine stimulation.
Adverse effects of the sympathomimetic mydriatics are primarily limited to temporary ocular effects and include burning, eye pain, and lacrimation.
β-Adrenergic Blockers
The antiglaucoma β-adrenergic blockers that reduce intraocular pressure include the β1-selective drug betaxolol, and the nonselective β-blockers levobunolol hydrochloride and timolol.
The ophthalmic β-blockers reduce both elevated and normal intraocular pressure. They reduce intraocular pressure by reducing aqueous humour formation.
Ophthalmic β-blockers are used to reduce elevated intraocular pressure in chronic open-angle glaucoma and ocular hypertension. They may also be used alone or in combination with a topical miotic, which may have an additive intraocular pressure-lowering effect.
Adverse effects of β blockers include transient burning and discomfort, blurred vision, pain, photophobia, lacrimation, blepharitis, keratitis, and decreased corneal sensitivity.
Carbonic Anhydrase Inhibitors
Ophthalmic carbonic anhydrase inhibitors include brinzolamide (Azarga®, Azopt®) and dorzolamide (Cosopt®, Trusopt®). These two drugs are available only in topical ophthalmic form. Both drugs are also sulfonamides and are chemically related to the sulfonamide antibiotics. They should be used with caution in patients who are allergic to sulpha antibiotics.
Carbonic anhydrase inhibitors work by inhibiting the enzyme carbonic anhydrase, which results in decreased intraocular pressure by reduction of aqueous humour formation.
Ocular carbonic anhydrase inhibitors are used primarily for the management of both open-angle and angle-closure types of glaucoma and may be used preoperatively to control intraocular pressure.
Patients with sulpha allergies may develop cross-sensitivities to carbonic anhydrase inhibitors.
Osmotic Diuretics
Osmotic medications may be administered intravenously, orally, or topically to reduce intraocular pressure; glycerin and mannitol are most commonly used.
Osmotic diuretics reduce ocular hypertension by causing the blood to become hypertonic, creating an osmotic gradient that pulls water from the aqueous and vitreous humours into the bloodstream, causing a reduced volume of intraocular fluid and decreased intraocular pressure.
Osmotic diuretics are used for acute glaucoma episodes and for reduction of intraocular pressure before or after ocular surgery. Glycerin is used first; if unsuccessful, mannitol is tried.
Osmotic diuretics are contraindicated with allergy, anuria, acute pulmonary edema, cardiac decompensation, and severe dehydration, because they can make all of these conditions worse..
The most frequent reactions to osmotic diuretics are nausea, vomiting, and headache. The most significant adverse effects are fluid and electrolyte imbalances.
Prostaglandin Agonists
The newest class of drugs used to treat glaucoma is the prostaglandin agonists, which include latanoprost (Xalacom®, Xalatan®), travoprost (Travatan Z®), and bimatoprost (Lumigan®).
Prostaglandins agonists reduce intraocular pressure in glaucoma by increasing the outflow of aqueous humour between the uvea and sclera, as well as through the trabecular meshwork.
Prostaglandin agonists are well tolerated. Adverse effects include foreign body sensation, punctate epithelial keratopathy (a dotted appearance of the cornea), stinging, conjunctival hyperemia, blurred vision, itching, and burning. Systemic effects occur infrequently.
In some people with hazel, green, or bluish-brown eyes, the eyes will turn permanently brown, even after the medication is discontinued.
Concurrent administration of prostaglandin agonists with any other eye drops containing the preservative thimerosal may result in precipitation.
Antimicrobials
A variety of infections can occur in the eyes; many are self-limiting. Topical antimicrobials used to treat ocular infections include antibacterial, antifungal, and antiviral drugs.
Topical antimicrobials used to treat infections of the eye work to destroy the invading organism; their actions are similar to those described for systemically administered drugs.
Ocular antimicrobials are used for known or suspected infection with one or more specific micro-organisms. Empiric treatment is based on clinical evaluation of presenting symptoms.
The most common adverse effects of ocular antibiotics are local and transient inflammation, burning, stinging, urticaria, dermatitis, angioedema, and drug hypersensitivity.
Concurrent use of antibiotics and corticosteroids may interact. Corticosteroids have immunosuppressive effects that may impede the therapeutic effects of ocular antimicrobials.
A large proportion of the inflammatory diseases of the eye are caused by viruses, and many ocular antimicrobials are available to treat bacterial, viral, and fungal eye infections, the most common of which include conjunctivitis, hordeolum, keratitis, uveitis, and endophthalmitis.
Anti-inflammatory Drugs
Anti-inflammatory ophthalmic drugs include corticosteroids and are used to inhibit inflammatory responses to mechanical forces, chemicals, and immunological reactions.
Anti-inflammatory drugs used systemically may also be used ophthalmically to treat various ocular inflammatory disorders and ocular surgery–related pain and inflammation. These drugs include both nonsteroidal anti-inflammatory drugs (NSAIDs) and corticosteroids.
Corticosteroids and NSAIDs are applied topically for symptomatic relief of many ophthalmic inflammatory conditions. They may be used to treat corneal, conjunctival, and scleral injuries from chemical, radiation, or thermal burns or from the penetration of foreign bodies during acute phases of the injury in order to prevent fibrosis and scarring, which result in visual impairment.
The most common adverse effect of corticosteroids is transient burning or stinging. Extended use may result in cataracts, increased intraocular pressure, and optic nerve damage.
Topical Anaesthetics
Topical anaesthetics are used to prevent pain to the eye and are beneficial during surgery, ophthalmic examinations, and removal of foreign bodies. The currently available topical anaesthetic used for ophthalmic purposes is tetracaine.
Application of topical anaesthetic drugs results in local anesthesia in less than 30 seconds; they are used to produce ocular anaesthesia for short corneal and conjunctival procedures.
Adverse effects are rare with ophthalmic anaesthetic drugs and are limited to local effects such as stinging, burning, redness, lacrimation, and blurred vision.
Diagnostic Drugs
Atropine sulphate solution and ointment are used as mydriatic and cycloplegic drugs to assist in eye examinations or to treat uveal tract inflammatory states.
Cyclopentolate solution (Cyclogyl®) is used as a diagnostic mydriatic and cycloplegic drug.
Fluorescein is a diagnostic dye used to identify corneal defects and foreign objects.
Antiallergic Drugs
Olopatadine (Patanol®) is an ocular antihistamine used for allergic conjunctivitis (hay fever), which can be seasonal or nonseasonal. It works by competing at receptor sites for histamine, which produces ocular symptoms such as itching and tearing.
Cromolyn sodium (Opticrom®), an antiallergic drug that inhibits the release of inflammation-producing mediators from sensitized inflammatory cells, is used to treat vernal keratoconjunctivitis.
Tetrahydrozoline (Allergy Eye Drops®, Visine®) is an ocular decongestant that promotes vasoconstriction of blood vessels in and around the eye to reduce the edema associated with allergic and inflammatory processes. It is specifically indicated to control redness, burning, and other minor irritations.
An array of over-the-counter products is available for providing lubrication or moisture to the eyes, which is helpful to patients with dry or otherwise irritated eyes.
NURSING PROCESS
Assessment
Document any redness, swelling, pain, excessive tearing, eye drainage or discharge, decrease in visual acuity, or other unusual symptoms.
Focus the nursing history on past or present systemic disease processes and exposure to any chemicals that could be topical irritants to the eye, skin, or mucous membranes, including past or present occupational and environmental exposures.
Implementation
Always inspect the solution, and administer only clear, unexpired products (drops, ointments, and solutions) to the eye.
Administer all ophthalmic preparations exactly as ordered. Always apply into the conjunctival sac. Safe and accurate application or instillation technique also includes avoiding contact of the eyedropper or tube to the eye to prevent contamination of the drug.
Applying pressure to the inner canthus after the instillation of the medication is needed to prevent or decrease systemic absorption and subsequent systemic adverse effects.
With abrasion or injury to the eye and the administration of the appropriate medications, patching the affected eye will prevent further injury resulting from the loss of the blink reflex because of the use of topical anaesthetic.
Evaluation
Patients need to report any increase in symptoms, such as eye pain or drainage and fever.
Therapeutic responses to miotics include decreased aqueous humour of the eye, with resulting decreased intraocular pressure and decreased signs, symptoms, and long-term effects of glaucoma.
β-Adrenergic blockers are therapeutic if intraocular pressure decreases. Adverse effects include weakness, eye irritation, rash, bradycardia, hypotension, and dysrhythmias.
Therapeutic responses to antibiotic, antifungal, and antiviral ophthalmic drugs include the elimination of the infection or condition and its complications, and the resolution of symptoms.
Therapeutic responses to ophthalmic anaesthetics include the prevention or relief of pain.
Adverse effects may include central nervous system excitation (e.g., dizziness, tremors, restlessness, and nervousness) if the drug is systemically absorbed. Anti-inflammatory ophthalmic solutions result in a decrease in allergic reactions (decreased itching, tearing, redness, and eye discharge). Potential complications of these solutions include swelling of the conjunctiva (chemosis).