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Concepts in Ocular Medication Administration
The main objective of ocular medication delivery is to maximize the amount of medication that reaches the ocular site of action in sufficient concentration to produce a beneficial therapeutic effect. This is determined by the dynamics of ocular pharmacokinetics: absorption, distribution, metabolism, and excretion.
Topical administration of ocular medications results in only a 1% to 7% absorption rate by the ocular tissues. Ocular absorp-tion involves the entry of a medication into the aqueous humor through the different routes of ocular drug administration. The rate and extent of aqueous humor absorption are determined by the characteristics of the medication and the barriers imposed by the anatomy and physiology of the eye. The natural barriers of absorption that diminish the efficacy of ocular medications in-clude the following:
· Limited size of the conjunctival sac. The conjunctival sac canhold only 50 μ L, and any excess is wasted. The volume of one eye drop from commercial topical ocular solutions typ-ically ranges from 20 to 35 μ L.
· Corneal membrane barriers. The epithelial, stromal, andendothelial layers are barriers to absorption.
· Blood–ocular barriers. Blood–ocular barriers prevent highocular tissue concentration of most ophthalmic medications because they separate the bloodstream from the ocular tis-sues and keep foreign substances from entering the eye, thereby limiting a medication’s efficacy.
· Tearing, blinking, and drainage. Increased tear productionand drainage due to ocular irritation or an ocular condition may dilute or wash out an instilled eye drop; blinking ex-pels an instilled eye drop from the conjunctival sac.
Distribution of an ocular medication into the ocular tissues in-volves partitioning and compartmentalizing of the medication between the tissues of the conjunctiva, cornea, lens, iris, ciliary body, choroid, and vitreous. Medications penetrate the corneal epithelium by diffusion by passing through the cells (intracellu-lar) or by passing between the cells (intercellular). Water-soluble (hydrophilic) medications diffuse through the intracellular route, and fat-soluble (lipophilic) medications diffuse through the in-tercellular route. Topical administration usually does not reach the retina in significant concentrations. Because the space be-tween the ciliary process and the lens is small, medication diffu-sion in the vitreous is slow. When high therapeutic medication concentration in the vitreous is required, intraocular injection is often chosen to bypass the natural ocular anatomic and physi-ologic barriers.
Aqueous solutions are most commonly used for the eye. They are the least expensive medications and interfere least with vision. However, corneal contact time is brief because tears dilute the medication. Ophthalmic ointments have extended retention time in the conjunctival sac and a higher concentration than eye drops. The major disadvantage of ointments is the blurred vision that results after application. In general, eyelids and eyelid margins are best treated with ointments. The conjunctiva, limbus, cornea, and anterior chamber are treated most effectively with instilled solutions or suspensions. Subconjunctival injection may be nec-essary for better absorption in the anterior chamber. If high medication concentrations are required in the posterior chamber, in-travitreal injections or systemically absorbed medications are con-sidered. Contact lenses and collagen shields soaked in antibiotics are alternative delivery methods for treating corneal infections. Of all these delivery methods, the topical route of administra-tion—instilled eye drops and applied ointments—remain the most common. Topical instillation, which is the least invasive method, permits self-administration of medication. It also pro-duces fewer side effects.
Preservatives are commonly used in ocular medications. Ben-zalkonium chloride, for example, prevents the growth of organ-isms and enhances the corneal permeability of most medications. Some patients are allergic to this preservative. This may be sus-pected even if the patient had never before experienced an aller-gic reaction to systemic use of the medication in question. Eye drops without preservatives can be prepared by pharmacists.
Common ocular medications include topical anesthetic, mydri-atic, and cycloplegic agents that reduce IOP; anti-infective med-ications, corticosteroids, NSAIDS, antiallergy medications, eye irrigants, and lubricants.
One to two drops of proparacaine hydrochloride (Ophthaine 0.5%) and tetracaine hydrochloride (Pontocaine 0.5%) are instilled before diagnostic procedures such as tonometry and gonioscopy and in minor ocular procedures such as removal of sutures or con-junctival or corneal scrapings. The nurse must instruct patients not to rub their eyes while anesthetized because this may result in corneal damage. Patients must never be allowed to take topical anesthetics home. Prolonged use can delay wound healing and can lead to permanent corneal opacification and scarring, result-ing in visual loss. Topical anesthetic is also used for severe eye pain to allow the patient to open his or her eyes for examination or treatment (eg, eye irrigation for chemical burns). Anesthesia occurs within 20 seconds to 1 minute and lasts 10 to 20 minutes.
Mydriasis, or pupil dilation, is the main objective of the adminis-tration of mydriatic and cycloplegic agents (Table 58-8). These two medications function differently and are used in combination to achieve the maximal dilation that is needed during surgery and fundus examinations to give the ophthalmologist a better view of the internal eye structures. Mydriatics potentiate alpha-adrenergic sympathetic effects that result in the relaxation of the ciliary muscle. This causes the pupil to dilate. This sympathetic action alone, however, is not enough to sustain mydriasis because of its short duration of action. The strong light used during an eye examination also stimulates miosis (ie, pupillary contraction). Cycloplegic medications are administered to paralyze the iris sphincter.
Patients are instructed about the temporary effects of mydri-asis on vision, such as glare and the inability to focus properly. Patients may not be able to read and should not drive. The effects of the various mydriatics and cycloplegics can last 3 hours to sev-eral days. Patients are advised to wear sunglasses (most eye clin-ics provide protective sunglasses) and to have a responsible adult drive them home.
Mydriatic and cycloplegic agents affect the central nervous system. Their effects are most prominent in children and elderly patients; these patients must be assessed closely for symptoms, such as rise in blood pressure, tachycardia, dizziness, ataxia, confusion, disorientation, incoherent speech, and hallucination. These medications are contraindicated in patients with narrow angles or shallow anterior chambers and in patients taking monoamine oxidase inhibitors or tricyclic antidepressants.
Therapeutic medications for glaucoma are used to lower IOP by decreasing aqueous production or increasing aqueous outflow. Because glaucoma calls for lifetime therapy, patients must be in-structed regarding both the ocular and systemic side effects of the medications.
Most antiglaucoma medications affect the accommodation of the lens and limit light entry through a constricted pupil. Visual acuity and the ability to focus may be affected. Factors to con-sider in selecting glaucoma medications are efficacy, systemic and ocular side effects, convenience, and cost.
Anti-infective medications include antibiotic, antifungal, and antiviral agents. Most are available as drops, ointments, or sub-conjunctival or intravitreal injections. Antibiotics include peni-cillin, cephalosporins, aminoglycosides, and fluoroquinolones. The main antifungal agent is amphotericin B. Side effects of amphotericin are serious and include severe pain, conjunctival necrosis, iritis, and retinal toxicity. Antiviral medications in-clude acyclovir and ganciclovir. They are used to treat ocular infections associated with herpesvirus and CMV. Patients re-ceiving ocular anti-infective agents are subject to the same side effects and adverse reactions as those receiving oral or parenteral medications.
The topical preparations of corticosteroids are commonly used in inflammatory conditions of the eyelids, conjunctiva, cornea, an-terior chamber, lens, and uvea. In posterior segment diseases that involve the posterior sclera, retina, and optic nerve, the topical agents are less effective, and parenteral and oral routes are pre-ferred. The topical eye drop preparation is prepared in suspen-sion; the patient is instructed to shake the bottle several times to obtain the maximum therapeutic effect of the medication.
The most common ocular side effects of long-term topical corticosteroid administration are glaucoma, cataracts, suscepti-bility to infection, impaired wound healing, mydriasis, and ptosis. High IOP may develop, which is reversible after cortico-steroid use is discontinued. To avoid the side effects of cortico-steroids, NSAIDS are used as an alternative in controlling inflammatory eye conditions and postoperatively to reduce in-flammation. NSAID therapy in combination with topical and oral preparations is an important adjunct therapy in managing uveitis.
Ocular hypersensitivity reactions, such as allergic conjunctivitis, are extremely common. These conditions result primarily from responses to environmental allergens. Most allergens are airborne or carried to the eye by the hand or by other means, although al-lergic reactions may also be drug induced. Corticosteroids are also commonly used as anti-inflammatory and immunosuppressive agents to control ocular hypersensitivity reaction.
Most irrigating solutions are used to cleanse the external lids to maintain lid hygiene, to irrigate the external corneal surface to re-gain normal pH (such as in chemical burns), to irrigate the corneal surface to eliminate debris, or to inflate the globe intra-operatively. These solutions have various compositions that in-clude sodium, potassium, magnesium, calcium, bicarbonate, glucose, and glutathione (ie, substance found in the aqueous humor). Sterile irrigating solutions, such as Dacriose, for lid hy-giene are available. Irrigating solutions are safe to use with an in-tact corneal surface; however, the corneal surface should not be irrigated in cases of threatened corneal perforation. For patients with severe corneal ulcer, specific orders must be obtained re-garding whether it is safe to irrigate the corneal surface or just to cleanse the external lids. Although it is good practice to promote hygiene, prevention of complications must be the primary con-cern. Normal saline solutions are commonly used to irrigate the corneal surface when chemical burns occur.
Lubricants, such as artificial tears, help to alleviate corneal irritation, such as dry eye syndrome. Artificial tears are topical preparations of methyl or hydroxypropyl cellulose that are prepared as eye drop solutions, ointments, or ocular inserts (inserted at the lower con-junctival cul-de-sac once each day). The eye drops can be instilled as often as every hour, depending on the severity of symptoms.
The objectives in administering ocular medications are to ensure proper administration to maximize the therapeutic effects and to ensure the safety of the patient by monitoring manifestations of possible systemic and local side effects. Absorption of eye drops by the nasolacrimal duct is undesirable because of the potential sys-temic side effects of ocular medications. To diminish systemic ab-sorption and minimize the side effects, it is important to occlude the puncta (Chart 58-10). This is especially important for patients most vulnerable to medication overdose, including elderly people, children, infants, women who are lactating or are pregnant, and patients with cardiac, pulmonary, hepatic, or renal disease.
A 30-second interval between eye drop instillations has a 45% rate of washout loss. A 1-minute interval between instillation of dif-fering types of ocular drops is recommended (Margolis et al., 2002).
Before the administration of ocular medications, the nurse should warn the patient that blurred vision, stinging, and a burn-ing sensation are symptoms that ordinarily occur after instillation and are temporary. Risk for interactions of the ocular medication with other ocular and systemic medications must be emphasized; therefore, a careful patient interview regarding medications being taken must be obtained.
Emphasis must be placed on hand-washing techniques before and after medication instillation. The tip of the eye drop bottle or the ointment tube must never touch any part of the eye. The medication must be recapped immediately after each use. If pa-tients who instill their own medications cannot feel the eye drops when they are instilled, the eye medication may be refrigerated, because a cold drop is easier to detect. A 5-minute interval be-tween successive eye drop administration allows adequate drug retention and absorption. The patient or the caregiver at home should be asked to demonstrate actual eye drop or ointment instillation and punctal occlusion
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