Ординатура / Офтальмология / Английские материалы / Ophthalmic Drugs Diagnostic and Therapeutic Uses 5th edition_Hopkins, Pearson_2007

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with Ps. aeruginosa that was resistant to the aminoglycoside antibiotics and found the method superior to frequent topical administrations. Similarly, Grossman & Lee (1989), who used iontophoresis to deliver high concentrations of ketoconazole (an antifungal agent) into the rabbit eye, found the method superior to subconjunctival injection.

HYDROGEL CONTACT LENSES

By their very nature, hydrogel lenses can take up solutions into the matrix of the lens; these can be released when the lens is placed in fresh solvent. It was assumed that this property would provide a means of applying a depot of drug to the eye so that it could be released over a period of time, giving a longer duration of action than a simple drop. For a variety of reasons, the soft contact lens has not become a routine method of drug delivery.

COLLAGEN SHIELDS

Collagen shields were originally prepared as a protective layer to place over a damaged cornea to reduce abrasion and promote healing (S¸ims¸ek & Kozer-Bilgin 1994, Spraul et al 1994). Like soft lenses, collagen shields will take up drugs from solutions and release them when applied to the eye. Friedberg et al (1991) found them more clinically acceptable than iontophoresis or constant delivery pumps. Reidy et al (1990) demonstrated their superiority over aqueous eyedrops using fluorescein in human volunteers and assessing the concentration in the aqueous humour. Willey et al (1991) used collagen shields to test the uptake of antiviral agents in mice as a model for screening these drugs. Being similar to contact lenses, some difficulty has been experienced in their administration. This has led Kaufman et al (1994) to use pieces of collagen shield to delivery drugs and agents for the relief of dry eye. Application was simpler and there was no blurring of vision.

OPHTHALMIC RODS

The Alani rod (Alani 1990, Alani & Hammerstein 1990) is a completely new concept in ophthalmic drug delivery and consists of a rod made from non-toxic plastic, which is dipped into an unpreserved solution of the drug to be administered. As the solution dries it forms a thin homogeneous coat on the rod, which can then be packed and sterilized by gamma irradiation. To release the drug, the tip of the rod is rubbed against the palpebral conjunctiva. The system has not received commercial interest.

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DRUG CLASSIFICATION

Ophthalmic drugs can be conveniently, if somewhat arbritarily, classified into two main groups: therapeutic and diagnostic. Although many ophthalmic drugs can be used for both therapeutic and diagnostic purposes (e.g. atropine, homatropine, physostigmine, pilocarpine), if their main interest for the optometrist is as a diagnostic agent, they will here be discussed in this grouping (see Part 2, Chapters 6–11) with appropriate references to their therapeutic applications.

DIAGNOSTIC AND PROPHYLACTIC DRUGS

Diagnostic ophthalmic drugs can be subdivided as follows:

•Cycloplegics: used to inhibit or paralyse the accommodation.

•Mydriatics: used to produce dilatation of the pupil.

•Miotics: used to constrict the pupil.

•Topical local anaesthetics: drugs applied to the surface of the mucous membrane of the eye to produce local insensitivity in this area.

•Staining agents: used to stain corneal or conjunctival abrasions, in applanation tonometry and contact-lens-fitting procedures.

•Decongestants: used as vasoconstrictors of congested conjunctival blood vessels.

•Prophylactic anti-infective preparations: these are therapeutic antiinfective drugs used to prevent pathological conditions developing after minor abrasions of the ocular epithelial tissues, which can occur in many situations including certain diagnostic procedures and contact lens practice.

THERAPEUTIC DRUGS

The principal groups of therapeutic drugs are:

•Drugs for the treatment of infections and inflammations.

•Drugs for the treatment of glaucoma.

•Artificial tears.

References

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OPHTHALMIC DOSAGE FORMS 81

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Chapter 5

Factors affecting the choice of diagnostic drugs

Unlike nearly every other group of pharmaceuticals, the range of available ophthalmic products has declined significantly over recent years. In most pharmaceutical groups, as products are discontinued they are more than replaced by new ones. However, many of the currently used diagnostic drugs have been in use for some time, and new ones are rarely introduced. The scarcity of new ophthalmic diagnostic drugs is a product of the high cost of introducing new compounds onto the market. Of the drugs that remain, the choice of concentrations is more limited than it was, as product rationalization results in the discontinuation of new drugs.

It is, however, difficult to bemoan the loss of many of the drugs that are now no longer available or can be obtained only by special manufacture. Many of them were historical oddities, with actions that are no longer appropriate for modern-day optometry and, as a result, without a clinical indication for their use.

CLASSIFICATION OF OPHTHALMIC DRUGS

•Cycloplegics: this group has suffered some attrition and, from an original list of seven agents, only four now remain, and one of these has little to recommend it.

•Mydriatic: from a long list of agents that have been used to dilate the pupil, only two are now seriously considered for optometric use. More potent mydriatics are used prior to surgery.

•Miotics: the greatest losses have occurred from this group. Miotics have a dual use, being therapeutic and diagnostic. A reduction in both uses means there is no longer a need for a large range of agents.

•Local anaesthetics: despite the use of the non-contact tonometer, the range of local anaesthetics has remained fairly steady. Cocaine, which was never really an option as far as the optometrist was concerned, is now rarely used medically because better agents are available.

•Stains: this is one area that, at one time, appeared to have growth. Nowadays, two stains are used routinely, and the use of one far exceeds the other.

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•It should have no local or systemic toxic effects.

•It should be pleasant and easy to use.

•It should be capable of being produced in stable and sterile ophthalmic form that is appropriate to the use for which it is intended: a cycloplegic ointment would make retinoscopy difficult if it were used immediately prior to the examination.

SELECTION OF PATIENTS

Every patient is different and the advantages and disadvantages of a proposed drug must be weighed in the light of the particular patient. In particular, the following questions should be addressed:

•Will the patient benefit from the use of a particular diagnostic drug?

•Has the particular drug been applied to this patient before? If so, were there any adverse reactions or allergic responses?

•Is the patient suffering from any ocular condition that would make the use of this drug inadvisable?

•Are there any systemic conditions that would be contraindications for the drug’s use?

•Is the patient currently taking any medicine that could interact with the diagnostic drug?

SELECTION OF DRUG AND CONCENTRATION

Based on the information gained from the patient’s history and the practitioner’s knowledge, a choice can be made as the most appropriate drug and concentration. Because the application of drugs in topical ophthalmic dosage forms is imprecise, the commonly used drug concentrations represent supramaximal doses. When lower concentrations of drugs were made available in the past, their use was poor and their production was discontinued. If there is any doubt as to which concentration to use, then the lower one should always be employed. Similarly, between two drugs, the weaker one with the shorter duration will often be the drug of choice.

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SECTION II

Diagnostic drugs and products used in optometry

Chapter 6

Cycloplegics

Cycloplegics are drugs that paralyse the ciliary muscle by blocking the muscarinic receptors that are normally stimulated by the release of acetylcholine from the nerve endings of the parasympathetic system. As the parasympathetic nervous system also innervates the pupil sphincter muscle, cycloplegia must be accompanied by mydriasis. It should be noted that mydriasis is not always evidence of an accompanying cycloplegia and merely indicates paralysis of the pupil sphincter. Cycloplegics are used to prevent or reduce accommodation during refraction, thus making latent refractive errors manifest. Cycloplegics are also used as adjunctive therapy in the treatment of certain diseases of the anterior segment of the eye (see Chapter 16).

INDICATIONS AND CONTRAINDICATIONS

INDICATIONS FOR CYCLOPLEGIC EXAMINATION

Cycloplegic examination can be desirable in some children and young adults but is most unlikely to be necessary in presbyopic adults. Use of a cycloplegic is indicated in the following cases:

•in children with constant or intermittent esotropia, on initial presentation and sometimes subsequently

•in children and young adults with asthenopia and esophoria, especially when a latent refractive error is suspected

•when retinoscopy suggests that accommodation is fluctuating significantly

•when the retinoscopy findings differ significantly from the results of subjective refraction

•in cases of anomalies of accommodation such as accommodative insufficiency, accommodative fatigue, accommodative inertia and spasm of accommodation

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•in cases where retinoscopy along the visual axis is very difficult due to lack of patient cooperation or mental handicap (Amos 1978)

•candidates for refractive surgery.

CONTRAINDICATIONS FOR CYCLOPLEGIC EXAMINATION

In optometric practice, cycloplegic examination is usually confined to children and young adults and there are unlikely to be any contraindications in this age group apart from abnormally shallow anterior chamber and dislocation or subluxation of the crystalline lens. If it is considered necessary to undertake cycloplegic refraction in a presbyopic adult, the contraindications are those relevant to the mydriasis that is a concomitant of cycloplegics. These contraindications are listed under mydriatics in Chapter 7.

IDEAL PROPERTIES OF CYCLOPLEGICS

•Quick in onset (the delayed onset of atropine puts special requirements on its dosing).

•Provides adequate depth of cycloplegia.

•Provides adequate duration of cycloplegia (a static level of cycloplegia must be achieved).

•Does not cause mydriasis (as mentioned earlier, this is unattainable so the mydriasis that invariably accompanies cycloplegia must be considered an unwanted side-effect that can cause photophobia).

•Causes no other pharmacological effect.

•Results in no local toxicity.

•Results in no systemic toxicity.

•Is stable.

•Is capable of presentation in single-use eyedrops.

•Causes no adverse subjective complaints such as ‘stinging’.

CYCLOPLEGIC REFRACTION – ADVANTAGES

Under cycloplegia, full static refraction can be estimated without interference from a tonic or clonic (fluctuating) contraction of the ciliary muscle. This is particularly important in the very young because of their large amplitude of accommodation, the latency resulting from this masking a large part of their full refractive error if it is of the hypermetropic type. The unreliability of subjective findings in the very young makes the retinoscopy results of paramount importance.

The full cycloplegic correction found is not necessarily given in any subsequent prescription that is deemed to be necessary. Knowledge of the cycloplegic findings is nevertheless of great importance, especially in young people and particularly where intermittent or constant strabismus, medium to high heterophoria or pseudomyopia are present, or where marked accommodative asthenopic symptoms exist.