Dermatologic Therapy and Procedures

Principles of Topical Therapy

A diverse group of medications is available in topical preparations, including antibiotics, antifungals, corticosteroids, acne preparations, sunscreens, cytotoxic agents, antipruritics, antiseptics, and pesticides. Topical therapy has the distinct advantage of delivering medications directly to the target organ. This route reduces the potential of systemic side-effects and toxicity seen with systemic therapy. The disadvantages of topical therapy are that it is time consuming, it can require large volumes of medication, it requires patient education in the technique of using topical medications, and, at times, it is not esthetically pleasing because of staining or greasy preparations.

For a medication to be effective topically, it must be absorbed into the skin. The main diffusion barrier of the skin is the stratum corneum, which is responsible for most of the protection offered by the skin against toxic agents, microorganisms, physical forces, and loss of body fluids.

Percutaneous absorption is influenced by (1) physical and chemical properties of the active ingredient; (2) concentration; (3) vehicle; and (4) variations in type of skin. Cutaneous penetration of an active ingredient is enhanced when it has a low molecular weight, is lipid soluble, and is nonpolar.

Substances move across the stratum corneum by passive diffusion and follow a dose–response curve. The higher the concentration applied, the greater is the quantity of medication absorbed.

The vehicle is nearly as important as the active agent in the formulation of topical medications. This was realized when investigators found that the release of drugs varied greatly with different vehicles. The more occlusive the vehicle, the greater is the hydration of the stratum corneum and penetration of the medication. In addition, occlusive vehicles increase local skin temperature and prevent mechanical removal and evaporation of the active agent. An ointment is the most occlusive vehicle.

Percutaneous absorption is also influenced by the location of the skin to which it is applied. Passive diffusion is slow through the stratum corneum but rapid through the viable epidermis and papillary dermis. Therefore, absorption is generally low on the palm and sole, in which the stratum corneum is thick, and high on the scrotum, face, and ear, in which the stratum corneum is thin. Breakdown of the barrier function of the stratum corneum by disease, chemicals (soaps or detergents), and physical injury results in increased permeability.

The selection of a topical preparation must involve not only the active agent but also its other ingredients. The formulation of many topical medications is complex. A water-based preparation (cream), for example, is composed of numerous ingredients, including the active agent, vehicle, and preservative, as well as an emulsifier to bring together the oil and water components of the preparation. As a general rule, it is better to select a commercially formulated preparation that is scientifically compounded than an extemporaneous preparation. The most frequently used vehicles are creams, ointments, lotions, foams, and gels.

Creams are semi-solid emulsions of oil in water that vanish when rubbed into the skin. They are white and nongreasy, and contain multiple ingredients. Preservatives are added to prevent the growth of bacteria and fungi. Ointments (oil-based) are emulsions of water droplets suspended in oil that do not rub in when applied to the skin. They are greasy and clear, and do not require preservatives. Ointments are selected when increased hydration, occlusion, and maximal penetration of the active ingredient are desired. Lotions are suspensions of powder in water that may require shaking before application. Calamine lotion is the classic example. Itching is relieved by the cooling effect of water evaporation, and a protective layer of powder is left on the skin. Other liquids such as solutions, sprays, aerosols, and tinctures are characterized by ingredients dissolved in alcoholic vehicles that evaporate to leave the active agent on the skin. These agents are particularly useful for hairy areas. Gels are transparent and colorless semi-solid emulsions that liquefy when applied to the skin.

Writing a Dermatologic Prescription

Writing a prescription for a topical medication involves more than simply requesting the active ingredient. In addition to the medication, the vehicle, concentration, and amount must be indicated, as well as the instructions for use. Several concentrations and vehicles may be available for a given topical drug. The physician should indicate which vehicle the pharmacist is to dispense. Patient compliance is often directly related to personal preference of vehicle. Greasy ointments on the face and hands can be unacceptable to the patient, and on the trunk or extremities may soak through clothing.

The type of error most frequently made in prescribing a topical drug probably involves the volume of medication to be dispensed. The size of the area being treated, the frequency of application, and the time between appointments or before predicted clearing of the eruption must all be taken into consideration when writing the prescription. An adequate quantity of medication is necessary to ensure the patienťs compliance, successful therapy, and cost savings. Smaller volumes of medication are comparatively more expensive than larger volumes. One gram covers an area approximately 10 × 10 cm. A single application of a cream or ointment to the face or hands requires 2 g; for one arm or the anterior or posterior trunk, 3 g; for one leg, 4 g; and for the entire body, 30 g. Prescribing 15 g to be applied twice a day to an eruption that involves large portions of the trunk and extremities would be unreasonable; the patient would be required to return for refills twice daily.

The physician needs to know the principles involved in writing a dermatologic prescription. For example, the patienťs eruption is moderately severe and requires an intermediate-strength topical steroid such as triamcinolone acetonide. Triamcinolone acetonide is available in three concentrations: 0.025%, 0.1%, and 0.5%. A 0.1% concentration is effective for moderately severe eruptions and can generally be used without concern for local or systemic side-effects. In this example, it is dispensed in a cream vehicle because the patient prefers a nongreasy preparation that rubs into the skin. The patient is going to use the medication on extensive areas of skin, requiring approximately 10 g per application twice a day. A prescription for 454 g (1 lb) of cream will last almost 2 weeks, and two refills will allow more than enough medication until the next appointment in 4 weeks.

Dressings and Baths

Dressings are useful as protective coverings over wounds. They prevent contamination from the environment, and many absorb serum and blood.

Dry dressings are used to protect wounds and to absorb drainage. They usually consist of absorbent gauze secured with adhesive tape. Adhesive tape can cause allergic contact dermatitis, in which case hypoallergenic tapes may be used. These are made of an acrylic plastic adhesive mass with a plastic or cloth backing. After surgery, the skin is often painted with an adhesive that contains benzoin, which may also be responsible for allergic contact dermatitis. Dry dressings may be nonadherent or adherent. Nonadherent dressings are used for clean wounds. When changed, they should not pull off newly formed epithelium. An example of a nonadherent dressing is petrolatum-impregnated gauze. Adherent dressings are used for debridement of moist wounds. The dressings may be dry or wet at first. For dry dressings, gauze is applied and changed regularly. For wet-to-dry dressings, water, saline, or an antiseptic solution is added to the dressing and allowed to dry. Accumulated debris is removed, although removal may be painful. Discomfort can be reduced if adherent dressings are first moistened (i.e., remoistened) before removal.

Wet dressings are used to treat acute inflammation. They consist of gauze, pads, or towels soaked continuously with water, an astringent (drying agent), or an antimicrobial solution. They soothe, cool, and dry through evaporation. In addition, when changed, they remove crusts and exudate. Water is the most important ingredient of wet dressings, but astringents such as aluminum acetate (Domeboro) and antiseptics such as povidone-iodine (Betadine) are frequently added. Impermeable covers such as plastic should not be placed over wet dressings because of the maceration that would ensue.

Occlusive dressings made of semipermeable plastic membranes (e.g., Duoderm) promote wound healing by maintaining a moist environment. They are frequently used on chronic ulcers (e.g., stasis ulcers). The moist environment allows migration of keratinocytes over the ulcer base to proceed more rapidly. In addition, occlusive dressings allow autodigestion of necrotic tissue by accumulation of inflammatory cells. For some wounds (e.g., donor graft sites), these dressings also significantly reduce pain.

Baths may be thought of as a form of wet dressing. They are effective in soothing, in decreasing itching, and in cleansing, and they are relaxing. They are used for acute eruptions that are crusting and weeping. They hydrate dry skin, but only if a moisturizer is applied immediately after the bath. Routinely used baths include tar emulsions (Cutar), colloidal oatmeal (Aveeno), and bath oils. Baths are limited to 30 minutes to prevent maceration, and are performed once or twice daily.

Topical Steroids (Glucocorticosteroids)

Perhaps no topical therapeutic modality is used more frequently than steroids because of their antiinflammatory effects. The use of glucocorticosteroids applied directly to diseased skin has resulted in a high therapeutic benefit with relatively little local and systemic toxicity. The mechanism of action of topical glucocorticosteroids is complex and is not thoroughly understood.

Potency

The potency of a topical glucocorticosteroid depends on its molecular structure. For example, triamcinolone acetonide is 100 times more potent than hydrocortisone. In addition, the vehicle carrying the steroid is important. For a steroid to be effective, it must be absorbed. Penetration of glucocorticosteroids through the stratum corneum (and, hence, increased activity) is optimized by using nonpolar, lipophilic glucocorticosteroid molecules compounded in vehicles that readily release the steroid.

Dozens of different topical glucocorticosteroids ( Fig. 4.1 ) have been formulated for use in skin disease, with many of these developed on the basis of potency assays. Measurement of the ability of glucocorticosteroids to induce vasoconstriction or blanching of the skin, vasoconstrictive assay , is the most frequently used method of estimating relative potency. The results of the vasoconstrictive assay parallel those found in clinical studies. Because this assay is much simpler to perform than complicated clinical studies, it is widely used to screen specific formulations before they are used in clinical trials.

Vasoconstrictive assay is the most common method for measuring potency.

Table 4.1 lists some topical glucocorticosteroids with different potencies. The percentage of the steroid present is relevant only when comparing percentages of the same compound. Thus, triamcinolone acetonide 0.5% is stronger than its 0.1% formulation, but hydrocortisone 1% is much weaker than triamcinolone acetonide 0.1%. In addition, potency depends on the vehicle. The same preparation tends to be more potent in an ointment base than in a cream base because of enhanced percutaneous penetration.

Table 4.1

Topical Steroids

Potency	Generic Name	(%)
Low	Hydrocortisone	1.0
Medium	Triamcinolone acetonide	0.1
High	Fluocinonide	0.05
Super-high	Clobetasol propionate	0.05

Guidelines for Topical Steroid Usage

A bewildering array of topical steroids is available in different vehicles. When prescribing a steroid preparation, one should consider several factors before making the selection of potency: vehicle, amount to be dispensed, and frequency of use. It is best to become familiar with one steroid in each class: lowest, medium, high, and super-high potency. By using only a few preparations, you will gain an enhanced appreciation of clinical efficacy, frequency of side-effects, available vehicles and volumes, and costs. Lowest-potency topical steroids are recommended for dermatoses that are mild and chronic, and involve the face and intertriginous regions. More potent steroids (medium and high) are used for dermatoses that are more severe and recalcitrant to treatment.

Once the appropriate potency has been selected, the vehicle ( Fig. 4.2 ) should be chosen. Acute and subacute inflammations characterized by vesiculation and oozing are best treated with non-occlusive vehicles in a gel, lotion, or cream. Ointments, because of their occlusive properties, are better for treating chronic inflammation characterized by dryness, scaling, and lichenification. Because of their greasy nature, ointments are less acceptable esthetically. However, they have less potential for irritation and allergic reaction. Lotions, solutions, foams, and gels are best used on hairy areas such as the scalp.

Use creams on weeping eruptions, ointments on dry lichenified skin, and gels, foams, or solutions on hairy areas.

Another consideration in topical steroid therapy is the frequency of application. The stratum corneum acts as a reservoir and continues to release topical steroid into the skin after the initial application. Applications once or twice a day are usually sufficient. Investigators have observed that chronic dermatoses, especially psoriasis, may become less responsive after prolonged use of topical steroids. This phenomenon is called tachyphylaxis . This diminished responsiveness after repeated applications has also been observed in vasoconstrictive assays.

Finally, the physician should instruct the patient in proper application and dispense sufficient medication to ensure adequate treatment. The need for continued treatment should be reviewed periodically.

A good rule is to use the smallest quantity and the weakest preparation that are effective for a particular eruption.