Topical therapy, the mainstay in the treatment of many dermatologic conditions, has traditionally been the bailiwick of dermatologists. Thus, it is no surprise that an understanding of the appropriate use and abuse of these agents has been either neglected or misunderstood by other health care professionals. The following is an attempt to provide a practical overview of topical therapy. The primary focus is on the use of topical steroids; other topical agents such as antibiotics and antifungals will be discussed in other sections of the book. (Topical antibiotics used for acne are listed in Chapter 1. Topical antibiotics for other cutaneous infections such as impetigo are described in Chapter 5.)
Topical therapy is generally safer than systemic therapy.
Creams are generally more popular with patients than ointments because they are less greasy and messy; however, they are usually less potent and more drying.
In addition to their enhanced potency, ointments are more moisturizing than creams, gels, and foams.
Gels and foams are greaseless; they spread more easily and are very practical for acne and for application to hairy areas of the body. They also dry on contact, leaving a thin, invisible film.
Solutions tend to be drying but cover large areas more easily than other preparations.
Lotions are moisturizing to some extent; however, those that contain propylene glycol may have drying effects. They are also easy to apply and can be used on any skin type.
Although foams may be somewhat drying, they are easy to spread, particularly on hairy areas such as the scalp as well as on the chests and backs of hirsute males. However, foams are expensive. Emollient (nonalcohol-based) foams have recently become available.
Patients’ choice should be taken into account because compliance is often related to patients’ personal preference for a vehicle.
The amount of a topical preparation applied does not affect its penetration or potency. The thicker the application, the greater the wastage—only the thin layer that is in intimate contact with the skin is absorbed; the remainder is rubbed off. More is not always better!
Once- or twice-daily applications are usually sufficient for most preparations (sunscreens and some moisturizers are generally applied more frequently).
The active drug is combined with a vehicle, or base. The vehicles vary in their ability to “deliver” to the target site in the skin.
The rate of penetration and of absorption of a topical preparation into the skin depends on how occlusive its vehicle is (discussion to follow) and on how readily the vehicle releases the active chemical.
The vehicle should be cosmetically acceptable and nonsensitizing.
There is some validity to the old adage: “If it’s dry, wet it; if it’s wet, dry it.”
Wet dressings help dry wounds, and they aid in the débridement of wounds by removing debris (e.g., serum, crusts). They also have a nonspecific antifungal and antibacterial effect, especially when chemicals such as aluminum sulfate, silver nitrate, acetic acid (vinegar is dilute acetic acid), and potassium permanganate are added.
For example, the application of Burow’s solution (aluminum sulfate and calcium acetate) helps dry out lesions that are weeping and oozing (e.g., poison ivy, tinea pedis, herpes simplex, herpes zoster) or impetiginized (e.g., impetigo, infected stasis ulcers). It is available without a prescription.
However, white vinegar in water, or even, plain unsterilized tap water may serve as less expensive, readily available—although probably less effective—alternatives, if Burow’s solution cannot be obtained.
SEE PATIENT HANDOUT, “Burow’s Solution” ON THE SOLUTION SITE
Mechanism of Action
Topical steroids have two basic mechanisms of action: anti-inflammatory and antimitotic.
Anti-inflammatory properties are of particular importance when they are used to treat eczematous and other primarily inflammatory conditions.
Antimitotic properties of topical steroids help to reduce the buildup of scale, as in the treatment of psoriasis, a condition that is both inflammatory and hyperproliferative (rapid cell division).
Topical steroids are used to treat most inflammatory dermatoses. They are the cornerstone of therapy in dermatology, and, when used properly, they are quite safe.
The unwanted effects of topical steroids are directly related to their potencies.
When possible, the lowest-potency steroid should be used for the shortest possible time. Conversely, one should avoid
using a preparation that is not potent enough to treat a particular condition.
For severe dermatoses, a very potent steroid may be used to initiate therapy, and a less potent preparation may be used afterward for maintenance (“downward titration”).
Occlusion, which involves placing medications under occlusive dressings produces increased hydration of the stratum corneum. Hydration increases penetration, which, in turn, increases efficacy and potency.
Tachyphylaxis (tolerance) occurs when the medication loses its efficacy with continued use. It is most often seen in the treatment of psoriasis and other chronic conditions when very strong topical steroids are used continuously for prolonged periods.
To help minimize tachyphylaxis, a high-potency preparation is applied until a dermatosis clears (“strong, but not long”), and a lower-potency topical steroid is prescribed for intermittent flares should they occur.
Fluorinated topical steroids are generally more potent than other topical steroids. For example, triamcinolone acetonide, which contains a fluoride ion, is 100 times more potent than nonfluorinated hydrocortisone.
When treating children and elderly patients, clinicians should take extra measures to avoid the use of potent fluorinated compounds, when possible.
Even without an added fluoride ion, certain molecular structural changes can increase potency in corticosteroids. For example, hydrocortisone valerate, which contains no fluoride ion, is almost as potent as triamcinolone acetonide.
Only nonfluorinated, mild topical steroids should be applied to the face and, ideally, only for short periods of time, to avoid atrophy and steroid-induced rosacea. (However, this rule can be broken. For example, for the treatment of severe acute contact dermatitis of the face, a superpotent topical steroid used briefly may be preferable to a mild, ineffective topical steroid or a systemic steroid.)
Thin eyelid skin requires the least potent preparations for the shortest periods of time. The intertriginous (skin touching skin) areas similarly respond to lower potencies because the apposition of skin surfaces acts like an occlusive dressing. The axillae and inguinal creases are particularly prone to higher absorption and resultant steroid atrophy (see Fig I.3).
Every potency class has a generic preparation that is generally more economical than the trade-name preparation; however, there is some concern about the bioequivalence between trade and generic formulations.
Delivery (Percutaneous Penetration)
Topical steroids and other topical preparations are ineffective unless they are absorbed into the skin. Percutaneous penetration varies among individual patients, and it can be manipulated by hydrating the skin, occlusive dressings, changing the drug, the vehicle, the length of exposure, or the anatomic surface area to which the drug is applied. It also depends on whether the skin is inflamed and therefore less of a barrier to penetration (e.g., eczematous skin).
The barrier to cutaneous penetration resides in the stratum corneum. The thicker the stratum corneum—as on the palms, soles, elbows, and knees—the more difficult it is for the topical agent to penetrate the skin. The ability to penetrate varies by anatomic site as follows:
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