White : Diseases of the Skin

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Gary M. White & Neil H. Cox

Diseases of the Skin

3	Topical Therapy

INTRODUCTION GENERAL PRINCIPLES

Topical therapy is the mainstay of dermatologic therapy. To be a dermatologist is to know the skin and how to improve it or cure it with topical therapy. While pills, injections, and infusions are becoming more and more important, the art and science of topical treatment is what most distinguishes dermatology from other medical disciplines. What vehicle to choose, how much to give, how often to apply it, etc. are described in this chapter. We also provide an overview of many of the more commonly used dermatologic topical therapies. Note, however, that some of these are more appropriately dealt with in disease-related chapters. For example, treatment of warts may include keratolytics, aldehydes, immunomodulators, and cytotoxic agents; discussion of the role of each of these is more conveniently placed in Chapter 25 in the section on warts.

The following discussion of vehicle, frequency, amount, etc. applies most directly to topical steroid use, but can often be extrapolated to other therapeutic categories.

Vehicle

It may be argued that the default vehicle is the cream. Creams are relatively easy to apply and are preferred by most patients. They can, however, have a drying effect with continued use. Creams are particularly appropriate for the face, neck, and groin, and for exudative conditions. Ointments are preferred in many situations, as they provide grease that is necessary to repair the barrier function of the skin (e.g. in eczema) and to reduce the appearance of scales (e.g. in psoriasis). The ointment also provides a measure of occlusion, thus increasing the potency of the agent. However, it should be noted that it is incorrect to use an active agent in ointment form primarily to benefit from the emollient action; this is discussed further later in this chapter. Ointments also have a potential advantage in that they are often preservative-free and thus less likely to cause allergy than are creams, which are water-based and therefore usually require addition of antimicrobials.

Solutions and lotions are easily spread over large areas, as well as being well suited for the scalp and other hair-bearing areas. They may cause stinging and burning, however. A thermolabile foam preparation is quite elegant and can be used on the scalp as well. Gels are often used on the face or in body folds. Aerosol sprays (with a tube attachment for the scalp) have been used for ease of application. For example, they may ease treatment of the legs in patients with limited mobility.

Frequency of application

Most prescription topical agents are applied once or twice a day. More frequent application is usually a waste of the medication. Frequency of application is usually category-specific. Topical retinoids are applied once a day, while topical steroids and macrolactam immununomodulators (tacrolimus and pimecrolimus) are usually applied twice daily (abbreviated to b.i.d.).

Amount to dispense

The amount of cream needed to cover the average adult's entire body is 20 – 30g. If one is going to treat a total body skin rash with topical therapy b.i.d. for 1 month, one would need 30 X 2 X 25g = 1500g or twenty-five 60-g tubes! Thus one can see that a patient with a total body rash needs systemic therapy. If, however, only 10% of the body surface area (BSA) needs to be treated, then 150g for 1 month is sufficient. One helpful rule of thumb for calculating BSA is that the surface area of one palm is about 0.5% of the total BSA over a broad range of ages (or a closed hand is 1%). Alternatively, the rule of nines, as used in burns treatment, can be applied to estimate larger areas (each arm, each lower leg or foot, each thigh, anterior trunk, posterior trunk, and head and neck each account for 9% of BSA). Neither method is accurate, however, for patients with a high body mass index.

For example, a hand dermatitis, affecting only the palms, requiring a topical steroid b.i.d. would require in a month

(30 days/month)X(2 applications/day)X
(1% BSA/application)X(25 g/100% BSA) = 15 g/month.

One can combine the last few terms to get

25 g/100% BSA per application X 0.5% BSA/palm = 0.125 g/palm per application

or 0.125 g/palm per application * 60 applications/month (b.i.d.) means the patient will need 7.5 g/palm per month for b.i.d. application.

In practice, the amounts applied vary between individuals, vary slightly between creams and ointments even for the same individual, and are significantly altered depending on whether use is sparing, as for a topical steroid, or more liberal, as for emollients. These issues are discussed further in the following sections.

PRACTICE POINTS

• Think about alternative vehicles besides cream. Ointments are more potent, more emolliating, and less likely to cause allergy.

• One palm (not including the fingers) is 0.5% of body surface area for a lean person over a broad range of ages.

• Assuming b.i.d. application, 7.5 g/month are needed for each palm-sized area.

Time of application

The best time to apply most medications is soon after washing or showering. The skin is clean, so application of a topical agent will not drive allergens and other irritants into the skin. The skin is also hydrated, increasing penetration and improving efficacy. For example, patients with eczema should apply their topical agents or emollients within a few minutes of getting out of the shower or bath. This locks in the moisture that the skin has absorbed. Waiting more than 5min allows the water to evaporate. However, if both a topical steroid and an emollient are to be used, the steroid should be applied first, and it may be best to apply the emollient after an interval of perhaps 30min.

PRACTICE POINTS

• In the shower or bath, the skin - Like a sponge - absorbs water. The skin is ‘sticky' when hydrated (increased coefficient of friction). This stickiness is lost 3 –
min after the shower or bath. Patients with atopic dermatitis should apply their medications within a 3 - 5 min time period.

• The active medication (e.g. steroid) should always be applied first. Some patients or their parents mistakenly apply the emollient before the steroid.

• Occlusion is an inexpensive way to augment the strength of a steroid. However, caution must be exercised, as the ultrapotent steroids may become too strong
with occlusion.

Occlusion

Occluding the topical agent increases penetration and efficacy. This can be done with plastic wrap covered by a sock for the feet (gloves for the hands), vinyl suit for the body, or shower cap for the scalp. Caution must be exercised with occlusion, as it can make the ultrapotent steroids too strong.