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Introduction to dermatology

An introduction to dermatological therapeutics

Objectives

To learn how to safely and appropriately use topical and systemic medications in patients with skin diseases

Key points

  • Correct selection of the base of a topical medication may be as important as the active ingredient.
  • Lotions are used for scalp.
  • Blisters are dried up by wetting them.
  • Ointments are used for dry skin conditions.
  • Creams are generally used elsewhere.
  • Topical medications may result in contact allergy, most often due to preservatives.
  • Potent topical steroids are used for short courses for flare-ups and should be avoided where the skin is very thin (eyelids and genitals).
  • Mild topical steroids may be used long term.
  • Oral antibiotics are used for acute bacterial infections and sometimes for chronic skin diseases complicated by bacterial infection.
  • The development of bacterial resistance is greater with topical antibiotics than with oral antibiotics.

Introduction

Skin diseases may be acute and short lived or chronic and incurable. Treatment is sometimes curative but is more commonly symptomatic, aiming to maintain comfort and function.

The range of treatments used by dermatologists include:

It is common for several items to be prescribed to patients with skin problems, as different preparations are required for different sites and stages of disease. The number and variety of items can be confusing so it is advisable to write down instructions.

Topical therapy

Normal stratum corneum is a significant barrier to diffusion of a drug through the epidermis into the target site, the papillary dermis. Penetration of the drug is greater if it is at high concentration in a lipid base and if the skin is warm and moist. The stratum corneum is a less effective barrier in diseased states such as eczema and psoriasis allowing greater drug penetration.

The choice of vehicle or base depends on the specific skin disease and the state of the skin in the affected site.

Base Properties
Tincture or solution Alcohol-based application. Often used for scalp problems or to apply to nail folds. May sting.
Lotion Alcohol or water-based lotions evaporate and cool inflammation; added glycerine or oil moisturises. Shake lotion has added powder, to dry weeping skin.
Spray Vehicle evaporates rapidly for scalp dermatoses; or for weeping inflamed skin to avoid having to touch it; or to cover large areas quickly (e.g. for insect repellent or sunscreen).
Gel Cooling preparation for oily areas i.e. face and scalp. May sting.
Cream Oil-in-water emulsion for cooling and moisturisation.
Fatty cream: water-in-oil emulsion for dry skin.
Ointment Occlusive emollient preparations that may be water-soluble (macrogols); or contain emulsifying agents; or contain oil/fat (water-repellent). Used for dry scaly skin.
Paste Powder added to oil or grease for protection and as emollient. Messy.
Powder Talc, zinc oxide or starch to reduce friction in skin folds.
Lacquer Adherent paint applied to nails.

There are numerous preparations marketed for skin diseases. Many include numerous ingredients of established or questionable value. Some of these are potential contact allergens so it is advisable to confine recommendations to well-known and relatively simple preparations.

Creams and lotions require preservatives because water-containing preparations encourage the growth of contaminating organisms. Unfortunately, preservatives are particularly prone to cause contact allergic dermatitis. If this is suspected, alcohol- or oil-based topical preparations may be more suitable. Common preservatives include:

Site of skin disease

Hair-bearing sites (particularly the scalp) are difficult to treat topically. Medicated shampoos are relatively straightforward to use but are in contact with the skin for only a few minutes then rinsed out so may not provide adequate therapeutic effect. Active ingredients include keratolytics such as salicylic acid, antimicrobials such as chlorhexidine and ketoconazole and anti-inflammatory agents such as tar. Clear alcohol or propylene glycol-based solutions and milky lotions are used to deliver leave-on topical steroids and calcipotriol. Thicker creams containing tar in various water-soluble bases are applied for one to eight hours, then washed out. Although effective, they are unpopular because of the mess and inconvenience.

Facial skin may be oily, especially in adolescence, in which case thick creams and ointments are undesirable and may provoke acne. Select 'non-comedogenic' products, ie. tested to ensure they are non-occlusive and do not cause comedones (blackheads and whiteheads). Older sun damaged skin tends to be dry, aggravated by solutions and gels. Many people have 'combination skin' ie an oily 'T' zone (forehead, nose, chin) and dry cheeks. Others are considered to have 'sensitive' skin, and complain of stinging, burning and redness provoked by numerous cosmetics, sunscreens and topical treatments. Facial creams are particularly liable to provoke a rash known as perioral dermatitis if applied to the nasolabial fold or chin. Skin is thin on the face, particularly eyelids. Long term use of topical steroids results in dermal atrophy so select low potency topical steroids for this site, used for short courses. Steroids may also aggravate telangiectasia.

Mucosal surfaces ie lips and anus are generally treated with ointments to provide lubrication, although creams are generally preferred for the genitals. These sites are very absorbent, as the stratum corneum is thin, so mild medications are necessary. Patients frequently complain of stinging.

Flexures, especially axillae and submammary folds, are naturally moist sites prone to intertrigo (an inflammatory rash caused by moisture, occlusion, friction and microbial proliferation). Thinner creams or lotions are preferred, and absorbent powders may be useful. The skin is thin, and potent topical steroids may result in striae (stretch marks) within a few weeks of use.

Hands and feet, especially palms and soles, have thick stratum corneum, which reduces absorption and effect of topical medications. Potent or ultrapotent topical steroids may be necessary. Hands in particular tend to dry out, especially if in water frequently or exposed to cold air. Select thicker bases such as fatty creams or ointments and apply emollients frequently.

Trunk and limbs can be treated with lotion, cream or ointment depending how dry the skin is and patient preference.

Stage of skin disease

The stage of various skin conditions (especially eczema) is frequently described as acute, subacute or chronic.

Acute skin conditions

Acute eruptions tend to be bright red and irritable. Examples are sunburn and acute eczema. Aim to cool and soothe with wet dressings, lotions and light creams. The patient may feel unwell and should rest. Blisters ('wet' skin) should be dried up using 'wet' astringent applications including dilute potassium permanganate solution, acetic acid or just water.

Some acute eruptions are due to bacterial infection such as impetigo and cellulitis. Oral antibiotics are indicated if the infection is severe but may not be necessary for localised self-limited disease.

Crusts and scale can be removed by soaking in a bath, applying cleanser or oil and gently rubbing with a soft cloth.

Wet wraps refer to the application of water and/or cream soaked tube gauze or compresses, which are covered with a dry dressing (sometimes plastic) and replaced twice daily. Most often used for extensive eczema, they are useful to cool and soothe inflamed skin, and to debride adherent crusts.

Chronic skin conditions

Chronic eruptions tend to be less red, with thicker drier skin. Examples are chronic plaque psoriasis and chronic dermatitis. Dry skin conditions should be moisturised using thicker ointments.

Chronic dermatitis may be linked to bacterial infection but in these cases, reducing bacteria may or may not prove helpful long term. About 5% of courses of antibiotics cause allergic reactions; these may be serious and even life threatening. Antibiotic use is also resulting in rising concern about resistant organisms. The resistance patterns of Staphylococcus aureus and Streptococcus pyogenes in Hamilton New Zealand in 2002 are shown in Table 1

Percentage of organisms resistant to an antibiotic (from Pathlab Medical Laboratory, July 2003).

Staph.aureus Strept. pyogenes
Penicillin V / amoxycillin 87 0
Flucloxacillin / amoxicillin-clavulanate / cefaclor 2 0
Erythromycin / clindamycin 12 2

How much for how long?

Even experienced dermatologists have great difficulty determining the quantity of topical agents to prescribe. The extent and severity of skin conditions can vary considerably from day to day, due to treatment or disease fluctuation. It may be applied thickly or thinly, once or many times daily.

In general, medications such as topical steroids are applied twice daily, whereas emollients should be used as often as is necessary to relieve dryness and itching, which might be twenty times daily.

The fingertip unit is a useful guide. One fingertip unit in an adult male from a standard nozzle provides 0.5g of ointment.

Always include a quantity in a prescription. In New Zealand, if you do not tell the pharmacy the quantity to prescribe the pharmacist will provide the patient with the smallest unit e.g. 15g tube. If you indicate a 3 months' supply, the pharmacist may provide repeats as required by the patient, which may be a much larger quantity than you had intended or is safe.

Instruct patients with acute skin conditions to advise you if and when they require more medications and monitor how much is used. Those with chronic conditions more frequently can request an appropriate amount for their needs but this will vary enormously between individuals.

Stronger topical steroids will be applied for a limited course of two to four weeks for acute conditions or flare-ups of chronic disease, with milder preparations used ad libitum in between.

Oral medication may be prescribed for regular use e.g. methotrexate 15g once weekly for 3 months, or flucloxacillin 500mg tid for 7 days, or to be taken 'as required' (PRN) e.g. promethazine 25mg at night for itch. Occasionally, a staged course of medication is prescribed, usually prednisone commencing at 40mg daily for a week or so then taken in slowly reducing dose over a period of several weeks or months.