Contact dermatitis

The skin normally performs its function as a barrier very effectively. If this is overcome by substances penetrating the epidermis an inflammatory response may occur leading to epidermal damage. These changes may be due to either (a) an allergic response to a specific substance acting as a sensitiser or (b) a simple irritant effect. An understanding of the difference between these reactions is helpful in the clinical assessment of contact dermatitis.

Common sources of allergic contact dermatitis

• Jewellery, clothing, wristwatch, scissors, cooking utensils

• Rubber gloves and boots

• Creams, ointments, cosmetics

• Nickel—and cobalt occasionally

• Paraphenylenediamine—used in hair dyes

• Rubber preservative chemicals

• Preservatives (parabenz, quarternium), balsam of Peru, fragrances, lanolin, neomycin, benzocaine in medicated ointments

Contact dermatitis—common sources

Allergic contact dermatitis

The characteristics of allergic dermatitis are:

• Previous exposure to the substance concerned.

• 48-96 hours between contact and the development of changes in the skin.

• Activation of previously sensitised sites by contact with the same allergen elsewhere on the body.

• Persistence of the allergy for many years.

The explanation of the sequence of events in a previously sensitised individual is as follows: The antigen penetrates the epidermis and is picked up by a Langerhans cell sensitised to it. It is then transported to the regional lymph node where the paracortical region produces a clone of T cells specifically programmed to react to that antigen. The sensitised T cells accumulate at the site of the antigen and react with it to produce an inflammatory response. This takes 48 hours and is amplified by interleukins that provide a feedback stimulus to the production of further sensitised T cells. Immunological response leading to contact dermatitis

Cloning

Interleukin 1

Proliferation

Interleukin 2

Inflammation

(Lymphokines) T helper cells T suppressor cells

Cloning

Interleukin 1

Proliferation

Interleukin 2

Inflammation

(Lymphokines) T helper cells T suppressor cells

T" lymphocyte

Lymph node Paracortical area

T" lymphocyte

Lymph node Paracortical area

Allergic contact dermatitis can be illustrated by the example of an individual with an allergy to nickel who has previously reacted to a wrist watch. Working with metal objects that contain nickel leads to dermatitis on the hands and also a flare up at the site of previous contact with the watch. The skin clears on holiday but the dermatitis recurs two days after the person returns to work.

Sensitisers in leg ulcer treatments

• Formaldehyde

• Chinaform (the "C" of many proprietary steroids)

Irritant contact dermatitis

This has a much less defined clinical course and is caused by a wide variety of substances with no predictable time interval between contact and the appearance of the rash. Dermatitis occurs soon after exposure and the severity varies with the quantity, concentration, and length of exposure to the substance concerned. Previous contact is not required, unlike allergic dermatitis where previous sensitisation is necessary.

Photodermatitis

Photodermatitis, caused by the interaction of light and i - luuju^ii- • j^i-i^ Allergic response to dithranol chemical absorbed by the skin, occurs in areas exposed to light.

It may be due to (a) drugs taken internally, such as sulphonamides, phenothiazines, and dimethylchlortetracycline, or (b) substances in contact with the skin, such as topical antihistamines, local anaesthetics, cosmetics, and antibacterials.

Morphology

The clinical appearance of both allergic and irritant contact dermatitis may be similar, but there are specific changes that help in differentiating them. An acute allergic reaction tends to produce erythema, oedema, and vesicles. The more chronic lesions are often lichenified. Irritant dermatitis may present as slight scaling and itching or extensive epidermal damage resembling a superficial burn, as the child in the illustration shows.

Pathology

The reaction to specific allergens leads to a typical eczematous reaction with oedema separating the epidermal cells and blister formation. In irritant dermatitis there may also be eczematous changes but also non-specific inflammation, thickening of keratin, and pyknotic, dead epidermal cells.

Photodermatitis

The distribution of the skin changes is often helpful. For example, an itchy rash on the waist may be due to an allergy to rubber in the waistband of underclothing or a metal fastener. Gloves or the rubber lining of goggles can cause a persisting dermatitis. An irritant substance often produces a more diffuse eruption, as shown by the patient who developed itching and redness from dithranol.

An allergy to medications used for treating leg ulcers is a common cause of persisting dermatitis on the leg.

Acute irritant dermatitis

Allergic response to elastic in underpants r

Allergic reaction to cosmetics

Substances commonly causing allergic occupational dermatitis

• Chromate—in cement and leather

• Biocides, for example, formaldehyde and isothiazolinones, used in cutting oils in engineering

• Epoxy resins (uncured monomers)

• Rubber chemicals

• Hair dressing chemicals—particularly dyes and setting lotions

• Plant allergens

Allergic reaction to epoxy resin

Patch testing

Patch testing is used to determine the substances causing contact dermatitis. The concentration used is critical. If it is too low there may be no reaction, giving a false negative result, and if it is too high it may produce an irritant reaction, which is interpreted as showing an allergy (false positive). Another possible danger is the induction of an allergy by the test substance. The optimum concentration and best vehicle have been found for most common allergens, which are the basis of the "battery" of tests used in most dermatology units.

The test patches are left in place for 48 hours then removed, the sites marked, and any positive reactions noted. A further examination is carried out at 96 hours to detect any further reactions.

Acutely infected eczema

It is most important not to put a possible causative substance on the skin in a random manner without proper dilution and without control patches. The results will be meaningless and irritant reactions, which are unpleasant for the patient, may occur.

Positive reactions marked

Test patches in place

Positive reactions marked

Test patches in place

Patches being removed after 48 hours
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