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Written by Dr Virginia Hubbard, specialist registrar in dermatology and Dr Malcolm Rustin, consultant dermatologist
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What is contact dermatitis? Contact dermatitis is inflammation of the skin that results from contact of an external substance with the skin. This can occur through one of two mechanisms: irritant or allergic.
Irritants cause approximately 80 per cent of cases of contact dermatitis. An irritant reaction is caused by the direct effect of an irritant substance on the skin. An irritant substance is one that would cause an inflammatory reaction in most individuals when applied in sufficient concentration for an adequate amount of time. Irritants are often encountered at work (occupational) although other common irritants are soaps, detergents, food and cement. Almost all workers in wet-work industries, such as hairdressing, cleaning, metal engineering, building-site work and horticulture develop some degree of irritant contact dermatitis. Both sexes are equally susceptible to irritant contact dermatitis. Those with dry skins or who have one or more of the 'atopic' conditions (eczema, asthma or hay fever) are more likely to develop an irritant contact dermatitis. An irritant dermatitis is most likely to occur on the hands.
Allergic contact dermatitis accounts for the remaining 20 per cent of cases. In this condition, the cause of skin inflammation is a hypersensitivity reaction, acting through the body's immune system, to a particular substance or group of related substances. Allergic contact dermatitis has the following features: The commonest allergens are: As with the irritant type, allergic contact dermatitis is more common in atopic individuals.
Irritants cause direct injury to the skin. This can occur over a short (acute) or long (chronic) period of time.
A single exposure to a substance causes an acute dermatitis, within minutes to hours after exposure. The course of events is:
This is due to multiple exposures, often to several irritants at low levels over time. This dermatitis can take many months or years to appear. The course of events is:
This is a type of immune reaction known as 'type IV' or 'delayed hypersensitivity' reaction. The characteristic feature of this immune response is a delay between first exposure to an allergen and the subsequent reaction. It, therefore, occurs in two stages, sensitisation and elicitation.
This starts with the substance penetrating the skin, which then binds to Langerhans' cells (a type of skin immune cell), which then leave the skin and travel to lymph nodes (glands) nearby. Here, the allergen is shown to another type of immune cell, T-lymphocytes, which proliferate and produce 'memory' cells that can remember that particular allergen.
Once sensitisation has occurred, subsequent exposure to the allergen causes the T-lymphocytes to recognise the allergen, which activates them and causes them to multiply. Inflammatory mediators are released that induce the features of inflammation and bring more T-lymphocytes to the site of exposure. This ongoing immune reaction results in the eczema-like inflammation of the skin at the site of contact. This phase occurs within 48-72 hours after exposure. Small amounts of allergen can be enough to cause an inflammatory reaction.
The appearance of acute irritant dermatitis can range from a mild reaction consisting of transient redness to a severe painful burn with blistering.
Chronic irritant dermatitis often begins with a few patches of dry, slightly inflamed skin that become thickened with time.
The features of eczema develop at the site of contact. For example, the first sign may be an itch under an earring or along a waistband that contains rubber. The itch can develop into an area of redness with swelling and even small blisters that weep. In contrast to irritant contact dermatitis, the reaction can extend beyond or occur in a different place from the site of contact. Occasionally, the appearance is that of urticaria (severely itchy raised red patches or wheals that can resemble insect bites, although these may be more irregular in shape). Rarely, swelling of the mouth and upper airways can occur, which is known as angioedema. This is serious and needs urgent medical attention.
If exposure to an allergen persists, the skin becomes drier, thicker and more scaly with a change in the pigmentation (colour). The most important factor in making a diagnosis is the suspicion by you or your doctor that a substance in your environment is causing the dermatitis. Doctors should always think about the possibility of contact dermatitis when managing a patient with an eczematous (eczema-like) reaction. Two features are key to differentiating contact dermatitis from other causes of an eczematous rash and to determine the offending substances: the timing of onset or exacerbations and the part of the body that is affected.
Allergic contact dermatitis usually occurs 48 to 72 hours after exposure, and will wax and wane depending on exposure. Improvement of dermatitis during weekends or holidays is in favour of an occupational origin for the offending substance. Occurrence or worsening at weekends suggests a hobby or environmental allergen. Seasonal variation of dermatitis is seen in particular with plant allergens, which can also be aggravated by light.
Contact dermatitis usually starts in and often remains localised to the region most in contact with the offending substance. The pattern of affected skin is a vital clue in the origin of the substance (eg, a rash shaped exactly like your metal belt buckle could indicate an allergy to nickel).
All people who have an unusual dermatitis or one that resists treatment should have patch testing to test whether certain contact allergens are aggravating the dermatitis. What else could it be? Several types of eczema-like reaction can produce a similar appearance:
Other skin conditions that should be considered are:
Once the diagnosis is made, and suspicious substances have been identified, your doctor will advise you how to avoid the suspected/offending substance. Avoidance often resolves the dermatitis but if this is difficult or if the dermatitis is long standing, you will need drug treatments. Corticosteroids in the form of creams and ointments can be applied to the affected area to reduce the inflammation. Antihistamine treatments can sometimes help with redness and itching, particularly with urticaria. What can your dermatologist do? For some patients, a referral to a skin specialist (dermatologist) is needed. The dermatologist will:
Whether the dermatitis will settle or recur depends on several factors.
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References Belsito DV. The diagnostic evaluation, treatment, and prevention of allergic contact dermatitis in the new millenium. J Allergy Clin Immunol 2000 Mar; 105(3): 409-20. Leow YH. Contact dermatitis due to topical traditional Chinese herbal medication. Clinics in Dermatology 1997 Jul-Aug; 15(4): 601-05. Mowad CM. Update on contact dermatitis. Advances in Dermatology 1999; 14: 61-86. |
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| Last updated 01.08.2005 |
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