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Contact dermatitis

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Contact dermatitis

Written by Dr Virginia Hubbard, specialist registrar in dermatology and Dr Malcolm Rustin, consultant dermatologist

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.

Irritant contact dermatitis 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

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:

previous exposure to the substance is needed to induce allergy.

the reaction is specific to one chemical or a group of similar chemicals.

all areas of skin that are in contact with the allergy-provoking substance (allergen) develop the rash.

avoidance of the allergen will result in resolution of the rash.

The commonest allergens are:

nickel

fragrances

rubber

some plants

formaldehyde

skin medications (including topical corticosteroids)

hairdressing chemicals.

As with the irritant type, allergic contact dermatitis is more common in atopic individuals.

What causes contact dermatitis?

Irritant contact dermatitis Irritants cause direct injury to the skin. This can occur over a short (acute) or long (chronic) period of time.

Acute irritant contact dermatitis A single exposure to a substance causes an acute dermatitis, within minutes to hours after exposure. The course of events is:

irritant substance penetrates the skin.

substance damages the membranes of skin cells.

cell damage prompts release of chemicals that trigger the immune system into action. This is called an inflammatory response. The chemicals involved (inflammatory mediators) include lysozymes, prostaglandins, histamine and kinins.

some inflammatory mediators cause increased blood flow while others attract further inflammatory mediators.

Chronic irritant contact dermatitis

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:

each exposure adds to the gradual disruption of the outer layer of skin.

each time, inflammatory mediators are released.

the top layer of skin (epidermis) gradually thickens.

the lipid (fat) layer in skin is gradually damaged.

affected skin loses its ability to function as a barrier, so further exposure to an irritant produces further damage, and a 'vicious cycle' ensues.

the final result is dryness, scaling and thickening of the skin.

Allergic contact dermatitis

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.

Sensitisation phase 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.

Elicitation phase

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.

What are the symptoms?

Irritant contact dermatitis

Acute The appearance of acute irritant dermatitis can range from a mild reaction consisting of transient redness to a severe painful burn with blistering.

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