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What are eczema and dermatitis?

The term ‘eczema’ can be used interchangeably with the term ‘dermatitis’. However both terms just describe a pattern of inflammation in the skin rather than a definitive diagnosis and what caused it. However, if possible the cause needs to be identified.

What is contact dermatitis?

Contact dermatitis is dermatitis caused by contact with an exogenous (outside) chemical (not food) interacting with the skin. If these chemicals cause an allergy they are referred to as allergens. There are however two pathological processes by which chemicals can damage the skin – irritant contact dermatitis and allergic contact dermatitis. In practice they often occur together resulting in the dermatitis.

Irritant contact dermatitis typically affects the hands and is an important cause of occupational skin disease. The most common irritants are prolonged or frequent contact with water (‘wet work’) soap, detergents and organic solvents. There is considerable individual variation in the threshold for skin irritation.

Allergic contact dermatitis (e.g. allergy to metals or fragrance chemicals or other cosmetic ingredients) involves a specific memory driven immune response, whereas in irritant contact dermatitis a non-specific inflammatory response is caused.

Some chemicals are recognized as having a greater potential to cause sensitisation (the capacity to cause an allergic response) and then on re-exposure to cause an allergic response in the skin resulting in dermatitis. These chemicals include many that we expose ourselves to on a daily basis e.g. cosmetic ingredients, metals plants etc. Therefore, allergic contact dermatitis can be a cause of someone’s eczema in its own right or can complicate another type of dermatitis e.g. atopic dermatitis, seborrhoeic dermatitis or discoid eczema.

Rarely it is possible to become allergic to the ingredients in treatment creams themselves, so that the treatment itself can perpetuate the dermatitis. This may be suspected if the eczema does not respond to treatment as expected.

If allergic contact dermatitis is suspected, then your dermatologist is likely to refer you for patch tests. This is the only test available for identifying if an allergen is causing or contributing to your eczema.

Why might I need a patch test?

Some common reasons for referring someone for patch tests include:

  • Certain distributions of eczema – eyelid, face, hands, feet, ears or genital areas.
  • Chronic hand eczema
  • If your eczema has become unstable or is involving different sites
  • Following a history of reacting to a particular product
  • To investigate why someone might not be responding to topical treatments as expected
  • To investigate certain drug reactions
  • To identify a cause of eczema when somebody has exposure to certain (sensitising) chemicals through their work or hobbies (occupationally induced allergic contact dermatitis).

The only way to diagnose allergic contact dermatitis is by patch testing.

What is occupational dermatitis?

Skin disease is the second commonest occupational disease in the European Union after musculoskeletal disorders. Contact dermatitis (eczema) accounts for 70-90% of all occupational skin disease. Most patients with this condition also present with hand eczema. Up to half of workers with occupational contact dermatitis (eczema) experience adverse effects on quality of life, daily function and relationships at home.

It is very important to perform patch test investigations where individuals have contact with recognized skin allergens, or work in jobs where work factors are likely to be influencing the eczema eg healthcare workers, hairdressers, chefs, engineers etc.

Allergic contact dermatitis can also complicate atopic eczema and other forms of eczema eg by becoming allergic to a topical steroid, a topical antibiotic or a chemical in a moisturiser prescribed to treat the eczema. This may be suspected if the eczema does not respond to treatment as expected. Patch tests can be used to identify if contact allergy is the “cause of” or “contributing to” eczema.

However, allergic contact dermatitis can be the cause of eczema (dermatitis) in its own right, eg developing an allergy to a hair dye chemical, a fragrance chemical or a preservative in a cosmetic product.