Allergic contact dermatitis

What is allergic contact dermatitis?

Allergic contact dermatitis is a form of dermatitis caused by an allergic reaction to a material (called an allergen) in contact with the skin. The allergen is harmless to people who are not allergic to it.

Allergic contact dermatitis is also known as contact allergy.

Acute weeping exudative allergic contact dermatitis due to a hair dye used on the beard 

Chronic lichenified allergic contact dermatitis due to leather in a sandal (ACD-patient1)

Allergic contact dermatitis due to a rubber accelerator (used in the rubber curing process) 

Acute allergic contact dermatitis due to lavender oil used on the pillow case to promote sleep 

Who gets allergic contact dermatitis?

Allergic contact dermatitis is common in the general population and specific employment groups.

• It is more common in women than men, primarily due to increased exposure to potential allergens such as:
  • Nickel, which is commonly found in jewellery and piercings, and increasingly in technology and equipment such as phone cases
  • Acrylates, which are widely used in nail cosmetics
• Children are also at an increased risk of nickel allergy, partly due to an immature skin barrier and increased prevalence of atopic dermatitis.
• In patients older than 70 years, allergic contact dermatitis to topical antibiotics is common.
• Allergic contact dermatitis, the most common type of occupational skin disease, is especially common in metal workers, hairdressers, beauticians, health care workers, cleaners, painters, and florists.

What causes allergic contact dermatitis?

Allergic contact dermatitis is a type IV (delayed) hypersensitivity reaction, which typically occurs 24–72 hours after exposure to an allergen. The mechanism involves CD4+ T-lymphocytes, which recognise an antigen (from an allergen) on the skin surface and release cytokines that activate the immune system and cause dermatitis.

Only a small number of people react to the specific allergen, which is harmless to those who are not allergic to it. This is because the person has become sensitised to the allergen, meaning it will trigger an immune response.

Factors that increase the likelihood of sensitisation include:

• Chemical factors eg, chemical size and concentration.
• Skin factors eg, skin disease, skin condition and integrity.
• Exposure factors eg, duration, frequency, and environmental conditions.

Note:

• Contact allergy occurs predominantly from an allergen on the skin rather than from internal sources or food (which is called systemic contact dermatitis)
• Patients may have been in contact with the allergen for years before becoming sensitised, without it causing dermatitis.
• Contact with tiny quantities of an allergen can induce dermatitis.
• Patients with impaired barrier function of the skin are more prone to allergic contact dermatitis eg. chronic irritant contact dermatitis, leg ulcers, perianal dermatitis.
• Patients with atopic dermatitis associated with defective filaggrin (a structural protein in the stratum corneum) have a high risk of also developing allergic contact dermatitis.

What are the clinical features of allergic contact dermatitis?

Allergic contact dermatitis usually appears 24-72 hours after contact with the allergen. It usually resolves over several days, provided the skin is no longer in contact with the allergen.

The typical appearance is often a well-demarcated pruritic eczematous eruption, which may be acute (blistering, weeping, and/or oedematous) or chronic (lichenified or scaly plaques).

Allergic contact dermatitis is generally well demarcated and localised to the site of contact with the allergen, but it may extend outside the contact area or become generalised.

Transmission of the allergen from the site of contact, for example via the fingers, can lead to contact dermatitis on other areas of the skin, including the eyelids and genitals.

Dermatitis is unlikely to be due to a specific allergen if the area of skin most in contact with that allergen is unaffected.

Common allergens that trigger contact dermatitis reactions include:

• Nickel in jewellery and piercings.
• Fragrances in perfumes and household items.
• Rosin in adhesive plasters.
• Paraphenylenediamine in permanent hair dye — causes swelling and blistering of the face and neck.
• Rubber accelerator chemicals used in the manufacture of rubber gloves.
• Methylisothiazolinone as a preservative in wash-off hair products and baby wipes — often causes an itchy red face.
• Acrylates in hair extensions and nail cosmetics — can result in fingertip dermatitis.
• Acrylates in dental implants.
• Topical medications eg, antibiotics — cause localised blistering at the site of the application.
• Plants eg, poison ivy or the Japanese wax tree Toxicodendron succedaneum — cause swelling and blistering on exposed sites.

There is a very long list of materials that have caused contact allergy in a small number of individuals.

Adhesive plaster reaction 

Sunscreen reaction 

Watch strap reaction 

Allergic contact dermatitis due to an adhesive in orthopaedic strapping 

See more images of allergic contact dermatitis

Specific allergic contact dermatitis images.

• Facial dermatitis images
• Hand dermatitis images
• Limb dermatitis images
• Truncal dermatitis images
• Patch test results images

Photoallergy

Sometimes, contact allergy arises only after the skin has been exposed to ultraviolet light. The rash is confined to sun-exposed areas even though the allergen may have been in contact with covered areas. This is called photoallergic contact dermatitis.

Examples of photoallergy include:

• Dermatitis due to a sunscreen chemical, affecting the top but not the under surface of the arm
• Dermatitis of the face, neck, arms, and hands due to antibacterial soap.

How do clinical features vary in differing types of skin?

Allergic contact dermatitis in patients with skin of colour more frequently presents with leathery, dark (hyperpigmented) patches. This is in contrast to the dry, cracked, erythematous skin typically seen in patients with fairer Fitzpatrick skin types.

In patients with darker skin, positive patch tests:

• Often appear with lichenification and hyperpigmentation, rather than erythema and vesicles
• Appear violaceous or faint pink, rather than bright red or pink.

Post-inflammatory pigmentary changes, including both hypo- and hyperpigmentation, are more common in people with darker Fitzpatrick skin types, which can lead to significant psychological distress.

What are the complications of allergic contact dermatitis?

Allergic contact dermatitis typically starts as a localised reaction to an allergen in contact with the skin, but severe reactions may generalise due to autoeczematisation and can lead to erythroderma.

Ingestion of a contact allergen may rarely lead to symmetrical drug-related intertriginous and flexural exanthema or systemic contact dermatitis.

How is allergic contact dermatitis diagnosed?

Allergic contact dermatitis is often a clinical diagnosis, without the need for specific testing. It is essential to take a detailed history that includes occupation, hobbies, products used at home and at work, sun exposure,allergens in use by consorts, and comorbidities. The morphology and location of the dermatitis are often the best indicators of the offending agent.

Patch testing is the gold standard in diagnosing allergic contact dermatitis and should be performed when contact allergy is suspected in patients with recurrent or chronic symptoms. Patch testing can establish the allergen(s) causing the rash to avoid future exposure.

Patients can also perform open application tests. This is done by applying the suspected product to a small area of sensitive skin (eg, inner aspect of upper arm) multiple times daily over several days, and then observing for a reaction. It can be done after patch testing to determine whether positive patch results lead to clinically significant reactions to products.

Other tests include:

• Fungal scrapings of skin for microscopy and culture can exclude fungal infection.
  The dimethylglyoxime test is available to ‘spot test’ if a product contains nickel.

A positive patch test to a piece of a suspect rubber glove 48 hrs after application on the skin 

What is the differential diagnosis for allergic contact dermatitis?

Irritant contact dermatitis:

• This is a form of dermatitis due to irritation or repetitive injury to the skin.
• Its mechanism differs from allergen exposure, the latter of which is immune-mediated.
• Irritants include water, soaps, detergents, solvents, acids, alkalis, and friction.
• Irritant contact dermatitis can affect anyone, provided they have had enough exposure to the irritant, but those with atopic dermatitis are particularly sensitive.
• Most cases of hand dermatitis are due to contact with irritants.
• Irritant contact dermatitis can occur immediately after a single injury or develop slowly after repeated exposure to an irritant.
• Other forms of dermatitis can mimic allergic contact dermatitis, including atopic dermatitis and inflammatory phytophotodermatitis.
• Contact urticaria
  • Rash appears within minutes of exposure and fades away within minutes to hours.
  • The allergic reaction to latex is the best-known example of allergic contact urticaria.
• Infections eg, fungal infections, soft tissue infections (eg, erysipelas, cellulitis), scabies, and impetigo.
• Drug eruptions

What is the treatment for allergic contact dermatitis?

General measures

Identification and avoidance of offending allergens and potential cross-reacting agents is the principal management strategy.

• Precisely identify causative allergens by having comprehensive patch tests.
• Identify where the allergen is found by reading product labels — note that many chemicals have several names eg, acrylic acid is also known as propene acid and acroleic acid.
• Be aware of cross-reactions eg, individuals allergic to nickel may also react to cobalt.
• For patients with known contact dermatitis, test new products by applying a small amount to a patch of skin to check for a reaction.
• Wear appropriate gloves to protect hands from touching allergenic materials. Some chemicals will penetrate certain gloves; seek the advice of a safety expert. Cotton gloves can be worn underneath rubber or leather gloves to prevent reaction to rubber accelerators and chromate.
• Seek dermatology input for further advice.

Specific treatments

Active dermatitis is usually treated with the following:

• Emollient creams
• Topical steroids
• Topical or oral antibiotics for secondary infection
• Oral steroids, usually short courses, for severe cases
• Phototherapy or photochemotherapy.
• Azathioprine, ciclosporin, or another immunosuppressive agent
• Tacrolimus ointment and pimecrolimus cream
• Retinoids for hyperkeratotic palmar involvement — Alitretinoin is approved for the treatment of chronic hand eczema.

What is the outcome for allergic contact dermatitis?

Sensitisation is usually life long. The prognosis is usually good and depends on patient education, adherence to allergen avoidance, and the treatment of active dermatitis.

The rash usually resolves if the allergen is avoided, but in some cases can persist indefinitely eg, chromate allergy. Furthermore, allergic contact dermatitis can recur even with minimal exposure to the allergen. Some allergens are difficult to avoid, with airborne allergens being particularly problematic eg, pollens and resins.

The longer a person suffers from severe allergic contact dermatitis, the longer it will take to clear after the diagnosis and cause are confirmed.