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Contact Dermatitis

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Contact dermatitis is an inflammation of the skin characterized by redness, itching, blistering and, in chronic cases, flaking of scales of skin, resulting from exposure of the skin to substances in the environment. The site and shape of the affected areas of skin are directly related to the area that has been exposed to the causative substance (click for picture).


Contact dermatitis is classified as:

Irritant contact dermatitis (Irritant CD)

Irritant CD develops following prolonged and repeated exposure to irritants such as caustic agents or detergents. This is not an IgE-mediated, allergic condition, and prior contact and sensitization to the causative substance is not necessary for the development of symptoms. Susceptibility to irritants varies, but, given sufficient exposure, most people would develop irritant CD.

Phototoxic reactions are the result of exposure to sunlight in combination with certain chemicals. All individuals with sufficient skin exposure to chemical photosensitizers such as psoralens (naturally occurring substances found in some plants) together with ultraviolet light (UV), would develop a phototoxic skin reaction. This reaction resembles sunburn and is not caused by a reaction of the immune system.

Allergic contact dermatitis (Allergic CD)

Allergic CD results from inflammation caused by a group of blood cells called allergen specific T lymphocytes. Prior exposure, which results in an allergic reaction, i.e., sensitization, is essential. Rapid development of skin inflammation, i.e., dermatitis, occurs following re-exposure to low concentrations of allergen, which would not cause a reaction in non-sensitized non-allergic individuals.

Photoallergic reactions result from new allergens created by the photochemical reaction of UV on certain chemicals and body proteins. The new allergens stimulate an allergic "immune" response which causes a skin reaction similar to that of allergic CD.


85%-95% of all occupationally-related skin disease in the working population of industrialized countries is due to contact dermatitis. In these countries hand dermatitis affects between 2% and 6% of the population.

Symptoms and Characteristics

The fundamental characteristic of contact dermatitis is the relationship to environmental exposure, which determines the site and shape of the skin reaction (click for picture). A sharp edge to an otherwise shapeless rash, or an abrupt cutoff, is a characteristic of contact dermatitis (click for picture).

Irritant CD
  • Irritant CD most frequently affects the hands (click for picture).
  • Phototoxic and photoallergic CD can occur due to a combination of drugs taken by mouth, a sensitizing agent (such as fragrance) applied to the skin, and subsequent exposure to sunlight. Patches of CD will appear only on the areas of skin exposed to the sun.
Allergic CD
  • Nickel allergy involves ears, skin under buckles and often the hands; accidental spread from the hands can affect the face.

    Hair products - dyes and sprays - affect the face, neck and ears.

    Dyes in socks and shoes affect the feet.

    Medications for the treatment of leg ulcers can cause dermatitis of the legs (click for picture).

  • In the case of an allergy, an itchy, red rash develops at the site of re-exposure to the causative substance within 6 to 12 hours. The reaction is worse after 48 to 72 hours, and, if exposure continues, the skin becomes scaly, weepy and flaky.
  • Strongly allergic people require very little contact with the causative agent for severe acute weeping eczematous reactions to occur.
  • Contact with the allergen can occur even through several layers of clothing. For example, nickel can be leached from keys, money or studs by perspiration.

Differential Diagnosis

It is important to obtain a correct diagnosis of either allergic or irritant CD, particularly when the dermatitis is caused by contact with a substance encountered at work, since the correct diagnosis of an irritant cause may allow the patient to continue at work with appropriate skin protection. A detailed history of all substances with which the individual comes into contact during domestic, occupational, sporting and leisure activities, together with the results of patch testing, is required.

Contact dermatitis can be similar in appearance to most inflammatory skin diseases. However the majority of skin diseases that are not related to contact have a symmetrical distribution, as is the case with IgE-mediated eczema (see atopic eczema/dermatitis syndrome), drug rashes, psoriasis and connective tissue diseases.

Fixed drug eruptions (a rash occurring repeatedly at the same place following ingestion of a medicine to which the individual is sensitized) can be single and round, and skin cells can be examined to ensure the eruption is not a symptom of a lymphoma (a form of cancer).

Allergens Causing Allergic CD


Nickel is the commonest cause of allergic CD in most countries. It is present in most metal alloys including 14-karat gold. Most nickel allergic individuals can avoid problems by wearing 18-karat gold and testing other suspect alloys with a dimethylglyoxime test. The role of dietary nickel in skin eruptions is controversial.

Rubber Accelerators: Thiurams, Carbamates, Mercaptobenzothiazole

Allergic CD to rubber usually results from sensitization to these compounds and occurs where rubber is worn next to or close to the skin. Examples are elastic bands in underclothes, and rubber in gloves, shoes and barrier contraceptives (click for picture). Carbamates are found not only in rubber products but also in garden fungicides. Allergy to black rubber may also be the result of sensitization to paraphenylenediamine dye.

Latex rubber can also cause so called allergic contact urticaria (hives), which is an IgE-mediated reaction to protein allergens from the latex. Patients with a history of contact urticaria should not undergo patch testing because there is a risk of anaphylaxis.


Paraphenylenediamine is a black dye used in permanent oxidative hair dyes and is used with cross-linkers to produce all hair dye colors. It is a major cause of allergic CD in hairdressers. Paraphenylenediamine is also used as an antioxidant in oils and greases, as a component in color film developers and as a dye for leather and rubber. It is an important cause of occupational dermatitis in a wide variety of trades.

Fragrances: Balsam of Peru and Cinnamic Aldehyde

Fragrances containing balsam of Peru and cinnamic aldehyde are present in many topical preparations, cosmetics, soaps, perfumes and toothpastes. Allergic patients must use "fragrance free" products - "unscented" products are not suitable since they may contain masking fragrances.

Adhesives and Varnishes

P-Tertiary-butylphenol formaldehyde resin (PTBP) is a phenolic contact adhesive used in the manufacture of plastics, plywood, cardboard boxes, varnish, rubber cements, leather products and lacquers. PTBT is a factor in dermatitis caused by some shoes, and in allergic CD caused by plastic products and oils.

Epoxy resins are widely used as adhesives in electronics, the construction industry and in marine paints and varnishes and are an important cause of allergic CD.

Formaldehyde and Formaldehyde Releasers

Formaldehyde is one of the most widespread allergens in the environment, being present in fixatives, adhesives, preservatives and disinfectants. Many cosmetics and disinfectants contain either formaldehyde or the formaldehyde releasers imidazolidinyl urea and quaternium-15 (click for picture). Formaldehyde is also used as a fabric treatment particularly in the "care- free" type of garments, which retain their shape. Many formaldehyde allergic patients can, however, tolerate garments that have been washed many times.


Exposure to potassium chromate is common in tanning of leather and in the construction industry due to cement (click for picture). Allergic CD to chromate in leather shoes may be seasonal as a result of the allergen being leached out by perspiration.


Benzocaine is a sensitizer and is present in many nonprescription medications such as preparations for the treatment of hemorrhoids and burns, and as a topical anesthetic. Cross-reactions with procaine occur.

The antibacterial agent neomycin is a common cause of allergic CD. Bacitracin may also cause sensitization.


Patch Testing

Patch testing requires three visits. On the first day, the allergens are applied to the back in a dermatitis-free area. These are applied by taping to the skin a number of small chambers containing the substances to be tested. Forty-eight hours later, the patches are removed and the skin is examined. The patient returns the following day for a final assessment of any skin reactions. Topical corticosteroids must not be applied to the test site, and oral corticosteroids should not be taken by the patient for at least 2 weeks prior to patch testing. Standardized allergens are used for patch testing whenever possible, since these are extensively tested and should not induce skin irritation or sensitization. It may be necessary on occasion to use non-standardized materials such as clippings of shoes and clothing. Non-standardized compounds that are generally safe for patch testing are cosmetics, moisturizers and medications that are applied directly to the skin.

Interpretation of patch test reactions

All patch test results will be interpreted in the light of the patient's clinical history and the size, shape and location of the affected areas of skin. Positive patch tests may not be relevant to the patient's present condition and may simply represent allergies that have occurred in the past (click for picture). Perspiration and moisture are important factors in the development of allergic CD, since they may be necessary to leach out the allergen from the article of clothing that is in contact with the skin. For this reason in some patients, the skin may only be affected during hot weather.

Standardized patch test allergen series

The standardized patch test allergen series consists of 20 commonly encountered contact sensitizers. The frequency with which these or indeed other contact allergens are encountered will vary between countries worldwide.

Group Allergen
Adhesives Epoxy resin 1%
Formaldehyde 1%
p-Tertiary-butylphenol formaldehyde resin 1%
Rubber compounds in rubber adhesives
Antimicrobials (Formaldehyde 1%)
Imidazolidinyl urea 2%
Neomycin sulfate 20%
Quarternium-15 2%
Fragrances Balsam of Peru 25%
Cinnamic aldehyde 1%
Hair dye Paraphenylenediamine 1%
Metals Nickel sulphate 2.5%
Rubber compounds Black rubber mix 0.6%
Mercaptobenzothiazole 1%
Mercapto mix 1%
(Paraphenylenediamine 1%)
Thiuram mix 1%
Topical medicaments Benzocaine 5%
Ethylenediamine dihydrochloride 1%
Lanolin alcohol 30%
(Neomycin sulfate 20%)
Rosins, waxes, polishes Colophony 20%
Cement and leather Potassium dichromate 0.25%

(….) Indicates allergens that are repeated in different groups.

False positive patch test reactions

Sometimes a false positive tests can occur because:

  • The petrolatum used to apply the allergens may cause a mild occasional skin reaction around hair follicles, some reddening, or a pus-filled spot.

  • Allergic or irritant reactions to the tape used to hold the allergen chambers in place are recognized by their relation to the site of the tape rather than to the area of the patch test.

  • A large skin reaction may overrun into the area of the next test, and, if any doubt exists about the allergen responsible for the skin reaction, the tests need to be repeated a greater distance apart.


Allergen/irritant avoidance

Contact dermatitis resolves spontaneously when the causative agent is identified by clinical history, site, appearance and patch testing and is removed.


Neither the itching nor the inflammation of contact dermatitis responds to antihistamine therapy. However, sedating antihistamines, such as diphenhydramine and hydroxyzine, are often administered at night to help sleep.

Topical corticosteroids

If allergen avoidance is not possible or on rare occasions not helpful, topical corticosteroids are the mainstay of therapy. Depending on the type of skin reaction, either a gel, ointment or cream may be recommended.

Potent corticosteroid preparations should not be used on the face because they can cause thinning of the skin.

Systemic corticosteroid treatment

This treatment may be prescribed if contact contact dermatitis involves a large area of the body, and interferes with essential aspects of daily life. Treatment is given for 14 days, at which time the corticosteroids are discontinued.

Patients should be informed of the possible unwanted effects of systemic corticosteroid treatment, because it can affect the immune system and lead to infection of the affected areas of skin. Blood pressure and glucose levels may be monitored while the patient is taking corticosteroid treatment.

Classification of the potency of topical corticosteroid preparations
Group Corticosteroid Indication/contraindication
1 Clobetasol propionate 0.05% cream or ointment
Dirlurasone diacetate 0.05% ointment
Betamethasone dipropionate 0.05% cream or ointment

Acute dermatitis responds best to Group 1 corticosteroids.
Pruritus (itching) improves in days and inflammation in a week.
Do not use on the face.

2 Fluocinonide 0.05% cream or ointment

As above

3 Triamcinolone 0.1% ointment
Amcinonide 0.1% ointment
Betamethasone valerate 0.1% ointment
As above
4 Triamcinolone 0.1% cream
Fluocinolone acetonide 0.025%
Can be used on the face for one week only
5 Hydrocortisone valerate 0.2% cream Can be used on the face for longer periods
6 Hydrocortisone 1% & 2.5% cream or ointment Advised for treatment of contact dermatitis in children