Allergic contact dermatitis is an allergic rash that develops when an allergen comes in contact with skin. It has many causes, it can be from a watch, snap on a pair of pants, or jewelry. It can be from the itch cream you put on something else or poison ivy.
It is a “cell mediated type IV hypersensitivity reaction“. It is also referred to as a delayed hypersensitivity reaction in that the rash presents 2-3 days after exposure. Irritant contact dermatitis looks a lot like this, however; the irritant type is not allergen mediated.
The incidence of allergic contact dermatitis varies by country. The U.S. reports 13.6 cases per 1,000 people. Sweden reports 2.7 per 1,000 and the Dutch 13 per 1,000. Females are more common in every country thought to be related to the nickel content in a lot of jewelry around the world.
Many substances can produce allergic contact dermatitis. Metals including Nickel, Gold, and Chromium are common. These may be experienced with jewelry, watch back plates, bands, and pant snaps or clasps.
Topical ointments can also cause the reaction. For example neomycin and bacitracin.
Other allergens include Colophony (sap, rosin or sawdust from spruce or fir trees), Cobalt, Formaldehyde, Thimersol (medicinal mercury compound), and additives to perfumes and cosmetics. Also oils from the toxicodendron plant family are notorious (poison ivy, sumac and oak).
The sequence of biochemical events and interactions that starts with a contact allergen and turns into an itchy blistered rash is complex. There are a lot of chemical players. The reaction can be thought of as occurring in two segments, the induction stage and the elicitation stage.
In the induction stage contact allergens get through the first skin layer and form protein bound complexes. Langerhans cells identify them and with the help from cytokines and chemokine’s transports the complexes through the lymphatic system to regional lymph nodes. There T cell antibody production occurs. The T cell multiplication allows a faster reaction when presented with the allergen again in the future.
The reaction occurs only at the site of allergen contact. The initial finding is itchy redness. Depending on the allergen and level of exposure it just may remain red or progress to papules (bumps), blisters and bullae (big blisters) which may rupture and weep. The erythematous (red) base may swell and become tender and slightly warm. Itching may progress to moderately severe with a burning quality. The weeping may form crusts of light honey colored material.
It is not possible to spread the rash by contact with the weeping. Only additional contact with the allergen can create more rashes.
Secondary infection is possible from scratching and inoculation from environmental bacteria.
Treatment of the problem
Treatment depends on the severity of the reaction. For mild erythema cool compresses or nothing may be required.
- Steroid creams, ointments or lotions are the mainstay of treatment. Antihistamine creams may be applied. An astringent white shake lotion or calamine lotion is frequently recommended. For weeping blisters an aluminum sulfate tetradecahydrate and calcium acetate monohydrate solution (Domeboro solution) can be applied as a compress as directed. Domeboro dries and soothes.
- Oral antihistamines may provide some comfort from itching and sedation.
- Oral steroids can be used in severe cases. They are typically given in decreasing doses.
- Immunomodulators block aspects of the allergic cascade, T-call activation for example. Topical brands include Elidel (Pimecrolimus) cream and Protopic (Tacrolimus) cream.
- Following resolution a good moisturizer should be used as the skin is often left thickened, dry and cracked.
- Phototherapy is sometimes used in chronic allergic contact dermititis. Both narrow band UV-B and psoralen plus ultraviolet-A (PPUA, psoralen is an oral photosensitizer taken before hand) are used.
Allergic versus Irritant Contact Dermatitis
Discriminating between the two can be difficult if the answer is not provided in the history. In general irritant contact dermatitis develops the rash immediately on contact as opposed to allergic which takes several days. This may be the only indication of the correct diagnosis.
In general the diagnosis of allergic contact dermatitis is made by clinical evaluation. The treatment for both allergic and irritant contact dermatitis is the same. If a severe reaction or if persistent recurrent rash presents several tests can be done to establish a definitive diagnosis.
Some tests offer information that exclude diagnosis arriving to the correct one by process of elimination.
- Potassium hydroxide test of a skin scraping can identify a fungal infection.
- Patch skin tests can identify the specific allergen(s).
- A Culture and Sensitivity (C & S) will assess for bacterial infection.
- Herpes simplex can be identified by culture of a vesicle or by serum testing (blood testing not definitive).
- Skin biopsy can also diagnose a fungal infection but gives a specific look at prevailing cell type and architecture. It can eliminate a skin T-cell lymphoma and note cells that would be congruent with allergic infiltration.
A Common Condition
There are nearly 8 1/2 million visits each year documented for allergic contact dermatitis. Understanding what it is and what to expect can help speed diagnosis and treatment.