Contact dermatitis occurs within hours of contact with the irritant or allergen.
The skin reaction persists for a few days if the skin no longer comes in contact with the allergen.

In more severe cases, allergic contact dermatitis may be followed by generalized rashes (throughout the body).
Generally, the ingestion of an allergen is harmless, but can cause:

  • The baboon syndrome (SDRIFE) that causes reddish skin on the buttocks, groin, sides of the back and upper thighs,
  • Systemic generalized contact dermatitis.

Contact dermatitis should be distinguished from a contact urticaria , in which a rashforms after a few minutes of exposure and passes in a few minutes or a few hours.
The  allergic reaction to latex is the most famous example of allergic contact urticaria.
Skin rashes caused by dermatitis can become infected if a person scratches himself many times and causes  skin lesions .



Types of dermatitis

There are two forms of contact dermatitis that are distinguished by the mechanisms that cause the inflammatory reaction, but the symptoms may be identical.

1. Irritant contact dermatitis (DIC) occurs where the irritant or toxic chemical has been applied to the skin.

2. Allergic contact dermatitis (CHD) is a itchy rash that occurs only in people who have developed an allergic sensitivity to a particular substance.
These substances are not necessarily irritating or toxic.
They can induce an immune response of the skin where direct contact occurs.
Dermatitis occurs only if there has been previous contact in the past (sensitization) with this substance. This allows the immune system to react to this substance when there is the next contact.


How does allergic contact dermatitis manifest?

Generally, dermatitis is limited to the site of contact with the allergen , although in more severe cases it may extend beyond the contact area or become generalized .
Sometimes the allergen is carried by the fingers to other parts of the body, such as:

The eyelids,
The genitals.

Some typical examples of allergic contact dermatitis are:

1. A wrist eczema under a watch strap because of nickel contact allergy.
2. An eczema in the leg when an ankle bandage is removed because of the adhesive allergy to the adhesive resin.
3. The hand dermatitis caused by chemicals used in the production of latex gloves.
4. The outside of the ear may have dermatitis because of earrings (allergic reaction to nickel).
5. Red spots on the face and itching due to allergy of contact with methylisothiazolinone, preservative in products to wash the hair and wipes.

How does irritative contact dermatitis manifest itself?

The irritant dermatitis reaction can be:

  • Acute , it looks like a burn with sudden redness and well-demarcated vesicles. It is the typical reaction caused by contact with a very toxic substance, such as an acid.
  • Chronic , it presents as a itchy skin rash and generalized skin peeling in the contact zone. The reaction is caused by repeated contact with mildly irritating substances such as soap or shampoo.


Causes of Irritant and Allergic Contact Dermatitis

The irritant contact dermatitis is the most common type (80% of cases).
It is caused by contact with:

  • Acids,
  • Alkaline materials, such as soaps and detergents, softeners,
  • Solvents or other chemicals.

Other irritants include:

1. Cement;
2. Hair dye;
3. Exposure to wet diapers can irritate the buttocks of children and newborns;
4. Pesticides or herbicides;
5. Latex gloves;
6. Some detergents, such as chlorine or skin cleansers, can also cause irritation to the genitals;
7. Shampoos and even water can cause dermatitis on the scalp.

Substances that cause irritant contact dermatitis can become allergens and cause hypersensitivity.

The allergic contact dermatitis is caused by exposure to a substance or material that the person is hypersensitive or allergic.
Contact dermatitis is not contagious.

The most common allergens are:
1. Adhesives, including those used for false eyelashes or wigs;
2. Antibiotics such as neomycin;
3. Balm of Peru (used in many personal and cosmetic products as well as in many foods and beverages);
4. Products for the treatment of fabrics and clothing, for example potassium dichromate found on the skin or leather;
5. Fragrances of perfumes, cosmetics, soaps and moisturizers;
6. Sweating , for example the intimate area may have a rash especially with synthetic fabric panties;
7. Enamel, hair dye and permanent solutions containing formaldehyde;
8. Nickel or other metals contained in:

  • Jewels,
  • Belts,
  • Metal zippers,
  • Bra hooks,
  • Coins,
  • Pocket knives,
  • Lipstick containers and compact powder base.

9. Poison ivy and other plants like:

  • Cypress grass
  • Ailanthus altissima,
  • Bosso,
  • Privet,
  • Peony,
  • Arnica.

11. Fig latex;
Rubber or latex gloves;
13. Some shoes, especially sneakers, by rubbers and resins used.

It is not possible to have an allergic reaction to a substance the first time exposure occurs, but regular use is required for a period of 5-21 days.

Photoallergic dermatitis 
Some products cause photoallergic dermatitis, that is, only with exposure to the sun.
Among them are:

  • The aftershave,
  • Solar creams,
  • Medications, such as:
    • Antibióticos (tetraciclinas, quinolonas)
    • Ointments with sulfas,
  • Some perfumes and deodorants that contain alcohol,
  • Oil of lemon peel, bergamot and lavender,
  • Products with tar and coal.

Some allergens in the air, such as ragweed or insecticide spray, can cause contact dermatitis.


Diagnosis and exams for contact dermatitis

The diagnosis is mainly based on:

  • In the appearance of the skin,
  • In the history of exposure to an irritant or allergen.

Contact Test
The contact test (or epicutaneous test) can determine which allergen is causing the reaction.
The contact test is used for some patients who have recurrences of chronic contact dermatitis.
Three consultations are required in the office and should be done by a health professional with the experience and skills to correctly interpret the results.

The reading is done between 48 and 96 hours after the application.

  1. At the first consultation , some small amounts of material composed of possible allergens are applied to the skin of the back with contenders (adhesive tapes).
  2. During the second consultation (after 48 hours) the tapes are removed to see the possible reaction.
  3. third consultation is performed about 2 days later (96 hours after application) to look for a delayed reaction.

The contact test can help distinguish irritating contact dermatitis from that of allergy:

  • The symptoms of irritative dermatitis disappear within 1-2 days,
  • Allergic dermatitis remains for many days.

Other tests may be done to rule out other possible causes, including cutaneous biopsy.

For the differential diagnosis one must consider:

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