Atopic Dermatitis vs. Contact Dermatitis

woman scratching her back
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In many instances, the difference between atopic dermatitis and contact dermatitis is quite obvious. In other cases, not so obvious. Some patients can even have both atopic and contact dermatitis at the same time, which is even more difficult to assess. While there are many similarities between these two common conditions, there are some important differences especially when it comes to the causes of the dermatitis.

Similarities

Both atopic and contact dermatitis are forms of eczema. Eczema is not a diagnosis. Rather, eczema refers to the actual rash that occurs as a result of these conditions. There are generally three different phases of eczema:

  1. Acute eczema is characterized by itchy blisters (vesicles) on inflamed, red skin.
  2. Sub-acute eczema is an itchy, dry, flaky, crusting, or oozing of the skin.
  3. Chronic eczema is characterized by lichenification, a leathery thickening of the skin that occurs as a result of chronic scratching.

A skin biopsy of both atopic and contact dermatitis will show similar features—namely spongiotic changes of the epidermis, a swelling of the epidermal skin cells that appears like a sponge under a microscope. Therefore, a skin biopsy will not differentiate between these two conditions.

Differences

There are many important differences between atopic and contact dermatitis, with the most important being the susceptibility of a person to develop the condition.

A person with atopic dermatitis often has a genetic mutation in a protein in their skin called filaggrin.

A mutation in filaggrin results in a breakdown in the barriers between epidermal skin cells. This leads to dehydration of the skin as well as the ability for aeroallergens, like pet dander and dust mites, to penetrate the skin.

Such aeroallergens result in allergic inflammation and a strong itching sensation. Scratching further disrupts the skin and causes more inflammation and more itching. 

An underlying propensity for allergy can also cause eczema to develop as a result of eating a food to which a person is allergic, causing T-lymphocytes (a type of white blood cell) to migrate to the skin and result in allergic inflammation. Without these underlying propensities, a person is unlikely to develop atopic dermatitis.

Contact dermatitis, on the other hand, is due to a reaction to a chemical exposure directly on the skin. It occurs among a majority of the population from interaction with poison oak (approximately 90 percent) and is also common when exposed to nickel, cosmetic agents, and hair dye. A person still must have the ability of their T-lymphocytes to recognize a chemical as a foreign substance and react to it in order to develop contact dermatitis.

The age of a person experiencing atopic dermatitis is an important distinction between these two conditions. Most people developing atopic dermatitis are five years of age or younger, while contact dermatitis is less common in young children.

The location of the eczema is an extremely important clue when differentiating between atopic and contact dermatitis.

Atopic dermatitis most classically involves the flexural locations of the skin, such as the folds of the elbows (antecubital fossa), behind the knees (popliteal fossa), front of the neck, folds of the wrists, ankles, and behind the ears.

The flexural areas are most often involved in older children and adults because these areas are easiest to scratch. Since atopic dermatitis is an itch, that when scratched, results in a rash, it makes sense that the locations easiest to scratch will be the areas that develop a rash. On the other hand, contact dermatitis occurs at the site of chemical exposure, and therefore can virtually be anywhere on the body.

Diagnosis

The diagnosis of atopic dermatitis involves the presence of eczema, the presence of itching (pruritus), and the presence of allergies. Allergies are diagnosed using skin testing or blood testing, and therefore atopic dermatitis is an allergic rash.

The diagnosis of contact dermatitis involves the presence of eczema, which is usually itchy, and the ability to determine the trigger with the use of patch testing. Contact dermatitis isn’t caused by an allergic process, but as a result of T-lymphocyte mediated delayed type hypersensitivity.

Regardless if the eczema is from atopic dermatitis or contact dermatitis, identifying and avoiding the cause if the main treatment modality.

Sources:

Beltrani VS, Bernstein IL, Cohen DE, Fonacier L. Contact Dermatitis: A Practice Parameter. Ann Allergy Asthma Immunol; 97:S1-38.

Atopic Dermatitis Practice Parameters. Ann Allergy Asthma Immunol; 93:S1-21.

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