Atopic Dermatitis Versus Contact Dermatitis

How to Tell the Difference

Eczema can be caused by different forms of dermatitis, most commonly atopic and contact dermatitis.

I see many patients in my office for both atopic dermatitis and contact dermatitis. In many instances, the difference between these two diseases is quite obvious; in other cases, not so obvious. Some of my 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 Between Atopic and Contact Dermatitis

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 – acute, sub-acute and chronic. Acute eczema is characterized by itchy blisters (vesicles) on inflamed, red skin. Sub-acute eczema is an itchy, dry, flaky, crusting or oozing of the skin. Lastly, 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 change of the epidermis, which is a swelling of the epidermal skin cells and appears like a sponge under a microscope. Therefore, a skin biopsy will not differentiate between these two conditions.

Differences Between Atopic and Contact Dermatitis

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

A person with atopic dermatitis often has a genetic mutation in a protein in the skin called filaggrin. A mutation in filaggrin results in a breakdown in the barriers between epidermal skin cells, leading to dehydration of the skin as well as the ability for aeroallergens (such as pet dander and dust mite) to penetrate the skin.

These 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 resulting in allergic inflammation in the skin.

Without these underlying propensities (filaggrin mutation and/or propensity for allergies), 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. Contact dermatitis occurs in the majority of the population to poison oak (approximately 90%), and is also common 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 5 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 of Atopic Versus Contact Dermatitis

The diagnosis of atopic dermatitis involves the present 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. 2006;97:S1-38.

Atopic Dermatitis Practice Parameters. Ann Allergy Asthma Immunol. 2004;93:S1-21.

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