Atopic vs. Contact Dermatitis: How They Differ

Atopic dermatitis and contact dermatitis are both very common types of eczema—a skin condition that can cause itchy, scaly, inflammatory rashes. While their symptoms are similar, the two have very different causes.

Atopic dermatitis is a chronic skin condition characterized by inflammation of the skin (dermatitis). Most cases of atopic dermatitis are thought to occur due to a combination of genetic and environmental factors.

Contact dermatitis develops when the skin comes in contact with something that triggers a reaction. Properly identifying the type of eczema is key to getting the correct treatment.

In some cases, the difference between the two is quite obvious; in other cases, it is not. Some patients can even have both atopic and contact dermatitis at the same time, making assessment more difficult.

Symptoms

Both atopic and contact dermatitis can go through eczema's three different phases.

During the acute phase, the first of the three, both types of dermatitis cause a red, itchy rash that may ooze or weep clear fluid. With contact dermatitis, small, fluid-filled blisters (called vesicles) are likely to develop, while weeping plaques (broad, raised areas of skin) are more common with atopic dermatitis. And while both conditions are extremely itchy during this phase, contact dermatitis is more likely to also cause pain and burning. If a case shows some distinction, it usually occurs in this phase.

It's during the next phase, the sub-acute phase, that atopic dermatitis and contact dermatitis are particularly hard to tell apart. In both cases, the rashes are rough, dry, and scaly, often with superficial papules (small, red bumps).

In both cases, the chronic stage is characterized by lichenification, a scaly, leathery thickening of the skin that occurs as a result of chronic scratching.

Given that these phases are not concrete and any contrasts may or may not be pronounced, telling contact dermatitis from atopic dermatitis based on the look of the rash alone can be a challenge. That's where some additional considerations come into play.

Location Differences

The location of the eczema rash 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), the front of the neck, folds of the wrists, ankles, and behind the ears.

Since atopic dermatitis begins as an itch that, when scratched, results in a rash, it makes sense that the locations easiest to scratch are those that are affected. The flexural areas are most often involved in older children and adults, but less so in babies, simply because they have trouble scratching these particular spots. In contrast, very young children tend to get atopic dermatitis on the face, the outside elbow joints, and the feet.

On the other hand, contact dermatitis occurs at the site of an allergen exposure, and therefore can be virtually anywhere on the body. These are often areas that aren't typically affected by atopic dermatitis; for example, on the stomach (due to nickel snaps on pants), under the arms (from antiperspirants), and on the hands (from wearing latex gloves).

Age Symptoms Appear

The age of a person experiencing an eczematous rash can be an important distinction between the two conditions as well. Most people who develop atopic dermatitis are 5 years of age or younger, while contact dermatitis is less common in young children.

While atopic dermatitis can appear for the first time in adulthood, contact dermatitis is much more common in adults.

While not a symptom itself, age can help put symptoms in context.

Causes

Perhaps the most significant difference between atopic and contact dermatitis is a person's susceptibility.

Atopic Dermatitis Mechanism

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 of 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 Mechanism

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, poison ivy, or poison sumac (approximately 80% to 90% of people react to contact with these plants). Contact dermatitis is also common when exposed to nickel, cosmetic agents, and hair dye.

Contact dermatitis isn’t caused by an allergic process, but as a result of T-lymphocyte-mediated delayed-type hypersensitivity.

Since it is not caused by the same allergic process as atopic dermatitis, contact dermatitis will not spread when you scratch it. Contact dermatitis should only affect the part of your skin that came in contact with the irritant. If the rash does spread, you are likely dealing with an allergic reaction.

Diagnosis

Despite similarities between the rashes, both atopic dermatitis and contact dermatitis are primarily diagnosed by visual inspection and review of a thorough medical history. Age of the person affected and the location of the rash, along with your healthcare provider's trained eye, are used to help differentiate between the two conditions.

In some instances, testing may be necessary.

The diagnosis of atopic dermatitis involves the presence of eczema rash, the presence of itching (pruritus), and the presence of allergies. Allergies are common in those with atopic dermatitis and can be diagnosed using skin testing or blood testing. There is no specific test to diagnose atopic dermatitis, however.

The diagnosis of contact dermatitis involves the presence of eczema rash, which is usually itchy, and the ability to determine the trigger with the use of patch testing.

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

Treatment

Treatment for both atopic and contact dermatitis is similar, with the goal of reducing inflammation and itching and preventing future breakouts.

Keeping the skin well-moisturized is recommended for both conditions, but it's critical for atopic dermatitis. Regular application of creams or ointments helps reduce and prevent flares. Moisturizing can help soothe the skin during an active contact dermatitis flare-up, but it will not prevent contact dermatitis.

Petroleum jelly, such as Vaseline, can be used to moisturize skin affected by dermatitis. Other moisturizers like Eucerin, CeraVe, or Aquaphor can also be helpful—just be sure to choose a bland product that does not contain any fragrances, essential oils, or other ingredients that could further irritate your skin.

Medications used to treat the conditions are similar as well, but there are differences in when and how they're used.

• Topical steroids: A mainstay of treatment for both atopic dermatitis and contact dermatitis, these medications reduce inflammation, irritation, and itching. Over-the-counter hydrocortisone is helpful for mild cases, while prescription steroids may be needed in others.
• Oral steroids: These drugs may be used in cases of contact dermatitis where the rash is severe or widespread. Oral steroids are rarely used for atopic dermatitis.
• Antihistamines: Although they don't clear up the rash in either condition, oral antihistamines can help relieve itching for some people.
• Phototherapy: Sometimes light therapy is used for adults with difficult-to-treat dermatitis.
• Topical calcineurin inhibitors: Elidel (pimecrolimus) and Protopic (tacrolimus) are nonsteroidal topical medications often used to treat atopic dermatitis in those ages 2 and older. They aren't often used for contact dermatitis, except in severe cases or in those who haven't responded to other treatments.
• Interleukin-4 receptor antagonists: Dupixent (dupilumab) is a biologic approved for the treatment of moderate-to-severe atopic dermatitis (not adequately controlled by topical prescription medications) in those six months and older. However, this medication is not approved to treat contact dermatitis.
• Dilute bleach baths: These are recommended in certain cases to help reduce Staphylococcus aureus bacteria on the skin. Dilute bleach baths may help improve atopic dermatitis but are generally not recommended for contact dermatitis. Evidence of their effectiveness is mixed; a 2018 review study found that bleach baths improved symptoms of atopic dermatitis. A 2017 review found bleach baths did decrease the severity of atopic dermatitis, but that plain water baths were just as effective.