Dermatitis is an inflammatory condition of the skin caused by outside agents (compare eczema, an endogenous disease in which such agents do not play a primary role). Irritant contact dermatitis may occur in anyone who has sufficient contact with such irritants as acids, alkalis, solvents, and (especially) detergents. It is the commonest cause of occupational dermatitis in hairdressers, nurses, cooks, etc.
In allergic contact dermatitis skin changes resembling those of eczema develop as a delayed (type IV) reaction to a particular allergen, which may be present at low concentrations. Common examples include nickel dermatitis (from costume jewellery, clothing fasteners, zips, etc.) and fragrance allergy (from toiletries, deodorants, perfumes, etc.). Treatment of dermatitis depends on removing the cause, which is not always possible.
Dermatitis herpetiformis is an uncommon very itchy rash with symmetrical blistering, especially on the knees, elbows, buttocks, and shoulders. It is associated with gluten sensitivity and responds well to treatment with dapsone or a gluten-free diet.
Dermatitis symptoms vary with all different forms of the condition. They range from skin rashes to bumpy rashes or including blisters. Although every type of dermatitis has different symptoms, there are certain signs that are common for all of them, including redness of the skin, swelling, itching and skin lesions with sometimes oozing and scarring. Also, the area of the skin on which the symptoms appear tends to be different with every type of dermatitis, whether on the neck, wrist, forearm, thigh or ankle. Although the location may vary, the primary symptom of this condition is itchy skin. More rarely, it may appear on the genital area, such as the vulva or scrotum. Symptoms of this type of dermatitis may be very intense and may come and go. Irritant contact dermatitis is usually more painful than itchy.
Although the symptoms of atopic dermatitis vary from person to person, the most common symptoms are dry, itchy, red skin. Typical affected skin areas include the folds of the arms, the back of the knees, wrists, face and hands.
Dermatitis herpetiformis symptoms include itching, stinging and a burning sensation. Papules and vesicles are commonly present. The small red bumps experienced in this type of dermatitis are usually about 1 cm in size, red in color and may be found symmetrically grouped or distributed on the upper or lower back, buttocks, elbows, knees, neck, shoulders, and scalp. Less frequently, the rash may appear inside the mouth or near the hairline.
The symptoms of seborrheic dermatitis on the other hand, tend to appear gradually, from dry or greasy scaling of the scalp (dandruff) to hair loss. In severe cases, pimples may appear along the hairline, behind the ears, on the eyebrows, on the bridge of the nose, around the nose, on the chest, and on the upper back. In newborns, the condition causes a thick and yellowish scalp rash, often accompanied by a diaper rash.
Perioral dermatitis refers to a red bumpy rash around the mouth.

A number of health conditions, allergies, genetic factors and irritants can cause different types of dermatitis:

Atopic dermatitis (eczema). This form of dermatitis is likely related to a mix of factors, including dry skin, a gene variation, an immune system dysfunction, bacteria on the skin and environmental conditions.
Contact dermatitis. This condition results from direct contact with one of many irritants or allergens — such as poison ivy, jewelry containing nickel, cleaning products, perfumes, cosmetics, and even preservatives in many creams and lotions.
Seborrheic dermatitis. This condition may be caused by a yeast (fungus) that is in the oil secretion on the skin. People with seborrheic dermatitis may notice their condition tends to come and go depending on the season.
Diagnosis of eczema is based mostly on the history and physical examination. However, in uncertain cases, skin biopsy may be useful.Those with eczema may be especially prone to misdiagnosis of food allergies.
Patch tests are used in the diagnosis of allergic contact dermatitis.

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