According to WebMD, atopic dermatitis is a skin condition that causes a scaling, itching rash behind the knees and on the face, arms and legs. Itching is the primary symptom of the condition and constant scratching leads to a scaling red rash that may ooze fluid. In some cases, continual scratching also causes skin to thicken. Atopic dermatitis is commonly referred to as eczema.
The Mayo Clinic explains that atopic dermatitis symptoms can begin as early as infancy. The rashes caused by atopic dermatitis are most commonly found in areas where the skin flexes, such as behind the knees and inside the elbows. Contact dermatitis is another form of dermatitis that occurs when the skin comes in contact with irritants or allergens. Symptoms of eczema include a burning rash and blisters.
According to MedlinePlus, the most common symptoms of eczema are dry skin and an itching rash that develops on the hands, feet and behind the knees. Medical professionals diagnose eczema based on family history and physical symptoms, as there is no single test to diagnose the condition. Allergic reactions to drugs or food can cause a rash anywhere on the skin that consists of red, itching bumps. Hives are also caused by stress and typically go away on their own.