Merck Manual

Please confirm that you are a health care professional

honeypot link

Definition of Dermatitis



Thomas M. Ruenger

, MD, PhD, Georg-August University of Göttingen, Germany

Reviewed/Revised Jan 2023

The meaning of the word "dermatitis" is inflammation of the skin. However, in clinical dermatology, dermatitis is used to describe a variety of different skin conditions that share the same inflammatory reaction pattern with similar clinical manifestations.

Histologically, lymphocytes extravasate into the dermis and then migrate into the epidermis, triggering intercellular epidermal edema (spongiosis), hyperproliferation, thickening, and hyperkeratosis. The dermatopathologic term for this process is spongiotic dermatitis or eczematous dermatitis. Although different types of dermatitis may differ in some minor histologic features, they cannot be clearly differentiated by histologic features alone.

Eczema is synonymous with dermatitis but is often used, mainly by non–health care practitioners, to mean atopic dermatitis (a specific type of dermatitis). Lichenoid dermatitis and interface dermatitis are histologic terms and do not indicate dermatitis clinically.

The term "exfoliative dermatitis" should no longer be used. It was previously used to describe erythroderma. Erythroderma Erythroderma Erythroderma is defined as erythema that covers more than 70% of the body surface area. It represents the maximum severity of various skin disorders. Diagnosis is by history and examination... read more Erythroderma can be caused by a dermatitis, but it can also be caused by nondermatitis skin disorders.

Symptoms and signs of dermatitis disorders result from their histologic features:

  • Erythema (due to dermal inflammation and increased blood flow)

  • Skin thickening (due to cellular epidermal infiltrate and edema)

  • Scaling (due to hyperproliferation of the epidermis and hyperkeratosis)

  • Pruritus (possibly due to histamine released during inflammation)

  • Erosions and possibly oozing, crusting, and secondary infection, all resulting from pruritus-induced scratching

The most prominent manifestations in the acute phase of dermatitis are erythema and scaling. The most prominent manifestations in the chronic phase of dermatitis are skin thickening and lichenification.

NOTE: This is the Professional Version. CONSUMERS: View Consumer Version
quiz link

Test your knowledge

Take a Quiz!