Definition and Epidemiology

Contact dermatitis is a type of rash that falls under the umbrella diagnosis of eczematous dermatitis, one of several types of dermatitis that share a similar “ill-defined scaling”¹ and histology. Others in the category include atopic dermatitis, nummular and dyshidrotic eczema, and id reactions. On histologic examination, they are characterized by spongiosis (intercellular edema). Contact dermatitis is further classified as irritant or allergic type.

Irritant contact dermatitis (ICD) is caused by the direct cytotoxic action of an agent on the cells of the epidermis and dermis. Cellular destruction and injury of the epidermal barrier result in inflammation and the nonimmunologic release of vasoactive peptides and proinflammatory cytokines. ICD is responsible for 80% of occupational contact dermatitis, most commonly on the hands. Irritants are mostly chemical in nature and include soaps, detergents, acids, and alkalis. Dermatitis may not develop until a threshold of exposure has been reached. Itching may be present, but the predominant symptoms are of pain and burning.

Allergic contact dermatitis (ACD) is a type 4 delayed hypersensitivity reaction, requiring prior sensitization for symptoms to be induced. After contact, the allergen combines with epidermal proteins that are taken up by Langerhans cells. T cells and macrophages then relay this information to precursor T cells in the lymph nodes, where sensitized T cells are produced.² Sensitization takes 10 to 15 days. Once an individual is sensitized, the dermatitis begins to erupt in 8 to 48 hours and, untreated, can persist for days or weeks.³ The most common allergens according to the North American Contact Dermatitis Group⁴ analysis of pooled data from January 1, 2009 to December 31, 2010 include substances such as nickel, neomycin, and bacitracin. In this report, the incidences of skin reactions to substances such as nickel and neomycin decreased, but reactions to allergens such as fragrances, propylene glycol, and benzocaine were reported more frequently. Workers in specific occupations associated with higher-than-average rates of ACD included hairdressers, printers, cement workers, painters, mechanics, food processors, and florists, among others.⁵

Clinical Presentation and Physical Examination

The distribution of the dermatitis is an important clue to the diagnosis. Both ICD and ACD cause a rash on the surface of the skin that was exposed. In ICD, symptoms may develop minutes to hours after exposure, or they may develop months after chronic exposure. A well-demarcated area of erythema, scaling, or crusting will occur at the site of the exposure. The hands are the most common area affected, followed by wrists and forearms.³ Contact dermatitis of the hand is more likely to be of the irritant than allergic type.⁶

Cumulative ICD is more common than the acute form and is characterized by hyperkeratosis (thickening), lichenification (accentuation of skin lines as a result of scratching), scaling, and fissuring. Weak irritants produce cumulative ICD, which may develop after months of continual exposure.⁵ Most reactions to cosmetics are of the irritant type. The dermatitis may occur on the face, eyelids, lips, and neck, often having been exposed by hand-to-face contact. Patients report burning and stinging sensations associated with the application of cosmetics.⁷

ACD is identified by its distribution, the acute nature of the symptoms, the complaint of itch, and the development of inflammation and vesicles. Rhus dermatitis (poison ivy or oak) is caused by a plant resin called urushiol. Exposure results in vesicles and bullae that develop for up to 2 weeks after contact and can last for up to 8 weeks if treatment is insufficient.⁸ To the patient, the dermatitis may seem to be spreading because it may not show up all at once. Once the resin is washed off, however, no further spread should occur, although full presentation may take several days.

Sensitivity to nickel can manifest as dermatitis around the neck from jewelry, around the umbilicus from metal waist fasteners, or under the breasts from bras with underwires. ACD of the hand can occur on the dorsum from allergy to rubber or gloves and on the palm from solid objects such as the leather grip of a tennis racket. Dermatitis that affects the hands and feet together may indicate an endogenous cause, such as a dietary metal sensitivity, but it could also represent other disorders, such as psoriasis or dermatomyositis.⁹ Pedal involvement without hand involvement suggests an ACD reaction to footwear, often resulting from allergy to the resins used in the leather tanning process. Table 48-1 lists possible irritants or allergens by anatomic location.

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