New rash on the right hand and neck
Diagnosis: contact dermatitis
The patient demonstrated how she applied perfume to her neck using the back of her right hand. She has acute contact dermatitis on top of previously undiagnosed eczema. Her history is positive for contact dermatitis in reaction to metal (probably nickel) in necklaces and to various fabrics.
Contact dermatitis is a common inflammatory skin condition in which erythematous and pruritic skin lesions result from contact with a foreign substance. The term describes 2 diagnostic categories—allergic contact dermatitis and irritant contact dermatitis. In this case, the chemicals in the perfume may have acted as irritants. The alcohol she used to treat the rash was probably an irritant as well.
A hypersensitivity reaction to irritants and allergens
The mechanism of allergic contact dermatitis is a delayed hypersensitivity reaction. A foreign substance comes into contact with skin and links to skin protein, forming an antigen complex recognized by the immune system. In the skin, the major antigen-presenting cell is the Langerhans cell. When the epidermis is re-exposed to the antigen, the sensitized T-cells initiate an inflammatory cascade leading to changes in the skin. The reaction generally occurs within 12 to 48 hours in a sensitized person. The differential diagnoses for this patient include atopic dermatitis, psoriasis, and fungal infection.
Cosmetics, perfumes, and chemicals in cleaning solutions are known to cause irritant contact dermatitis. The most typical locations are hands or feet, with the dorsum affected more often than the palm or the sole. The lesions are usually erythematous and not well demarcated. The lesions may be macular, papular, or vesicular.
Allergic contact dermatitis often occurs after contact with allergens such as nickel in jewelry, or resins in plants such as poison ivy or poison oak. Vesicles may appear, and the hands are most commonly affected. The lesions may form a linear pattern, as in the case of plant dermatitis.
