Rash or Itching

Joslyn Sciacca Kirby and Christopher J. Miller

Rash is a nonspecific term used to describe any inflammatory reaction of the skin. Inflammation of any component of the skin, including the epidermis, dermis, or subcutaneous fat can cause a rash. Rashes that involve the epidermis usually manifest surface changes, such as pustules, vesicles, or scales (Table 52.1). By contrast, inflammation of the dermis and subcutaneous fat may change the color (usually red or purple) or thickness of the skin without surface changes.

A vast array of immunologic, infectious, genetic, and nutritional etiologies can cause rashes of the face and neck. For practical purposes, this chapter limits discussion to common rashes of the face and neck. Uncommon etiologies that mimic more common rashes are discussed selectively. Identifying the primary morphology of the rash (eg, papule, plaque, blister), distribution, and secondary changes of the skin (eg, scaling or crusting) will assist the practitioner in distinguishing among the many rashes that can occur on the face and neck. This chapter is divided into three sections: (1) scaling papules, plaques, and patches; (2) vesicles, bullae, and pustules; and (3) swelling.

Scaling Papules, Plaques, and Patches (Table 52.2)

Eczematous rashes/dermatitis: The term eczema describes rashes whose primary histologic feature is intercellular edema of the epidermis. These eczematous rashes are often itchy and have a wide range of presentations.

Contact dermatitis: A form of eczema due to a substance that causes either an irritant or allergic response when applied to the skin. Irritant contact dermatitis results when substances, such as acids or alkalis, cause a direct toxic effect to the skin. Irritant contact dermatitis usually results within hours of the insult. By contrast, allergic contact dermatitis represents a delayed type IV hypersensitivity immunologic response from contact with any number of potential allergens. The immunologic response requires at least 1 to 4 days after contact before a rash arises. Common allergens causing dermatitis on the head and neck include topical antibiotics (Fig. 52.1), nickel in costume jewelry, and fragrances. Contact dermatitis frequently itches. Acute rashes usually present with small or large blisters, juicy papules, and crusting on a background of redness. Chronic dermatitis can progress to thick scaling plaques after frequent rubbing and scratching. Sharp delineation of the edges of the rash, geometric or linear configuration, and correlation with a known irritant or allergen may help diagnose contact dermatitis. Both irritant and allergic dermatitis will improve by avoiding the insulting agent.

**Table 52.1** Terms of Morphology
Term	Description	Size
Papule	Raised lesion	< 5 mm
Patch	Flat, broad lesion	> 5 mm
Plaque	Raised, broad lesion	> 5 mm
Vesicle	Small blister with clear fluid	< 5 mm
Bulla	Large blister with clear fluid	> 5 mm
Pustule	Small blister with cloudy fluid	< 5 mm