Distribution of Skin Lesions

Although it is possible to recognize some skin eruptions by their patterns of distribution, the type and shape of the lesion, which have already been discussed, are much more reliable criteria in diagnosis. Inasmuch as the type, shape, and arrangement of lesions and their distribution pattern constitute the important tetrad in dermatologic diagnosis, it is important that the physician be acquainted with some of the more characteristic distribution patterns that are presented in the discussions of individual diseases throughout this book. Skin disorders can be classified as localized (isolated), regional, or generalized; the term total (universal) denotes involvement of the entire skin, the hair, and the nails.

As a first step in the examination of the skin, the physician should view the disrobed patient from a distance. After a survey of the entire skin surface and a close-up inspection of the type and shape of lesions, the distribution pattern can be put in perspective. For example, if eczematous patches are found on the wrist, earlobes, and neck, there is a clue to a metal contact dermatitis from a metal watchband, metal earrings, and a metal necklace.

When an eruption occurs in a bilateral and symmetric distribution, the cause is often endogenous or systemic. This pattern suggests hematogenous dissemination of the pathologic stimulus and is most often indicative of hypersensitivity reactions (e.g., drug sensitization and “allergic” vasculitis), viral exanthems, and certain other skin disorders, such as atopic eczema, dermatitis herpetiformis, etc.

In most cases, the reason for the localization of skin lesions to certain areas is unknown. A few factors, however, account for sites of predilection. Diseases caused or exacerbated by exposure to sunlight are localized to exposed areas such as the dorsa of the hands and arms, the neck, and face, a so-called “photo-distribution.” Areas of the face that are usually spared include the skin of the top part of the upper eyelids and the skin of the hair-covered scalp. Cutaneous (discoid) and systemic lupus erythematosus are predominantly localized in exposed areas but may also appear in completely light-shielded areas, such as the skin of the hair-covered scalp, ears, mouth, and feet.

Areas of minor and repeated trauma and areas where skin rubs against skin account for the distribution of lesions of epidermolysis bullosa and some of the lesions in vitiligo and psoriasis. Trauma in combination with light exposure accounts for the skin fragility and bullae localized to the backs of the hands and the face in porphyria cutanea tarda.

Hidradenitis suppurativa consists of abscesses of apocrine sweat glands and is therefore localized to axillae, nipples (in females), and anogenital areas.

Rosacea is usually confined to the “blush” area of the face, and factors that induce blushing are thought to be precipitative; these include alcoholic beverages, certain spicy condiments, hot beverages, and possibly emotional stress.

Candidiasis (moniliasis) is predominantly localized to areas where the skin is warm and moist (axillary, inframammary, and inguinal regions, the intergluteal cleft, the vaginal area, and the mouth). Candida albicans is a frequent resident of the gastrointestinal tract and reaches some of these sites by direct contact.

Herpes zoster occurs in a dermatomal pattern because the virus moves along the sensory nerves to the skin.

Lesions may be associated with openings of follicles, as in the follicular keratoses of keratosis pilaris, pityriasis rubra pilaris, and vitamin A deficiency. There is a follicular pattern of involvement in acne, lichen planopilaris, psoriasis, some drug eruptions, fungal infections (particularly Trichophyton rubrum and Trichophyton verrucosum), various forms of bacterial folliculitis, and some cases of atopic eczema.

During the time taken to survey the distribution pattern of the dermatosis, it is pertinent to review the history, occupation, various forms of exposure (e.g., to light or to airborne and contact allergens), and history of drug ingestion.