A burrow (Fig. 2-2) is an elevated channel in the superficial epidermis produced by a parasite, such as the mite Sarcoptes scabiei. Scabies burrows characteristically are located on the wrists and in fingerwebs; the diagnosis is confirmed by demonstrating the mite microscopically in skin scrapings. The human hookworm may also produce a serpiginous burrow; however, demonstrating this organism is much more difficult.

14. What is a comedo?

A comedo (Fig. 2-3) is a folliculocentric collection of sebum and keratin. Comedonal acne characteristically consists of both open (blackheads) and closed comedones (whiteheads). When the contents of a closed comedo are exposed to air, a chemical reaction occurs, imparting the black color of an open comedo.

15. What is the difference between petechiae and purpura?

The size of the lesion is the major difference. Both petechiae and purpura result from extravasation of red blood cells into the dermis; hence, they do not blanch with pressure. Petechiae, however, are much smaller than purpura; they are <5 mm in diameter.

16. What are targetoid (target) lesions?

Targetoid lesions typically consist of three zones. The first zone consists of a dark or blistered center (bull’s-eye) that is surrounded by a second, pale zone. The third zone consists of a rim of erythema. Target lesions classically are found in patients with erythema multiforme (Fig. 2-4).

17. List some of the additional descriptive adjectives used in dermatology that refer to color or pigmentation.

• Depigmented: Absence of melanin, a lack of color. Depigmented macules and patches are commonly found in vitiligo.

• Hypopigmented: Lighter than normal skin color; normal number of melanocytes but decreased production of melanin by the melanocytes. The ash-leaf macule of tuberous sclerosis is an example of a hypopigmented macule.

• Hyperpigmented: Darker than normal skin color. Junctional nevi and café-au-lait macules (neurofibromatosis) are hyperpigmented macules.

• Erythematous: Showing redness of the skin.

18. How do atrophy and lichenification differ?

Atrophy (Fig. 2-5) is thinning of the epidermis, dermis, or subcutis (fat). Epidermal atrophy leads to a fine, cigarette-paper wrinkling of the skin surface, whereas dermal and fat atrophy cause a depression in the skin surface.

Table 2-1. Primary Skin Lesions

Table 2-2. Secondary Skin Lesions

SECONDARY LESION	DEFINITION	MORPHOLOGY
Crust	A collection of cellular debris, dried serum, and blood; a scab Antecedent primary lesion is usually a vesicle, bulla, or pustule

Erosion A partial focal loss of epidermis; heals without scarring

Ulcer A full-thickness, focal loss of epidermis and dermis; heals with scarring

Fissure Vertical loss of epidermis and dermis with sharply defined walls; crack in skin

Excoriation Linear erosion induced by scratching

Scar A collection of new connective tissue; may be hypertrophic or atrophic
Scar implies dermoepidermal damage

Scale Thick stratum corneum that results from hyperproliferation or increased cohesion of keratinocytes

Figure 2-1. Telangiectasia.

Figure 2-2. Scabies.

Figure 2-3. Comedo.

Figure 2-4. Targetoid lesions. Patient with classic erythema multiforme demonstrating lesions resembling a “bull’s-eye” Note that some lesions demonstrate a small, centrally placed blister.

(Courtesy of the Fitzsimons Army Medical Center teaching files.)

Figure 2-5. Atrophy.

A lichenified lesion (Fig. 2-6) is a focal area of thickened skin produced by chronic scratching or rubbing. The skin lines are accentuated, resembling a washboard (Fig. 2-7).

19. Do skin diseases have characteristic arrangements or configurations?

Some, but not all, cutaneous diseases demonstrate characteristic arrangements or configurations of lesions. Commonly used adjectives include:

• Annular: Used to describe lesions that are ring shaped. Annular plaques are typical findings in granuloma annulare, tinea corporis (ringworm), and erythema marginatum.

• Gyrate: From the Latin gyratus, which means “to turn around in a circle,” gyrate skin lesions are rare presentations. Gyrate erythema that resembles wood grain or topographic maps is seen in erythema gyratum repens, which usually heralds the presence of an internal malignancy.

• Dermatomal: Used to describe lesions that follow neurocutaneous dermatomes. The classic example is herpes zoster (shingles), which demonstrates grouped vesicles on an erythematous base in a dermatomal distribution.

• Linear: More than 20 diseases may demonstrate linear configurations. One example is allergic contact dermatitis to poison ivy; it characteristically demonstrates linear erythematous papules or vesicles.

• Grouped: Papules, pustules, or blisters (vesicles or bullae) may demonstrate grouped configurations. A typical example is herpes simplex, which demonstrates grouped vesicles on an erythematous base.

20. What is the Koebner phenomenon?

Traumatizing the epidermis of a patient with a certain preexisting skin disease will cause the same skin disease to form in the traumatized skin. Noticing this skin finding is helpful when creating a differential diagnosis. Only certain diseases are associated with a Koebner phenomenon; lichen planus, lichen nitidus, and psoriasis (Fig. 2-8) are examples.

21. Do skin diseases characteristically occur in certain locations?

Yes. This is the reason that a complete skin examination should be performed on all patients. Seborrheic dermatitis characteristically occurs on the scalp, nasolabial folds, retroauricular areas, eyelids, eyebrows, and presternal areas; it tends to spare the extremities. Psoriasis may resemble seborrheic dermatitis, but it characteristically demonstrates a different distribution, usually involving the extremities (elbows, knees), intergluteal fold, scalp, and nails.