Epidermis

The outermost layer of the skin, the epidermis, is composed primarily of keratinocytes to form a stratified epithelial tissue that functions primarily to protect against the external environment and prevent water loss. The innermost layer of the epidermis consists of the basal cells at the dermal–epidermal junction. Basal cells divide to form keratinocytes, which undergo an evolution by initially flattening out to form the stratum granulosum. The keratinocytes eventually die and form the outermost barrier, the stratum corneum, which is continually replenished. The epidermis also has melanocytes, the melanin-producing cells of the skin, Langerhans’ cells, which are dendritic cells derived from macrophages and perform immune surveillance, as well as Merkel’s cells, which are mechanosensory touch receptors.

Dermis

The dermis is composed primarily of a matrix of fibroblasts that produce the support structure of the skin by synthesizing collagen and elastic fibers. The part of the dermis closest to the epidermis is the more cellular layer, the papillary layer. The papillary dermis supports unmyelinated nerve endings that transmit sensations of pain, itch, and temperature. Deep to this is the reticular layer, which is dense in collagen and elastic fibers. Both layers contain blood and lymphatic vessels. In addition, the dermis supports most of the skin’s appendages, including the hair follicle, apocrine and eccrine sweat glands, and sebaceous glands.

Subcutaneous Tissue

The subcutaneous tissue of the skin serves to conserve body heat and as a protective cushion. The subcutaneous tissue is composed primarily of adipocytes, which play important roles in glucose and fat metabolism. In addition, factors released by adipocytes contribute to wound healing, vascular remodeling, and the inflammatory and immune responses. A fibrous network anchors the adipocytes to deeper structures, such as muscular fascia and periosteum.

Primary Lesions

Primary lesions (Figure 122-2) are characterized by their diameter and depth. A macule is a flat lesion that can be seen by changes in skin color but cannot be felt. The border may be well circumscribed or may gradually blend into the surrounding skin. It may be of any size, but the term is generally used to describe lesions smaller than 1 cm. Flat lesions larger than 1 cm are termed patches. Similar to macules, papules are small (<1 cm) lesions but are palpable with the greatest mass above the surface of surrounding skin. Larger elevated skin lesions are termed plaques. Plaques may be formed by a confluence of papules or can be the primary lesion. Palpable, solitary lesions whose mass is primarily below the surface, in the dermis and subcutaneous tissue, are termed either nodules (0.5-2 cm) or tumors. Tumors may be benign or malignant.

Figure 122-2 Primary lesions.

Primary lesions are also characterized by the presence of fluid or debris-filled cavities. For example, a small (<1cm) fluid-filled lesion is termed a vesicle. Larger fluid-filled lesions are bullae. Discrete, elevated lesions that contain purulent debris are pustules. The contents may be infectious or reactive. A larger purulent collection that is palpable but may contain deeper components is an abscess.

Other primary lesions include a wheal (hive), which is a firm, elevated lesion that is secondary to dermal edema. Wheals vary in size and shape, are usually pink to red, and may be transient. A cyst is a well-circumscribed, deep lesion that is covered by normal epidermis. It may contain fluid or semisolid debris.

Secondary Lesions

Primary lesions may undergo changes caused by evolution, irritation, infection, or application of treatments. Although it is important to recognize the secondary lesions, these changes often have less diagnostic utility than the primary lesion. Scales are layers of the stratum corneum that have desquamated but still remain attached to the skin surface. Crusts are thick accumulations of cellular debris, blood, pus, or serum. Erosions are superficial (involving only the epidermis) losses of tissue, resulting in depression of the surface. They generally heal without scar formation. In contrast, deeper tissue loss that extends into the dermis and even subcutaneous tissue is called an ulcer. Ulcers often heal with scarring. Ulceration or erosions that are linear and result from scratching are termed excoriations. Fissures form at sites of chronic inflammation. They are sharply demarcated, linear disruptions in the epidermis with extension into the dermis. Lichenification is a marked thickening of the epidermis that results in exaggeration of skin markings (Figure 122-3). Lichenification is a result of chronic irritation caused by inflammation, rubbing, or scratching.

Figure 122-3 Flexural lichenification.

Healing of lesions may result in secondary changes as well. For example, atrophic changes of the epidermis or dermis result in depressions in the skin surface. Epidermal atrophy is demonstrated by very thin, nearly translucent skin with loss of markings. Such areas are susceptible to mechanical damage. Dermal atrophy manifests as depressions with overlying normal skin color and markings. Scars may form at the sites of injury. A scar is a permanent change in the skin after an injury that results in fibrosis. Scars may be hypertrophic, which is an exaggerated response to skin damage that remains within the boundaries of the original injury. Hypertrophic scars are differentiated from keloids, which continue to grow long after the injury and can grow well outside the boundary of the initial insult.

Color of Lesions

Another important characteristic of lesions is how they compare with the patient’s normal skin color. Lesions that are darker than the surrounding skin may be hyperpigmented. Inflammation can lead to hyperpigmentation. Other examples of hyperpigmentation include nevi, transient pustular melanosis of the newborn, and café-au-lait spots (see Chapters 124 and 126). Increased dermal pigmentation often results in a bluish discoloration of lesions, such as seen in dermal melanocytosis (colloquially termed Mongolian spots). Lesions may also be hypopigmented, such as postinflammatory hypopigmentation, tinea versicolor, or ash leaf spots of tuberous sclerosis. Depigmented lesions have lost all pigment and can be differentiated from hypopigmented lesions by Wood’s lamp examination. An example of depigmented disorders is vitiligo. Whereas lesions that are pink to red may be inflammatory in origin, more intense red to purple lesions are often vascular.

Configuration of Lesions

Additional diagnostic information can be gained from correct identification of the configuration or shape of individual or grouped lesions. Helpful terminology not only provides a specific description of the lesions’ shape but also helps indicate the underlying process. Whereas discrete lesions describe those that remain as distinct and separated from surrounding areas of disease, confluent describes lesions that have coalesced or merged. Clustered lesions are grouped on one area. Linear lesions occur in a band or line and suggest a reactive dermatitis, such as poison ivy, or may follow developmental lines, such as an epidermal nevus.

Specific description of the shape of lesions provides further information. To label a lesion round gives little specific information, but describing it as annular (Figure 122-4) specifies a ring-shaped lesion with a raised or erythematous border and central clearing. Common examples of annular lesions include tinea infections, erythema migrans, and granuloma annulare. Discoid lesions are also round but tend to be more solid in nature. Other round lesions include nummular (Figure 122-5), targetoid (containing-concentric rings), and guttate (droplike) lesions; the latter is commonly used to describe a form of psoriasis in children that occurs after acute streptococcal infection. Umbilicated lesions have a central depression; common examples include molluscum contagiosum and varicella.

Figure 122-4 Annular lesion.

Figure 122-5 Nummular eczema.

Serpiginous describes lesions that have linear and curving elements as though following the track of a snake. Reticulated lesions have a netlike or lacy configuration, such as cutis marmorata or livedo reticularis. Morbilliform is a term that refers to a measles-like eruption. It consists of red macules and papules that may be discrete or confluent on large areas of the body surface. Common examples of morbilliform eruptions include Kawasaki’s disease and drug eruptions. Lesions that have a variety of shapes are described as multiform.

Distribution of Lesions

Many cutaneous lesions have predilections for specific areas. Although these are discussed in more detail with specific disease processes, some generalizations about lesion distribution may be helpful in initial diagnosis. Linear eruptions may follow a dermatomal pattern, thus affecting the skin supplied by a single spinal nerve. Linear lesions may also follow lines of Blaschko, lines of embryologic development of the skin (Figure 122-6). These cutaneous lesions represent a form of genetic mosaicism; examples include epidermal nevi, lichen striatus, and some congenital disorders. Lesions that arise only in sun-exposed areas may represent a disorder of photosensitization or one precipitated by sun exposure.

Figure 122-6 Linear lesions.