Chapter 19 The Skin
Dermatology is a visual discipline, and it relies on accurate macroscopic and microscopic description of skin lesions. Thus it is important to pay attention to the basic characteristics of each lesion, including its:
In clinical practice, it is customary to classify skin lesions as primary or secondary. Primary skin lesions include macules, papules, patches, plaques, nodules, cysts, wheals, vesicles, bullae, and pustules. Secondary skin lesions are related to the progression of the disease but may also be caused by scratching, trauma, or treatment. This group of lesions includes crusts, erosions, ulcers, fissures, excoriations, scars, and scales (Figs. 19-1 and 19-2).
Figure 19-1 Primary skin lesions.
(From Fitzpatrick JE, Aeling JL: Dermatology Secrets. Philadelphia, Hanley & Belfus, 1996, pp. 9–10.)
Macules are flat lesions presenting as a change in the color of skin, whereas papules are slightly elevated. For example, a freckle (ephelis) is a macule, whereas a congenital mole (intradermal nevus) is a papule. Some childhood exanthemas, such as measles, present in the form of a widespread maculopapular rash—that is, red macules, which are flat, and papules, which are slightly raised. Very large macules are called plaques.
Papules are small, slightly elevated solid skin lesions measuring less than 0.5 cm. In practice, this means they can be covered with the flat end of a pencil. Nodules are deeper seated and larger, measuring more than 0.5 cm in diameter. Most skin tumors present as nodules, but obviously some can be diagnosed early in their development while they are still smaller than 0.5 cm. (Continued on p. 390)
Like papules, plaques are slightly elevated skin lesions, but they measure more than 0.5 cm in diameter. Plaques often arise by a confluence of multiple papules. In contrast to nodules, plaques appear relatively flat and do not have a deeper dermal component. Plaques are typical of eczema, a common skin lesion that may have many mechanisms.
Wheals are solitary or grouped, raised, soft skin lesions. Wheals are larger than papules and more elevated than a plaque. In contrast to nodules, which are firm and develop over a longer period of time, wheals are soft, edematous, and of recent onset. Wheals are often pruritic, red or pale, and often tend to disappear without any consequences. Wheals are typically caused by allergies. Dermal edema due to a release of histamine from mast cells is the underlying cause of wheals in dermographism.
Vesicles are slightly raised skin lesions filled with fluid. Grouped vesicles are typical of herpes simplex infection. Larger vesicles measuring more than 0.5 cm are called bullae. In common parlance, bullae are called blisters.
Cysts are cavities lined by an epithelial layer. Cysts develop from invaginations of surface epithelium or obstruction of excretory ducts of skin appendages. They may contain fluid and thus have some common features with bullae. However, bullae do not have a defined epithelial lining and typically develop fast, whereas cysts develop over a prolonged period of time. Epidermal inclusion cysts filled with desquamated epithelium and keratin may resemble nodules.
Erosions are superficial defects of the epidermis above the epidermodermal basement membrane. Ulcers are deeper defects involving both the epidermis and the dermis. Erosions heal without scarring, whereas the healing of ulcers involves granulation tissue and scar formation. Linear erosions caused by scratching are called excoriations.
Like all other diseases, skin disease can be classified as acute or chronic. Acute diseases may heal, recur, or progress to chronic diseases. Chronic diseases can be preceded by an acute stage, but many of them begin insidiously without a distinct acute phase, lasting without an obvious end. The best example of such a chronic disease, psoriasis, is a common chronic disease that has no obvious beginning and rarely, if ever, any end.
Skin diseases may be localized and limited to an area of the body, or they may involve the entire body. For example, bacterial folliculitis may be limited to the face, whereas viral exanthemas, such as measles, involve the entire body. Many systemic diseases, such as systemic lupus erythematosus (SLE), have skin manifestations and at the same time involve many internal organs.
Pathologists can classify most skin diseases into several categories, which then must be correlated with clinical data to arrive at the final clinical–pathologic diagnosis. General categories of skin diseases recognized in skin biopsies are too broad. For example, chronic dermatitis may be caused by immune mechanisms, infections, or irritation. It is thus common to classify skin diseases according to the way they involve the skin. These categories include:
The clinical term eczema (from the Greek word meaning to “boil over”) is probably the best example of the vagueness of some dermatologic terms. In acute stages, it presents in the form of erythematous, oozing, or crusted papules, which then become crusted in the chronic stages. Eczema is a reaction pattern and may be a manifestation of:
Figure 19-3 Herpetic vesicles. The vesicles (bullae) are intraepidermal. In addition to fluid, these vesicles contain enlarged keratinocytes, which are typically multinucleated and have “ground-glass” nuclei filled with viral particles (inset). Viral particles can be seen only by electron microscopy, but they can also be demonstrated by immunohistochemistry with antibodies to herpes virus.
Human papillomaviruses are DNA viruses that are classified into more than 70 subgroups. Some of these viruses have a predilection for certain anatomic sites, causing clinically identifiable skin lesions known as verrucae or warts (Fig. 19-4).
Figure 19-4 Human papillomavirus-induced lesions. A, Verruca vulgaris. Histologically, the lesion shows papillomatosis (i.e., formation of papillae that have a central vascular connective tissue core covered with epithelium), acanthosis (i.e., thickening of the epidermis), thick granular layer, and surface hyperkeratosis. B, Plantar wart is flat and shows hyperkeratosis and acanthosis.
Impetigo is a superficial skin infection caused by gram-positive bacteria. Staphylococcus aureus is the most common cause. Most often, the disease presents in the form of superficial pustules on the faces of small children. Histologically, the pus accumulates in the upper part of the epidermis. The pustules rupture easily and heal without scarring.
Folliculitis is a bacterial infection of the hair follicles characterized by an accumulation of pus around the hair shaft. Infection is most often caused by S. aureus and involves hairy areas of the skin, such as the buttocks and thighs, bearded part of the face, and the scalp. Suppuration extending into the perifollicular soft tissue may lead to formation of abscesses, which are known as furuncles or boils. Confluent boils form indurated masses that are dark bluish black and are thus known as carbuncles (name derived from a cognate for carbon). See Fig. 19-5.
Figure 19-5 Skin lesions caused by purulent bacteria. A, Impetigo is characterized by the formation of intraepidermal pustules. B, Folliculitis is an inflammation of hair follicles. Pus fills the follicle. C, Furuncle is characterized by the extension of the inflammation to the surrounding connective tissue (i.e., formation of an abscess around the hair follicle). D, Carbuncle forms through the confluence of several furuncles.
Cellulitis is diffusely spreading suppurative infection of the dermis and soft tissues. Most often, it is caused by Streptococcus pyogenes. This beta-hemolytic group A streptococcus secretes lytic enzymes (streptokinase, DNAse, and hyaluronidase), which facilitate the spreading of the infection through the tissues.
Erysipelas is a superficial cellulitis with prominent involvement of lymphatics, usually caused by group A beta-hemolytic Streptococcus pyogenes. Most often, it occurs on the face, arms, and legs. Redness and swelling of the affected area are accompanied by regional lymphadenitis and systemic symptoms, such as high fever and chills.
Dermatophytes produce tinea, commonly known as ringworm. Tinea cruris is known as jock itch, and tinea pedis is known as athlete’s foot. Tinea capitis, a fungal infection of the scalp most often found in children, may cause loss of hair (alopecia). Onychophytosis is nail infection caused by fungi of the Trichophyton species.
Infections of the deeper portions of the skin may occur because of the entry of invasive fungi through damaged skin or hematogenously during a fungemia. Such infections are caused by Blastomyces, Coccidioides, Histoplasma, Cryptococcus, and other fungi that affect internal organs.
Mycetomas are chronic suppurative infections of the subcutis and soft tissues of feet, arms, or back characterized by marked swelling and formation of pus-draining sinus tracts. Despite their name, which implies that these lesions are a manifestation of mycotic infections, only a minority of mycetomas are caused by fungi. The most common causes of mycetomas are filamentous bacteria such as Nocardia and Actinomyces.