Diseases of the Skin

The skin serves as an important mirror of many internal disorders that do not actively involve the skin, but are simply reflected in it. The vascular system of the skin, for example, enables it to demonstrate passively such hematologic and circulatory disorders as anemia, polycythemia, jaundice, and shock. The cutaneous nervous system, by means of numbness or paresthesia, provides evidence of certain vitamin deficiencies or of lesions in the brainstem.

In other cases, the skin is involved actively in a reaction to an internal disorder. A wide spectrum of cutaneous vascular changes arise from allergic reactions to various foods, medications, infecting organisms, or disease-altered tissue. Keratin, for example, may be altered by a variety of nutritional deficiencies, and the pigmentary system responds to hormonal abnormalities such as Addison's disease or hypothyroidism, or reduced thyroid function.

A third category of cutaneous manifestations comprises those pathologic processes that take place both in the skin and in other organs. Some of these are fundamentally noncutaneous diseases that occasionally directly involve the skin. Leukemia, a hematologic disorder, occasionally forms malignant infiltrates in the skin, and coccidio-mycosis, a fungous infection usually limited to the lungs, may rarely involve the skin. Both of these diseases may provoke cutaneous allergic reactions. Skin involvement in very important in this category since the fortuitous appearance of the disease on the body surface may permit its rapid diagnosis, afford an opportunity to obtain a specimen for culture or biopsy, and lead to early treatment. Conversely, other disorders are primarily cutaneous conditions that may involve other organs. Examples of skin diseases that may also involve other organs are lichen planus, which may occasionally involve the gastrointestinal tract, and mastocytosis, which may rarely involve the bones, liver, and other internal organs.

The final category includes those diseases whose involvement is, as far as can be determined, limited to the skin. Acne and psoriasis are examples.

All diseases affecting the skin, like those affecting other organs, can be caused by a wide variety of agents, and any particular disease may reflect a combination of factors. Cutaneous tuberculosis and leprosy, for instance, involve not only bacterial infection but a degree of allergy to the respective causative microorganism.

The major known causes of diseases affecting the skin may be summarized as follows: (1) injury by physical or chemical agents, such as, mechanical force, heat, cold, sunlight, x-irradiation, or various chemicals applied to the skin; (2) infection and infestation with bacteria, viruses, rickettsia, fungi, and even some algae as well as some other lower forms of life that may parasitize the skin; (3) nutritional deficiencies, hormonal imbalances, and other disorders of body chemistry, causing such diseases as scurvy, hirsutism, and gout; (4) allergy, exhibited in several ways, including hives, and contact dermatitis (certain autoimmune diseases are also thought to result from a hypersensitivity to normal or body tissues); (5) abnormalities of the blood and blood vessels; (6) abnormalities of the nervous system and psyche; (7) benign and malignant tumors; and (8) genetic diseases, such as albinism and ichthyosis.

Skin diseases or cutaneous manifestations of systemic diseases and disorders can affect any part of the skin, including the epidermis, the dermis and its connective tissue and various other components, the pigment-producing system, the subcutaneous adipose tissue, the vascular system, the neural system, the sweat glands, the sebaceous glands, and the hair and nails.

Whatever the cause or whatever the target tissue involved, the direct result of a disease involving the skin is a lesion. Those that arise as a direct result of the disease process are called primary lesions. The appearance, shape, arrangement, distribution, and location of primary lesions are very important in the diagnosis of diseases affecting the skin.

The major types of primary lesions are: (1) mactile, a spot that is neither raised above nor depressed below the skin surface but differs from the surrounding normal skin in color or texture (vitiligo, a disease of the pigmentary system, is composed of macules); (2) papule, a lesion under 5 mm (0.2 inch) in diameter, which is raised above the skin surface, as in lichen planus; (3) nodule, a papule over 5 mm in diameter; (4) plaque, a relatively flat-surfaced raised lesion usually formed by a confluence of papules or nodules; (5) wheal, or hive, a particular type of papule that is itchy and usually lasts only several hours, formed by the extravasation of serum into the dermis (a mosquito bite produces a wheal); (6) vesicle, a lesion or small blister, under 5 mm in diameter and filled with clear liquid, usually serum, that will drain when punctured (contact dermatitis from poison ivy is often vesicular); (7) India, a large vesicle; and (8) pustule, a pus-containing lesion, which may be thin-walled, as in the pus-filled vesicles of chicken pox, or which may be relatively thick-walled, as in the pus-filled pimples of acne.

As a result of either natural evolution of primary lesions or the artificial alteration of them, so-called secondary lesions sometimes occur. The major types of secondary lesions are: (1) Scale, a flake of "dead skin' actually composed of abnormal epidermal cells that lack the adherent properties of normal stratum corneum. Both psoriasis and seborrheic dermatitis produce scales. (2) Crust, dried and congealed exudation that may accumulate on the surface of a lesion. Although usually composed primarily of serum, a crust may also contain dried pus or blood. (3) Erosion, the absence of epidermis, usually resulting from mechanical trauma. An excoriation is an erosion produced by scratching or scraping. Since the underlying dermis is left intact, no scar is produced. (4) Scar, new connective tissue that forms when there has been injury to or alteration of the dermis. (5) Ulcer, a deep erosion that extends into or even beneath the dermis. Impaired venous circulation in the legs is a notorious cause of ulceration. (6) Fissure, a split in the skin resulting from diminished flexibility. (7) Atrophy, an alteration of the skin resulting from a diminution of either the epidermis or the dermis. The' former produces a characteristic fine wrinkling and transparency of the skin, while the latter produces a slight depression.