Although most diseases affecting the skin originate in the layers of the skin, such abnormalities are also important factors in the diagnosis of a variety of internal diseases. There is some truth in the belief that the skin mirrors a person’s internal health. Often, the visibility and accessibility of skin make it the first organ of the body to show detectable signs of underlying disease. Abnormalities of the skin frequently suggest metabolic, malignant, and glandular diseases.
Like other tissues, skin is afflicted by all types of pathological changes, including hereditary, inflammatory, benign and malignant (neoplastic), endocrine, hormonal, traumatic, and degenerative processes. Emotions affect the health of the skin as well. The reaction of the skin to these diseases and disorders differs from that of other tissues in many ways. For example, extensive inflammation of the skin may affect metabolism within other organs and systems of the body, causing anemia, circulatory collapse, disorders of body temperature, and disturbance of water and electrolyte balance in the blood. The skin has such vigorous healing properties, however, that widespread injury, as in thermal burns, may be followed by a marked degree of regrowth of the injured or diseased areas, with a disproportionally small degree of scarring.
The skin has an inherent region-specific anatomical diversity that may profoundly modify the appearance of a rash. This is apparent when skin transplanted from one area of the body to another (other than a symmetrically opposite area) retains the morphological characteristics of the donor area. Thus the morphology of eczema or lichen planus on the palms and soles may bear little or no resemblance to the same disease in the same individual on the face or scalp. In these instances a biopsy shows the abnormalities of the cells of the skin and the pattern and distribution of any invading cells. The ability to identify immunoreactants (immunoglobulins, or antibodies, that react with specific invading agents, or antigens) in skin biopsies has greatly increased the accuracy of the diagnosis of inflammatory disorders and has clarified their immunologic basis, especially in the blistering disorders.
Generalized Skin Diseases
The terms eczema and dermatitis are often used interchangeably to denote an inflammatory process in the skin that involves the upper dermis and epidermis. The epidermis exhibits swelling of the keratinocytes and accumulation of fluid between them (spongiosis). In the severe form of spongiosis, blisters form within the epidermis. Childhood eczema in black children frequently is seen as a follicular eruption of pinhead-sized papules. In chronic forms of eczema or dermatitis the prominent changes are thickening of the epidermis and marked hyperkeratosis (thickening of the outer horny layer of the epidermis). These changes lead to lichenification (see above), roughening and scaling of the skin surface, and itching. The function of the horny layer as an impervious barrier may be seriously impaired, with two important consequences: loss of water from the skin leads to desiccation of the horny layer, which in turn leads to cracking, increased scaling, and soreness; and loss of the barrier function causes increased absorption of medications applied to the surface of the skin. For example, the enhanced absorption of topically applied corticosteroids may cause toxic changes in the skin and in distant organs and tissues.