Porth's Essentials of Pathophysiology, 4e

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Integumentary Function

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melanocytes, or, in some cases, melanocytes that no longer produce melanin. Although there are many treat- ment regimens for vitiligo, none is curative. Self-tanning lotions, skin stains, and cosmetics are used for cam- ouflage. Micropigmentation (tattooing) has been done on smaller, recalcitrant areas, but it is often difficult to attain a correct color match. If extensive skin surfaces are involved, the treatment may be reversed and the pig- mented areas bleached to match the remainder of the skin color. A melanocytotoxic agent is used to remove remaining melanocytes from skin areas. Melasma is a disorder characterized by darkened mac- ules on the face. It is common in all skin types, but most prominent in brown-skinned people from Asia, India, and South America. It occurs in men but is more common in women, particularly during pregnancy or while using oral contraceptives. It may or may not resolve after giving birth or discontinuing hormonal birth control. Melasma is exacerbated by sun exposure. Treatment measures are palliative, mostly consisting of limiting exposure to the sun and using sunscreens. Bleaching agents containing 2% to 4% hydroquinone are standard treatments. Pruritus Pruritus is an unpleasant sensation of itch leading to the desire to scratch. Symptoms of pruritus range from mild to so severe that it interrupts sleep and the general qual- ity of life. While pruritus most commonly occurs in skin disorders, it may also occur with systemic disorders, such as chronic kidney disease, diabetes, and biliary disease. Pruritus is multidimensional and has been classified into four types: (a) pruritoceptive (generated in the skin: bug bites), (b) neurogenic (generated in the central nervous system: cholestasis), (c) neuropathic (due to lesions in the central nervous system blocking normal responses), and (d) psychogenic (emotional or stress related). Pruritus originates within the skin’s free nerve end- ings, is carried by small myelinated type C nerve fibers to the dorsal horn of the spinal cord, and is then transmit- ted to the somatosensory cortex through the spinotha- lamic tract (see Chapter 35). Many chemicals have been found to produce the itch sensation, including histamine, serotonin, and cytokines. Substances such as bradykinin and bile salts act locally to stimulate the itch sensation. Prostaglandins are modulators of the itch response, low- ering the threshold for other mediators. Opioids produce pruritus in a number of patients who receive them while narcotic antagonists have been used successfully to relieve pruritus. Regardless of cause, pruritus is often exacer- bated by skin inflammation, dry or hot ambient tempera- tures, skin vasodilation, and psychological stressors. Scratching, the well-known response to itch, is a spi- nal reflex response that to varying degrees can be con- trolled by the individual. While providing momentary relief, scratching increases inflammation and stimulates nerve endings, leading to more itching and scratching. Repeated scratching also causes undesirable changes in the skin such as lichenification. Successful treatment of pruritus requires interruption of this cycle.

The focus of treatment measures for pruritus can be grouped into two categories: specific and nonspecific. Specific treatment involves finding and correcting the underlying disorder, thereby eliminating the itch sensa- tion. Nonspecific measures involve the use of measures to relieve the itch sensation and prevent complications that arise from intense and persistent scratching. Measures such as using the entire hand to rub over large areas and keeping the fingernails trimmed often can relieve itch and prevent skin damage. Self-limited or seasonal cases of pruritus may respond to treatment measures such as moisturizing lotions, bath oils, and the use of humidifiers. Because vasodilation tends to increase itching, cold applications may provide relief. Cool show- ers before bed, light sleepwear, and cool home tempera- tures also may be helpful. Also helpful can be cooling over-the-counter anti-itch agents containing menthol, camphor, or phenol. These substances stimulate nerve fibers that transmit the sensation of cold, thereby mask- ing the itch sensation. Mild cutaneous disorders, such as bug bites, are mediated by histamine; therefore, topi- cal antihistamines tend to be the treatment of choice. Topical corticosteroids are effective as antipruritics, par- ticularly when used for urticaria (hives) or insect bites. However, because most cases of pruritus are not his- tamine related, their management should be directed at the underlying cause. For example, systemic antihis- tamines and corticosteroids may be indicated for per- sons with severe pruritus or atopic dermatitis. Topical capsaicin cream and topical aspirin have been used for localized chronic pruritic disorders. Other modalities that have been used for pruritus with varying degrees of success are phototherapy, acupuncture, antidepressant medications, behavior modification, and alternative therapies (herbal, nutritional, and reflex therapies). In persons with pruritus due to a systemic cause, itching gradually recedes as the primary condition improves. Dry Skin Dry skin, also called xerosis, may be a natural occur- rence, as in the drying of skin associated with aging, or it may be symptomatic of an underlying systemic disease or skin disorder such as contact dermatitis. Most cases of dry skin are caused by dehydration of the stratum corneum. The effects of aging on skin dryness include a change in the composition of sebaceous gland secretions and a decrease in the secretion of moisture from the sweat glands. Aging is also accompanied by a decrease in skin capillaries as well as a flattening of the dermal rete ridges, resulting in less surface area for exchange of fluids between the dermis, epidermis, and skin surface. Dry skin appears rough and scaly; there may be increased wrinkles or lines. Persons with dry skin often experience severe pruritus, most commonly of the extremities, back, abdomen, or waist. They may resort to scratching, resulting in cracking, fissuring, and a number of other skin maladies. Moisturizing agents are the cornerstone of treatment for dry skin. These agents exert their effects by repair- ing the skin barrier, increasing the water content of the