Porth's Essentials of Pathophysiology, 4e

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Integumentary Function

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population and occurs worldwide, although the inci- dence is lower in warmer, sunnier climates. The aver- age age of onset is in the third decade; its prevalence increases with age. Approximately one third of persons have a genetic history, indicating a hereditary factor. Childhood onset of the disease is more strongly associ- ated with a family history than psoriasis occurring in adults older than 30 years of age. In rare cases, psoriasis is associated with arthritis (see Chapter 44). Etiology and Pathogenesis. Although the primary cause of psoriasis is uncertain, it is thought to be an immunolologic disorder in persons with a genetic pre- disposition. It is not known if the inciting antigen is of self or environment origin. A genetic component is sup- ported by population studies indicating a greater inci- dence of psoriasis among relatives of persons with the disease than among the general population. There is a strong association between psoriasis and the human leu- kocyte antigen-C (HLA-C), particularly the HLA-Cw6 allele. 40 Immunologically, T lymphocytes may be the key to the pathogenesis of psoriasis. 40 Eruption of pso- riatic lesions coincides with T-cell infiltration into the epidermis, and resolution of the lesions follows disap- pearance or reduction in epidermal T cells, suggesting a T-cell–mediated release of cytokines and growth fac- tors that stimulate abnormal growth of keratinocytes and dermal blood vessels. Accompanying inflamma- tory changes are caused by infiltration of neutrophils and monocytes. Environmental stimuli may trigger the release of cytokines and growth factors from keratino- cytes and other cells, with ensuing immune and inflam- matory responses that lead to the full development of psoriatic lesions. For example, psoriatic lesions can be induced in susceptible individuals by trauma, a process known as the Köbner phenomenon . Clinical Features. Histologically, psoriasis is character- ized by increased epidermal cell turnover with marked epidermal thickening, a process called hyperkeratosis . The granular layer (stratum granulosum) of the epider- mis is thinned or absent, and neutrophils are found in the stratum corneum. There also is an accompanying thinning of the epidermal cell layer that overlies the tips of the dermal papillae, and the blood vessels in the dermal papillae become tortuous and dilated. These capillary beds show permanent damage even when the disease is in remission or resolved. The close proximity of the vessels in the dermal papillae to the hyperkera- totic scale accounts for multiple, minute bleeding points that are seen when the scale is lifted. Psoriasis most frequently affects the elbows, knees, scalp, lumbosacral area, and intragluteal cleft. The primary lesions are sharply demarcated, thick, red plaques covered by a silvery scale that varies in size and shape (Fig. 46-16). In darker-skinned persons, the plaques may appear purple. There may be excoriation, thickening, or oozing from the lesions. A differen- tial diagnostic finding is that the plaques bleed from minute points when removed, which is known as the Auspitz sign .

Stevens-Johnson syndrome. Lateral pressure causes the surrounding skin to separate easily from the dermis ( Nikolsky sign ). The tracheobronchial mucosa and con- junctiva may be involved in severe cases with resultant scarring. Ophthalmologic consultation is required when ocular involvement in present. Treatment. Treatment of erythema multiforme minor and less-severe cases of Stevens-Johnson syndrome includes relief of symptoms using compresses, antipru- ritic drugs, and topical anesthetics. Corticosteroid ther- apy may be indicated in moderate cases, although its use is controversial. For severe cases of Stevens-Johnson syndrome and toxic epidermal necrolysis, hospitaliza- tion is required for fluid replacement, respiratory care, administration of antibiotics and analgesics, and appli- cation of moist dressings. When large areas of skin are detached, the care is similar to that of a thermal burn. Intravenous immunoglobulin may hasten the healing response of the skin. Generally, healing is a slow pro- cess, taking 6 weeks or more to regenerate skin. The mucous membranes heal slowly and follow-up treat- ment is often needed for ophthalmologic and mucous membrane sequelae. Discontinuation of the responsible drug and chemically related compounds is imperative, as are measures to prevent future exposures to the drug. Papulosquamous Dermatoses Papulosquamous dermatoses are a group of skin dis- orders characterized by scaling papules and plaques. Among the major papulosquamous diseases are psoria- sis, pityriasis rosea, and lichen planus. Psoriasis Psoriasis is a common, chronic, inflammatory skin dis- ease characterized by raised erythematous plaques with silvery scales. 37–40 It affects approximately 2% of the FIGURE 46-15. Erythema multiforme. Characteristic targetlike lesions are noted here. (From Goodheart HP. Goodheart’s Photoguide to Common Skin Disorders. Philadelphia, PA: Wolters Kluwer Health | Lippincott Williams &Wilkins; 2009:311.)