Porth's Essentials of Pathophysiology, 4e

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Integumentary Function

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or ringworm, are confined to the epidermis and its integ- uments, the hair and nails. Most of the superficial mycoses are dermatophytoses— dermatoses caused by the dermatophytes, a group of closely related fungi classified into three genera: Microsporum , Epidermophyton , and Trichophyton . 2,4 Some dermatophytes are anthropophilic; that is, they are parasitic in humans and are spread by other infected humans. These tend to cause chronic infections that are difficult to treat. Zoophilic species cause parasitic infections in animals, some of which can be spread to humans. Geophilic species originate in the soil, but may infect animals, which in turn serves as a source of infec- tion for humans. These species tend to cause inflamma- tory lesions that respond well to therapy or may even resolve spontaneously. 2,4 The fungi that cause superficial mycoses emit an enzyme that enables them to digest keratin, which results in superficial skin scaling, nail disintegration, or hair breakage, depending on the location of the infec- tion. Deeper reactions involving vesicles, erythema, and infiltration are caused by the inflammation that results from exotoxins liberated by the fungus. Diagnosis of superficial fungal infections is primar- ily done by microscopic examination of skin scrapings for fungal spores, the reproducing bodies of fungi. 2 Potassium hydroxide (KOH) preparations are used to prepare slides of skin scrapings. Potassium hydroxide disintegrates human tissue and leaves behind the thread- like filaments, or hyphae, that grow from the fungal spores. Cultures also may be done using a dermatophyte test medium or a microculture slide that allows for direct microscopic identification. The Wood light is an ultraviolet (UV) light that can assist with the diagnosis of tinea, as some types of fungi fluoresce yellow-green when the light is directed onto the affected area. Topical agents are commonly used in the treatment of tinea infections; however, success often is limited because of the lengthy duration of treatment, poor compliance, and high rates of relapse at specific body sites. The principal agents are the azoles (ketoconazole, miconazole, clotrimazole, etc.) and the allylamines (naf- tifine and terbinafine). Both act by inhibiting the syn- thesis of ergosterol, which is an essential part of fungal cell membranes. 2,3 Topical corticosteroids may be used in conjunction with topical antifungal agents to relieve itching and erythema secondary to inflammation. The systemic (i.e., oral) antifungal agents include gris- eofulvin, the azoles, and the allylamines. 2,3 Griseofulvin is an antifungal agent derived from a species of penicil- lium, whose only use is in the systemic treatment of der- matophytosis. It acts by binding to the keratin of newly formed skin, protecting the skin from new infection. It must be administered for 2 to 6 weeks to allow for skin and hair replacement, and nail infections often require months of treatment. Systemic azoles and allylamines are also used. In contrast to griseofulvin, the synthetic agents are fungicidal (i.e., kill the fungus) and therefore are more effective over shorter treatment periods. Some of the oral agents can produce serious side effects, such as hepatic toxicity, or interact adversely with other medications.

Tinea of the Body or Face. Tinea corporis (ringworm of the body) can be caused by any of the dermatophyte species. Although it affects all ages, children seem most prone to infection. Transmission is most commonly from kittens, puppies, and other children who have infections. The most common types of lesions are oval or circular patches on exposed skin surfaces and the trunk, back, or buttocks (Fig. 46-1). Less common are foot and groin infections. The lesion begins as a red pap- ule and enlarges, often with a central clearing. Patches have raised red borders consisting of vesicles, papules, or pustules. The borders are sharply defined, but lesions may coalesce. Pruritus, a mild burning sensation, and erythema frequently accompany the skin lesion. Tinea faciale, or ringworm of the face, is typically caused by one of the Trichophyton species. Tinea faci- ale may mimic the annular, erythematous, scaling, pru- ritic lesions characteristic of tinea corporis. It also may appear as flat erythematous patches. Topical antifungal agents usually are effective in treating tinea corporis and tinea faciale. Oral antifungal agents may be used in resistant cases. Tinea of the Scalp. Tinea capitis, the most common type of fungal infection in children, is an infection of the scalp and hairshaft. Children between the ages of 3 and 14 years are primarily affected. The primary lesions vary from grayish, round, hairless patches to balding spots. The lesions vary in size and are most commonly seen on the back of the head (Fig. 46-2). Mild erythema, crust, or scale may be present. The individual usually is asymptomatic, although pruritus may exist. Treatment is with oral griseofulvin or synthetic antifungal agents that penetrate the hair shafts. 5–7 Topical ointments or shampoos are sometimes indicated. The inflammatory type of tinea capitis is caused by virulent strains of T. mentagrophytes, T. verrucosum, and M. gypseum . The onset is rapid, and inflamed lesions usually are localized to one area of the head. The inflam- mation is believed to be a delayed hypersensitivity reac- tion to the invading fungus. The initial lesion consists

FIGURE 46-1. Tinea of the arm due to the dermatophytic fungus trichophyton rubrum. (From the Centers for Disease Control and Prevention Public Health Image Library. No. 4811.)