Atlas of pathophysiology, 2 Edition

Part II - Disorders

Skin Disorders

Fungal infections

Fungal infections of the skin are often regarded as superficial infections affecting the hair, nails, and stratum corneum (the dead top layer of the skin). Fungi infect and survive only on the nonviable keratin within these structures. The most common fungal infections are dermatophyte infections (tineas) and candidiasis (moniliasis).

Tinea infections are classified by the body location in which they occur—for example:

·   capitis, scalp

·   corporis, body

·   pedis, foot

·   cruris, groin.

Some forms infect one gender more commonly than the other. For example, tinea cruris is more common in males. Obesity and diabetes predispose to tinea and candida infection.

Age Alert

Children develop tinea scalp infections, young adults more commonly develop infection in the interiginous areas, and older adults develop onchomycosis.

Candidiasis of the skin or mucous membranes is also classified according to the infected site or area:

·   intertrigo—axilla or inner aspect of thigh

·   balanoposthities—glans penis and prepuce

·   vulvitis

·   diaper dermatitis

·   paronychia—folds of skin at the margin of a nail

·   onychia—nail bed

·   thrush—mouth.

Candida organisms may be normal flora of the skin, mouth, GI tract, or genitalia.

The prognoses for tinea infection and candidiasis are very good. They usually respond well to appropriate drug therapy and resolve completely. It's important to reduce risk factors to obtain a good outcome from the infection. Antifungal therapy usually resolves candidiasis, but if risk factors aren't avoided, a chronic condition can develop.

Causes

Tinea infection

·   Microsporum, Trichophyton, or Epidermophyton organisms

·   Contact with contaminated objects or surfaces

Risk factors for tinea infection

·   Obesity

·   Atopy, immunosuppression

·   Antibiotic therapy with suppression of normal flora

·   Softened skin from prolonged water contact, such as with water sports or diaphoresis

Candidiasis

·   Overgrowth of Candida organisms and infection due to depletion of the normal flora (such as with antibiotic therapy)

·   Neutropenia and bone marrow suppression in immunocompromised patients (at greater risk for the disseminating form)

·   Candida albicans, normal GI flora (causes candidiasis in susceptible patients)

·   Candida overgrowth in the mouth (thrush)

Pathophysiology

Dermatophytes, which grow only on or within keratinized structures, make keratinases that digest keratin and maintain the existence of fungi in keratinized tissue. The pathogenicity of dermatophytes is restricted by the cell-mediated immunity and antimicrobial activity of the polymorphonuclear leukocytes. Clinical presentation depends on fungal species, site of infection, and host susceptibility and immune response.

In candidiasis, the organism penetrates the epidermis after binding to integrin receptors and adhesion molecules and then secretes proteolytic enzymes, which facilitate tissue invasion. An inflammatory response results from the attraction of neutrophils to the area and from activation of the complement cascade.

Signs and symptoms

Tinea infection

·   Erythema, scaling, pustules, vesicles, bullae, maceration

·   Itching, stinging, burning

·   Circular lesions with erythema and a collarette of scale (central clearing)

Candidiasis

·   Superficial papules and pustules; later, erosions

·   Erythema and edema of epidermis or mucous membrane

·   As inflammation progresses, a white-yellow, curdlike material over the infected area

·   In thrush—white coating of tongue, buccal mucosa, and lips, which can be wiped off to reveal a red base

·   Severe pruritus and pain at the lesion sites (common)

Diagnostic test results

·   Microscopic examination of a potassium hydroxide-treated skin scraping reveals the offending organism.

·   Culture determines the causative organism and suggests the mode of transmission.

·   Wood's lamp examination in a darkened room demonstrates fluorescence.

Treatment

Tinea infection

·   Topical fungicidal agents, such as imidazole or an allylamine product

·   If no response to topical treatment—oral agents, such as allylamines or azoles

Candidiasis

·   Intertrigo, balanitis, vulvitis, diaper dermatitis, paronychia—nystatin or imidazoles

·   Oral candidiasis (thrush)—azoles, imidazoles

·   Systemic infections—I.V. amphotericin B or oral ketonazole

P.383

SKIN SCRAPINGS FROM FUNGAL SKIN INFECTION

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FUNGAL INFECTION OF NAIL

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