Figure 7.1. Leading scale and scaly surface of tinea pedis. Also note distal lateral subungual onychomycosis on the great toe.

    Definitive diagnosis of a fungal or yeast infection is made by performing a potassium hydroxide (KOH) wet mount and seeing characteristic hyphae or pseudohyphae in the scale. Alternatively, a fungal culture can be sent but can take 3–4 weeks for definitive results. Diagnosis of onychomycosis also can be done by sending nail clippings for periodic acid Schiff staining and histopathology. Treatment for limited cutaneous infection is topical. Use of topical steroids will worsen infection, although it mitigates associated pruritus in many cases. A systemic agent should be considered for extensive surface involvement, tinea capitis, or nail infections. The most common oral therapies consist of terbinafine, which inhibits fungal production of ergosterol. Fluconazole also can successfully treat dermatophyte infections, both of the nail and the skin, but is not FDA-approved for these indications and should be used for candidal infections. Topical therapies include terbinafine, azole derivatives (i.e., ketoconazole), and ciclopirox.

SYSTEMIC FUNGAL INFECTIONS

There are multiple invasive fungi that can infect the skin. Infection is through local trauma to the skin, through IV catheters, or by inhalation. In the United States, coccidioidomycosis, histoplasmosis, and blastomycosis are the most likely infections to be seen, although rarely are they causes of dermatologic disease. In immunosuppressed individuals, cryptococcal infections of the skin may occur. Coccidioidomycosis is caused by the dimorphic fungi Coccidioides immitis (limited geographically to the San Joaquin Valley in California) and Coccidioides posadasii (in the desert Southwest of the United States, Mexico, and Central/South America).

    Symptoms are similar for both species. Infection is by the respiratory route with skin manifestations being rare, usually limited to reactive findings such as erythema nodosum or erythema multiforme. Histoplasma capsulatum is another dimorphic fungus endemic to the central United States (Ohio, Missouri, and Mississippi River valleys) that causes respiratory infections and rare skin infections. North American blastomycosis is caused by Blastomyces dermatitidis and is found in similar distributions to Histoplasma. Both Histoplasma and Blastomyces rarely cause skin findings in immunocompetent individuals, but cutaneous infection can be found in HIV-infected patients. Cutaneous histoplasmosis may present as erythematous papules, ulcerations, or acneiform or molluscum-like lesions, whereas blastomycosis presents as a disseminated morbilliform eruption. Cutaneous cryptococcosis (Cryptococcus neoformans) also may occur in HIV-infected individuals with multiple presentations including cellulitis, papules/plaques/ulcerations, or lesions similar to molluscum contagiosum. Diagnosis is established by tissue biopsy and/or culture.

CUTANEOUS DEEP FUNGAL INFECTIONS

Chronic fungal infection caused by direct infection of the skin can occur due to a variety of organisms. Sporothrix schenckii lives on decaying organic material and is implanted most often in the skin of an extremity (characteristically by prick from a rosebush thorn). Fungal infection develops and spreads along lymphatic channels, causing erythematous nodules in a linear lymphatic distribution. Mycetoma is caused by a wide variety of fungal species including Nocardia spp., Pseudallescheria boydii (the most common cause worldwide), Acremonium spp., and Madurella spp. Men are more affected than women and presentation is usually on the foot, caused by traumatic inoculation. This is rarely seen in the United States. Diagnosis is by KOH, fungal culture, and/or biopsy as well as examination of the type of “grains” produced by the infection. Black grains suggest a common fungal infection such as Madurella; small white grains suggest Nocardia. Grains with red coloration are due to Actinomadura pelletieri. Larger yellow-white to white grains are either fungal or actinomycotic. Other fungal infections to consider include zygomycetes (such as Mucor) or chromoblastomycosis caused by multiple organisms including Phialophora verrucosa, Cladosporium carrionii, Rhinocladiella aquaspersa, and Fonsecaea spp.

CUTANEOUS BACTERIAL INFECTIONS

Bacterial infection of the skin can present in a variety of ways. Superficial infection in the epidermis by gram-positive organisms, usually staphylococcal or streptococcal, causes impetigo. The primary lesion is a superficial pustule that rapidly is traumatized/erupts, and a honey-colored crust forms (Figure 7.2). When this crust is removed, the base is glistening and moist. A bullous variant also exists, most often caused by staphylococci. Deeper infection, usually ulcerated lesions on the lower extremities, is called ecthyma. Treatments for both types of infection are gentle local debridement and cleansing, topical antibiotics such as mupirocin, or appropriate oral antibiotics.

    Cellulitis and erysipelas are bacterial infections with deeper cutaneous involvement. Cellulitis is infection of the dermis and subcutaneous tissues. It presents as spreading, erythematous, tender and hot patches and plaques (Figure 7.3). Men have infections more often than women, and the lower extremity is frequently involved. Streaking along lymphatics is called lymphangitis. Cellulitis with a violaceous color and bullae suggest infection by Streptococcus pneumoniae. Erysipelas is more superficial than cellulitis and is usually caused by group A beta-hemolytic streptococci. It involves the local lymphatics and is characterized by the induration and sharp cutoff notable on palpation. It is found most commonly on the lower extremities, followed by the face.

    Staphylococcal scalded skin syndrome (SSSS) is caused by phage group 2 Staphylococcus aureus toxin production. Toxin production (A and B toxin) cause cleavage of desmoglein 1 in epidermal desmosomes, resulting in superficial cleavage of the epidermis at the granular cell layer and below. This results in skin desquamation, bullae formation, and erosions. This must be differentiated expeditiously from Stevens-Johnson syndrome or toxic epidermal necrolysis—usually by a skin biopsy, which should be processed by frozen section that allows for immediate pathologic examination. SSSS is common in infants and children (98% of cases are <6 years old) and has a lower mortality (1–5%) in this age group. In adults it is an uncommon finding associated with a much higher mortality (up to 50%). Adults with SSSS may be immunosuppressed.

CUTANEOUS VIRAL INFECTIONS

Many viral infections cause skin manifestations. Diseases of childhood, and occasionally adulthood, with skin manifestations are summarized in table 7.1. Human herpes viruses, frequent causes of human disease, are summarized in table 7.2. Nonspecific morbilliform exanthems are associated with many viral infections, although the most common cause is enteroviruses. While many of these are considered diseases of childhood, it is important to consider the cause of a morbilliform eruption because many of these also affect adults.

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