Subcutaneous Tissue Infections and Abscesses

Etiology

The cause of abscesses and subcutaneous infections in children is not always clear. Microbial etiology can vary both by predisposing host factors (e.g., primary or secondary immunodeficiency, various genetic conditions) and route of infection (e.g., hematogenous, superficial trauma allowing infection from skin flora, penetrating trauma inoculating environmental microbes). S. aureus, including both methicillin-susceptible and -resistant strains (MSSA and MRSA), is the most commonly isolated pathogen overall. Infection can be due to a single pathogen or polymicrobial.

Identification of the pathogens causing a particular subcutaneous soft tissue infection or, in some cases, exclusion of a variety of noninfectious conditions in the differential diagnosis of subcutaneous tissue infections ( Box 73.3 ), requires proper use of diagnostic tests and interpretation of results. If the surface of a wound or site of infection is sampled with a swab, several organisms can be isolated that are colonizing the area but may not be contributing to the disease process. If an organism is suggested by Gram stain (or special stains in the case of fungi) and identified by culture, the likelihood of its pathogenicity is increased. Chances of identifying the true pathogen are improved further if culture specimens are obtained by decontaminating the skin and performing an incision to express material or fine-needle aspiration, by swabbing the exudate from a site of suppuration, or by performing a punch or excisional biopsy.

Cultures should be obtained for routine and fastidious, aerobic and anaerobic organisms when appropriate. Sampling to identify fungi and viruses, including herpes simplex virus (HSV) and cytomegalovirus (CMV), particularly in immunocompromised patients, usually is appropriate. Susceptibility testing should be performed on isolated pathogens to guide antibiotic selection.

This chapter describes infections that lead to abscess formation and tissue necrosis involving subcutaneous tissues as deep as the inner layer of the superficial fascia. Infections that commonly extend into the deep fascia and muscle are discussed elsewhere (see Chapter 75 ).

Abscesses

An abscess is a localized, usually inflamed, walled-off collection of purulent exudate formed by a collection of white blood cells (WBCs), bacteria, and disintegration or necrosis of tissue. An abscess is recognized clinically as a firm, tender, erythematous nodule that is fluctuant. The initial skin lesion commonly can be mistaken for a spider bite.

A subcutaneous abscess most commonly evolves by local extension of a primary infectious process from the epidermis or dermis, such as a cutaneous abscess originating from a skin appendage such as a pilosebaceous unit (e.g., furuncle, carbuncle, infundibular cyst, periporitis ) or secondarily from a site of skin disease or injury. A subcutaneous abscess also can arise by direct traumatic implantation or invasion of pathogens into subcutaneous tissue or occasionally by hematogenous spread.

In the patient with an abscess, constitutional symptoms usually are absent initially, unless the process has extended into deeper tissues or the bloodstream or has originated from the bloodstream. Often, no predisposing factor can be identified, although several processes that disrupt the integrity of the barrier function of the skin or the integrity of local immunologic processes, particularly neutrophil function, are associated with abscess formation. Patients with hyperimmunoglobulin E syndrome and chronic granulomatous disease are prone to abscesses. Despite the extensive list of immunodeficiency diseases associated with abscess formation, most people with recurrent abscesses lack evidence of immunodeficiency.