Soft Tissue Infections – Incision, Drainage, Debridement

Overview

Necrotizing skin infections and necrotizing soft tissue infections (NSTIs) are the most severe form of bacterial infection in the soft tissues, and they represent a wide spectrum of disease. Common synonyms that have been applied to this disease process include necrotizing fasciitis, Fournier gangrene, clostridial myonecrosis, gas gangrene, Meleney ulcers, and “flesh eating” infections. What all these have in common is aggressive bacterial infection of the deep spaces of the soft tissue and/or muscle that result in tissue necrosis, which can be rapidly progressive and highly lethal. The blood supply to the fascia and subcutaneous fat is more tenuous than the skin or underlying muscle and is thus more vulnerable to an infectious process. Bacteria invade the soft tissues and track along either the superficial or deep fascial planes. The combination of the invading organisms and the infiltration of polymorphonuclear cells can lead to microvascular thrombosis, which results in tissue ischemia and necrosis. Some organisms, such as Clostridium perfringens , also produce toxins that can cause necrosis of otherwise well-perfused tissue, such as muscle. The average mortality rate for NSTI in recently published series is 22%, but this varies based on the patient population. The severity and progression of the disease depends largely on the site of infection, the medical comorbidities of the host, and the pathogenic organisms involved.

Diagnosis

Because these infections can spread rapidly in the deep tissues, skin findings may be minimal, and there is frequently a delay in the diagnosis of NSTI. Several studies have associated a delay in diagnosis and treatment of NSTI of greater than 12 hours with increased mortality. Thus, it is paramount that clinicians maintain a high index of suspicion for this disease process, especially in high-risk populations. Patients at increased risk for the development of NSTI include those with a history of injection drug use, deep penetrating injuries, morbid obesity, diabetes mellitus, peripheral vascular disease, and immunosuppression. Common infections and sites involved include perineal or perianal infections, perirectal abscesses, foot or lower extremity ulcerations, penetrating wounds, bite wounds, injection drug sites, post-surgical wounds, pressure ulcers, and gastrointestinal pathology with perforation. In up to 10% of cases, however, there is no identifiable portal of entry for these organisms.

Clinical signs and symptoms include pain out of proportion to the physical exam, induration and edema with or without erythema, skin blistering or sloughing, patches of skin necrosis showing a blue-black discoloration, crepitus, and thin gray drainage from the wound ( Figures 57-1-1 to 57-1-3 ). Plain radiographs may reveal evidence of gas in the soft tissues, but the absence of this finding does not rule out the disease. Evidence of systemic sepsis including tachycardia, hypotension, low urinary output, and high fluid requirements are generally late signs that suggest advanced disease. NSTI should also be considered in patients with soft tissue infections that continue to progress despite antibiotic therapy. Several authors have focused on the utility of basic laboratory studies to raise concern for NSTI in the setting of a soft-tissue infection. These include leukocytosis or leukopenia, hyponatremia, and evidence of early renal insufficiency that includes elevated blood urea nitrogen or creatinine. Very high white blood cell counts have been associated with severe clostridial infections. In one study, a presenting white blood cell count over 40,000 carried a 50% predicted mortality rate.

The diagnosis of NSTI should depend on the clinical presentation combined with the initial laboratory studies. Additional imaging with computed tomography or magnetic resonance imaging may be helpful in identifying deep abscesses in equivocal cases but should not delay operative exploration when there is high suspicion. In most cases these studies will merely show fat stranding in the subcutaneous tissue, which is nonspecific for infection. Tissue biopsies and frozen-section analysis has been described, but they are likely to delay surgical exploration and may not be definitive. When in doubt, the best course of action is to proceed to the operating room for exploration. The surgical incision should extend down to the deep muscle fascia with inspection of all tissue layers for necrotic tissue, and the wound should be probed to identify any tracking in the fascial planes. If necrotic tissue is encountered, the diagnosis is confirmed, and debridement can be initiated.

Microbiology

To select appropriate antibiotic treatment strategies, it is important to understand the variety of pathogens involved in these infections. Most NSTIs are polymicrobial with an average of 4.4 organisms isolated per infection. These polymicrobial infections can cause significant local tissue necrosis but tend to be more indolent and carry a lower mortality risk than infections caused by a limited number of highly virulent pathogens, such as Streptococcus pyogenes (group A β-hemolytic Streptococcus ), Clostridium species, and community-acquired methicillin-resistant Staphylococcus aureus (MRSA). Due to the unique nature of these infections, each will be individually addressed.