Although uncommon, necrotizing fasciitis (NF) is an infectious diseases emergency. Predisposing conditions include diabetes mellitus, surgery, trauma, peripheral vascular disease, and injection drug use. Classification of this life-threatening infection has been proposed based on the microbiology of infection . Type 1 necrotizing fasciitis refers to a mixed aerobic and anaerobic infection (ie, diabetic foot infection and Fournier's gangrene, for example). Facultative streptococci, staphylococci, enterococci, aerobic
Gram-negative bacilli, such as Escherichia coli, Klebsiella, Pseudomonas, Enterobacter, and Proteus, and anaerobes, such as Peptostreptococcus, Bacter-oides, and Clostridium spp are the characteristic pathogens associated with these infections. Type II infections are monomicrobial, classically associated with the "flesh-eating" bacteria S pyogenes. Other bacteria capable of producing monomicrobial infection include Streptococcus agalactiae, V vulnificus, Clostridium spp, and S aureus [28,41]. Of note, Aeromonas spp- and Streptococcus pneumoniae-associated NF have been reported in patients who have underlying collagen vascular diseases [42-44].
Usually associated with antecedent skin infection or trauma, NF is rapid in onset with the sequential development of erythema, extensive edema, and severe, unremitting pain, often despite antibiotic therapy. Hemorrhagic bul-lous lesions, skin necrosis, and crepitus associated with gas in soft tissues also may develop. Systemic toxicity manifest by fever, hypotension, tachycardia, delirium, and multiple organ dysfunction is characteristic of severe cases. Anesthesia localizes at the infection site because of associated nerve necrosis. The legs and other extremities are the most common locations for NF, although any site can be involved, including the perineum, trunk, head and neck, and buttocks [45,46].
Early recognition of NF is essential. Plain films and CT scans can be helpful in demonstrating gas in the soft tissues when infection is attributable to anaerobic bacteria or the Enterobacteriaceae. Magnetic resonance imaging is reported to be more sensitive in identifying necrotizing skin and soft tissue infections and the extent of involvement [47-49]. No laboratory test is specific for the diagnosis of NF. A diagnostic scoring system for distinguishing these infections based on routine laboratory tests has been proposed. Titled the Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC) score, it uses factors that have been reported to be independently predictive of NF (total white cell count, hemoglobin, sodium, glucose, serum creati-nine, and C-reactive protein) to create a score to assist physicians in the evaluation of NF . The negative predictive value and positive predictive value of a cut-off score of 6 is reported to be 96% and 92%, respectively. The authors conclude that patients who have an LRINEC score greater than or equal to 6 should "be carefully evaluated for the presence of necrotizing fasciitis'' . Tissue oxygen saturation monitoring also has been reported to be helpful in identifying NF of the lower extremities in patients who do not have chronic venous stasis, peripheral vascular diseases, shock, or systemic hypoxia. A cut-off value of less than 70% for the involved tissue oxygen saturation demonstrated a sensitivity of 100% and a specificity of 97% .
When NF is suspected, surgical evaluation is needed. Definitive diagnosis of NF is made by visualization of affected fascia. The passage of a probe or finger without resistance across soft tissue that typically is adherent to deep fascial planes is characteristic . Needle aspiration of involved soft tissue or blood cultures may yield the putative pathogen but is not specific for diagnosing necrotizing skin infections [53,54]. Frozen section full-thickness biopsy performed early in the course of infection may provide histopathologic confirmation of NF and prompt definitive surgical debridement . Culture of intraoperatively-obtained tissue is most helpful in determining the microbial cause.
Surgery is not only diagnostic but also therapeutic. It is intended to preserve viable skin while removing necrotic tissue and ensuring hemostasis. Repeat intervention is recommended and multiple debridements may be required until evolving necrosis is no longer appreciated. Plastic surgical reconstruction with split-thickness skin grafting often is needed. In addition to surgery, polymicrobial-associated NF should be treated with a broad-spectrum antibiotic regimen consisting of ampicillin-sulbactam or piperacillin-tazobactam and ciprofloxacin and clindamycin . Group A streptococcus-associated NF is best treated with penicillin plus clindamycin. The potential presence of S aureus may dictate the need for vancomycin because of the emergence of methicillin resistance in this organism. Overall mortality rates range from 17% to 40% and are increased when there is a delay in surgery or when toxic shock syndrome with NF secondary to S pyogenes is present [45,46,54,56].
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