(See also Overview of Bacterial Skin Infections.)
There are three subtypes of necrotizing soft tissue infection (NSTI):
Type I NSTI, typically involving the torso and perineum, results from a polymicrobial infection usually including group A streptococci (eg, Streptococcus pyogenes) and a mixture of aerobic and anaerobic bacteria (eg, Bacteroides species). These organisms typically extend to subcutaneous tissue from a contiguous ulcer or infection, or after trauma. Streptococci can arrive from a remote site of infection via the bloodstream. Perineal involvement (also called Fournier gangrene) is usually a complication of recent surgery, perirectal abscess, periurethral gland infection, or retroperitoneal infection resulting from perforated abdominal viscera. Patients with diabetes are at particular risk of type I NSTI. Type I infections often produce gas in the soft tissue, making its manifestation similar to that of gas gangrene (clostridial myonecrosis), which is a monomicrobial infection (1).
Type II NSTI is most commonly caused by group A beta-hemolytic streptococci; Staphylococcus aureus is the second most common pathogen. Patients tend to be younger with few documented health problems but may have a history of IV drug use, trauma, or recent surgery. The infection has the potential for rapid local spread and systemic complications such as toxic shock.
Type III NSTI is usually associated with aquatic injuries sustained in warmer coastal areas. Vibrio vulnificus is the usual pathogen. Type III infections share clinical similarities to type II infections and may spread rapidly.
Necrotizing soft tissue infection causes tissue ischemia by widespread occlusion of small subcutaneous vessels. Vessel occlusion results in skin infarction and necrosis, which facilitates the growth of obligate anaerobes (eg, Bacteroides) while promoting anaerobic metabolism by facultative organisms (eg, Escherichia coli), resulting in gangrene. Anaerobic metabolism produces hydrogen and nitrogen, relatively insoluble gases that may accumulate in subcutaneous tissues.
The primary symptom of necrotizing soft tissue infection is intense pain. In patients with normal sensation, pain out of proportion to clinical findings may be an early clue. However, in areas denervated by peripheral neuropathy, pain may be minimal or absent. Affected tissue is red, hot, and swollen and rapidly becomes discolored. Bullae, crepitus (resulting from soft-tissue gas), and gangrene may develop. Subcutaneous tissues (including adjacent fascia) necrose, with widespread undermining of surrounding tissue. Muscles are spared initially. Patients are acutely ill, with high fever, tachycardia, altered mental status ranging from confusion to obtundation, and hypotension. Patients may be bacteremic or septic and may require aggressive hemodynamic support. Streptococcal toxic shock syndrome may develop.
Diagnosis of NSTI, made by history and examination, is supported by leukocytosis, soft-tissue gas on x-ray, positive blood cultures, and deteriorating metabolic and hemodynamic status.
During surgical exploration, there is a gray exudate, friable superficial fascia, and absence of pus.
Treatment of early necrotizing soft tissue infection is primarily surgical, which should not be delayed by diagnostic studies.
IV antibiotics are adjuncts, usually including 2 or more drugs; an empiric regimen pending results of Gram stain and culture includes penicillin G 4 million units every 4 hours combined with clindamycin 600 to 900 mg every 8 hours or ceftriaxone 2 g every 12 hours. Evidence of bullae, ecchymosis, fluctuance, crepitus, and systemic spread of infection requires immediate surgical exploration and debridement. The initial incision should be extended until an instrument or finger can no longer separate the skin and subcutaneous tissue from the deep fascia. The most common error is insufficient surgical intervention; repeat operation every 1 to 2 days, with further incision and debridement as needed, should be carried out routinely.
Amputation of an extremity may be necessary.
IV fluids may be needed in large volumes before and after surgery. Antibiotic choices should be reviewed and adjusted based on Gram stain and culture of tissues obtained during surgery. Adjuvant hyperbaric oxygen therapy may also be of benefit; however, evidence of its effectiveness is inconclusive. IV immune globulin has been suggested for streptococcal toxic shock syndrome with NSTI.
Necrotizing soft tissue infection (NSTI) can develop from a contiguous ulcer or infection, hematogenous spread, or after trauma.
Consider NSTI in patients with characteristic findings or pain out of proportion to clinical findings, particularly patients with diabetes or other risk factors.
Arrange surgical therapy while instituting IV fluid and antibiotic therapy and without delaying for testing.