Necrotizing Fasciitis (cont.)
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Necrotizing Fasciitis Causes
Necrotizing fasciitis is caused by bacteria in the vast majority of cases, though fungi can also rarely lead to this condition as well. Many cases of necrotizing fasciitis are caused by group A beta-hemolytic streptococci (Streptococcus pyogenes), either individually or along with other bacterial pathogens. Group A streptococcus is the same bacteria responsible for "strep throat," impetigo (skin infection), and rheumatic fever. In recent years, there has been a surge in cases of necrotizing fasciitis caused by community-acquired methicillin-resistant Staphylococcus aureus (MRSA), often occurring in intravenous drug abusers. Most cases of necrotizing fasciitis are polymicrobial and involve both aerobic and anaerobic bacteria. Additional bacterial organisms that may be isolated in cases of necrotizing fasciitis include Escherichia coli, Klebsiella, Pseudomonas, Proteus, Vibrio, Bacteroides, Peptostreptococcus, and Clostridium, among others. A rarely encountered bacterium in humans, Aeromonas hydrophila has recently been implicated in a case of necrotizing fasciitis in a young female who cut her leg in a zip line accident. This organism, generally found in freshwater and brackish water in warmer climates, typically causes infections in fish and amphibians.
In many cases of necrotizing fasciitis, there is a history of prior trauma, such as a cut, scrape, insect bite, burn, or needle puncture wound. These lesions may initially appear trivial or minor. Surgical incision sites and various surgical procedures may also serve as a source of infection. In many cases, however, there is no obvious source of infection or portal of entry to explain the cause (idiopathic).
After the bacterial pathogen gains entry, the infection can spread from the subcutaneous tissues to involve deeper facial planes. Progressive spread of the infection will ensue, and it can sometimes involve adjacent soft tissues as well, including muscle, fat, and skin. Various bacterial enzymes and toxins lead to vascular occlusion, resulting in tissue hypoxia (decreased oxygen) and ultimately tissue necrosis. In many cases, these tissue conditions allow anaerobic bacteria to proliferate as well, allowing for the progressive spread of infection and continued destruction of tissue.
Individuals with underlying medical problems and a weakened immune system are also at increased risk of developing necrotizing fasciitis. Various medical conditions, including diabetes, renal failure, liver disease, cancer, peripheral vascular disease, and HIV infection, are often present in patients who develop necrotizing fasciitis, as are individuals undergoing chemotherapy and those taking corticosteroids for various reasons. Alcoholics and intravenous-drug abusers are also at increased risk. Many cases of necrotizing fasciitis, however, also occur in otherwise healthy individuals with no predisposing factors.
For classification purposes, necrotizing fasciitis has been subdivided into three distinct groupings, primarily based on the microbiology of the underlying infection; type 1 NF is caused by multiple bacterial species (polymicrobial), type 2 NF is caused by a single bacterial species (monomicrobial) which is typically Streptococcus pyogenes, and type 3 NF (gas gangrene) is caused by Clostridium spp. Infection caused by Vibrio spp (frequently Vibrio vulnificus) is a variant form often occurring in individuals with liver disease, typically after ingesting seafood or exposing skin wounds to seawater contaminated by this organism.
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