Necrotizing Fasciitis Early Diagnosis: Etiologic Review
Steven Lee, Jennifer Veldhuyzen, Anh Tuan Vo
Biomedical Engineering Dept, School of Engineering and Applied Science
University of Virginia
415 Lane Road, Charlottesville, VA 22908
Etiology and Pathogenesis of Necrotizing Fasciitis
Necrotizing Fasciitis (NF) is a disease characterized by serious infection of the fascia and subcutaneous tissue. The diagnosis of this disease presents a challenge to medical personnel because the early stages of the disease bear resemblance to other skin infections. NF pathogens include a wide range of microorganisms, mostly bacteria, although incidences of fungal NF have been reported. Polymicrobial infections are common, with aerobic, anaerobic, gram negative and gram positive species represented. Literature review indicates that bacterial cell wall components trigger neutrophil hyperresistinemia, and many bacteria, especially Staphylocuccus aureus and Streptococcus pyogenes, release superantigens into tissue that trigger T-cell activation and lead to uncontrolled cytokine dumping, promoting inflammation. Other toxins released include Panton-Valentine Leukocidin, and superantigens work in conjunction with the gram negative lipopolysaccharide to increase tissue damage. Surgical debridement and intravenous broad spectrum antibiotics, still the primary treatment for NF, can be supplemented with alternative treatments such as hyperbaric oxygen therapy and intravenous immunoglobulin G. Surgical debridement as the primary treatment is invasive and damaging; therefore, this review emphasizes the importance of early diagnosis and establishment of an antigen-specific treatment plan for NF.
Keywords: fascia, subcutaneous tissue, debridement, hyperbaric oxygen therapy, immuglobulin G, superantigens, leukocidin, lipopolysaccharides
Necrotizing Fasciitis (NF) is a rare, deadly disease associated with widespread necrosis of the fascia, subcutaneous tissue, and in severe cases, the skin and muscles1,2. Fascias are soft, connective tissues that serve to separate the muscles from the skin and protect them. Subcutaneous tissues, also known as hypodermis, store fat and regulate temperature. NF, named by Wilson in 19523, occurs mostly in adults, but infants are also susceptible. Annually, there are approximately 0.08 per 100,000 children worldwide who are reported with NF4. Also, it was reported in 1995 that there are approximately 0.49 cases per 100,000 population5. In the United States, NF has an incidence of 500 to 1500 reported cases per year3,6. The prognosis of this disease is entirely dependent upon the time of detection and treatment of the disease; however, mortality rates can range from 30% to 60%2,7.
There is a wide variety of pathological organisms and/or bacteria that can cause the infection. Medical personnel have classified NF into two types: Type 1 and Type 21. Type 1, polymicrobial infection, occurs with greater frequency than Type 2, monomicrobial infection or infection due to only one organism1. NF can also be classified by the microbe causing it. For instance, the type of NF that is caused by Streptococcus Pyogenes, also known as group A streptococcus, is called acute streptococcal gangrene2,8.
Various risk factors contribute to NF susceptibility. Diabetes mellitus is one such predisposing factor for this infection because a relatively high blood sugar level provides an environment that is low in oxygen content: an environment in which bacteria thrive9. Any injury, such as a small lesion in the hand, can also increase one’s likelihood of accruing NF, and patients who have undergone past surgeries are prone to develop this disease via surgical area infection10. Although NF can affect any age group, the elderly, age of 60 and older, are at a higher risk than those that are younger,1,11.
The use of anti-inflammatory or immunosuppressive drugs may also increase the risk of NF11,12. The use of anti-inflammatory drugs inhibits symptoms of infection; thus promoting a delay in treatment time11. Immunosuppressive drugs lower the body’s normal immune response, thereby increasing the susceptibility to infections. A case study presented the effects of one such immunosuppressive, Infliximab, used to treat rheumatoid arthritis by inhibiting tumor necrosis factor-alpha (TNF-α). Study showed that the patient who received Infliximab suffered more severe sepsis or an increase in infections related to a suppressed immune system12.
Moreover, patients associated with injection drug use are also at a higher risk of contracting the infection, as are obese patients with a body mass index greater than 3013. Amongst newborns, circumcision has reportedly been cited as a risk factor, but there is little support for this hypothesis14. Other factors that can increase the risk of susceptibility of the lethal disease include gender, past history of trauma and/or heart disease, malnutrition, and alcohol abuse11.
NF’s resemblance to other less-threatening skin infections promotes misdiagnosis and heavily impacts survival rates. Misdiagnosis as cellulitis, a milder skin infection, is common for NF patients1,15. Patients with these skin infections both share a common symptom of a mild edema, or small inflamed area of the skin15 at the site of infection. As a result, medical personnel have difficulties in detecting NF until the infection worsens. NF is suspected when a bulla begins to form at the site of infection, and signs of rotting skin are evident.3
Newborn babies are also susceptible to NF and its subsequent misdiagnosis. In toddlers, physicians mistakenly diagnose NF for omphalitis, which is an infection of the umbilical cord in newborn babies. Omphalitis and NF both present with a similar edema and practitioners generally cannot differentiate between the two infections until the area of infection begins to discolor, indicative of NF16,17. A study of three Costa Rican infants demonstrated that delay in both detection and treatment of NF resulted in early deaths,16 further establishing that as with adult patients, early detection is essential to decreasing morbidity and mortality rates.
In summary, early detection of this disease at its benign and less-threatening stages presents a challenge to many medical officials, causing a delay in administration of appropriate treatment needed to prevent the disease from spreading throughout the body1. As a consequence, mortality and morbidity rates increase.
There are specific symptoms that differentiate NF from other infections, however. In addition to the characteristic rotting, discolored edema, many affected individuals also exhibit symptoms of high fever and sepsis syndrome18,19, which occurs when the human body produces a systemic response against an infection characterized by abnormal cytokine release and subsequent organ degredation20. Unbearable pain at the site of infection is another common sign18,19, signifying a systemic bacterial toxin release degrading body tissue19; however, this symptom is only expressed in the later stages of the disease. This lack of pain heavily diminishes the likelihood of early diagnosis.
Many medical technologies are also available to promote NF diagnostic accuracy early on. Such medical technologies include ultrasounds, frozen section biopsies, and magnetic resonance imaging. Although ultrasounds and frozen section biopsies can be used to appropriately diagnose NF14, magnetic resonance imaging, or MRI, is frequently used to provide more specific information about an infection. The study conducted by Schmid, Kossmann, and Duewell found that MRI scans are very sensitive in detecting skin infections, especially in analyzing edemas22. Finally, as a non-invasive medical test, MRIs also provide practical and psychological ease in diagnosis because they do not require physicians to penetrate the patients’ body.
Laboratory tests such as the Laboratory Risk Indicator for Necrotizing Fasciitis (LRINEC) can also be used to determine whether an individual has contracted NF.23 The LRINEC has a scoring system that measures a patient’s level of C-reactive protein, total white cell count, and the levels of sodium, glucose, creatinine, and haemoglobin. Generally, a LRINEC score of 6 or greater (range 0-13) provides a good indication that the patient is at a high risk of contracting NF23. Thus far, the LRINEC has been proven to be very effective in helping medical personnel detect NF at the early stage24. This is mainly due to the fact that, based on the study conducted by Dr. Chin-Ho Wong, the LRINEC has a high specificity but a low sensitivity. In other words, the LRINEC has a high success rate in detecting early NF24.
Microbiology of Infectious Agents
A variety of bacterial species have been implicated in causing NF, notably including common gram-positive bacteria such as Staphylococcus aureus and Streptococcus pyogenes. Streptococcus pyogenes infections in general are reportedly on the rise since the 1980s, with seasonal fluctuations and population density affecting bacterial strain and rate of infection.26
Variability of the N terminus of emm genes appears to play some role in bacterial strength: Strep. strains with alleles emm 1 and 3 cause NF more frequently and more severely than other strains do, with emm 3 considered the more fatal. While it is not known precisely what causes greater emm 3 fatality, it is known that the emm gene does code for variability in the M protein on the surface of the bacteria. If the M protein plays a role in cell to cell adherence or tissue infiltration,26,27 emm 3 strain bacteria binds more fiercely to host cells and moves more swiftly through the body, thus increasing the severity and spread of the resultant NF.
Study of the evolutionary and location history of these bacteria may prove as essential as genetic analysis, as Strep. resistance to antibiotics in certain regions may correlate with the high use of the particular antibiotic in that region, indicating rapid defensive species adaptation. Staphylococcus aureus and epidermidis both occur in greater frequency than methicillin-resistant strains (MRSA)26;however, the MRSA strain is becoming a more serious factor as it migrates between community-acquired and hospital-acquired cases, and its resistance increases the difficulty in treatment.28, 29
A number of gram negative bacteria have also been cultured in NF cases,30 and soft tissue infections are often caused by a mixture of gram positive, gram negative, aerobic, and anaerobic bacteria.14, 31 The most common anaerobic bacteria cultured by far appears to be Prevotella, with E.Coli presenting as the most prevalent gram-negative aerobic bacteria.14,30,31 The more common polymicrobial and less common monomicrobial infections tend towards the same mortality rate,14, 31 indicating that the competition between the bacteria has little to no effect on its potency.
NF is not, however, limited to only bacterial infection, nor is it limited to predominantly terrestrial species. Cases of NF have been reported following bacteremia due to the sea-dwelling protobacteria Shewanella algae,32 the toxin in a Hymenotera bee sting33, and even zygomycotic fungi34, demonstrating that the mechanism of attack is not dependent strictly on one specific toxin excreted into the tissue. Treatments of these other microbial infections tend to follow the same procedure as bacterial infections, 32,33,34 namely extensive debridement and antibiotics. Fungal NF has been successfully treated with posaconazole, a simple antifungal agent,35,36 but the more common bacterial NF is complicated with strong, swiftly evolving antibiotic resistance. With any infection, there is always the possibility that broad scale antibiotic treatment will promote the development of a superinfection, in which toxins have eliminated all weaker bacteria, leaving more powerful strains without competition. The organism variety in NF infections mandates careful determination of the exact biology of each patient’s separate case for appropriate administration of antibiotics.
Microbial Toxin Release
Microbes entering the host through a lesion in the skin release toxins as they spread throughout the area of entry. One of the toxins causing the muscle soreness present in early MRSA-induced NF is Panton-Valentine Leukocidin (PVL), a controversial antigen causing apoptosis of phagocytes. Because PVL commonly induces cytolysis, and study has shown no difference in cytolytic activity between PVL active and inactive MRSA strains, it appeared that PVL did not in fact play a large mechanistic role in NF infection. However, Tseng et. al found that PVL-antibodies, administered early, do have an effect in limiting soft tissue infection. Their experimental modeling also demonstrated that PVL destroys muscle tissue and induces inflammation via immune-cell over-activation and cytokine dumping. This mechanism of PVL attack explains why older experimental candidates tended to fare better in the study; part of PVL’s negative impact involves the host’s own immune system’s poor reaction, and older immune systems do not respond so extensively to bacterial challenge.37 This is not to say that older patients have a higher NF survival rate, merely that older individuals may survive PVL toxin release better than younger individuals. In addition to PVL, NF bacteria also release compounds that promote acidosis, disrupting normal tissue function. Substances released by bacteria also cause dying and high risk patients to present with elevated potassium and creatinine serum, as well as an understandably high white blood cell count.14
Superantigen Behavior and T-cell Autoimmune Response
In addition to releasing waste into tissue, consuming resources needed by local tissue and bearing foreign antigens and toxins that do not belong in the human body, bacteria such as S. pyogenes release superantigens which alter the immune system’s normal response.38 S. pyogenes specifically operates via antigen speA and speC, with emm1 and 3 strains operating with high levels of speA.26,27 SpeF, a heat resistant antigen noted for its possible role in bacterial dissemination, is also documented.38
Superantigens, which are important in a number of severe bacterial infections, trigger high levels of T-cells at the infection site, summoning elevated immune response and pro-inflammatory release of cytokines by promoting the production of immune-cell homing chemicals such as CLA. Study shows elevated CD4 levels at S. pyogenes infection sites38 and since CD4 is a receptor protein primarily used by helper T-cells (Th) for binding to MHC-II occurring on antigen-presenting immune cells, an elevated level is indicative of the presence of Th. CD-44 is another CD protein with elevated levels in NF S. pyogenes infections. This protein causes inflammatory T-cells to dump cytokines when activated and bound to hyaluronic acid. It is believed that superantigens take on this activation role, and because the bacteria infecting the tissue contain hyaluronic acid in their capsules, it is possible that the active CD-44 actually binds to the bacteria. This raised bacterial adherence increases subsequent damage to T-cells, accelerating the disease progression38. Normally, any bacterial antigens would go through digestion via a monocyte first and then undergo display on a MHC II for the T-cells to bind to and investigate. When instead chemicals bind externally to the T-cell receptors that read MHC II, large scale cytokine dumping occurs and this further causes large-scale activation of other T-cells.39
Superantigens are also thought to interact with the surface toll-like receptors (TLR) on monocytes involved in the innate immunity. These TLRs normally help differentiate between different bacterial types, often operating in conjunction with lipopolysaccharide (LPS) receptors, which recognize the LPS present in the cell wall of gram-negative bacteria. Streptococcal and staphylococcal superantigens in conjunction with LPS have been able to up-regulate monocyte LPS signaling in infection and triggering massive signaling of TLR-2 receptors when nothing else in the bacteria will. Superantigens do not seem to actually bind to these TLRs, but depend heavily on some kind of MHC-II action in order to function. In vivo TLR up-regulation only occurs in the presence of S. pyogenes, which could be due to the fact that S. pyogenes has more genes coding for superantigens and produces stronger superantigens than staphylococcal species. Unlike in previously described MHC and superantigen reactions, it is not clearly posited what result TLR up-regulation has on infection progression, as monocyte response against the bacteria is only enhanced at incredibly high concentrations of TLR-ligand. Since monocytes exposed to TLR-ligands ex vivo demonstrate a significantly diminished potency, TLR up-regulation probably negatively impacts the monocyte cytokine release40.
Another unfortunate immune system phenomenon occurring in NF involves the body’s anti-prokaryotic neutrophils. NF presents with a great deal of mononuclear cell inflammation from extensive cytokine activity caused by lymphocytes early on41, but despite T-cell and monocyte abundance, affected areas have been known to present with an unusual absence of neutrophils38. Experiments have shown that although neutrophils successfully phagocytose and engulf invasive bacteria, invasive S. pyogenes is actually resistant to death via phagocytosis. In fact, in severe cases the bacteria actually succeeds in paralyzing and then killing polymorphonucleocytes (PMN) before they can reach the affected homing area. On contact with the bacteria, PMNs come into contact with streptolysin O, which are released by the bacteria to lyse eukaryote cell membranes. Even if the bacteria do not kill PMNs, bacterial envelope proteinases such as ScpC do destroy homing cytokines such as IL-8, preventing neutrophils from migrating.42
NF bacteria can also induce neutrophils to secrete chemicals that enhance microbial activity, such as Protein M1, which presents itself in S. pyogenes infections. It binds to monocytes and forms a complex with the molecule fibrinogen, stimulating the monocyte secretion of heparin-binding protein (HBP). In the presence of this molecule, M1 triggers additional widespread cytokine and chemokine dumping from polymorphonucleocytes such as neutrophils, ultimately using the host’s immune system to promote greater intensity cycles of inflammation. Coincidentally, M1 uses the afore-mentioned TLRs to bind to host immune cells.43
In both strep-related and gram-negative induced infections M1 also triggers macrophages and neutrophils to release resistin into the system, spreading the problem much farther than the original site of infection, as resistin levels correlate with septic shock. A greater fraction of the neutrophils than the macrophages release resistin, and a small group of unknown cells also contribute to the release. Resistin pre-occurs in neutrophils, unlike the cytokines produced in response to the bacteria, and may have some role in glucose metabolism. Bacterial proteins such as LPS and M1 trigger resistin’s widescale release, with M1 mobilizing 60%. It appears that this process also has to do with M1’s ability to bind to fibrinogen.44
Surgical debridement of necrotic tissue is the primary treatment for NF. Many studies have shown that prompt and adequate initial debridement greatly decreases the risk of death4,45. Adequate debridement requires the surgeon to remove all necrotic tissue until healthy tissue can be seen on the debridement site. The first incision is made over the infected area parallel to neurovascular bundles and extended until healthy bleeding tissue is reached52. Any necrotic tissue left over from debridement serves as an incubation area for bacteria51. Amputation of a limb should be considered if debridement involves removing muscle groups from a limb that would leave the limb inoperable, or if the infection has progressed to the joints47. The wound must be carefully monitored and re-evaluted for signs of necrotic tissue since multiple debridement surgeries are commonly required before the progress of the infection can be controlled46,47,48.
Careful management of the extensive wounds resulting from this debridement is necessary in order to prevent secondary infections and encourage healthy tissue formation52. One report suggested using hydrogel dressings because they promote the formation of granulated tissue. The report also suggested that wet to dry dressings be discontinued51. A different study recommended the use of wet to dry dressings be used in combination with zinc saline when surgical debridement is not an option, preventing secondary infection63. Foam dressings can be used to fill cavities that result from debridement. Instead of hydrogel dressings, vacuum assisted closure dressings could also be used to aid in tissue granulation57. There does not appear to be a definitive preference in favor of hydrogel dressing over vacuum assisted closure dressings. The wound can be closed using skin flaps or split thickness grafts once the infection has been controlled and healthy tissue has formed52.
Patients diagnosed with NF are put on intravenous broad spectrum antibiotics as soon as possible. Although current broad spectrum antibiotics have little effect on the infected site because of poor blood flow and inadequate antibiotic delivery to necrotic cells, antibiotics can help control the spread of the infection and should be used as a supplement to debridement46,52. Various regiments of antibiotics can be used to cover Streptococci, Staphylcocci, and anerobes before laboratory results. Clindamycin should be involved in the antibiotic regiments if group A streptococci are suspected of being involved in the infection54,55,56 because it inhibits production of exotoxins and the M protein by shutting down bacterial ribosome function46. Vancomycin and linezolid are not usually used unless MRSA is thought to be present. Bellapianta et al suggest that if clusters of gram-positive cocci are present on the gram stain then this provides adequate suspicion that MRSA is present and vancomycin and linezolid should be used46.
Reading of the literature provides an overall recommendation for the use of localized hyperbaric oxygen (HBO) as a therapeutic treatment for NF. Various studies have reported that there are survival benefits from the use of HBO while others report that there are no survival benefits58. One retrospective study showed that patients treated with HBO had a mortality rate that was 11% higher than patients without HBO treatment. Patients treated with HBO on average required 3.3 surgical debridements while patients without HBO treatment only required an average of 1.5 surgical debridements59. The results of another study showed that patients treated with HBO had a 22% lower mortality rate than patients who were not treated with HBO64.
Patients with NF caused by group A Streptococcus can undergo intravenous immunoglobin G treatment (IVIG). IVIG is thought to inhibit the binding of superantigens from GAS onto T-cells which would then inhibit the release of pro-inflammatory cytokines by the T-cells. Patients can respond to the over production of pro-inflammatory cytokines by going into a toxic shock syndrome60. Inhibiting the release of these pro-inflammatory cytokines limits this risk of toxic shock syndrome in NF patients. Studies confirm the mechanism of IVIG, and while the mechanism of IVIG appears to be very promising, there is a lack of randomized studies on actual patient outcome with IVIG61. As a result there is insufficient data to support or discourage the use of IVIG for patients with NF.
Protein C inhibitor (PCI) is a potential future treatment for NF. PCI possesses a broad antibacterial property that works by triggering a membrane disruption in bacteria and causing an outflow of intracellular matrix and killing the bacteria without damaging eurokaryotic cells. One study found that there was increased PCI concentration around an infection site. The study determined that PCI could be transported on the surfaces of platelets, and during an infection, PCI was delivered to the infection site once the platelets were activated. The authors of this study also supported the use of PCI as a treatment option because processing PCI was not necessary since the full protein had antibacterial properties62.
Another field of research focuses on creating a vaccine using protease secreted by group A Streptococcus. One study mutated Streptococcal Pyrogenic Exotoxin B and fused it with Streptococcal Pyrogenic Exotoxin A. The fused proteins were then injected into mice and the mice were protected from the Steptococcus pyrogene infection65. The study suggested that the fused proteins could have a potential role in protecting against Steptococcus pyrogene infections.
Review of the available literature reveals NF as a highly complex, widely unexplained system failure. A disease in which some sectors of the immune system are up-regulated while others, namely the neutrophil front, are completely immobilized, presents enormous treatment difficulties. This review has uncovered several clear principles, however. Firstly, because primary treatment involves debridement and an intense antibiotic regimen, early diagnosis is essential to preserving as much of the patient’s body as possible. Secondly, accurate identification of microbial agents allows for the possibility of a specific, narrow-spectrum antibiotic regimen, and treatment should ideally proceed away from broad-spectrum antibiotics in the direction of specific antibiotic development that is effectively delivered to the infection source.
Current treatments deal with removing the necrotic cells along with the toxins through the excessive debridement of necrotic and healthy tissues. The overall literature consensus encourages surgeons to remove a large portion of healthy vascularized tissue around the necrotic area in hopes of controlling the spread of NF, but a large percentage of NF patients must undergo second and third debridement surgeries. We propose a new technique in which toxins are removed without requiring the removal of healthy tissue. Our device would encapsulate the wound site and circulate a solution across the wound site. The goal of the circulation is to provide a gradient for the toxins to diffuse into the circulating solution from the extracellular fluid. A filter could be placed on the other end of the wound to remove the toxins from the solution and allow solution to re-circulate across the wound site, enhancing additional toxin removal from the body. In addition to containing all of the necessary ions in the extracellular fluid, the solution could be incorporated with antibiotics. Delivering antibiotics in this way directly to the infected site would stop the progression of NF. With this method, we hope to shift the treatment from relying on debridement surgeries to relying on antibiotics and dialysis. We envision debridement surgeries will remain necessary, but the mentality will shift towards using debridement only to remove necrotic tissue, preserving healthy tissue while the antibiotics eliminate infectious agents.
Additionally, toxins that destabilize the hosts’ immune system could be directly treated via the engineering of specific inhibitors and denaturing solutions. Denaturing superantigens released by the bacteria would address this issue, but researchers would have to confront the concern of denaturing necessary human enzymes as well. Furthermore, many superantigens are able to withstand greater wider ranges of temperature and pH than human proteins can, reducing the likelihood of success for any chemo-type activity. It would be optimal to find an enzyme that could break down the proteinase that dissolves the homing cytokines for neutrophils, or an enzyme that directly digests M1. It is probably more possible to insert alternative, harmless ligands into the system that would bind to TLR and compete with M1, preventing neutrophils from releasing resistin. However, any M1-like substance may have the capacity to cause resistin release. To prevent this, researchers could design the competing molecule so that it binds to the neutrophil site, yet can be modified by another molecule in such a way as to prevent effective resistin release.
In closing, thorough review of the current literature illustrates that addressing NF on a biological level, and driving the focus away from blunt removal of tissue, would greatly enhance patient comfort and survival under the ravages of this rare and deadly infection. Early diagnosis and continued microbial research continues to provide hope for decreasing the mortality and morbidity rates of this disease.
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