Necrotising soft tissue infections (NSTIs) are a collection of severe infections characterised by rapidly progressive soft tissue inflammation and necrosis.
Specific examples of NSTIs include:
NSTIs are usually rapidly progressive, resulting in extensive tissue destruction. They are associated with high morbidity and mortality. Accordingly, prompt diagnosis and treatment are essential.
NSTIs may start as a local infection from an abrasion, laceration or bite. Patients may present with cellulitis, which rapidly progresses with pain disproportionate to the clinical picture. Remember, pain may precede skin changes by 24-48hrs.
Diagnosis of necrotising infections is clinical. Effective management necessitates early recognition, resuscitation and early initiation of antibiotics. Urgent surgical debridement remains the predominant factor affecting mortality. Post-operatively, patients are typically managed within an intensive care setting.
NSTIs are rare, but carry high morbidity and mortality rates.
Although rare, evidence suggests the incidence of NSTIs in England have doubled in the last 20 years and now sit around 1,100 cases per year. The cause for this increase is not clear, however may reflect greater awareness (and thereby increased reporting), increasing bacterial virulence and/or increased antimicrobial resistance. Despite this increase in incidence, age-standardised mortality remains at around 16%.
NSTIs typically occur in the lower extremities, trunk and perineum but rarely involve the face. Commonly there is an inciting cause, such as a laceration or pre-existing infection.
Patients presenting with NSTIs often have some predisposition to infection. Immunocompromise, increasing age, peripheral vascular disease, and obesity are some predisposing factors.
NSTIs are commonly classified according to the microbial source of the infection.
Types I and II are responsible for the majority of cases of NSTI in the UK. Types III and IV are extremely rare.
Constitute the majority of NSTIs in the UK, approximately 70-80%. These infections are polymicrobial (typically mixed anaerobes & aerobes, on average four or more organisms) and more frequently occur in the perineal region and trunk.
Patients are typically immunocompromised, diabetic and/or with multiple co-morbidities (peripheral vascular disease, obesity, chronic renal disease, chronic alcohol/drug abuse, HIV etc). Mortality is dependent on time to diagnosis and pre-existing comorbid state. Importantly, NSTIs in these patient groups may present somewhat more indolently and therefore diagnosis may require a higher degree of clinical suspicion.
Caused by group A streptococcus (Strep. pyogenes +/- Staph. aureus). More commonly affecting the limbs, but truncal involvement has also been reported. Type II infections are associated with toxic shock syndrome. Type II NSTIs can occur in healthy, young, immuno-competent individuals. Frequently, there is a history of recent trauma.
Gram-negative monomicrobial infection. Typically associated with Vibrio species infection from seafood ingestion or water contamination of wounds. Carries high mortality despite early recognition.
Fungal infection (typically Candida species, zygomycetes). Typically occur in patients with large traumatic wounds or burns who are severely immunocompromised. Infections tend to be particularly aggressive with mortality of up to 50%.
Knowledge of skin and soft tissue anatomy is key.
Knowledge of the anatomy of the skin and fascial planes is key to understanding the pathogenesis of NSTIs and their clinical manifestations.
Disproportionate pain compared with physical findings is typical. Pain often precedes skin changes by 24-48hrs.
NSTIs may be initially misdiagnosed as cellulitis or a superficial skin infection. The most distinctive symptom is disproportionate pain compared with physical findings.
Pain often precedes skin changes by 24-48hrs.
Skin changes of NSTIs typically occur in three stages:
Systemic features include fever, shock and acute kidney injury (indicated by reduced / no urine output).
Diagnosis of NSTIs is essentially clinical.
The gold standard is surgical exploration and tissue biopsy.
The presence of fascial necrosis with loss of natural tissue planes is diagnostic. IF SUSPECTED DO NOT DELAY SURGICAL EXPLORATION.
Laboratory Risk Indicator for Necrotising Fasciitis (LRINEC) score.
A scoring system designed to distinguish between necrotising infections and other soft tissue infections (e.g. cellulitis). A score greater than 6 is highly suggestive of an NSTI (positive predictive value: 92%; negative predictive value: 96%).
The score is based upon:
Prompt resuscitation, broad-spectrum antibiotics, aggressive surgical dedribement & supportive care are essential.
Patients should be managed by an experienced multidisciplinary team - critical care, surgeons (plastics, orthopaedics, general surgeons), anaesthetists and microbiologists.
Patients' should be optimised with the aim of establishing adequate tissue perfusion and oxygenation. Resuscitation should be commenced early and be 'goal-directed'; invasive monitoring and central venous access may be warranted. Given the significant risk of multiorgan failure, patients should be managed in an intensive care facility.
Antibiotics should be perceived as an adjunct to surgical debridement. Broad-spectrum antibiotics should be initiated without delay according to Trust guidelines. Commonly used agents include:
Due to the complex microbiology of NSTIs, early discussion with a senior microbiologist is crucial.
Timing and adequacy of the initial surgical debridement are key.
Due to the thrombogenic nature of NSTIs, early and aggressive surgical debridement remains the priority. Antibiotics are unable to penetrate infected tissue effectively - thus all necrotic tissue must be excised. Deeper tissues (e.g. muscle) should be inserted thoroughly and excised if needed. Timing and adequacy of the initial debridement is a major determinant of mortality. Debridement may need to be so radical that limb amputation is warranted - with this in mind, it is always wise to coordinate with the appropriate surgical teams early (e.g. Orthopaedics).
Surgical debridement physically removes the nidus of infection and associated toxins. Multiple tissue and fluid samples are often taken at the time of surgery and sent to the microbiologists for analysis. This allows for subsequent refinement of the antibiotic regimen.
Infection is rarely eradicated after a single procedure and serial debridements are often needed (wounds are commonly re-explored in theatre at 24-48 hours time). Further debridement of devitalised tissue may be undertaken at this time.
Effective management of NSTIs necessitates aggressive surgical debridement followed by prolonged supportive care and eventual reconstruction.
Additional considerations include:
Reconstructive procedures should only be considered when the infection is completely eradicated.
Reconstruction should be delayed until the infection has clinically (and microbiologically) resolved. Multiple swabs and tissue samples are often sent over a prolonged period for culture and subsequent analysis.
Following multiple debridements, patients can often represent a reconstructive challenge. Methods of reconstruction typically follow the reconstructive ladder, with methods employed including:
Selection of the most appropriate reconstructive technique typically depends not only on the site and nature of the defect but also on the viability of the underlying and surrounding tissues.
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