Surgical Site Infections

Surgical site infection (SSI) is a type of surgical infection that occurs at or near a surgical incision within 30 days of the procedure or within 90 days if prosthetic material is implanted. Surgical site infections are classified according to the depth of involvement as superficial, deep, or organ/space. Diagnosis relies on clinical findings and may require diagnostic imaging. Management involves antibiotics as well as surgical drainage/debridement as necessary.

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Overview

Definition

Surgical site infection (SSI) is an infection that occurs at or near a surgical incision or deep to it within 30 days of the procedure or within 90 days if prosthetic material is implanted.

Classification of surgical site infections

  • Superficial: involves skin and subcutaneous tissue
  • Deep: involves fascial and/or muscle layers
  • Organ/space: involves organs or spaces deep to the incision (e.g., intraabdominal abscess)

Classification of surgical wounds

Surgical wounds are classified from clean to dirty on the basis of the increasing risk of infection.

Table: Classification of surgical wounds
ClassDescription
Clean
  • No inflammation encountered
  • The respiratory, GI, or genitourinary tract is not entered.
  • There is no break in aseptic technique.
Clean-contaminated
  • The respiratory, GI, or genitourinary tract is entered.
  • No significant spillage
Contaminated
  • Acute inflammation (without pus) is encountered or there is visible contamination of the wound
  • Examples:
    • Spillage from a hollow viscus during the procedure
    • Compound/open injuries < 4 hours old
Dirty
  • Presence of pus
  • Previously perforated hollow viscus
  • Compound/open injuries > 4 hours old

Pathophysiology

Contamination of surgical wounds occurs to some degree in all surgical procedures. However, in most cases, the infecting agents do not overcome host defenses. For SSI to occur, the bacterial colonization must trigger an immune response.

Risk factors

Procedural factors:

  • Violations of sterile technique (i.e., poor skin preparation, contamination of instruments, inadequate handwashing/scrubbing)
  • Inadequate or ill-timed antibiotic prophylaxis
  • Prolonged procedure time
  • Local tissue necrosis
  • Hematoma formation
  • Use of foreign material in wounds (e.g., drains, mesh)
  • Preoperative shaving
  • Prolonged preoperative hospital stay (↑ nosocomial infection)
  • Contamination with bowel contents

Patient factors:

  • Advanced age (> 65 years)
  • Prior surgical infection
  • Malnutrition
  • Obesity
  • Diabetes mellitus
  • Hypothermia
  • Hypoxemia
  • Hyperglycemia
  • Immunosuppression
  • Therapy with corticosteroids
  • Chronic inflammation
  • Smoking
  • Renal failure
  • Peripheral vascular disease (PVD)
  • Anemia
  • Previous radiation of surgical site
  • Chronic skin disease
  • Carrier state (e.g., chronic Staphylococcus carriage)

Pathogenesis

When the virulence factors of a microorganism are sufficient to overcome the innate and adaptive immune responses, a clinically important infection occurs in the following manner:

  1. Once microbes enter, host defenses (i.e., tissue barriers, iron-sequestering lactoferrin and transferrin, and fibrinogen) act to limit and/or eliminate all pathogens.
  2. Resident macrophages secrete cytokines (tumor necrosis factor α (TNF-α); IL-1β, 6, and 8; and interferon (IFN-γ)) within the tissue milieu and the systemic circulation.
  3. Microbial opsonization, phagocytosis, and extracellular and intracellular microbial destruction occur via cellular ingestion.
  4. At the same time, the classical and alternative complement pathways are activated and vascular permeability increases.
  5. Inflammatory fluid and neutrophils flow into the area of infection.
  6. Possible outcomes:
    • The microorganism is eradicated.
    • Containment in the parenchyma (i.e., abscess)
    • Localized infection, displaying local symptoms (i.e., cellulitis)
    • Systemic infection (bacteremia or fungemia) with its accompanying presentation (i.e., fever, tachycardia)
Sites of bacterial colonization

Sites of bacterial colonization and common colonizers

Image by Lecturio. License: CC BY-NC-SA 4.0
Factors required for the development of an infection

Factors required for the development of an infection

Image by Lecturio. License: CC BY-NC-SA 4.0

Clinical Presentation

Systemic inflammation:

  • Fever
  • Tachycardia
  • Tachypnea

Local inflammation:

  • Local erythema
  • Tenderness
  • Purulent discharge
  • Peritoneal signs in organ/space abdominal infections
  • Abscesses close to the skin are painful, and the overlying skin is swollen, erythematous, and warm.

Septic shock:

  • Hypotension
  • Pallor
  • Sweating
  • Rigors
  • Confusion (altered mental status) 

Diagnosis

The diagnosis of SSI relies primarily on clinical findings. Diagnostic imaging and laboratory studies are helpful for identifying deep tissue and organ/space infections.

Physical exam

Clinical findings (local):

  • Purulent exudate draining from the surgical site
  • A surgical site that has reopened in the setting of ≥ 1 clinical sign of infection (pain, swelling, erythema, warmth) 
  • Erythema, swelling, fluctuance, and/or induration around the incision
  • Organ/space infections: 
    • Fever/chills/signs of sepsis
    • Localized abdominal tenderness with peritoneal signs

Sepsis findings (systemic):

  • Criteria for systemic inflammatory response syndrome (SIRS):
    • Temperature > 38°C (100.4°F) or < 35°C (95°F)
    • HR > 90/minute
    • RR > 20 or partial pressure of carbon dioxide (PaCO2) < 35 mm Hg
    • WBC > 12,000 (leukocytosis) or < 4000 (leukopenia)
  • Sequential Organ Failure Assessment (SOFA):
    • Mortality prediction score 
    • Used to track a person’s status during the stay in an ICU
    • Based on an assessment of the degree of dysfunction of 6 organ systems. 
    • The score is calculated at admission and every 24 hours until discharge.
    • Quick SOFA (qSOFA) is an abbreviated assessment tool (poorly sensitive, but decently specific for the risk of death):
      • RR ≥ 22/minute
      • Altered mentation
      • Systolic BP ≤ 100 mm Hg

Laboratory

  • CBC: WBC > 10,000 (leukocytosis)
  • Serum creatinine
  • BUN
  • CRP
  • Microbiology: 
    • Gram staining and culture of a wound or aspiration
    • Blood cultures in patients who are febrile or have suspected sepsis

Diagnostic imaging

  • Deep tissue or organ/space SSIs may require radiologic imaging for definitive diagnosis.
  • Prosthetic infection (e.g., hernia mesh) may also require imaging to support the diagnosis.
  • CT, MRI, or ultrasonography can identify abdominal, pelvic, and soft tissue abscesses.
  • Imaging is needed for percutaneous or surgical drainage planning.
Axial computed tomography of the pelvis showing pelvic abscess

Postsurgical pelvic abscess

Image: “Axial computed tomography of the pelvis showing pelvic abscess” by Plastic and Reconstructive Surgery Department, Cork University Hospital, Wilton, Cork, Ireland. License: CC BY 3.0

Management

The management of SSIs may include antibiotics, abscess drainage (open or percutaneous), and removal of infected prosthetic material.

Antibiotic therapy

  • Reserved for patients with significant cellulitis, SIRS, or organ/space or prosthetic infections
  • Antibiotic choice depends on the procedure performed/type of wound:
    • Infections associated with skin flora mostly require gram-positive coverage as well consideration for possible methicillin resistance.
    • Contamination with intestinal flora requires gram-negative and anaerobic coverage.
Table: Antibiotic choice based on potential organism involved
Organism1st choiceAlternative
Methicillin-sensitive Staphylococcus aureusCefazolinClarithromycin
Methicillin-resistant Staphylococcus aureusVancomycinLinezolid or daptomycin
Coagulase-negative staphylococciVancomycinLinezolid or daptomycin
Streptococcus pneumoniaeBenzylpenicillinClarithromycin
Streptococcus pyogenes (group A β-hemolytic streptococcus)Benzylpenicillin, clindamycinClarithromycin
EnterococciAmoxicillinVancomycin
Bacteroides spp.MetronidazoleAmoxicillin–clavulanate
Escherichia coliPiperacillin–tazobactamMeropenem
Haemophilus influenzaeAmoxicillinAmoxicillin–clavulanate
Klebsiella spp.Amoxicillin–clavulanateMeropenem
Proteus spp.Amoxicillin–clavulanateMeropenem
Pseudomonas aeruginosaPiperacillin–tazobactamMeropenem
Clostridium spp.Benzylpenicillin, metronidazoleMetronidazole

Surgical therapy

  • Superficial wound infection/abscess:
    • Opening the wound 
    • Allowing the wound to heal by secondary intention with daily packing/dressing changes 
  • Deep wound infection with fascia/muscle involvement:
    • Opening the wound
    • Debridement of infected/ devitalized tissue may be needed.
    • Daily packing/dressing changes for the wound; may benefit from vacuum wound therapy
  • Organ/space infection:
    • Very small abscesses (< 3 cm) may resolve with antibiotic therapy alone.
    • Larger abscesses usually require either percutaneous or operative drainage.
  • Infections involving prosthetic materials:
    • Initial therapy is antibiotics with drainage of all associated abscesses.
    • Salvage may be attempted if there is good clinical response to antibiotics and drainage, but most of the time, removal of the prosthetic material is required.

Prevention

  • Adherence to sterile technique
  • Appropriate antibiotic prophylaxis
  • Bowel preparation (cleansing) when feasible in cases involving bowel resection/anastomosis
  • Optimizing patient’s condition (smoking cessation, adequate nutrition, blood glucose control, avoiding hypothermia in the OR)

References

  1. Gossain S, Hawkey PM. (2018). Infections and antibiotics. In Garden, O. James, CBE BSc MB ChB MD FRCS(Glas) FRCS (Ed) FRCP(Ed) FRACS (Hon) FRCS Can(Hon) FACS(Hon) FRCS(Hon) FCSHK (Hon) FRCSI(Hon), & Parks, Rowan W., MB BCh BAO MD FRCSI FRCS(Ed) (Eds.), Principles and practice of surgery, pp. 48–59.
  2. Owens CD, Stoessel K. (2008). Surgical site infections: epidemiology, microbiology and prevention. J Hosp Infect 70(Suppl 2):3–10. https://www.ncbi.nlm.nih.gov/pubmed/19022115
  3. Bulander RE, Dunn DL, Beilman GJ. (2019). Surgical infections. Brunicardi F, Andersen DK, Billiar TR, et al. (Eds.), Schwartz’s Principles of Surgery, 11th ed. McGraw-Hill.
  4. Bjerknes S, Skogseid IM, Sæhle T, Dietrichs E, Toft M. (2014). Surgical site infections after deep brain stimulation surgery: frequency, characteristics, and management in a 10-year period. PLoS One 9(8):e105288. https://doi.org/10.1371/journal.pone.0105288
  5. Quick C, Biers S, Arulampalam T. (2020). Immunity, Inflammation, and Infection, Essential Surgery: Problems, Diagnosis, and Management, 6th ed. Edinburgh: Elsevier. 
  6. Young PY, Khadaroo RG. (2014). Surgical site infections. Surg Clin North Am94:1245–1264. https://www.ncbi.nlm.nih.gov/pubmed/25440122
  7. Berríos-Torres SI, Umscheid CA, Bratzler DW, et al. (2017). Centers for Disease Control and Prevention guideline for the prevention of surgical site infection, 2017. JAMA Surgery 152:784–791. doi:10.1001/jamasurg.2017.0904

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