Wound Infection Clinical Presentation

Surgical site infection is a difficult term to define accurately because it has a wide spectrum of possible clinical features.

The Centers for Disease Control and Prevention (CDC) have defined SSI to standardize data collection for the National Nosocomial Infections Surveillance (NNIS) program.^{[11, 12]} SSIs are classified into incisional SSIs, which can be superficial or deep, or organ/space SSIs, which affect the rest of the body other than the body wall layers.

• Definitions of surgical site infection (see image below) Definitions of surgical site infection (SSI).

  • Superficial incisional SSI: Infection involves only skin and subcutaneous tissue of incision.
  • Deep incisional SSI: Infection involves deep tissues, such as fascial and muscle layers. This also includes infection involving both superficial and deep incision sites and organ/space SSI draining through incision.
  • Organ/space SSI: Infection involves any part of the anatomy in organs and spaces other than the incision, which was opened or manipulated during operation.
• Superficial incisional SSI is more common than deep incisional SSI and organ/space SSI. Superficial incisional SSI accounts for more than half of all SSIs for all categories of surgery. The postoperative length of stay is longer for patients with SSI, and when adjusted for other factors influencing length of stay.

According to a report by the NNIS program,^[13] surgical site infections are defined as follows:

• Superficial incisional SSI

  • Occurs within 30 days after the operation
  • Involves only the skin or subcutaneous tissue
  • At least 1 of the following:
    • Purulent drainage is present (culture documentation not required).
    • Organisms are isolated from fluid/tissue of the superficial incision.
    • At least 1 sign of inflammation (eg, pain or tenderness, induration, erythema, local warmth of the wound) is present.
    • The wound is deliberately opened by the surgeon.
    • The surgeon or clinician declares the wound infected.
  • Note: A wound is not considered a superficial incisional SSI if a stitch abscess is present; if the infection is at an episiotomy, a circumcision site, or a burn wound; or if the SSI extends into fascia or muscle.
• Deep incisional SSI

  • Occurs within 30 days of the operation or within 1 year if an implant is present
  • Involves deep soft tissues (eg, fascia and/or muscle) of the incision
  • At least 1 of the following:
    • Purulent drainage is present from the deep incision but without organ/space involvement.
    • Fascial dehiscence or fascia is deliberately separated by the surgeon because of signs of inflammation.
    • A deep abscess is identified by direct examination or during reoperation, by histopathology, or by radiologic examination.
    • The surgeon or clinician declares that a deep incisional infection is present.
• Organ/space SSI

  • Occurs within 30 days of the operation or within 1 year if an implant is present
  • Involves anatomical structures not opened or manipulated during the operation
  • At least 1 of the following:
    • Purulent drainage is present from a drain placed by a stab wound into the organ/space.
    • Organisms are isolated from the organ/space by aseptic culturing technique.
    • An abscess in the organ/space is identified by direct examination, during reoperation, or by histopathologic or radiologic examination.
    • A diagnosis of organ/space SSI is made by the surgeon or clinician.

All surgical wounds are contaminated by microbes, but in most cases, infection does not develop because innate host defenses are quite efficient in the elimination of contaminants. A complex interplay between host, microbial, and surgical factors ultimately determines the prevention or establishment of a wound infection. Factors that affect surgical wound healing are classified in the chart below.

• Microbiology: Microbial factors that influence the establishment of a wound infection are the bacterial inoculum, virulence, and the effect of the microenvironment. When these microbial factors are conducive, impaired host defenses set the stage for enacting the chain of events that produce wound infection.

  • Most SSIs are contaminated by the patient's own endogenous flora, which are present on the skin, mucous membranes, or hollow viscera. The traditional microbial concentration quoted as being highly associated with SSIs is that of bacterial counts higher than 10,000 organisms per gram of tissue (or in the case of burned sites, organisms per cm² of wound).^[14]
  • The usual pathogens on skin and mucosal surfaces are gram-positive cocci (notably staphylococci); however, gram-negative aerobes and anaerobic bacteria contaminate skin in the groin/perineal areas. The contaminating pathogens in gastrointestinal surgery are the multitude of intrinsic bowel flora, which include gram-negative bacilli (eg, Escherichia coli) and gram-positive microbes, including enterococci and anaerobic organisms. See Table 1 for pathogens and their frequencies. Gram-positive organisms, particularly staphylococci and streptococci, account for most exogenous flora involved in SSIs. Sources of such pathogens include surgical/hospital personnel and intraoperative circumstances, including surgical instruments, articles brought into the operative field, and the operating room air.
  • The most common group of bacteria responsible for SSIs are Staphylococcus aureus. The emergence of resistant strains has considerably increased the burden of morbidity and mortality associated with wound infections.
  • Methicillin resistant Staphylococcus aureus (MRSA) is proving to be the scourge of modern day surgery. Like other strains of S aureus, MRSA can colonize the skin and body of an individual without causing sickness, and, in this way, it can be passed on to other individuals unknowingly. Problems arise in the treatment of overt infections with MRSA because antibiotic choice is very limited. MRSA infections appear to be increasing in frequency and are displaying resistance to a wider range of antibiotics.^[15]
  • Of particular concern are the vancomycin intermediate Staphylococcus aureus (VISA) strains of MRSA. These strains are beginning to develop resistance to vancomycin, which is currently the most effective antibiotic against MRSA. This new resistance has arisen because another species of bacteria, called enterococci, relatively commonly express vancomycin resistance.

Table 1. Pathogens Commonly Associated with Wound Infections and Frequency of Occurrence^[11] (Open Table in a new window)

• Risk factors (other than microbiology)

  • Decreased host resistance can be due to systemic factors affecting the patient's healing response, local wound characteristics, or operative characteristics.
    • Systemic factors include age, malnutrition, hypovolemia, poor tissue perfusion, obesity, diabetes, steroids, and other immunosuppressants.
    • Wound characteristics include nonviable tissue in wound; hematoma; foreign material, including drains and sutures; dead space; poor skin preparation, including shaving; and preexistent sepsis (local or distant).
    • Operative characteristics include poor surgical technique; lengthy operation (>2 h); intraoperative contamination, including infected theater staff and instruments and inadequate theater ventilation; prolonged preoperative stay in the hospital; and hypothermia.
  • The type of procedure is a risk factor. Certain procedures are associated with a higher risk of wound contamination than others. Surgical wounds have been classified as clean, clean-contaminated, contaminated, and dirty-infected (see Table 2).

Table 2: Surgical Wound Classification and Subsequent Risk of Infection (If No Antibiotics Used)^{[11, 16]} (Open Table in a new window)