Infectious Disease and Microbiology – Surgical site infections

Surgical site infection (SSI), formerly called a surgical wound infection, is an infection occurring at or near an operative site. It may be recognized by purulent drainage, positive cultures from a primarily closed surgical site, reopening of the wound because of infection, or a clinician’s diagnosis of infection.

SSIs are commonly classified as superficial incisional, deep incisional, or organ/space infections, according to the depth and anatomical structures involved.

A superficial incisional SSI involves only the skin and subcutaneous tissue of the incision and typically occurs within the postoperative surveillance period. Findings may include purulent superficial drainage, recovery of microorganisms from an aseptically collected specimen, or characteristic clinical evidence of infection.

A deep incisional SSI involves deeper soft tissues such as fascia and muscle. It may present with deep purulent drainage, abscess formation, wound dehiscence, or radiologic or operative evidence of deep infection.

An organ/space SSI affects an anatomical structure that was entered or manipulated during surgery but lies deeper than the incision. Examples include intra-abdominal abscesses, infected joint spaces, mediastinal infections, and pelvic collections.

Epidemiology

Surgical site infections are among the most important healthcare-associated infections and account for a substantial proportion of infections occurring in surgical patients.

Most SSIs involve the incision itself, while a smaller proportion involve deeper organs or spaces accessed during surgery.

SSIs can substantially prolong hospitalization, increase the likelihood of readmission, and markedly increase healthcare costs.

Many infections become apparent after hospital discharge, making appropriate patient education and postoperative surveillance important.

Risk factors

The risk of infection depends on the number and virulence of contaminating organisms, the patient’s underlying health, and the quality of the surgical technique.

Patient-related risk factors include diabetes, obesity, smoking, malnutrition, extremes of age, immunosuppression, systemic corticosteroid use, prolonged preoperative hospitalization, and active infection at another body site.

Colonization with Staphylococcus aureus, particularly MRSA, can increase the risk of postoperative infection.

Procedure-related factors include a long duration of surgery, foreign material or prosthetic devices, drains, excessive tissue trauma, poor hemostasis, and inappropriate hair removal.

Shaving the operative field substantially before surgery can produce microscopic skin injuries and increase infection risk.

Prevention

Prevention begins with appropriate infection-control practices, surgical asepsis, effective sterilization, operating-room ventilation, careful surgical technique, and appropriate perioperative antimicrobial prophylaxis.

Whenever possible, infections at sites distant from the planned operation should be identified and treated before elective surgery.

Good perioperative blood glucose control is particularly important in patients with diabetes.

Smoking cessation should be encouraged before elective surgery because smoking impairs wound healing and increases postoperative complications.

The preoperative hospital stay should be kept as short as reasonably possible.

Maintaining normal body temperature during and after surgery also helps decrease the risk of SSI.

When antimicrobial prophylaxis is indicated, the drug should be selected according to the type of operation and expected organisms and administered so that effective tissue concentrations are present at the time of incision.

Routine vancomycin prophylaxis is generally inappropriate, but it may be considered in selected patients with a substantial risk of MRSA infection or colonization.

Etiology

Most surgical site pathogens originate from the patient’s own skin, gastrointestinal, or genital flora, depending on the operative site.

The most frequently encountered organisms include Staphylococcus aureus, coagulase-negative staphylococci, Streptococcus species, Enterobacteriaceae, and Enterococcus species.

Operations involving the gastrointestinal or female genital tract may additionally produce infections caused by anaerobic organisms such as Bacteroides species.

Foreign bodies dramatically reduce the bacterial inoculum required to establish infection, which explains the particular concern surrounding prosthetic joints, vascular grafts, cardiac devices, and other implanted materials.

Antimicrobial-resistant organisms, including MRSA, are increasingly important causes of SSI.

Fungal infection, including Candida, may occur in selected high-risk patients.

Unusual organisms or clusters of postoperative infections should raise concern for a common contaminated source or infection-control problem and should be reported to the relevant infection-prevention team.

Clinical presentation

Patients may present with fever, chills, increasing postoperative pain, localized tenderness, or drainage from the operative wound.

Deep incisional or organ/space infection may present primarily with persistent unexplained fever, even when the superficial incision initially appears normal.

Physical examination

The surgical wound should be examined carefully for erythema extending beyond the wound margin, warmth, tenderness, induration, fluctuance, purulent drainage, or separation of the wound edges.

Wound dehiscence can result from infection but may also occur because of mechanical failure or impaired wound healing.

Sternal wounds after cardiac surgery require particular attention because deep infection can lead to mediastinitis or sternal osteomyelitis.

A patient with persistent fever and sternal instability or rocking after cardiac surgery may have a deep infection even when the skin surface appears relatively normal.

Diagnosis

Diagnosis is based on clinical examination together with microbiologic and radiologic evaluation when indicated.

Persistent leukocytosis may support the presence of infection but is nonspecific.

Purulent drainage or aspirated material should be sent for Gram stain and aerobic and anaerobic cultures whenever feasible.

Blood cultures should be obtained when there are signs of systemic infection, bacteremia, sepsis, or deep organ involvement.

Imaging

Imaging is particularly useful when a deep collection or organ/space infection is suspected.

Computed tomography is commonly used to identify postoperative abscesses, fluid collections, fascial involvement, and deeper extension of infection.

Ultrasound may be useful for superficial or intra-abdominal collections and can help guide aspiration or drainage.

A suspected abscess generally requires drainage and microbiologic sampling rather than antimicrobial treatment alone.

Treatment

Management depends on the depth and severity of infection, the operative site, the presence of foreign material, and the patient’s clinical condition.

A superficial incisional infection may sometimes be managed by opening the wound, providing local wound care, and using antibiotics when surrounding cellulitis or systemic manifestations are present.

When antibiotics are required for superficial infections, therapy should primarily cover common skin organisms, especially S. aureus and streptococci.

Deep incisional and organ/space infections usually require broader empiric antimicrobial therapy, followed by narrowing of treatment once culture and susceptibility results are available.

Operations involving the gastrointestinal or female genital tract require coverage of gram-negative enteric organisms and anaerobes in addition to gram-positive bacteria.

Empiric choices in severe infections may include agents such as ampicillin-sulbactam, piperacillin-tazobactam, or an appropriate broad-spectrum cephalosporin or carbapenem, depending on the operative site, patient risk factors, and local antimicrobial-resistance patterns.

MRSA-active treatment should be added when the patient has relevant risk factors or when MRSA is identified.

The duration of antimicrobial therapy depends on source control, infection depth, the responsible organism, presence of prosthetic material, clinical response, and whether drains or residual infected tissue remain.

Surgical management

Source control is essential in many surgical site infections.

An infected wound may need to be opened to permit drainage, debridement, and removal of necrotic tissue.

Deep abscesses generally require percutaneous or operative drainage.

Re-exploration of the surgical site may be necessary when there is persistent infection, inadequate drainage, an anastomotic leak, tissue necrosis, or failure to improve with antimicrobial treatment.

When an implanted prosthetic device becomes infected, definitive management may require removal of the device, depending on the type of implant, organism, duration of infection, and feasibility of salvage.

Wound management

Heavily contaminated surgical wounds may sometimes be left open initially and allowed to heal by secondary intention or undergo delayed primary closure.

Open wounds may require regular irrigation, dressing changes, packing, or negative-pressure wound therapy depending on their size and complexity.

Follow-up

Patients should be instructed to seek medical evaluation if they develop fever, increasing wound pain, spreading redness, swelling, purulent drainage, wound separation, or new systemic symptoms after surgery.

Because many SSIs develop after discharge, postoperative follow-up is important even when the initial hospital course was uncomplicated.

Prognosis

Superficial infections usually have a good outcome when recognized and treated promptly.

Deep and organ/space infections are associated with substantially greater morbidity, prolonged hospitalization, readmission, repeat surgery, and mortality.

Complications

Possible complications include wound dehiscence, deep abscess formation, bacteremia, sepsis, delayed wound healing, and recurrent infection.

Following cardiac surgery, deep infection may result in mediastinitis or sternal osteomyelitis, both of which are serious and potentially life-threatening.

Infection surrounding a prosthetic device can progress to prosthesis infection, frequently requiring prolonged antimicrobial therapy and sometimes removal or replacement of the device.

High-Yield Pattern

Purulent drainage or spreading erythema from a surgical incision → suspect superficial SSI

Deep pain, wound separation, fever, or deep purulent drainage → suspect deep incisional SSI

Persistent postoperative fever with a deep collection on imaging → suspect organ/space SSI

SSI + abscess or necrotic tissue → source control with drainage/debridement is essential

Prosthetic material + persistent infection → consider device-associated infection and possible removal