Surgical site infection: Clinical sciences

Surgical site infection, or SSI for short, is a common postoperative complication. This is most commonly caused by gram-positive bacteria living on the skin, but can also be caused by other pathogens such as anaerobic gut bacteria encountered during bowel surgery. Depending on the depth of infection, SSI is subdivided into three types. Superficial incisional SSI is the least invasive type that’s limited to the skin and subcutaneous tissue; deep incisional SSI affects deeper tissues like muscle and fascia layers; and organ space SSI, which is deep within the organ or body cavity where the surgery occurred.

The first step in approaching someone suspected to have SSI is to obtain a focused history and physical examination. The person’s history is especially important. Most cases of SSI develop between 4 and 30 days after surgery, but this does not include necrotizing infections like Group A strep or Clostridia, which would present within 48 hours and progress rapidly. Individuals with SSI may report pain or tenderness at the surgical site. Importantly, the person’s surgical history will provide details to help determine their risk of infection.

Next, you can use surgical wound classification to identify those at risk for SSI. Surgical wound classification is based on the degree of contamination and includes four main categories: Clean, Clean-contaminated, Contaminated, and Dirty. The likelihood of SSI increases drastically across these groups. Classification depends on infectious risk factors, such as location, trauma history, or breaks in sterile technique. For example, wounds in colonized areas like the mouth or urinary tract are at a much higher risk for developing SSI, as are open traumatic wounds.

Some high yield facts to keep in mind! One major consideration when evaluating SSI is any history of surgical implant, such as joint replacement surgery or mesh hernia repair. Implants are a big risk factor for SSI, since bacteria can cling to the foreign material and cause infection.

Moving on to the physical exam, there might be peri-incisional signs of infection such as swelling, warmth, and erythema, or purulent drainage from the surgical site. Sometimes, the incision can split open at the skin, which is called dehiscence. There could also be induration where the skin becomes harder and thicker due to inflammation, and fluctuance which is a collection of pus under the skin, giving it a “boggy” feel on palpation. Additionally, there might be systemic signs of infection, such as fever, tachycardia, and hypotension, suggesting the infection has spread throughout the body.

Point of care ultrasound, or POCUS is often done during the physical exam to find abscesses that can’t be detected clinically. If the wound is open and has peri-incisional signs of infection, a wound swab should be sent for culture and gram stain. If there are systemic signs of infection, a complete blood count and blood cultures should also be sent.

After the history, physical and labs have been obtained, it’s time to determine the type of infection. The first type you might diagnose is superficial incisional SSI. This occurs up to 30 days after surgical intervention and is limited to the superficial incisional skin and subcutaneous tissues. You will see peri-incisional signs of infection with purulent drainage or a positive wound culture. Peri-incisional pain or tenderness is also common. Because these infections are so superficial, it is rare to see systemic signs of infection.

When it comes to treatment, the surgical team will perform bedside wound opening. In this procedure, sutures or staples are removed from the skin to open the affected part of the incision and allow the infection to drain. The wound is probed to explore the depth and extent of tissue involvement. Next, any necrotic tissue is debrided, and infected fluid is collected for culture and gram stain.