Abscess

A pus-containing, confined structure within an inflammatory wall, which requires drainage by whichever means available.

• Contrast that to contaminated peritoneal fluid or loculated collections, which do not have a wall

Pathophysiology

• Abscesses represent an intermediate natural outcome of infection - not quite cleared, not quite progressive
• Natural history is progression to overwhelming sepsis unless drained
• Microbiology
  • Typically poly-microbial
  • Secondary abscesses usually have mixed aerobic-anaerobic flora
  • Obligate anaerobes are usually involved with late abscess formation, such as Bacteroides fragilis
  • Primary abscesses are often mono-bacterial, especially Staph

Classifications

• Visceral vs non-visceral
• Primary vs secondary
• Spontaneous vs post-operative
• Intra-peritoneal vs retroperitoneal
• Simple vs complex (multiple, multi-loculated, communication with bowel, associated with necrotic tissue, associated with cancer)
• Anatomical location

Presentation

• Post-operatively, often presents as sepsis and ileus

Diagnosis

• CT provides the best anatomical information, but USS is also good, especially for RUQ
• Imaging during the first post-operative week cannot distinguish between a sterile fluid collection and an infected fluid collection - would need to do a diagnostic aspiration in this context
• CT features suggestive of abscess are a contrast-enhancing, well-defined rim, and the presence of gas bubbles

Management

• Clinical context is key for detected post-op fluid collections
• True abdominal abscesses should be drained by the least invasive method possible
  • Factors favouring percutaneous: hostile abdomen, accessible location, source already controlled, visceral location, single abscess, thin viscosity, stable patient
  • Factors favouring surgery: easy abdominal access, interloop, multiple abscesses, loculated, bowel communication, associated necrosis, associated malignancy, thick debris, critically ill patient, failed percutaneous drainage
• Complex abscesses are more likely to require true surgical drainage - although can still temporise with percutaneous.
• Clinical improvement should be seen within 24-72 hours. Ongoing sepsis indicates treatment failure.
• Catheter drainage, as opposed to just aspiration, is probably more effective. Small simple abscesses containing thin fluid or within solid organs are more likely to be manageable with just aspiration. Evidence indicates 7Fr drains are just as good as 14Fr, and it can always be upsized later.
  • Regularly flush tubes with saline to keep them patent
  • Remember to check that the drain is properly secured - radiologists don't always secure it properly
  • Regularly clean and observe the drain site to avoid necrotizing infections
  • No true evidence around removal - typically when daily output <30mL (subtracting injected saline) which often takes 5-7 days
• Re-imaging - can give some contrast down the tube to further delineate location and size of persisting cavity. Abscesses which do not collapse, tend to recur.
• The source of the abscess must also be dealt with
• Antibiotics are of secondary importance, and may not actually be evidence-based for abdominal abscesses! Perhaps their most sensible usage is to help control sepsis while drainage is pending, then to cover bacteraemia and spillage after drainage, then to be weaned as soon as pus has been evacuated and the patient improves. Don't need to continue antibiotics just because a drain is in place.

Open drainage

• Attempt a direct anatomical approach rather than exploratory laparotomy, where possible
  • Subphrenic and subhepatic abscesses can be approached extra-peritoneally through a subcostal incision or (if posterior) through the bed of the 12th rib
  • Pericolic, appendicular and retroperitoneal abscesses are usually best approached through a loin incision
• Try to 'fill' the cavity with omentum or adjacent structures
• With adequate surgical drainage and source control, no drains are necessary