What is the recommended diagnostic workup and management for a subserosal fistula, particularly when it is associated with Crohn’s disease?

Subserosal Fistula in Crohn's Disease

Subserosal fistulas (also termed sinus tracts) in Crohn's disease represent wall defects extending outside the bowel wall but not reaching adjacent organs or skin, and should be evaluated with CT or MR enterography followed by medical optimization with biologics (infliximab or adalimumab) and surgical resection reserved for symptomatic cases with strictures, abscesses, or malabsorption. 1

Diagnostic Workup

Imaging Modalities

• CT enterography (CTE) or MR enterography (MRE) are the primary diagnostic tools with moderately high accuracy for detecting penetrating Crohn's disease complications including sinus tracts, fistulas, inflammatory masses, and abscesses 1

• Subserosal fistulas appear as extra-enteric tracts extending from the bowel wall, with or without internal air or fluid, typically arising from within or just proximal to a stricture 1

• Look for associated findings including bowel loop angulation, tethering of adjacent bowel, and upstream dilation, as these nearly always accompany penetrating disease 1

• MRI enterography is preferred when endoscopy cannot reach affected segments and can identify fibrostenotic strictures, anastomotic narrowing, and bowel wall changes 2

Critical Imaging Assessment

• Describe the origin of the tract and its relationship to surrounding structures, even though it doesn't connect to another epithelial surface 1

• Evaluate for concurrent strictures systematically, as penetrating disease strongly associates with stricture formation and the proximal end of inflamed stenotic segments should be scrutinized for sinus tract formation 1

• Assess for abscess formation, as more than two-thirds of patients with fistulizing disease have associated abscesses that must be drained before definitive intervention 1

• In postoperative patients, consider anastomotic leak in addition to fistulizing Crohn's disease when examining extra-enteric tracts near surgical anastomoses 1

Management Strategy

Medical Therapy

• Infliximab is the first-line biologic for penetrating Crohn's disease, with strong evidence for fistula closure (moderate certainty evidence) 1

• Adalimumab is an alternative biologic option though evidence is less robust than infliximab (low certainty evidence) 1

• Thiopurines (azathioprine 2.0-3.0 mg/kg/day or 6-mercaptopurine 1.5 mg/kg/day) should be used for maintenance therapy, either alone or in combination with biologics 1

• Antibiotics alone (metronidazole 750-1500 mg/day or ciprofloxacin 1000 mg/day) are not recommended as monotherapy for fistula closure, though they may be used as adjunctive therapy 1

• Medical therapy to control disease-related inflammation is imperative before considering surgical intervention to increase the likelihood of tract healing 1

Surgical Indications

• Surgical resection is strongly recommended when subserosal fistulas are associated with:

  • Abscess formation requiring drainage 1
  • Bowel stricture causing obstruction 1
  • Excessive diarrhea or malabsorption 1
  • Failure of medical therapy 1

• Asymptomatic subserosal fistulas without complications do not require surgical treatment 1

• Interval surgical resection after abscess drainage is not always necessary and should be determined through joint medical-surgical discussion 1

Surgical Approach

• Resective surgery targeting the diseased bowel segment is the definitive treatment when surgery is indicated 1

• The most conservative approach should be adopted to avoid soft tissue damage and prevent extensive scarring 1

• If abscess is present, drainage must precede definitive surgery using either percutaneous drainage or surgical drainage 1

Common Pitfalls

• Do not confuse subserosal fistulas with complete fistulas that connect two epithelial surfaces, as management differs 1

• Do not overlook concurrent strictures, as they are nearly always present with penetrating disease and influence surgical planning 1

• Avoid using the term "phlegmon" for inflammatory masses, as it is ambiguous and doesn't clarify whether there is a drainable abscess component 1

• Do not proceed with definitive surgery without first draining any associated abscess, as this increases morbidity 1

• Ensure complete imaging of the anal sphincter complex at every CTE/MRE examination, as perianal disease may coexist and alter management 1

Clinical Algorithm

1. Confirm diagnosis with CTE or MRE to characterize the subserosal fistula and identify complications 1

2. Assess for abscess - if present, drain percutaneously or surgically before proceeding 1

3. Evaluate for symptoms (pain, obstruction, malabsorption) and associated strictures 1

4. If asymptomatic without complications: Initiate or optimize medical therapy with infliximab or adalimumab plus thiopurines for maintenance 1

5. If symptomatic or complicated: Joint medical-surgical discussion for timing of resection, with medical optimization before surgery 1

6. Monitor response clinically with decreased symptoms; repeat imaging if clinical response is unclear 1