Is a CT (computed tomography) scan of the leg indicated after failed outpatient therapy for leg cellulitis?

Management of Failed Outpatient Therapy for Leg Cellulitis

CT imaging of the leg is indicated after failed outpatient therapy for leg cellulitis to rule out deep soft-tissue abscesses, sinus tracts, or osteomyelitis that may require surgical intervention. 1

Evaluation of Treatment Failure

When outpatient therapy for leg cellulitis fails, a systematic approach is needed to identify the underlying cause:

1. Reassess the diagnosis: Consider alternative diagnoses that can mimic cellulitis, including:

   • Lymphoma or other malignancies 2
   • Deep vein thrombosis
   • Contact dermatitis
   • Venous stasis dermatitis

2. Identify risk factors for treatment failure:

   • Fever (temperature >38°C) at initial presentation (4.3× higher risk) 3
   • Chronic leg ulcers (2.5× higher risk) 3
   • Chronic edema or lymphedema (2.5× higher risk) 3
   • Prior cellulitis in the same area (2.1× higher risk) 3
   • Cellulitis at a wound site (1.9× higher risk) 3
   • Larger surface area of infection 4
   • Previous failed oral therapy 4

Diagnostic Approach

Imaging Studies

• CT scan is specifically indicated to detect:

  • Deep soft-tissue abscesses
  • Sinus tracts
  • Potential osteomyelitis 1

• Additional imaging options:

  • Ultrasonography (especially high resolution) may help detect abscesses or sinus tracts 1
  • MRI is best for detecting bone involvement and provides anatomic information about sinus tracts, abscesses, or muscle involvement 1
  • Plain radiographs should be obtained first to evaluate for bone involvement 1

Microbiological Assessment

• Obtain appropriate wound specimens for culture before initiating new antibiotics 1
• Wound sampling methods (in order of preference):
  1. Tissue specimens from debrided base by curettage or biopsy (A-I evidence) 1
  2. Needle aspiration of purulent collections
  3. Properly collected swab specimens (only if other methods unavailable) 1
• Blood cultures should be performed if the patient has a severe infection or is systemically ill 1

Management Algorithm

1. Hospitalization assessment:

   • Consider hospitalization if any of the following are present:
     • Systemic toxicity (fever, leukocytosis)
     • Metabolic instability
     • Rapidly progressive or deep-tissue infection
     • Substantial necrosis or gangrene
     • Critical ischemia
     • Requirement for urgent diagnostic/therapeutic interventions 1

2. For hospitalized patients:

   • Obtain appropriate specimens (wound and blood) for cultures
   • Perform CT imaging to evaluate for deep soft-tissue involvement
   • Initiate parenteral antimicrobial therapy
   • Consider surgical consultation for potential debridement 1

3. Wound management:

   • Perform thorough debridement of necrotic tissue
   • Measure and record wound size, extent of surrounding cellulitis, and quality/quantity of drainage 1
   • Use a sterile, blunt metal probe to assess wound depth and detect foreign bodies, abscesses, or bone involvement 1

Common Pitfalls and Caveats

• Diagnostic error: Up to 20.5% of patients with cellulitis experience treatment failure 3, often due to misdiagnosis or unrecognized complications.

• Inadequate initial assessment: Failure to identify risk factors for treatment failure at initial presentation leads to inappropriate outpatient management.

• Delayed imaging: Waiting too long to obtain imaging in patients with failed outpatient therapy can lead to progression of deep infections.

• Inappropriate antibiotic selection: Consider whether the initial antibiotic regimen covered the likely pathogens, particularly if there are risk factors for resistant organisms.

• Overlooking underlying conditions: Chronic edema, venous insufficiency, and tinea pedis are common portals of entry that need to be addressed to prevent recurrence 5.

• Recurrence risk: Patients with one episode of cellulitis have a high risk of recurrence, suggesting that longer courses of antibiotics or prophylaxis might be beneficial in selected cases 5.

By following this systematic approach with appropriate imaging and culture-guided therapy, you can effectively manage patients with failed outpatient therapy for leg cellulitis and improve outcomes.