What are the guidelines for diagnosing subacute appendicitis using Ultrasound (USG)?

Ultrasound Diagnostic Guidelines for Subacute Appendicitis

Ultrasound should be used as the first-line imaging modality for diagnosing subacute appendicitis, with specific diagnostic criteria including appendiceal diameter >6mm, non-compressible appendix, and wall thickness ≥3mm. 1, 2

Diagnostic Criteria for Subacute Appendicitis on USG

Primary Diagnostic Findings

• Appendiceal diameter: Greater than 6mm (outer wall-to-outer wall measurement)
• Appendiceal wall thickness: Equal to or greater than 3mm
• Compressibility: Non-compressible appendix under graded compression
• Periappendiceal changes: Presence of complex mass or fluid collection

Secondary Supportive Findings

• Wall hyperemia: Increased blood flow on color Doppler
• Appendicolith: Presence of shadowing calcification
• Periappendiceal fat stranding: Increased echogenicity of surrounding fat

Diagnostic Algorithm

1. Visualization technique:

   • Use graded compression technique to displace bowel gas and improve visualization
   • Scan from the right lower quadrant, identifying the ascending colon and tracing inferiorly
   • Identify the appendix at the base of the cecum

2. Measurement protocol:

   • Measure maximum outer diameter (MOD) in transverse view
   • Assess wall thickness in longitudinal view
   • Apply gentle pressure to assess compressibility

3. Interpretation based on findings:

   • Definite appendicitis: MOD >8mm, non-compressible, wall thickness ≥3mm
   • Probable appendicitis: MOD 6-8mm with secondary signs (hyperemia, periappendiceal fluid)
   • Possible appendicitis: MOD 6-8mm without secondary signs (borderline case)
   • Normal appendix: MOD <6mm, compressible, wall thickness <2mm

Special Considerations

Borderline Cases (6-8mm)

For appendices with borderline diameter (6-8mm), additional sonographic findings should be evaluated 3:

• Color Doppler assessment for hyperemia
• Spectral Doppler for increased flow
• Periappendiceal fat changes
• Loss of normal layered appearance of appendiceal wall

Limitations and Pitfalls

• False negatives may occur in early appendicitis or retrocecal position
• False positives may occur with other inflammatory conditions 4
• Visualization may be limited by:
  • Patient obesity
  • Bowel gas
  • Operator experience
  • Retrocecal position of appendix

Differential Diagnosis

Be aware of conditions that can mimic appendicitis on ultrasound 4:

• Crohn's disease
• Tubo-ovarian abscess
• Typhilitis
• Sigmoid diverticulitis
• Cecal carcinoma
• Appendiceal tumors

Population-Specific Recommendations

Children

• Ultrasound should be the initial imaging modality
• Use CT only when ultrasound is inconclusive 1, 2

Elderly Patients

• CT scan with IV contrast is recommended for elderly patients with Alvarado score ≥5
• Ultrasound alone is not sufficient to exclude appendicitis in elderly patients 1

Pregnant Women

• Ultrasound is the first-line imaging modality
• MRI is recommended if ultrasound is inconclusive 2

Accuracy and Performance

The combined criteria of appendix with muscular wall thickness ≥3mm and visualization of a complex mass separate from adnexa in females provides:

• Sensitivity: 68%
• Specificity: 98% 5

While ultrasound has limitations in sensitivity (reported ranges from 29-84% for perforated appendicitis), it remains valuable as the first-line imaging tool due to its safety profile and availability 1.

For cases where ultrasound is inconclusive, CT scan provides superior diagnostic accuracy with sensitivity and specificity >95% 2, 6.