Appendix Ultrasound: RLQ Pain Protocol and Appendicitis Criteria

Right lower quadrant pain is one of the most common emergency presentations, and appendicitis is the diagnosis you can't miss. Ultrasound is the first-line imaging in children and pregnant women, and plays an important role in adults as well. The technique — graded compression — is learnable but requires practice.

Indications

• RLQ pain (primary indication)
• Suspected appendicitis in children, pregnant women, and young adults
• Evaluation of RLQ pain in women (to also evaluate for ovarian pathology)
• Follow-up of known appendicitis managed non-operatively
• Evaluation for periappendiceal abscess

Equipment and Setup

Probe: 5–12 MHz linear transducer. Switch to curvilinear if the appendix is deep.

Patient position: Supine. No prep required — do not delay for bowel prep.

Key principle — graded compression: Apply slow, progressive pressure with the probe. This displaces overlying bowel gas, moves the probe closer to the appendix, and reveals the appendix sitting behind the compressed bowel. Don't just press hard — push slowly and progressively, releasing and repositioning between compressions.

Finding the Appendix

This is the technical challenge. The appendix arises from the cecum and has no fixed position — it can be retrocecal (behind the cecum), pelvic (low), or lateral. Most often it's at McBurney's point: 1/3 of the way from the right anterior superior iliac spine (RASIS) to the umbilicus.

Systematic approach:

1. Start in the RLQ at McBurney's point in transverse
2. Apply graded compression — compress, hold, release, compress again
3. Identify the cecum: a large hypoechoic blind-ending structure with a haustra wall
4. Follow the cecum to its tip — the appendix arises here
5. The appendix is a blind-ending, non-peristalsing tubular structure
6. Follow it from cecal tip as far as you can

Landmark structures:

• Psoas muscle: Large hypoechoic structure posterior to the vessels — the appendix often lies anterior or medial to it
• Iliac vessels: Just medial — avoid pressing directly on them
• Terminal ileum: Can look like the appendix — distinguish by peristalsis (ileum moves, appendix does not) and by tracing back to the cecum

Diagnostic Criteria for Appendicitis

Primary Criteria

Non-compressible appendix ≥ 6 mm outer diameter:

• Measure outer wall to outer wall in transverse cross-section
• Normal appendix: < 6 mm, compressible with graded pressure
• Appendicitis: ≥ 6 mm, non-compressible
• The 6 mm threshold has been standard since the original Puylaert description

Appendicolith:

• Echogenic focus with posterior shadowing within the appendix lumen
• Highly specific for appendicitis when combined with inflammation
• May be present without appendicitis but warrants close clinical correlation

Secondary Signs

These findings support the diagnosis when seen alongside the enlarged appendix:

• Periappendiceal fat stranding: The echogenic fat around the appendix is thickened, hyperechoic, non-compressible
• Free fluid: Small amounts adjacent to the appendix or in the right iliac fossa
• Hyperemia on color Doppler: Increased flow in the appendix wall — the "ring of fire" or circumferential vascular pattern
• Loss of mural layers: In gangrenous appendicitis, the normal wall architecture disappears

Signs of Perforation

• Abscess: Complex fluid collection containing debris or gas echoes
• Free fluid: Extending into the pelvis or beyond the RLQ
• Loss of appendiceal wall integrity: The wall is discontinuous
• Phlegmon: Heterogeneous inflammatory mass surrounding the appendix, poorly defined borders

When the Appendix Is Not Visualized

This is common — reported non-visualization rates range from 20–40% depending on patient habitus and operator experience.

What to do:

1. Always document that you looked systematically
2. Assess for secondary signs even without seeing the appendix — fat stranding, free fluid, and RLQ tenderness on compression all matter
3. Check the pelvis in women — ovarian torsion, ruptured cyst, and tubo-ovarian abscess all cause RLQ/pelvic pain
4. Report: "Appendix not visualized. No secondary signs of appendicitis identified. If clinical concern persists, CT of the abdomen and pelvis is recommended."

Do not call the study normal if you didn't see the appendix. That is the most important documentation principle in appendix ultrasound.

Mimics of Appendicitis on Ultrasound

Mesenteric adenitis: Enlarged mesenteric lymph nodes (> 10 mm short axis) in the RLQ, normal appendix. Common in children after viral illness.

Ovarian pathology: Ovarian torsion, ruptured follicular cyst, tubo-ovarian abscess — all cause acute pelvic and RLQ pain. Always evaluate both ovaries in women.

Crohn's disease: Terminal ileal wall thickening with hyperemia, mesenteric fat wrapping, "creeping fat" — may cause focal inflammation resembling appendicitis.

Epiploic appendagitis: Small echogenic mass with hypoechoic rim, adjacent to the colon, non-compressible — this is a self-limiting condition.

Omental infarction: Similar to epiploic appendagitis, larger, usually near the greater omentum.

Pediatric Considerations

• Children have less intraperitoneal fat, making graded compression easier and appendix visualization rates higher
• The appendix is proportionally larger in children relative to body size
• Perforation rates are higher in children (appendix is thin-walled, perforates quickly)
• Consider mesenteric adenitis (common and benign) in children with RLQ pain and a normal appendix

Pregnancy

• The appendix migrates superiorly as pregnancy progresses — in the second and third trimester, look for it more laterally and superiorly than usual
• MRI is often used for inconclusive ultrasound in pregnancy
• Perforation carries higher risk for preterm labor — don't delay imaging or clinical decision

Reporting Language

• "Appendix visualized, measuring 4.5 mm outer diameter, compressible with graded pressure. No periappendiceal fat stranding or free fluid. No evidence of appendicitis."
• "Non-compressible appendix identified measuring 9 mm outer diameter. Periappendiceal echogenic fat stranding and a small amount of adjacent free fluid. Color Doppler demonstrates circumferential mural hyperemia. Findings are consistent with acute appendicitis."
• "Appendix not visualized despite graded compression technique. No periappendiceal fat stranding or free fluid identified. Ovaries appear normal bilaterally. Appendicitis cannot be excluded sonographically. CT of the abdomen and pelvis recommended for further evaluation."

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SonoBuddy's Protocols section includes the appendix ultrasound protocol with graded compression technique steps.