Pediatric Abdominal Ultrasound: Pyloric Stenosis, Appendicitis, and Intussusception

Pediatric abdominal ultrasound presents unique challenges — small anatomy, uncooperative patients, and clinical urgency. But it's also an area where ultrasound performs exceptionally well, often being the definitive diagnostic modality without radiation. These are the three most critical pediatric abdominal studies to know.

General Pediatric Scanning Tips

Use high-frequency probes: Pediatric anatomy is small and superficial. 8–15 MHz linear transducer for most pediatric abdominal work. The higher frequency gives dramatically better resolution on a 2 kg infant than the curved probe you'd use on an adult.

Warm everything: Cold gel on an infant causes immediate unhappiness and movement. Warm the gel, warm your hands.

Scan during feed or post-feed: Neonates are calmer during feeding. Pyloric stenosis scanning is actually better after a feed (you can watch gastric emptying delay in real time).

Enlist the parent: A parent's hand on a toddler is worth more than any restraint. The parent in the exam, holding the child's hand, talking to them — this transforms the scan from impossible to manageable.

Scan fast when the child is calm: If you have a 10-second window of stillness, use it for your key measurement. Don't spend it adjusting the machine. Be ready before you put the probe on.

Hypertrophic Pyloric Stenosis (HPS)

Clinical Context

HPS presents in infants 3–6 weeks old (range: 2 weeks to 5 months). Classic presentation: progressive non-bilious projectile vomiting, "hungry vomiter" (feeds and immediately vomits, wants to feed again), palpable "olive" mass in the right upper quadrant.

Ultrasound is the gold standard for diagnosis — it has replaced the UGI series in most institutions.

Scanning Technique

• Feed the infant first if possible (or scan immediately before a scheduled feed)
• High-frequency linear probe (8–15 MHz)
• Transverse epigastric approach: the pylorus sits to the right of midline, just inferior to the liver
• The pylorus connects the stomach (fluid-filled when feed has been given) to the duodenum
• Sagittal and transverse views of the pyloric channel

HPS Diagnostic Criteria

Measurement	Abnormal Threshold	Notes
Pyloric wall thickness	≥ 3 mm	Measured single wall, muscularis layer
Pyloric channel length	≥ 17 mm	AP length of the channel
Pyloric diameter	≥ 11–14 mm	Overall AP diameter of the pyloric mass

Most reliable single measurement: muscle thickness ≥ 3 mm.

Normal pylorus: Thin wall, food/fluid passes through readily within a few minutes after feeding.

HPS: Thick, elongated channel. On longitudinal view, the "cervix sign" — the pylorus indents the antrum, looking like a cervix. On transverse, the "donut sign" — circular echogenic muscle around a small anechoic/echogenic channel.

Real-time observation: If you give the infant a feed during the scan, watch for:

• Gastric distension
• Absent or minimal pyloric opening
• Absence of gastric emptying over several minutes
• "Antral nipple sign" — pyloric mucosa protruding into the antrum

What It's Not

• A normal pylorus in a vomiting infant: document muscle thickness carefully and note that the pyloric channel was seen to open and allow flow
• Pylorospasm: transient narrowing — the key is the muscle wall thickness. If < 3 mm and you see intermittent opening, spasm is more likely than HPS. Follow clinically.

Appendicitis

Clinical Context

Appendicitis is the most common surgical emergency in children. Diagnosis is clinical + imaging. Ultrasound is the first-line imaging study (no radiation). CT is the backup when ultrasound is inconclusive.

Technique

• High-frequency linear 9–15 MHz
• Graded compression technique: place the probe over the right lower quadrant at the point of maximum tenderness, and apply progressive gentle compression to displace bowel loops
• The appendix is a blind-ending tubular structure arising from the cecum
• Landmark: the cecum (look for the termination of the ileum into the cecum)

Normal vs Abnormal Appendix

Feature	Normal	Appendicitis
Outer diameter	≤ 6 mm	> 6 mm
Wall	Thin, compressible	Thickened, non-compressible
Lumen	Compressible, may contain air	Non-compressible, may contain appendicolith
Periappendicular fat	Echogenic, not tender	Hyperechoic (fat stranding), tender

Diagnostic criteria: Non-compressible appendix > 6 mm outer diameter = appendicitis.

Appendicolith: Echogenic focus with posterior shadowing in the appendix = stone. Its presence in a dilated appendix is highly specific.

Perforated appendicitis: Free fluid, loculated collection, appendix may not be identifiable (it's burst), prominent fat stranding.

When You Can't Find the Appendix

This is common — the appendix is not always identifiable. A retrocecal appendix, significant bowel gas, and patient obesity all limit visualization.

Documentation: "Appendix not identified. Limited examination due to [body habitus/bowel gas/patient cooperation]. If clinical suspicion remains, CT is recommended."

Do not say "appendix is normal" if you did not see it.

RLQ Pain Differentials in Children

• Mesenteric adenitis: multiple enlarged lymph nodes in the mesentery (> 5 mm short axis), no appendiceal abnormality — usually viral etiology
• Ovarian pathology (girls): ovarian torsion, cyst, abscess
• Meckel's diverticulitis: rare; diverticulum on the ileum
• Ileocecal intussusception: see below

Intussusception

Clinical Context

Intussusception = one segment of bowel telescopes into an adjacent segment, causing obstruction and ischemia. Most common in children 6 months to 3 years. Classic triad: intermittent colicky pain, "currant jelly" stool, palpable abdominal mass — but the triad is only present in 20–25%.

Most common site: ileocolic (terminal ileum telescopes into the colon).

Ultrasound Appearance

The telescoped bowel creates a characteristic appearance:

• Transverse view: "Target" or "donut" sign — concentric rings of alternating echogenicity, like a bulls-eye. The outer rings = the receiving bowel (intussuscipiens). The inner echogenic center = the invaginated bowel + mesentery.

• Longitudinal view: "Sandwich" or "pseudokidney" sign — the loops of bowel in cross-section look like a kidney (kidney-shaped hypoechoic mass). On long axis, the mesenteric fat appears as hyperechoic tissue sandwiched between layers of bowel.

• Size: The intussusception mass is typically > 3 cm in diameter.

Doppler Assessment

Color Doppler of the intussusception: look for vascularity within the intussuscepted bowel wall.

• Present flow: bowel is viable
• Absent flow: ischemic bowel — surgical emergency, not suitable for pneumatic reduction

Treatment Implication

Intussusception is treated by pneumatic (air) or hydrostatic (saline) enema reduction under fluoroscopic or ultrasound guidance. Surgical emergency if reduction fails or if ischemia is suspected.

Communicate urgently: Any intussusception finding must be communicated to the clinical team immediately. This is a time-sensitive diagnosis.

Other Pediatric Abdominal Pathologies to Know

Inguinal hernia: Bowel or omentum in the inguinal canal — look for peristalsis within the hernia sac. Color Doppler to assess vascularity if incarcerated (no flow = strangulated).

Ovarian torsion: Enlarged ovary (> 4 cm), heterogeneous texture, absent or decreased Doppler flow (but presence of flow does not exclude torsion). This is a clinical emergency requiring surgical exploration.

Meckel's diverticulum: Blind-ending tubular structure arising from the ileum, approximately 2 feet from the terminal ileum. Rarely identified on ultrasound unless complicated (diverticulitis, intussusception as lead point).

Hydrocolpos/Hydrometrocolpos: Distended vagina ± uterus with fluid in neonates — look for a midline cystic mass in the pelvis.

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SonoBuddy's Measurements section includes pediatric reference values for organ sizes and appendix diameter — useful for quick reference during pediatric exams.