Intussusception and pyloric stenosis on ultrasound
- Finding
Two common causes of vomiting in babies, a telescoped bowel and a thick stomach outlet muscle, are both diagnosed with ultrasound, without radiation.
What it is
Intussusception and hypertrophic pyloric stenosis are the two classic paediatric diagnoses made with ultrasound. In intussusception a segment of bowel (the intussusceptum) telescopes into the next (the intussuscipiens), most often the terminal ileum into the colon (ileocolic). It peaks between about six months and three years, with colicky pain, drawing up of the legs, vomiting and, late, «redcurrant jelly» stools; ultrasound is the test of choice and image-guided enema can both confirm and treat it. Pyloric stenosis is thickening of the pyloric muscle that blocks gastric emptying in infants of about two to eight weeks, typically with forceful non-bilious vomiting after feeds and a hungry baby; ultrasound measures the muscle and watches whether anything passes. Both studies are quick, painless and free of radiation.
What it looks like
- Intussusception, transverse view: target or doughnut sign, concentric rings of bowel wall and mesenteric fat, usually in the right abdomen under the liver.
- Intussusception, longitudinal view: pseudokidney sign, layered bowel that looks like a kidney.
- An ileocolic intussusception is usually about 2.5 cm or more across; small, short small-bowel-only intussusceptions are often transient and self-resolve.
- Pyloric stenosis: a thickened muscle wall, commonly 3 mm or more thick, and a long pyloric channel, commonly 15 mm or more, that does not open during the scan.
- Pyloric stenosis signs: target sign in cross-section, «cervix sign» in long section, and mucosa bulging into the antrum (antral nipple sign).
- Plain film in intussusception may be normal, show little gas in the right lower quadrant, a soft-tissue mass, a crescent of gas around the head, or obstruction.
Why it looks that way
- The target appearance comes from the layers of bowel wall of the inner and outer segments, with dragged mesenteric fat between them.
- In pyloric stenosis the hypertrophied muscle is hypoechoic (dark) and surrounds the bright mucosa, so in cross-section it looks like a doughnut.
- Persistent vomiting of gastric acid explains the classic hypochloraemic, hypokalaemic metabolic alkalosis of pyloric stenosis.
Differential and pitfalls
- Bilious (green) vomiting in an infant is not pyloric stenosis: think malrotation with volvulus and arrange an urgent upper contrast study.
- Pylorospasm can mimic pyloric stenosis briefly; a muscle that relaxes and lets fluid through during the scan is not stenosis, and a repeat scan can help.
- In older children, or with recurrent intussusception, look for a pathological lead point (Meckel diverticulum, polyp, lymphoma, enlarged nodes).
- Absent blood flow in the bowel wall or trapped fluid on ultrasound suggests ischaemia and lowers the chance of successful reduction.
- When ultrasound is not available, an upper GI study shows the narrow pyloric channel (string sign) and the impression of the muscle on the antrum (shoulder sign).
Next step
- Suspected intussusception: urgent abdominal ultrasound; inform the paediatric surgeon early.
- Confirmed intussusception in a stable child: image-guided air or fluid enema reduction by the radiologist, with a surgeon available.
- Peritonitis, free air or shock: no enema; resuscitate and call the surgeon for operation.
- Suspected pyloric stenosis: ultrasound of the pylorus, blood gas and electrolytes; correct fluids and alkalosis before surgery.
- Bilious vomiting in a neonate or infant: call the surgeon and request an urgent upper GI contrast study.
Test yourself
Think of the answer, then tap the question to see it.
What does intussusception look like on a transverse ultrasound image?
A target or doughnut: concentric rings of bowel wall and mesenteric fat, usually in the right abdomen near the liver.
When is enema reduction of an intussusception contraindicated?
When there is peritonitis, free intraperitoneal air (perforation) or shock; these children go to surgery after resuscitation.
Which ultrasound findings support hypertrophic pyloric stenosis?
A thickened pyloric muscle (commonly 3 mm or more), a long channel (commonly 15 mm or more) and no gastric contents passing during the scan.
Why does bilious vomiting in an infant point away from pyloric stenosis?
The pyloric block lies above the entry of bile, so the vomit is not green; bile means obstruction beyond the ampulla, such as malrotation with volvulus.
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Connections
Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.
Sources: Child Intussusception (StatPearls (NCBI Bookshelf), 2023); Pyloric Stenosis (StatPearls (NCBI Bookshelf), 2023).
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Representative model, for educational purposes only.