Intususepsi

01Pathophysiology

Intussusception occurs when one segment of the intestine telescopes into the lumen of an immediately adjacent segment, much like a collapsible telescope folding into itself. The segment that invaginates inward is called the intussusceptum, and the receiving outer segment is the intussuscipiens. The ileocolic junction is the most common location because the relatively mobile terminal ileum prolapses through the ileocecal valve into the cecum and ascending colon.

In infants and young toddlers, the trigger is almost always idiopathic lymphoid hyperplasia of the Peyer patches in the terminal ileum. A preceding viral upper respiratory infection or gastroenteritis, most often caused by adenovirus, leads to reactive hypertrophy of these lymphoid aggregates. The enlarged Peyer patches create a mass effect that the normal peristaltic wave catches and pushes distally, dragging the proximal bowel wall into the distal lumen.

Once telescoped, the mesentery of the intussusceptum is compressed between the two bowel walls. This compression obstructs venous outflow first (veins are low-pressure and collapse easily), causing venous congestion, mucosal edema, and oozing of blood and mucus into the intestinal lumen. This is the origin of the classic "currant jelly stool", a mixture of blood and mucus that resembles red currant jelly. If left untreated, ongoing compression eventually occludes arterial inflow, leading to ischemia, necrosis, and ultimately perforation with peritonitis.

The episodic colicky pain reflects intermittent peristaltic waves attempting to push the intussusceptum further forward. Between waves, the child may appear deceptively calm or even lethargic. This intermittent pain-free interval is a hallmark feature tested repeatedly in vignettes. Lethargy as a presenting complaint is a well-known atypical presentation and should raise suspicion for intussusception in a child of the right age group.

In children older than 5 years and in adults, idiopathic intussusception is uncommon. A pathologic lead point should be assumed until proven otherwise. In older children this is most often a Meckel diverticulum or lymphoma; in adults, consider polyps, lipomas, or metastatic disease. Henoch-Schonlein purpura (IgA vasculitis) causes submucosal hemorrhage that can act as a lead point, creating a well-known association tested in exam settings.

02Classification and Clinical Manifestation

Clinical features by stage of progression:

03Diagnostic Workup

When a child between 6 and 36 months presents with episodic colicky pain, vomiting, and bloody stools, the best initial test is an abdominal ultrasound. Ultrasound is noninvasive, does not use radiation, and has a sensitivity and specificity both exceeding 95% for intussusception. The finding you need to recognize is the "target sign" on transverse imaging, which represents the concentric rings of the telescoped bowel layers seen in cross-section. On longitudinal view, the layered appearance resembles a kidney, hence the term "pseudokidney sign."

If ultrasound confirms intussusception and there are no signs of peritonitis or perforation, the next step is a therapeutic air enema (pneumatic reduction) or hydrostatic reduction under fluoroscopic or ultrasound guidance. This step is simultaneously diagnostic and therapeutic: it confirms the diagnosis and attempts to reduce the intussusception by pushing the intussusceptum back out of the intussuscipiens using controlled air pressure.

Plain abdominal radiographs are often the first imaging obtained in the emergency setting, but they are neither sensitive nor sufficient to confirm the diagnosis. Their primary role is to rule out perforation (look for pneumoperitoneum / free air under the diaphragm) before proceeding to an enema reduction. If free air is present, enema reduction is absolutely contraindicated and the patient proceeds directly to surgery.

In children older than 5 years or in any adult with suspected intussusception, a CT abdomen with contrast should be obtained to evaluate for a pathologic lead point. The CT will show the target sign and may reveal a mass, lymph node enlargement, or Meckel diverticulum.

04Management and Treatment

Acute stabilization begins with standard resuscitation measures. The child should receive IV fluid resuscitation (normal saline or lactated Ringer, 20 mL/kg bolus if signs of dehydration or shock are present), nasogastric tube placement if there is significant vomiting or abdominal distention, and nothing by mouth. Blood work including a complete blood count, basic metabolic panel, and type and screen should be sent in anticipation of possible surgery.

Once the child is stabilized and perforation has been excluded by plain film, the first-line treatment is pneumatic (air) enema reduction. This is performed by a radiologist under fluoroscopic or ultrasound guidance. Air is insufflated into the rectum under controlled pressure. Successful reduction is confirmed when there is free reflux of air into the terminal ileum across the ileocecal valve. The success rate is high, roughly 80 to 95% in experienced centers.

If enema reduction fails after up to three attempts, or if the child deteriorates during the procedure, surgical intervention is required. At laparotomy, the surgeon attempts gentle manual reduction by compressing the intussuscipiens distally (never pulling the intussusceptum proximally, as this risks perforation). If the bowel is necrotic or a pathologic lead point is found, segmental resection with primary anastomosis is performed.

Contraindications to enema reduction that should prompt immediate surgical referral include: signs of peritonitis (rebound tenderness, guarding, rigid abdomen), pneumoperitoneum on plain film, prolonged symptoms beyond 48 hours with hemodynamic instability, or evidence of a pathologic lead point requiring resection.

After successful enema reduction, the child is admitted for observation for at least 24 hours because the recurrence risk is approximately 5 to 10%. Parents should be counseled to return if the child develops recurrent episodic pain, vomiting, or bloody stools.

05Differential Diagnosis and Distractors

06Traps and High-Yield Pearls

The single most common way students lose points on intussusception questions is by failing to recognize atypical presentations, particularly lethargy. A vignette may describe a previously well infant who now appears limp, pale, and intermittently inconsolable without mentioning bloody stool at all. The classic triad of colicky pain, vomiting, and currant jelly stool is present in fewer than half of all cases at initial presentation, so you cannot wait for all three features to be present before suspecting the diagnosis.

Another frequent trap involves the age-dependent approach to lead points. When a vignette describes intussusception in a child older than 5 years or in an adult, the expected answer is not simply "order ultrasound and perform enema reduction." Instead, the question is testing whether you recognize the need to search for a pathologic lead point with CT imaging and potentially proceed to surgical resection rather than enema reduction alone.

Students also confuse the diagnostic and therapeutic roles of the enema. The best initial test is ultrasound, not an enema. However, the enema serves as both the confirmatory procedure and the treatment. If the question asks "what is the next best step after ultrasound confirms intussusception in a stable child?" the answer is therapeutic enema reduction, not surgery. Surgery is reserved for failed enema reduction, peritonitis, or perforation.

Finally, watch for the contraindication trap: if the vignette describes free air under the diaphragm, peritoneal signs, or hemodynamic instability, enema reduction is absolutely contraindicated. The correct next step in that scenario is emergent surgical exploration, not enema. The core competency being tested is your ability to risk-stratify the patient: stable children get enema reduction, unstable or perforated children go straight to the operating room.