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Prolapse of a segment of bowel and its mesentery into the distal bowel segment. Three concentric layers of bowel wall: the outer layer (of intussuscipiens) and two inner layers of intussusceptum (the bowel folded back on itself). Vascular compromise induced by mesenteric compression may lead to bowel ischemia and necrosis.

  1. Ileocolic intussusception: 90% of all cases
  2. Ileoileocolic
  3. Ileoileal
  4. Colocolic intussusception is extremely rare

Most intussusceptions are idiopathic. About 10% are caused by an identifiable lead point. Lead points are more common in newborns up to 3 months of age and in patients older than 3 1/2 years. Lead points: Meckel's diverticulum, polyp, lymphoma. Idiopathic intussusception probably occurs because of hypertrophied lymphoid tissue in the terminal ileum, induced by a preceding viral illness.

Intussusception is a common cause of acute abdomen in children between the ages of 6 months and 2 years.

The classic triad of symptoms is: 1. colicky abdominal pain and vomiting 2. palpable abdominal mass (followed hours to days later) with 3. bloody currant jelly stool Less than 50% of patients present with all three; most have colicky abdominal pain with vomiting. Diagnosis of intussusception must be made as expeditiously as possible to prevent ischemic changes in the involved bowel wall. Given the clinical presentation, abdominal radiographs will often be ordered to rule out intussusception. Plain radiographs are diagnostic in 29–50% of cases. The best results are obtained by performing at least two views including supine and prone or left lateral decubitus views. These latter two views force air into the cecum unless there is intussusception. Common radiographic signs of intussusception include: 1. Soft tissue mass in the right upper or lower quadrant 2. Paucity of gas in the right lower quadrant Other radiographic signs of intussusception include: 3. Lateralization of the ileum 4. Air crescent sign—gas around part of the intussusceptum (although rare, this is highly specific for the diagnosis) 5. Rim or Target sign — soft-tissue mass containing a faintly visible circle of fat density (the mesenteric fat) 6. Small bowel obstruction — this may be present but is not specific for intussusception 7. Pneumoperitoneum — very rare and would necessitate urgent surgical reduction 8. Decreased volume of colonic stool 9. Paucity of small bowel gas Ultrasound has become the first line modality for intussusception in most centers. For intussusception, ultrasound is 98.5–100% sensitive and 88–100% specific, and it is able to identify anatomic lead points, making it an extremely valuable screening tool. Many will perform ultrasound on all cases, reserving radiography for sonographically positive cases prior to air enema (to rule out small bowel obstruction or pneumoperitoneum). Findings on ultrasound include: 1. Target or donut — transverse view (the outer hypoechoic ring represents the edematous intussusceptum folded back on itself) 2. Crescent or donut — the entrapped mesentery (often containing nodes) within the intussusception 3. Sandwich or pseudokidney — on longitudinal view 4. Potential presence of multiple lymph nodes 5. Fluid entrapped in the intussuscipiens — may predict an unsuccessful reduction by enema 6. Free fluid — more often with a longer history 7. Lack of blood flow in the wall on color Doppler (this may predict non-reducibility) CT scan may identify intussusception incidentally and show the target sign with alternating rings of soft tissue and lucency representing the bowel and mesenteric fat respectively. This transient intussusception is asymptomatic. A similar appearance could be seen on MRI. The treatment of intussusception is with air enema or barium enema. The only true contraindications to air or barium enema reduction include peritonitis and perforation. The rate of successful reduction is identical with both procedures (50–91%). Those patients who fail enema reduction go to surgery where the intussusception is reduced, necrotic bowel (if present) is resected and the bowel is examined for a lead point. Recurrent intussusception, if reduced successfully by air enema, will require repeat enema, with no limit to the number that can safely be performed (bearing in mind that each attempt at reduction exposes the child to additional radiation). Delayed repeat enema may successfully reduce another 50% of cases. Bowel perforation is a potential complication from enemas. Barium and air have similar perforation rates (0.5%—2.8%), although air in considered safer because the size of the bowel tear is smaller and the degree of peritonitis from fecal soiling is less. Barium may be more sensitive in detecting lead points, but lead points are rare. Despite a lead point, enema reduction should still be attempted in a clinically stable patient. Before an enema, the child should be thoroughly assessed by a pediatric surgeon, ideally present during the procedure. Air enema is performed using an air enema kit consisting of a sphygmomanometer attached to a rectal tube. Air is pumped into the bowel encountering the soft tissue mass, which is gently pushed back under continuous observation with fluoroscopy. A puff of air into the terminal ileum and disappearance of the mass will signal successful reduction. A residual soft tissue mass may represent edema of the ileocecal valve or incomplete reduction. The recurrence rate for intussusception is 10% and is highest within the first 2 days following reduction. The rate of failed reduction is highest when there is small bowel obstruction, the history is prolonged (beyond 24 hours), the site of obstruction is distal or in the presence of a lead point. There is no definite imaging algorithm for intussusception. Some centers perform air enema and abdominal radiographs on all cases of suspected intussusception while others screen every child with ultrasound and reserve radiography and enema for the positive cases. This latter strategy results in two tests being performed in a large number of patients. Cost-effectiveness advocates propose stratifying the group by clinical parameters. The cases with a classical history (with the clinical triad) go directly to enema whereas those of lower suspicion go to ultrasound. This will reduce the number of unnecessary enemas and the number of unnecessary costly tests.