• Peak incidence: 4 to 24 months ^[1]. The majority of patients are younger than 2 years ^[2].
• Uncommon before 3 months and after 6 years of age. When it occurs outside this window, a pathological lead point should be strongly suspected ^[2][3].
• In adults, intussusception is rare and almost always secondary to a structural lesion (polyp, lipoma, GIST, carcinoma) ^[5].

• Male > Female (approximately 3:2) ^[1][2].

• In Hong Kong, intussusception remains a common paediatric surgical emergency. The typical patient is a well-nourished infant aged 6–12 months presenting in autumn/winter (coinciding with peak viral gastroenteritis season). Awareness of this condition among parents and primary care physicians allows earlier presentation and higher rates of successful non-operative reduction.
• Rotavirus vaccine (Rotarix®, RotaTeq®) is part of the HK childhood immunisation schedule. There is a small but recognised increased risk of intussusception in the first 1–2 weeks following the first dose of rotavirus vaccine, though the absolute risk is very low (~1–6 per 100,000 vaccinated infants) ^[4].

Understanding why intussusception occurs most often at the ileocaecal junction requires appreciating the anatomy:

• Terminal ileum: The last ~20–30 cm of the ileum is particularly rich in lymphoid tissue (Peyer's patches) — aggregated lymphoid follicles in the submucosal layer. These are part of gut-associated lymphoid tissue (GALT) and are most prominent in childhood, gradually atrophying with age.
• Ileocaecal valve (ICV): A physiological sphincter at the junction of the ileum and caecum. It acts as a one-way valve. The abrupt change in calibre (narrow ileum → wide caecum) and the lip-like valve structure create a natural "funnel" into which the ileum can prolapse.
• Mesentery: The ileal mesentery is relatively mobile and long in children, allowing the ileum to prolapse into the caecum and ascending colon.

The combination of:

1. Abundant lymphoid tissue (prone to reactive hyperplasia → acts as a "lead point")
2. Calibre change at the ICV (creates a natural telescoping point)
3. Mesenteric mobility in young children

...explains why ~85–90% of paediatric intussusceptions are ileocolic (ileocaecal) ^[4].

When the intussusceptum is dragged into the intussuscipiens:

1. Mesentery is compressed between the two layers of bowel → venous obstruction first (veins are thin-walled and compress before arteries)
2. Venous congestion → oedema of bowel wall → bloody mucus secretion ("red currant jelly stool")
3. If untreated, progressive oedema → arterial compromise → ischaemia → necrosis → perforation → peritonitis
4. Luminal obstruction → proximal bowel dilatation, vomiting (initially non-bilious if the obstruction is distal; bilious if proximal to ampulla of Vater or as condition progresses)

• May be preceded by viral infection ^[1].
• 75% of cases are idiopathic — no clear pathological lead point is found ^[2].
• Proposed mechanism: Viral infections (URTI, gastroenteritis — particularly adenovirus, rotavirus, enterovirus) cause reactive lymphoid hyperplasia of Peyer's patches in the terminal ileum ^[2][3].
  • Enlarged Peyer's patches (90%) act as a "lead point" — the hypertrophied lymphoid tissue protrudes into the lumen and is caught by normal peristalsis, which propels it distally, dragging the proximal bowel with it ^[4].
  • This explains the seasonal variation (autumn/winter peaks coinciding with viral season) and the age predilection (Peyer's patches are most prominent in infancy/early childhood).

A pathological lead point is a structural lesion within the bowel wall or lumen that acts as the "anchor" dragged by peristalsis. It is more common in children > 3 years and in adults. In adults, intussusception is ALWAYS associated with pathological lead-points (usually intraluminal lesions in SB), e.g., polyp, submucosal lipoma, Meckel's diverticulum, GIST, carcinoma ^[5].

• Occurs after abdominal or retroperitoneal surgery (1–21 days post-op).
• Caused by uncoordinated peristaltic activity during recovery or traction from suture lines/feeding tubes ^[2].
• Usually ileo-ileal (not ileocolic), making it difficult to diagnose and not amenable to pneumatic/hydrostatic reduction.
• Diagnosis requires a high index of suspicion in any child with post-operative abdominal distension and vomiting.

• Rotavirus vaccine (particularly the earlier RotaShield® vaccine, now withdrawn) was associated with intussusception.
• Current vaccines (RotaTeq®, Rotarix®) carry a very small increased risk (~1–6 extra cases per 100,000 vaccinated infants), predominantly within 1 week of the first dose ^[4].
• The benefit of vaccination (preventing severe rotavirus gastroenteritis) far outweighs this small risk, and vaccination remains recommended.

The sequence of events in intussusception follows a logical, step-wise progression:

1. Why venous obstruction before arterial?

   • Veins are thin-walled, low-pressure vessels. When the mesentery is compressed between the intussusceptum and intussuscipiens, veins are occluded first while arterial inflow continues → venous engorgement and haemorrhagic oedema of the bowel wall.

2. Why "red currant jelly" stool?

   • Venous congestion → transudation of blood and mucus into the bowel lumen. The mixture of blood and mucus gives the characteristic dark red, jelly-like appearance (like redcurrant jam). This is a late sign and indicates significant mucosal ischaemia — it means the condition has been present for hours ^[4].

3. Why colicky (intermittent) pain?

   • Each peristaltic wave attempts to propel the intussusceptum further into the intussuscipiens → intense visceral pain during the peristaltic contraction → relief between contractions.
   • As the condition progresses, the pain may become constant (suggesting ischaemia/peritonitis).

4. Why bilious vomiting?

   • Bilious vomiting occurs because the obstruction is distal to the ampulla of Vater (in ileocolic intussusception, the obstruction is at the ileocaecal junction) → bile-stained vomiting indicates intestinal obstruction ^[4].
   • Initially vomiting may be non-bilious (reflex vomiting from visceral stimulation), but as obstruction progresses, it becomes bilious.

5. Why a "sausage-shaped" mass?

   • The telescoped bowel forms an elongated, firm, curved mass in the right upper quadrant (the intussusceptum advances from the ileocaecal region along the ascending colon toward the hepatic flexure and transverse colon) ^[4].
   • Sausage-shaped mass in RUQ ^[4].

6. "Dance's sign" (emptiness in RIF)

   • As the caecum is drawn up with the advancing intussusceptum, the right iliac fossa becomes empty to palpation (the caecum has been "pulled up"). This is called Dance's sign.

Intussusception causes mechanical small bowel obstruction. The general pathophysiology of intestinal obstruction applies ^[4]:

• Proximal dilatation: Initially, increased peristalsis attempts to overcome the obstruction. Soon, gas accumulates (swallowed air + bacterial fermentation) and fluid accumulates (failed absorption + secretion from oedematous wall).
• Dehydration and electrolyte imbalance from:
  • Reduced oral intake
  • Vomiting
  • Defective intestinal absorption (oedematous bowel wall)
  • Transudation of fluid into the peritoneal cavity (third-spacing)
• Strangulation: Unlike simple obstruction, intussusception causes strangulated obstruction (mesenteric compromise) → ischaemia → necrosis → perforation → faecal peritonitis and sepsis.

The clinical presentation of intussusception is one of the most classic and exam-tested presentations in paediatric surgery. However, the classical triad (intermittent colicky abdominal pain + red currant jelly stool + sausage-shaped mass) is present in fewer than 50% of cases ^[4].

Understanding the chronological evolution helps you assess severity:

This is one of the most important differential groups because red currant jelly stool is a hallmark of intussusception but is a late sign. Several other conditions also cause rectal bleeding in children ^[2][4]:

Key distinguishing point for rectal bleeding: Intussusception bleeding is painful, small amount, with mucus ("red currant jelly"), whereas Meckel's diverticulum bleeding is painless, large amount, altered blood ^[4].

This is a critically important differential group because intussusception can present primarily with lethargy, and the abdominal pathology may be overlooked ^[2]:

When intussusception presents late with peritonitis, or when the history is atypical, it can mimic conditions that cause RLQ pain and peritoneal irritation ^[3][4]:

Blood tests: Blood count, renal and liver function, amylase, clotting profile, arterial blood gas, type and screen ^[10]

Non-operative reduction works by applying retrograde pressure (air or fluid) into the colon via the rectum, which pushes the intussusceptum back through the intussuscipiens in the reverse direction of the original telescoping. Think of it as "un-telescoping" the bowel by pushing from below.

There are two main techniques, and two main guidance modalities:

Fluoroscopic-guided pneumatic reduction (75–95% success rate): instill gas with 18–22 Fr Foley → maintain pressure ~100–120 mmHg × 3 min → observe for gush of air into terminal ileum ^[4].

1. Preparation: Child is resuscitated, IV access secured, NG in situ. Surgeon and anaesthetist must be on standby (in case of perforation requiring emergency laparotomy). Erect CXR done beforehand to exclude pre-existing pneumoperitoneum.
2. Catheter insertion: 18–22 Fr Foley catheter inserted into the rectum; balloon inflated to create a seal (prevents air leak) ^[4].
3. Air insufflation: Air is insufflated via a hand pump or controlled insufflator, connected to a pressure gauge.
4. Pressure maintenance: Maintain pressure ~100–120 mmHg for ~3 minutes ^[4]. The pressure must not exceed 120 mmHg (risk of perforation rises steeply above this).
5. Monitoring: Under fluoroscopy, the intussusceptum is seen as a soft tissue mass outlined by air. As reduction proceeds, the mass retreats toward the ileocaecal valve.
6. Endpoint of successful reduction: Observe for gush of air into terminal ileum — this is the definitive sign that the intussusception has been completely reduced ^[4]. Other markers of success include: appearance of water and bubbles in terminal ileum, free flow of contrast or air into terminal ileum, relief of symptoms, disappearance of abdominal mass ^[2].
7. If unsuccessful: A single attempt may be repeated (up to 3 attempts total is the general consensus). Between attempts, allow a rest period of 15–30 minutes (the oedema may settle slightly, improving the chance of success on the next attempt).

• Increasingly popular in Hong Kong and globally as it avoids radiation entirely.
• Ultrasound is now the intervention of choice for guidance ^[2].
• Warm normal saline is infused per rectum under gravity (height of the saline bag provides the hydrostatic pressure, typically hung ~100 cm above the patient).
• The USG operator watches the intussusception reduce in real-time — the "target sign" progressively disappears, and eventually free fluid is seen flowing into the terminal ileum.
• Advantages over fluoroscopy: No radiation, real-time monitoring of bowel wall viability (Doppler), better visualisation of complete reduction, ability to detect residual intussusception or lead points.

Post-reduction management: Observation with hospitalisation for 12–24 hours ^[2]:

Surgical reduction: indicated if ^[4]:

1. Failed pneumatic reduction (after up to 3 attempts)
2. Peritonitis / necrosis
3. Suspected pathological lead points

Additional surgical indications ^[2]: 4. Suspected intussusception who is critically ill ^[2] 5. Suspected bowel perforation ^[2] 6. Refractory to non-operative reduction ^[2] 7. Post-operative (ileo-ileal) intussusception — not amenable to enema reduction 8. Adult intussusception — adults usually require formal resection to r/o CA ^[5]

Manual reduction at operation is attempted in most cases but resection with primary anastomosis may be needed if manual reduction is not possible or a lead point is identified ^[2].

The operative steps follow a logical sequence:

1. Operative decompression (always performed first):

   • Milk SB content in retrograde manner to stomach → insert orogastric tube for aspiration → replace aspirated volume as measured ^[13].
   • This decompresses the dilated proximal bowel, improving surgical access and reducing the risk of post-operative ileus.

2. Manual reduction (Hutchinson's manoeuvre):

   • The surgeon applies gentle, sustained retrograde pressure on the intussuscipiens (the distal "receiving" bowel) to push the intussusceptum back out — like squeezing toothpaste back into the tube.
   • Critical rule: Never PULL on the intussusceptum (this will tear the oedematous, friable bowel). Always PUSH from below.
   • Continue squeezing gently until the intussusceptum is fully delivered back through the ileocaecal valve.

3. Assessment of bowel viability after reduction:

   • Inspect the reduced bowel for colour (pink = viable; dark purple/black = necrotic), peristalsis, mesenteric pulsations.
   • Wrap in warm saline-soaked packs and wait 10–15 minutes to reassess if borderline.

4. Decision regarding resection:

5. Incidental appendicectomy: Some surgeons perform a prophylactic appendicectomy during the procedure, as the caecum is already mobilised. This is not universally practised.

6. Histological examination: All resected specimens (bowel, lead points) should be sent for histology to exclude lymphoma, GIST, or other pathology.

Why it happens: The trapped mesentery between the intussusceptum and intussuscipiens undergoes progressive vascular compression. Veins (low-pressure, thin-walled) occlude first → venous congestion and oedema → if untreated, the swelling eventually compresses arteries (high-pressure, thick-walled) → arterial ischaemia → transmural necrosis (gangrene) ^[2][8].

Clinical features suggestive of strangulation ^[8]:

• Clinical signs: Fever, tachycardia, peritoneal signs (guarding, rigidity, rebound tenderness)
• Clinical symptoms: Continuous or worsening abdominal pain (loss of the classic pain-free intervals — the pain becomes constant because the ischaemia is now continuous, not just during peristaltic waves)
• Biochemical: Leucocytosis, metabolic acidosis, raised lactate → intestinal ischaemia ^[8]
• Radiological: Pneumatosis intestinalis (gas within the bowel wall from gas-producing bacteria infiltrating necrotic tissue), portal venous gas (gas entering mesenteric venous drainage and travelling to the portal system — an ominous sign), bowel wall thickening, reduced or lack of bowel wall enhancement on CT ^[8][15]

Significance: Morbidity and mortality are dependent on duration of ischaemia and its extent. Any length of ischaemic bowel can cause significant systemic effects secondary to sepsis and dehydration ^[8]. This is why early diagnosis and treatment are critical.

Prognosis of intestinal obstruction: Mortality 2% (non-strangulated) vs 10–30% (strangulated) ^[13].

Why it happens: Transmural necrosis weakens the bowel wall → the necrotic segment gives way → intestinal contents (faeces, bacteria, gas) spill into the peritoneal cavity.

Consequences:

• Faecal peritonitis: Spillage of enteric bacteria (E. coli, Bacteroides, Enterococcus) into the sterile peritoneal cavity triggers a massive inflammatory response → peritonitis ^[2].
• Pneumoperitoneum: Free gas visible under the diaphragm on erect CXR, or as Rigler's sign (both sides of bowel wall visible) on supine AXR ^[15].
• Septic shock: Bacterial contamination → systemic inflammatory response → multi-organ dysfunction.

Clinical features: Sudden worsening of pain (which may paradoxically briefly "improve" as the distended bowel decompresses on perforation — a false reassurance), followed by signs of generalised peritonitis (board-like rigidity, absent bowel sounds, fever, tachycardia, hypotension) ^[8].

Why it happens: Follows from two mechanisms:

1. Perforation → direct faecal contamination of peritoneum (see above).
2. Bacterial translocation → even without frank perforation, the ischaemic, oedematous bowel wall loses its barrier function → bacteria and endotoxins translocate across the mucosa into the mesenteric lymphatics and portal circulation → bacteraemia and endotoxaemia ^[2].

Pathophysiology of sepsis from bowel ischaemia: Endotoxin (lipopolysaccharide from Gram-negative bacteria) triggers a cytokine cascade (TNF-α, IL-1, IL-6) → systemic vasodilation, capillary leak, myocardial depression → septic shock → multi-organ dysfunction (renal failure, DIC, ARDS).

Clinical features: High fever, tachycardia, hypotension, altered consciousness, poor capillary refill, oliguria. In infants, sepsis may present subtly with lethargy, poor feeding, and temperature instability (hypothermia as well as hyperthermia).

Why it happens: The intestinal obstruction component of intussusception causes fluid and electrolyte losses through multiple mechanisms ^[8]:

Electrolyte consequences:

• Hypokalaemia: Loss of K⁺ in vomitus + metabolic alkalosis shifts K⁺ intracellularly + renal K⁺ wasting from RAAS activation (hypovolaemia → aldosterone → Na⁺ reabsorption and K⁺ secretion in collecting duct)
• Hypochloraemia: Loss of HCl in vomiting
• Metabolic alkalosis: Loss of H⁺ in vomiting (high SBO/gastric outlet pattern)
• Metabolic acidosis (late): If ischaemia develops → lactic acidosis overrides the alkalosis
• Hyponatraemia: Dilutional (from inappropriate ADH secretion in the setting of hypovolaemia) and loss in vomitus

Why it happens: If extensive bowel necrosis has occurred by the time of surgery, a large segment of bowel may need to be resected. In neonates and infants, the total small bowel length is shorter to begin with (~200–250 cm at birth), so losing even a moderate segment represents a proportionally larger loss.

Definition: Short bowel syndrome occurs when the remaining functional small bowel is insufficient to maintain adequate nutrient and fluid absorption (generally < 100 cm in children, or < 30 cm of jejunum + ileum if the ileocaecal valve is lost).

Consequences: Malabsorption, failure to thrive, dependence on parenteral nutrition, micronutrient deficiencies. Loss of the terminal ileum specifically impairs bile salt reabsorption (→ fat malabsorption, steatorrhoea) and vitamin B12 absorption.

Relevance to intussusception: This complication underscores the importance of early diagnosis and non-operative reduction — the longer the delay, the more bowel may need to be resected.

If a pathological lead point was the cause of intussusception, the complications of the lead point itself must be considered: