Small bowel obstruction

Small Bowel Obstruction: Aetiology, Presentation & Management

Bowel obstruction represents one of the most frequent surgical admissions in Australia, accounting for approximately 15% of all emergency presentations for acute abdominal pain. Rapidly differentiating mechanical obstruction from paralytic ileus, identifying the anatomical level of blockage, and detecting early bowel strangulation are fundamental clinical skills.

Aetiologies of Small Bowel Obstruction (SBO)

The causes of mechanical small bowel obstruction are anatomically classified into three categories:

1. Extrinsic Lesions (Most Common):
   • Postoperative Adhesions (60% to 70%): Previous open or laparoscopic abdominopelvic surgery (particularly appendicectomy, colorectal resections, and gynaecological procedures) induces peritoneal fibrinous adhesions that mature into fibrous bands, tethering or kinking small bowel loops.
   • Incarcerated / Strangulated Hernias (15% to 20%): External abdominal wall hernias (indirect/direct inguinal, femoral, umbilical, incisional) and internal hernias (mesenteric defects, paraduodenal). In elderly females, an incarcerated femoral hernia is easily overlooked and carries an exceptionally high risk of strangulation due to the rigid, unyielding boundaries of the femoral canal.
   • Extrinsic Compression / Carcinomatosis: Peritoneal metastases from ovarian, gastric, or colorectal malignancies.
2. Intrinsic Mural Lesions:
   • Strictures secondary to Crohn's disease, radiation enteritis, ischaemic strictures, or chronic NSAID enteropathy.
   • Primary neoplasms: Small bowel adenocarcinoma, gastrointestinal stromal tumors (GIST), carcinoid/neuroendocrine tumors, lymphoma.
3. Intraluminal (Intraluminal Obstruction):
   • Gallstone Ileus: A large gallstone (typically $> 2.5\text{ cm}$) erodes through the inflamed gallbladder wall into the adjacent duodenum via a cholecystoduodenal fistula, traveling down the gastrointestinal tract until it impacts at the narrowest anatomical point—the ileocaecal valve. Pathognomonic radiological triad on plain radiograph or CT is the Rigler triad: (1) pneumobilia (air in the biliary tree), (2) mechanical small bowel obstruction, and (3) an ectopic radiopaque gallstone in the right iliac fossa.
   • Intussusception, ingested foreign bodies, bezoars (phytobezoars/trichobezoars), heavy ascaris burden.

Clinical Presentation: The Cardinal Tetrad

1. Colicky Abdominal Pain: Centred in the periumbilical region (midgut), occurring in rhythmic paroxysms corresponding to vigorous peristaltic waves attempting to overcome the mechanical barrier. In uncomplicated obstruction, pain softens between spasms; continuous, unremitting, focal pain indicates developing strangulation and ischaemia.
2. Vomiting: In proximal (jejunal) SBO, vomiting occurs early and is profuse, consisting of gastric secretions and bile. In distal (ileal) SBO, vomiting occurs later and becomes feculent (orange-brown with a foul odour) resulting from bacterial overgrowth and decomposition of stagnant intraluminal contents.
3. Abdominal Distension: Minimal or absent in high jejunal obstruction; prominent, generalized, and progressive in distal ileal obstruction.
4. Absolute Constipation (Obstipation): The failure to pass both flatus and faeces. Crucially, patients may continue to pass residual flatus and bowel motions stored distal to the transition point for the first 12 to 24 hours following obstruction onset.

Examination Findings

• General inspection reveals dehydration, sunken eyes, dry mucous membranes, tachycardia, and peripheral vasoconstriction.
• Abdomen is distended with visible peristaltic waves in thin patients. Palpation reveals diffuse tenderness without localized peritonitis in uncomplicated cases. Rebound tenderness, involuntary guarding, or localized rigidity signals transmural ischaemia or perforation.
• Auscultation reveals high-pitched, tinkling, hyperactive bowel sounds with "rushes and borborygmi" during early obstruction, which progresses to a silent abdomen as bowel exhaustion or strangulation develops.
• Mandatory Examination Rule: The clinician must systematically expose and palpate all hernial orifices (both groins for inguinal and femoral hernias, umbilicus, surgical scars for incisional hernias). A digital rectal examination (DRE) must be performed to assess for rectal masses, faecal impaction, or rectal blood.

Diagnostic Imaging

• Plain Abdominal Radiography (Supine and Erect):
  • Multiple dilated centrally located loops of small bowel ($> 3\text{ cm}$ diameter).
  • Plicae circulares (valvulae conniventes): Thin, closely spaced mucosal folds that completely traverse the entire width of the luminal diameter (distinguishing small bowel from large bowel).
  • Ladder-like configuration of fluid-filled loops with air-fluid levels on erect view.
  • String-of-pearls sign: Tiny pockets of trapped gas along the superior border of fluid-filled, vertically oriented small bowel loops.
  • Paucity or complete absence of gas within the colon and rectum.
• Contrast-Enhanced CT of the Abdomen and Pelvis: The gold standard investigation (sensitivity $> 95\%$). It precisely localizes the transition point (dilated proximal bowel transitioning to collapsed distal bowel), identifies the specific aetiology (adhesional band, hernia, tumor, gallstone), and evaluates for life-threatening complications:
  • Closed-Loop Obstruction: A segment of bowel occluded at two adjacent points along its length (e.g., twisting around an adhesional band or within an internal hernia ring). Lacks proximal and distal venting; intraluminal pressure escalates exponentially, precipitating rapid venous infarction.
  • Strangulation / Ischaemia: Decreased or absent bowel wall enhancement following IV contrast, bowel wall thickening ($> 3\text{ mm}$), mesenteric fluid, mesenteric haziness ("swirl sign" of twisted vessels), pneumatosis intestinalis, or free intraperitoneal air.

Management Principles: "Drip and Suck" Protocol

Approximately 70% to 80% of patients with uncomplicated adhesive small bowel obstruction resolve successfully with non-operative, conservative management:

1. Aggressive Intravenous Fluid Resuscitation ("Drip"): Protracted vomiting and massive third-space fluid sequestration into the bowel lumen cause profound hypovolaemia, hypokalaemic hypochloraemic metabolic alkalosis, and prerenal acute kidney injury. Administer balanced crystalloids (Hartmann solution or 0.9% normal saline supplemented with potassium chloride), titrating to urine output $\ge 0.5\text{ mL/kg/h}$.
2. Nasogastric Tube Decompression ("Suck"): Insert a wide-bore ($14-16\text{ Fr}$) Salem sump nasogastric tube to low continuous or intermittent suction. This empties gastric secretions, decompresses proximal bowel, prevents pulmonary aspiration, and decreases wall tension.
3. Water-Soluble Contrast Challenge (Gastrografin): Administer $50-100\text{ mL}$ of water-soluble, hyperosmolar contrast (diatrizoate meglumine / Gastrografin) via the NGT. In addition to being diagnostic (if abdominal radiograph at 8 to 24 hours demonstrates contrast within the caecum/colon, non-operative resolution occurs in $> 95\%$ of cases), Gastrografin is therapeutic: its extreme hypertonicity draws fluid into the bowel lumen, diluting content, reducing bowel wall oedema, and stimulating peristalsis.
4. Indications for Immediate Emergency Surgery (Laparotomy / Laparoscopy):
   • Clinical or radiological signs of strangulation or bowel ischaemia (fever, persistent tachycardia, unremitting localized pain, localized peritonitis, leukocytosis, elevated serum lactate).
   • Closed-loop obstruction or volvulus on CT.
   • Incarcerated / strangulated external hernia (urgent operative reduction and defect repair).
   • Complete mechanical obstruction failing to resolve after 48 to 72 hours of conservative therapy.

• Laplace relation: For an ideal thin cylindrical structure, wall tension per unit length relates to pressure × radius, and wall stress also depends inversely on thickness. The spherical pressure × radius/2 relation should not be substituted indiscriminately. Clinical perforation risk also depends on duration and tissue condition.

Large bowel obstruction (LBO) accounts for approximately 20% to 25% of all intestinal obstructions. In contrast to SBO, LBO is a disease predominantly of older adults, with distinct anatomical and physiological mechanics.

Primary references (checked 7 October 2026): National bowel screening.