Small bowel obstruction



  • Around 20% of patients with acute abdominal pain are due to intestinal obstruction, of which 80% are small intestine.

Risk factors/aetiology/pathophysiology


  • The most common cause of SBO is post-surgical adhesions (60%)
    • Tend to present <4 weeks post op if acute but can present months-years later with features of chronic obstruction
  • The next most common cause is a strangulated hernia (20%)
  • Other causes include malignancy (usually of the caecum) (5%) and volvulus (5%)
  • Rarely- ileus can cause a pseudo-obstruction (functional obstruction)

SBO leads to dilatation of the intestine proximal to the obstruction due to a build up of GI secretions, contents and swallowed air.  This dilatation can further stimulate secretory and peristaltic activity (to try and wash out the blockage).  This is the reason for early vomiting and diarrhoea in SBO.  The pressure builds such that lymphatic drainage is blocked and lymphoedema develops (worsening the problem further).  In severe cases, pressure grows to compress venules and there is increased venous pressure at the capillary bed, resulting in further loss of fluid (dehydration and hypovolaemia may ensue).  If the pressure continues to build, venous ischaemia and eventually micro-arterial occlusion and ischaemia can result.  However, it is more common for dilated bowel to loop on itself and cut off the mesenteric arterial supply, resulting in rapid necrosis.




The presentation of the obstruction is dependent on its location e.g. proximal (high) SBO usually presents with vomiting (of semi-indigested food); distal (low) SBO can present with faeculant vomiting, but more commonly present with abdominal distention and constipation.


  • History
    • Crampy, intermittent abdominal pain (simple); may be constant and severe (strangulated)
    • Nausea and vomiting
    • Abdominal distention
    • Total Constipation (although diarrhoea may be an early sign)
  • Examination
    • Abdominal distention
    • Hyperactive/tinkling bowel sounds or, if late/strangulated there may be no bowel sounds (abnormal in any case)
    • Fever and tachycardia may be late signs associated with strangulation
    • Always examine for hernias



  • Bloods- important ones include U&Es (particularly if vomiting is severe); FBC to check any infective causes (often presentation is not dissimilar from infective gastritis) and to look for anaemia (?blood loss/cancer); CRP is often raised; Amylase should be checked to rule out pancreatitis (perhaps not relevant to all patients); lactate may also be raised in ischaemic bowel
  • Imaging
    • AXR- distended loops of small bowel (may be able to see air-fluid levels in an upright film); lack of colonic air.
      • As a general rule, the maximum diameter of small bowel should be 3cm (large bowel 6cm and caecum 9cm- 3/6/9 rule); and the small bowel can be differentiated from large bowel by the presence of Valvulae conniventes- mucosal folds extending the entire cirumference of small bowel
      • NB may not always appear like this- early closed-loop obstruction may be invisible/gasless
English: An upright Xray demonstrating a small...
English: An upright Xray demonstrating a small bowel obstruction ( note multiple air fluid levels ) (Photo credit: Wikipedia)


    • CXR is done to look for signs of perforated bowel (free air under diaphragm)
    • CT is done to confirm the extent of SBO and identify regions of obstruction/strangulation etc- and is necessary for any patient requiring surgery



  • Resuscitation
    • Often require extensive resuscitation with saline or Hartmann’s due to hypovolaemia secondary to reduced intake, vomiting, loss of fluids in the lumen of the bowel (increased excretion)
    • May also require supplementation with electrolytes e.g. potassium, magnesium
    • Catheterisation to monitor fluid balance is often required
  • Mx of obstruction
    • NG suction- most patients will benefit from this
    • In simple, partial obstruction (particularly if due to adhesions secondary to previous surgery), the majority of patients will just be managed conservatively as above.  Other reasons for conservative management include metastatic disease, IBD etc.
    • Indications for surgery include:
      • Any chance of strangulation (severe abdo pain, abnormal inflammatory markers, evidence of closed-loop obstruction); peritonitis/visible perforation; irreducible hernia
      • Failure to improve on conservative treatment

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