Bowel Obstruction

The colloquial term bowel obstruction usually refers to a mechanical blockage of the bowel, whereby a structural pathology physically blocks the normal passage of intestinal contents. Around 15% of acute abdomen cases are found to have a bowel obstruction

When the bowel is not mechanically blocked but does not work properly, for example because of inflammation, electrolyte derangement or recent surgery, this is known as functional obstruction or paralytic ileus.


The most common causes of bowel obstruction depend on location:

  • Small bowel – adhesions and herniae
  • Large bowel – malignancy, diverticular disease, and volvulus
    • A large bowel obstruction should be considered to be caused by a GI cancer until proved otherwise

The full list of causes of bowel obstruction can otherwise be divided into extrinsic, intramural, and intraluminal causes, as described in Table 1.

Intraluminal Gallstone ileus, ingested foreign body, faecal impaction
Mural Carcinoma, inflammatory strictures*, intussusception**, diverticular strictures, Meckel’s diverticulum, lymphoma
Extramural Hernias, adhesions, peritoneal metastasis, volvulus

Table 1 – Causes of Bowel Obstruction *especially in CD patients **most common in children


Once the bowel segment has become occluded, there is gross dilatation of the proximal limb of bowel, which in turn results in increased peristalsis of the bowel. This leads to secretion of large volumes of electrolyte rich fluid into the bowel (sometimes termed ‘third spacing’). Urgent fluid resuscitation and close attention to fluid balance is essential.

If there is a 2nd obstruction proximally (as occurs if the obstruction is due to a twist in the bowel or in large bowel obstruction if the ileocaecal valve is competent) this is termed a “closed-loop” obstruction. This is a surgical emergency as the bowel will continue to distend, stretching the bowel wall until it becomes ischaemic and ultimately perforates

Fig. 1 - Sigmoid volvulus, showing severely ischaemic bowel requiring surgical resection

Figure 1 – Sigmoid volvulus, showing severely ischaemic bowel requiring surgical resection; this is an example of a closed-loop obstruction

Clinical Features

The cardinal features of bowel obstruction are:

  • Abdominal pain – colicky or cramping in nature, secondary to the bowel peristalsis.
    • Any pain originally colicky that is now constant in nature or worse on movement should be a “red flag” that ischaemia may be developing
  • Vomiting – initially of gastric contents, before becoming bilious and then eventually faeculent (a dark-brown foul-smelling vomitus)
  • Abdominal distension
  • Absolute’ constipation – failure to pass flatus and faeces

It is important to remember that patients may not display all of these features at once. For example, in patients with very distal obstructions vomiting will develop late, if at all (and in closed loop large bowel obstruction will not develop at all). Likewise, the more proximal the obstruction, the later the constipation will develop.

On examination, patients may show evidence of the underlying cause (e.g. surgical scars, cachexia from malignancy, or obvious hernia) and abdominal distension. Assess for any clinical signs of dehydration or sepsis.

Palpate for focal tenderness* (including guarding and rebound tenderness on palpation). Percussion may reveal a tympanic sound and auscultation may reveal ‘tinkling’ bowel sounds, both signs characteristic of bowel obstruction.

*Focal tenderness indicates ischaemia and is a key warning sign. Patients with bowel obstruction may be uncomfortable on palpation due to the discomfort from pressing on a distended abdomen, but there should be no focal tenderness, guarding, or rebound tenderness unless ischaemia is developing

Differential Diagnosis

The differential diagnoses for bowel obstruction include:

  • Paralytic ileus
  • Toxic megacolon
  • Constipation


Laboratory tests

All patients with suspected bowel obstruction require FBC, CRP, U&Es (hypokalaemia and high urea may be present), and a Group and Save (G&S).

An venous blood gas can be useful to evaluate the signs of ischaemia (high lactate) or for the immediate assessment of any metabolic derangement (secondary to dehydration or excessive vomiting). 


CT imaging is the imaging modality of choice in suspected bowel obstruction and shift in modern practice is moving towards CT scanning as the initial imaging used where possible.

CT imaging (Fig. 2) is more useful than AXRs as they are (1) more sensitive for bowel obstruction; (2) can differentiate between mechanical obstruction and pseudo-obstruction; (3) can demonstrate the site and cause of obstruction (hence extremely useful for operative planning); and (4) may demonstrate the presence of metastases if caused by a malignancy (which is likewise extremely useful in operative planning).

Figure 2 – CT imaging demonstrating features of small bowel obstruction

A plain abdominal radiograph (AXR) is still used in some settings as the initial investigation for bowel obstruction. The AXR findings (Fig. 3) seen in a patient with bowel obstruction are:

  • Small bowel obstruction:
    • Dilated bowel (>3cm)
    • Central abdominal location
    • Valvulae conniventes visible (lines completely crossing the bowel)
  • Large bowel obstruction:
    • Dilated bowel (>6cm, or >9cm if at the caecum)
    • Peripheral location
    • Haustral lines visible (lines not completely crossing the bowel, ‘indents that go Halfway are Haustra’)

An incompetent ileocaecal valve in a large bowel obstruction may show concurrent large and small bowel dilatation on AXR. An erect chest x-ray may also be requested to assess for free air under the diaphragm if clinical features suggest a bowel perforation.

Fig 2 - Bowel obstruction on AXR; (1) Small bowel obstruction, showing valvulae conniventes crossing a dilated, centrally-located bowel; (2) Large bowel obstruction, with peripherally located dilated bowel segments.

Figure 3 – Bowel obstruction on AXR; (1) Small bowel obstruction, showing valvulae conniventes crossing a dilated, centrally-located bowel; (2) Large bowel obstruction, with peripherally located dilated bowel segments

Contrast fluoroscopy can also be useful in small bowel obstruction caused by adhesions from previous surgery. It has been shown to predict quite reliably whether or not the obstruction will settle and some studies have shown a therapeutic benefit of the contrast itself (although this is controversial)


The definitive management of bowel obstruction is dependent on the aetiology and whether it has been complicated by bowel ischaemia, perforation, and/or peritonism.

These patients are often very fluid deplete.  All patients therefore need urgent fluid resuscitation and careful attention paid to fluid balance (often 4-5 litres may be required in the first 24 hours).  Most require a urinary catheter.

Patients with closed loop bowel obstruction or evidence of ischaemia (pain worsened by movement, focal tenderness and pyrexia) require urgent surgery.

Conservative Management

In the absence of signs of ischemia or strangulation, initial management is essentially conservative and is often referred to as a ‘drip and suck’ management:

  • Make the patient nil-by-mouth (NBM) and insert a nasogastric tube (NG tube) to decompress the bowel (‘suck’)
  • Start IV fluids and correct any electrolyte disturbances (‘drip’)
  • Urinary catheter and fluid balance
  • Analgesia as required with suitable anti-emetics


Adhesional small bowel obstruction resulting from previous surgery is treated conservatively in the first instance (unless there is evidence of strangulation / ischaemia), with a success rate of around 80%.

A water soluble contrast study should be performed.  If contrast does not reach the colon by 6 hours then it is very unlikely that it will resolve and the patient should be taken to theatre.

Large bowel obstruction or small bowel obstruction in a patient who has not had previous surgery (termed a “virgin abdomen”) rarely settles without surgery.

Fig 2 - A nasogastric tube should be inserted as part of management of post-operative ileus.

Figure 4 – A nasogastric tube should be inserted as part of management of bowel obstruction.

Surgical Management

Surgical intervention is indicated in patients with:

  • Suspicion of intestinal ischaemia or with a closed loop bowel obstruction
  • Small bowel obstruction in a patient with a virgin abdomen
  • A cause that requires surgical correction (e.g. a strangulated hernia or obstructing tumour)
  • If patients fail to improve with conservative measures (typically after ≥48 hours)

The nature of surgical management will depend on the underlying cause but generally involves a laparotomy. If resection of bowel is required, the re-joining of obstructed bowel is often not possible and a stoma may be necessary.


The complications of bowel obstruction include:

  • Bowel ischaemia
  • Bowel perforation, leading to faecal peritonitis (high mortality)
  • Dehydration and renal impairment

Key Points

  • Any pain originally colicky in a suspected case of bowel obstruction that is now constant in nature or worse on movement should be a “red flag” that ischaemia may be developing
  • Focal tenderness in patients with bowel obstruction indicates ischaemia and is a key warning sign
  • A closed loop bowel obstruction is a surgical emergency
  • Early recognition of those patients with impending strangulation and ischaemia is essential as early surgery will prevent the need for bowel resection
  • Patients with bowel obstruction lose large volumes of electrolyte rich fluid and frequently develop hypokalaemia and acute kidney injury


Question 1 / 7
Which of the following is a typical symptom of small bowel obstruction?


Question 2 / 7
Which clinical feature is most likely to be absent in a 'closed-loop' obstruction?


Question 3 / 7
Which of the following examination signs indicates that ischaemia may be developing?


Question 4 / 7
Which measurement in an arterial blood gas is most indicative of ischaemia?


Question 5 / 7
What is an indication for surgical intervention in bowel obstruction?


Question 6 / 7
Which of the following is not associated with elevated lactate levels in a patient with small bowel obstruction?


Question 7 / 7
Which of the following is most likely to result in a complete bowel obstruction?


Further Reading

Acute GI Obstruction
Best Pract Res Clin Gastroenterol

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