Bowel Obstruction

star star star star star
based on 93 ratings

Last updated: December 26, 2021
Revisions: 33

Last updated: December 26, 2021
Revisions: 33

format_list_bulletedContents add remove

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

Once the bowel segment has become occluded, gross dilatation of the proximal limb of bowel occurs, resulting in an increased peristalsis of the bowel. This leads to secretion of large volumes of electrolyte-rich fluid into the bowel (often termed ‘third spacing’). Urgent fluid resuscitation and careful fluid balance is required.

*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

Closed Loop Obstruction

If there is a second obstruction proximally (such as in a volvulus or in large bowel obstruction with a competent ileocaecal valve) 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 or perforates.


The most common causes of bowel obstruction depend on location:

  • Small bowel – adhesions and herniae
  • Large bowel – malignancy, diverticular disease, and volvulus

The full list of causes of bowel obstruction can otherwise be divided into extrinsic, intramural, and intraluminal causes (Table 1)




Gallstone ileus, ingested foreign body, faecal impaction


Cancer, inflammatory strictures*, intussusception**, diverticular strictures, Meckel’s diverticulum, lymphoma


Hernias, adhesions, peritoneal metastasis, volvulus

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

Clinical Features

The cardinal features of bowel obstruction are:

  • Abdominal pain – colicky or cramping in nature (secondary to the bowel peristalsis)
  • Vomiting – occurring early in proximal obstructions and late in distal obstructions
  • Abdominal distension
  • Absolute constipation – occurring early in distal obstruction and late in proximal obstruction

*Initially of gastric contents, before becoming bilious and then eventually faeculent

On examination, patients may show evidence of the underlying cause (e.g. surgical scars, cachexia from malignancy, or obvious hernia) or abdominal distension. Ensure to assess the patient’s fluid status, as third-spacing can occur in bowel obstruction.

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.

*Patients with bowel obstruction may have abdominal tenderness, however should not have features of guarding or rebound tenderness, unless ischaemia is developing

Figure 1 – Causes of Bowel Obstruction (A) Small Bowel Obstruction secondary to adhesions (B) Large Bowel Obstruction secondary to malignancy

Differential Diagnosis

The differential diagnoses for bowel obstruction include pseudo-obstruction, paralytic ileus, toxic megacolon, and constipation


Laboratory tests

All patients with suspected bowel obstruction require routine urgent bloods on admission, including FBC, CRP, U&Es, LFTs, and a Group and Save (G&S); important to monitor for electrolyte changes and third-space losses

A 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). 


A CT scan with IV contrast of the abdomen and pelvis is the imaging modality of choice in suspected bowel obstruction and a 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 useful for operative planning); and (4) may demonstrate the presence of metastases if caused by a malignancy (which is likewise useful in operative planning).

Figure 2 – CT scan 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

Water soluble contrast study (also termed contrast fluoroscopy, typically using gastrograffin) 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 intravascularly fluid deplete. All patients therefore need urgent fluid resuscitation and careful attention paid to fluid balance (often several litres of intravenous fluid may be required in the first 24 hours). Most will 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 ischaemia 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, Fig. 4) 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

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

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 in cases that do not resolve within 24 hours conservative management. 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.

Surgical Management

Surgical intervention is indicated in patients with:

  • Suspicion of intestinal ischaemia or closed loop bowel obstruction
  • A cause that requires surgical correction (such as 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 will warrant 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

  • Small bowel obstruction is commonly caused by adhesions or herniae, and large bowel obstruction by malignancy, diverticular disease, or volvulus
  • Any colicky pain in a suspected case of bowel obstruction that becomes constant in nature or worse on movement should be a “red flag” that ischaemia may be developing
  • 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