A small bowel obstruction (SBO) is defined as a blockage in the small intestine, interrupting the normal flow of intra-luminal gastric contents. Small bowel obstructions are increasingly common, accounting for >50% of emergency laparotomies yearly in the UK. The potential complications (including bowel necrosis, perforation and sepsis) have the potential to cause significant morbidity and mortality, therefore must be recognised and treated rapidly to reduce the impact and significant healthcare burden this condition poses.


  • Incidence: 20.00 cases per 100,000 person-years
  • Peak incidence: 40-50 years
  • Sex ratio: 1:1
Condition Relative
Acute appendicitis5.50
Paralytic ileus2.50
Acute pancreatitis2.00
Small bowel obstruction1
Large bowel obstruction0.30
<1 1-5 6+ 16+ 30+ 40+ 50+ 60+ 70+ 80+


The most common cause of small bowel obstruction is adhesions, in approximately 68% of patients. The second most common cause is incarcerated hernias, with an incidence of 12%.

An easy way to remember the causes of small bowel obstructions is via the mnemonic “HANG IVs”
  • Hernias 2%
  • Adhesions
    • From previous surgery with formation of intra abdominal adhesions, commonly colorectal and gynaecological surgery
  • Neoplasms (malignant, benign, primary or secondary) (5%)
  • Gallstone ileus
  • Intussusception
  • Volvulus
  • Strictures (eg Crohn’s disease (6%), ischaemia)

Very rare causes include radiation enteritis (1%), intra abdominal abscesses and foreign bodies.

In children, the most common causes of SBO include:
  • Appendicitis
  • Intussusception
  • Intestinal atresia
  • Volvulus


Small bowel obstructions are caused by a blockage in the lumen of the gastrointestinal tract, meaning the normal passage and digestion of food contents are interrupted. As a result, the bowel distal to the obstruction will collapse. The bowel proximal will continue to dilate with the continuous build up and stasis of luminal contents and gas. It is the combination of the dilation of the intestine, and the continuing intestinal peristalsis, which leads to the colicky nature of the abdominal pain.

As luminal contents fail to pass through the bowel, it results in subsequent nausea and vomiting.
  • Often bilious in nature
  • Leads to loss of fluid, Na+, K+, H+ and Cl-
  • Results in a metabolic alkalosis, hypokalemia and hypovolemia

With time, bowel wall oedema forms, which compresses the intestinal veins and lymphatics, reducing the venous drainage of the bowel. As this occurs, it compresses intestinal arterioles and capillaries, preventing arterial perfusion to the bowel wall. This can lead to subsequent bowel ischaemia, infarction, necrosis, and perforation.

Clinical features

  • Absence of passing flatus (90%) or stool (80%)
  • Abdominal pain (90%)
    • Ranges in severity
    • Often cramping in nature
    • Intermittent every 3-4 minutes. Constant pain may indicate bowel ischaemia.
    • Often precedes vomiting (constant pain may indicate bowel ischaemia)
  • Nausea and vomiting (80%)
    • May be bilious
  • Abdominal distension (65%)

  • Bowel sounds
    • Initially high pitched (tinkling)
    • Increased frequency
    • In delayed presentations, bowel sounds can be reduced due to secondary ileus
  • Abdominal tenderness
  • Fever and tachycardia (if hernia strangulation is the cause of obstruction)

In more proximal bowel obstructions, patients tend to present earlier. Characteristically, abdominal pain and vomiting are the predominant symptoms.

In distal small bowel obstructions, patients usually present after 2-3 days of abdominal pain. The predominant symptoms are abdominal distension and constipation.

In the case of a severe, complete and/or complicated obstruction, the common features include:
  • Complete obstipation
  • Severe lethargy
  • Fevers and rigors
  • Bilious vomiting
  • Tachycardia and tachypnoea


Abdominal X-Ray

The ideal initial investigation for a suspected SBO is plain radiography, as it is a usually fast and reliable test to confirm the diagnosis.
  • Upright image
    • To assess for air-fluid levels and pneumoperitoneum
    • If not possible, UpToDate suggests a patient should be placed in lateral decubitus position to show free air and/or fluid levels
  • Supine image

Findings on x-ray consistent with SBO include:
  • Dilated bowel loops with/without air-fluid levels
    • Considered dilated if small bowel is >3cm diameter
  • Proximal bowel dilation and distal bowel collapse
  • Absence of gas in the abdomen
    • Due to complete filling of bowel loops with sequestered fluid

CT Scan
  • BMJ indicate that plain radiography can be poorly sensitive for early and low grade SBO
  • If there is a high clinical suspicion from the history and examination, and x-rays are negative, BMJ recommend ordering an abdominal CT scan also
  • Where possible, if available, BMJ suggest that CT scan should be used instead of plain radiography due to higher sensitivity as well as ability to identify the cause of the obstruction in the first place

Findings consistent with SBO include:
  • Dilated bowel loops proximal to the transition point
    • Small bowel diameter >3cm
  • Bowel wall thickening
    • >3mm
  • Collapsed bowel distal to transitional point
  • Pneumoperitoneum
    • Indicates perforation and therefore increased urgency of management
  • Submucosal oedema/haemorrhage

Additional investigations
  • Small bowel follow through (barium or gastrografin)
    • May also be therapeutic for SBO as well as diagnostic
    • If concerned about perforation, a water soluble contrast only should be used (e.g. gastrografin)
    • Presence of contrast in the large bowel at 24 hours post-administration predicts the resolution of a SBO with 97% sensitivity and 96% specificity

Differential diagnosis

There are a number of differential diagnosis for SBO, as while many patients will present with the classical symptom profile, the signs/symptoms can also be similarly present in a number of other conditions.

Large bowel obstruction
  • Similarities

  • Differences
    • LBO more frequently caused by malignant tumours and volvulus
    • Faeculent vomiting is more frequently associated with LBO (late symptom)
    • Abdominal distention is more common in LBO
    • Typically the interval at which cramping pain occurs is longer

Paralytic ileus
  • Similarities
    • Present with abdominal pain, vomiting and constipation
    • Both commonly caused by abdominal surgery

  • Differences
    • More frequently caused by recent abdominal surgery and medications (e.g. opioids)
    • Presents with diffuse, continual abdominal pain (rather than colicky abdominal pain)
    • Bowel sounds tend to be absent

  • Similarities
    • Present with abdominal pain, nausea, and vomiting

  • Differences
    • Vomiting is non-bilious in nature
    • Usually are febrile, whereas SBO usually must be complete/complicated to cause a low-grade fever
    • Typically in younger patients with no previous surgical history


  • Differences
    • Often have chronic constipation, with diarrhoea (paradoxically) during acute episodes
    • Often a chronic condition with recurrent episodes


Most importantly, BMJ state that it is imperative to note that SBO is a medical emergency, therefore requires rapid recognition, admission and subsequent management.

Non-operative treatment tends to be reserved for patients with a partial SBO, with subsequent surgical management if there is poor response to this management after 48-72 hours, according to BMJ best practice. Approximately 1/4 of patients will not respond to non-operative management, and therefore will eventually require surgery.

Non-operative treatment
  • Fluid + electrolyte resuscitation
    • This is particularly important if the patient is vomiting and there are large sequestrations of fluid in the intestinal space
    • Intravenous lines and large volumes of IV fluids are required
    • UpToDate suggest use of isotonic crystalloid such as Ringer's lactate, or normal saline
    • A Foley catheter should be used to monitor fluid status and urine output, with >0.5mL/kg/hour being adequate
  • Nasogastric tube
    • Decompression of the upper gastrointestinal tract should be initiated early to avoid vomiting, as well as to reduce gastric and small bowel distention
    • Will also act as a powerful anti-emetic
  • Diet
    • Patients should become nil by mouth until resolution of SBO
    • UpToDate suggest a small subset of patients with only a partial bowel obstruction may tolerate small volumes of oral hydration
  • Both BMJ best practice and UpToDate discourage the use of prophylactic broad-spectrum antibiotics in non-operative treatment regimes. However, if surgery is required, antibiotic prophylaxis can be considered.

Operative treatment
  • Indications for operative management include:
    • Bowel compromise (e.g. ischaemia, perforation, necrosis), generally occurring in complete bowel obstructions
    • Surgically correctable causes (e.g. volvulus, incarcerated hernia, gallstone ileus, foreign body ingestion, tumour)
  • Prophylactic antibiotics
    • If surgery is being undertaken, patients should have antibiotic prophylaxis
    • BMJ suggests the use of cefoxitin, or ampicillin plus gentamicin
  • Surgical treatment often involves correction of the underlying cause


If left untreated, small bowel obstructions can cause serious, life-threatening complications, leading to significant morbidity and mortality.

Bowel ischaemia
  • A change in the character of the pain (e.g. intermittent to continuous pain, increase in severity) or worsening of abdominal signs (e.g. rebound tenderness, guarding) should highlight the possibility of complications and the need for immediate intervention
  • If SBO is managed promptly, resolution tends to occur within 24-48 hours. However, beyond this time frame, without resolution, the risk of vascular compromise increases significantly.
  • Strangulation is the arguably one of most severe complications of SBO, occurring when bowel wall oedema compromises the perfusion of the intestine and necrosis follows
  • If this progresses further, it can result in bowel wall perforation, causing leakage of contents into the abdominal cavity, resulting in peritonitis, abdominal sepsis and potentially death

  • Increasingly more common if bowel perforation is present, sepsis can be a life-threatening condition and has a significant morbidity and mortality
  • Can result in multi-organ failure and therefore death

Short bowel syndrome
  • This complication occurs if surgical management of SBO is necessary, and requires a large portion of small intestine to be removed for appropriate management
  • With reduction in the surface area and distance for absorption of gastric nutrients, it essentially may lead a patient to require lifelong supplemental nutrition or intestinal transplantation