Small Bowel Obstruction

Overview

Bowel obstruction describes obstruction leading to the prevention of normal movement of the contents in the small and large bowels. The obstruction may be mechanical (e.g. adhesions or tumours) or functional (e.g. where peristalsis is decreased, see Ileus).

Both small and large bowel obstructions are emergencies that require urgent diagnosis and treatment as they can lead to bowel ischaemia, perforation, peritonitis, sepsis, and death.

In the developed world, small bowel obstruction (SBO) is most commonly caused by adhesions (often due to previous abdominal surgery, see below). 

Adhesions

Adhesions are fibrous bands that form between organs and surrounding tissue, ‘adhering’ them together. This is part of the normal healing process in a similar way to how scar tissue forms.

Previous abdominal surgery can cause the formation of adhesions within hours, however, they may not produce symptoms for up to years and decades after, or sometimes never. Adhesions can cause the small bowel to twist, become pulled on, and kink, leading to impaired flow of material. Adhesions can also form in the pelvis (e.g. Asherman’s syndrome) and the shoulder joint (causing adhesive capsulitis).

Causes

Overview

Causes of SBO include:

• Adhesions from previous surgery – the most common cause
• Abdominal hernias that contain bowel tissue
• Crohn’s disease – due to inflammation causing adhesions or strictures
• Appendicitis – if an abscess forms, this can cause SBO
• Malignancy
• Volvulus

In children, other causes include:

• Intestinal atresia
• Pyloric stenosis
• Intussusception

Epidemiology

• Both small and large bowel obstructions make up 20% of hospital admissions for acute abdominal pain. SBO is more common than large bowel obstructions.
• In the UK, SBO is most commonly due to adhesions after surgery, followed by hernias

Presentation

Overview

The classic features of SBO include:

• Severe intermittent abdominal pain:
  • Often episodes of colic lasting a few minutes (due to increased peristalsis attempting to remove obstructed material) and central
  • May improve with vomiting as material is ejected
• Abdominal distention:
  • More likely in lower SBO
• Nausea and vomiting:
  • Vomiting may be:
    • Bilious (green) – suggests upper SBO
    • Faeculent (thick and foul-smelling) – suggests distal SBO
  • Vomiting may occur before constipation:
    • The higher the obstruction, the earlier vomiting develops even if no food is eaten as saliva and secretions are still made and enter the stomach
• Constipation:
  • Not always absolute, some people may still be able to pass flatus and small amounts of stool
• High-pitched ‘tinkling’ bowel sounds – more common in early bowel obstruction
  • Trapped material and gas in the bowels lead to increased peristalsis in an attempt to clear the obstruction. Over time, as obstruction worsens, peristalsis decreases, resulting in reduced/absent bowel sounds.
  • The ‘tinkling’ sound is due to the rapid movement of material
• Features of Peritonitis may be present if perforation occurs

Investigations

Overview

Key investigations include:

• Abdominal X-ray:
  • Often performed first and may show dilated small bowel loops (>3 cm in diameter)
• CT abdomen:
  • Confirms SBO

Other investigations are to look for the consequences of SBO, such as:

• Full blood count (FBC):
  • May show leukocytosis suggesting inflammation or infection
• Urea and electrolytes (U&Es) and blood gases:
  • May show metabolic derangements, such as:
    • Hypochloraemic, hypokalaemic metabolic alkalosis due to severe vomiting:
    • Elevated urea and creatinine in dehydration
    • Increased lactate, suggesting intestinal ischaemia

Differential Diagnoses

Large bowel obstruction (LBO)

• Pain is generally felt lower in the abdomen and tends to be continuous or colicky with longer intervals. It is less prominent as the large bowel contracts less than the small bowel.
• Constipation occurs earlier in LBO and is more severe
• Abdominal distention is more common and prominent in LBO
• In LBO, vomiting is less prominent of a feature compared to constipation.
  • Vomiting suggests a more proximal lesion.
• In LBO, if vomiting is present, this tends to be feculent rather than bilious
• There may be clinical features of colorectal cancer
• See Large Bowel Obstruction

Ileus

• Pain is less severe or may be sent, and is not colicky
• Bowel movements are absent, constipation is present, and no passage of flatus
• Bowel sounds are absent on auscultation (whereas in SBO, this is a late sign)
• There may be an associated cause (e.g. postoperative ileus, see Ileus for more)

Management

Overview

The initial management of all patients with bowel obstruction includes:

• Nil-by-mouth – to reduce the amount of material entering an obstructed bowel
• IV fluids
• Nasogastric tube with free drainage (‘drip and suck’) to remove stomach contents and decompress the bowel

Some patients are managed with supportive treatment (such as correcting electrolyte abnormalities) as in many cases, adhesions ‘loosen up’ and the obstruction resolves. Others require surgery, such as if evidence of bowel ischaemia, perforation, or peritonitis are present.

Complications

As SBO progresses, blood flow to the bowel decreases, leading to ischaemia, necrosis, and perforation, followed by the leakage of bowel contents into the peritoneum, which can result in peritonitis, sepsis, shock, and death.

If large amounts of bowel need removal, patients may develop short bowel syndrome, resulting in inadequate nutrient absorption, leading to patients requiring long-term supplemental nutrition orally or parentally.

Prognosis

• The prognosis of SBO is good for most patients (around 3%) if treated early within 24 hours. If delayed, this can increase past 10%.
• People with SBO secondary to adhesions may have recurrent SBO due to the formation of more adhesions.