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Bowel obstruction
Essentials
- 1
Small bowel obstruction is most often caused by adhesions, hernias or neoplasms. Large bowel obstruction more commonly results from neoplasms, volvulus or strictures.
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The common clinical features of bowel obstruction are paroxysms of poorly localized abdominal pain, constipation/obstipation, abdominal distension, nausea, vomiting and hyperactive or high-pitched bowel sounds. Examination for hernias is essential.
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On abdominal x-rays, dilated loops of bowel with multiple air-fluid levels can confirm the diagnosis of bowel obstruction. Where clinical suspicion is high and plain radiography is negative, a computed tomography scan is recommended.
- 4
Initial treatment consists of correction of dehydration and electrolyte abnormalities, decompression, analgesia and further assessment (particularly to identify strangulating bowel obstruction).
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Strangulating bowel obstruction and/or perforation are indications for urgent surgery.
Introduction and pathophysiology
Bowel obstruction is the interruption of the normal progression of intestinal contents. It can result from a mechanical obstruction or from a failure of intestinal motility without obstruction. It can affect the small or large bowel, may be partial or complete and can be strangulating or non-strangulating.
The ‘ABC’ mnemonic—Adhesions, Bulge, Cancer/Crohn’s—can be used to remember the common causes of small bowel obstruction (SBO): adhesions (60% to 85%), hernias (2% to 3%), neoplasms (2% to 5%), and Crohn disease (5% to 7%). Less common causes include gallstones, foreign bodies, strictures, radiation, diverticulitis, endometriosis and abscesses. Ninety percent of large bowel obstructions (LBOs) are caused by adenocarcinoma of the colon and rectum, volvulus or strictures from diverticulitis.
The pathophysiology of mechanical bowel obstruction relates to rising intraluminal pressure, mucosal injury, bacterial overgrowth and inflammatory response. Bowel proximal to the obstruction distends with gas, fluid and electrolytes; thereafter hypersecretion escalates, bowel absorptive ability decreases and progressive systemic volume losses occur. Vomiting ensues more quickly the more proximal the bowel obstruction is and worsens the dehydration and electrolyte disturbances.
If obstruction persists, then the intraluminal pressure rises and local vascular compromise can occur, especially venous stasis. As pressures rise and blood flow diminishes, the bowel can strangulate and necrosis may follow, with consequent perforation and sepsis. A closed-loop obstruction implies both proximal and distal obstruction (e.g. strangulating hernia or volvulus) and, typically, leads to vascular compromise more quickly and therefore to a higher risk of strangulation, ischaemia and perforation.
Functional obstruction, where there is failure of intestinal motility but no mechanical cause, is seen postoperatively as adynamic ileus and may occur with the use of opiates. Acute colonic pseudo-obstruction (Ogilvie syndrome), which is also a functional obstruction, can also produce small bowel dilatation; risk factors for this include antikinetic drugs (e.g. calcium channel blockers, anticholinergic drugs, phenothiazines, or anti-parkinsonian medications), severe electrolyte disturbances, neurological disorders, thyroid disorders and major acute medical illnesses or recent surgery.
Clinical features
History
In early bowel obstruction, abdominal pain is poorly localized and colicky; later it may become more constant and, if severe, suggests ischaemia, strangulation, perforation or peritonitis. Patients with proximal SBO tend to have more profuse vomiting, more frequent paroxysms of pain and less abdominal distension than those with more distal obstructions. With LBO, periumbilical or hypogastric pain and abdominal distension are the most common presenting features. Vomiting is more common in SBO and is a late symptom in LBO. Faeculent vomiting suggests a more high-grade SBO. Obstipation (inability to pass flatus and stool) was formerly thought to be typical, but the passage of flatus and stool may continue .
The gastrointestinal and surgical history helps differentiate causes of mechanical obstruction and drug history and systems enquiry may identify potential causes of functional obstruction.
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