Small Bowel Obstruction (SBO) and Large Bowel Obstruction (LBO)
In this article, SBO and LBO are discussed together rather than in separate sections. Although SBO and LBO have distinct anatomical and pathological differences, they often present with similar clinical features, distinguishing between them is critical for diagnosis and management.
Definition
The term bowel obstruction typically refers to mechanical bowel obstruction, where a physical blockage prevents the normal passage of intestinal contents through the bowel lumen
SBO vs LBO:
- SBO: obstruction at the level of the duodenum / jejunum / ileum
- LBO: obstruction at the level of the caecum / colon / rectum
Paralytic ileus (functional bowel obstruction) is something completely different, see separate section below.
Aetiology
There are 2 main ways to classify the causes of bowel obstruction.
Aetiology by Site of Obstruction (SBO vs LBO)
~80% cases of bowel obstruction are SBO [Ref]
| Type | Common causes | Rarer causes |
|---|---|---|
| SBO |
|
|
| LBO |
|
|
Paediatric: [Ref]
| Type | Causes |
|---|---|
| SBO |
|
| LBO |
|
Aetiology by Pathophysiological Mechanism
| Category | Description | Causes |
|---|---|---|
| Extraluminal bowel obstruction | Extrinsic compression of the bowel |
|
| Intramural bowel obstruction | Underlying cause arises from the intestinal wall |
|
| Intraluminal obstruction | Underlying cause is within the intestinal lumen |
|
Clinical Features
Symptoms
Typical presentation: [Ref1][Ref2]
- ColickyColicky abdominal pain is defined as pain that is intermittent, crampy, and typically occurs in waves.
In the context of bowel obstruction, colicky pain arises because peristalsis persists despite the mechanical blockage; each peristaltic wave increases pressure proximal to the obstruction, resulting in a transient, cramping pain that recurs at regular intervals. As the obstruction progresses and bowel distension worsens, peristalsis may eventually cease, and the pain can become constant rather than intermittent.
abdominal pain (often peri-umbilical or generalised, but can vary) - Nausea and vomiting
- Constipation or obstipationObstipation is defined as the complete absence of both passage of stool and flatus, typically indicating a more severe and advanced form of bowel dysfunction than ordinary constipation.
It is most commonly associated with mechanical bowel obstruction.
Complete vs partial bowel obstruction:
- Complete bowel obstruction causes obstipation (inability to pass stool and gas)
- Partial bowel obstruction causes constipation +/- intermittent passage of flatus or overflow diarrhoea, and is associated with more gradual symptom progression
Signs
Typical examination findings: [Ref1][Ref2]
- Abdominal distension
- Tympanic percussion
- Altered bowel sounds
- Early: high-pitched and tinkling
- Late: absent bowel sounds
- Signs of dehydration (e.g. dry mucous membrane, reduced skin turgor, tachycardia, hypotension)
- Dehydration in bowel obstruction is caused by:
- 1) Vomiting and reduced oral intake
- 2) 3rd spacing in oedematous bowel
- 3) Impaired intestinal water absorption
- Dehydration in bowel obstruction is caused by:
SBO vs LBO
The predominance of certain clinical features can point towards SBO or LBO. [Ref]
| Clinical feature | SBO | LBO |
|---|---|---|
| Nausea and/or vomiting | Early onset and prominent
Bilous vomiting |
Less prominent |
| Abdominal distension | Less prominent | Early onset and prominent |
| Constipation or obstipation | Late onset | Early onset |
In summary, SBO presents with nausea and vomiting +++ while LBO presents with abdominal distension and constipation / obstipation +++
Compliations
The following complications may occur in order:
- Bowel ischaemia
- Bowel perforation
- Peritonitis and sepsis
Investigation and Diagnosis
Imaging
1st line (and gold standard): CT abdo-pelvis with IV contrast [Ref]
- The presence of a transition point (sudden narrowing of the bowel lumen) confirms bowel obstruction
- CT also allows identification of complications
2nd line:
- Abdominal X-ray – only performed if patient is unstable / CT is not readily available as it has limited diagnostic accuracy
- Water-soluble contrast study (e.g. Gastrograffin) – both diagnostic and therapeuticA Gastrografin study in the context of bowel obstruction refers to the administration of Gastrografin, a hyperosmolar water-soluble contrast agent either orally or via a nasogastric tube, followed by serial abdominal radiographs to track the passage of contrast through the gastrointestinal tract. This protocol is used primarily in cases of suspected partial small bowel obstruction.
Delayed appearance of gastrograffin in the colon (taking >24 hrs) is indicative of complete bowel obstruction. Gastrografin is also considered therapeutic because its hyperosmolar properties draw fluid into the bowel lumen, which can reduce bowel wall edema and stimulate peristalsis, thereby promoting resolution of the obstruction.
Differentiating SBO and LBO on abdominal x-ray (common in exams):
| Feature | Small Bowel Obstruction (SBO) | Large Bowel Obstruction (LBO) |
|---|---|---|
| Location of dilation | Central | Peripheral |
| Mucosal folds | Valvulae conniventes (aka plicae circulares): transverse mucosal folds that span the full width of bowel lumen (more prominent with obstruction) | Haustral folds: incomplete, irregularly spaced indentations that do not span the entire lumen of the colon |
| Bowel dilation | >3cm | >6cm (transverse colon), >9cm (caecum) |
| Bowel visibility | Large bowel is usually not visible | Small bowel is often visible alongside dilated colon |
If clinical features suggest complicated bowel obstruction → proceed to surgery without awaiting imaging [Ref]
The 3-6-9 rule can be used to remember the diameter of different bowel segments:
Small bowel: should not exceed 3 cm in diameter.
Transverse colon: should not exceed 6 cm in diameter.
Cecum: should not exceed 9 cm in diameter
Diameters that exceed the above thresholds are in keeping with SBO/LBO and a higher risk of perforation.
Work-Up
A standard blood test work-up for an acute abdomenThe acute abdomen is defined as a clinical syndrome characterized by sudden onset of severe abdominal pain that may be diffuse or localized. typically includes:
- FBC and CRP – to assess for inflammation / infection
- U&E
- LFT
- Amylase / lipase – to assess for acute pancreatitis
- Lactate – to assess for acute mesenteric ischaemia
- Clotting panel + Group and save – in case surgery is necessary
- Blood cultures – if sepsis is suspected
Management
Initial management: drip and suck [Ref]
- IV fluids
- NG tube insertion (for bowel compression)
- Make the patient NBM
Subsequent definitive management depends on the cause and severity (less frequently examined, compared to initial management).
Definitive management of common causes of bowel obstruction: [Ref]
| Cause of bowel obstruction | Management |
|---|---|
| Adhesive SBO | A general approach:
|
| Hernia |
|
| Malignancy |
|
| Volvulus |
|
If a bowel obstruction progresses to bowel perforation with diffuse peritonitis, sepsis, or haemodynamic instability, an urgent exploratory laparotomy via a midline incision is required.
See the Gastrointestinal (GI) Perforation article for more details.
Volvulus
Definition
Volvulus is defined as the twisting of a loop of intestine around its mesentery, which can cause bowel obstruction, strangulation, gangrene and perforation
There are 2 main types of volvulus: [Ref]
- Sigmoid volvulus (2/3 cases)
- Caecal volvulus
Aetiology
Clinical Features
Both sigmoid and caecal volvulus cause LBO (see above for clinical features of LBO)
- Sigmoid volvulus typically presents with a more subacute course
- Caecal volvulus typically presents more acutely
Both sigmoid and caecal volvulus have a significant risk of complications (e.g. ischaemia, gangrene, perforation)
Investigation and Diagnosis
1st line (and gold standard): CT abdo-pelvis with IV contrast [Ref]
- The presence of the whirl sign (twisting of mesenteric vessels) confirms a volvulus
Abdominal X-ray findings are commonly featured in data interpretation exam questions
- Sigmoid volvulus: coffee bean sign (a large, dilated, inverted U-shaped loop of the sigmoid colon with the apex pointing toward the right upper quadrant)
- Caecal volvulus: a kidney-shaped loop of bowel in the mid-abdomen
Management
Sigmoid Volvulus
Initial management: endoscopic detorsion with flexible sigmoidoscopy +/- placement of decompression tube [Ref]
- Recurrence is very common (up to 75%) after non-operative management
- Elective sigmoid colectomy is recommended after successful detorsion to prevent recurrence
If a volvulus progresses to bowel perforation with diffuse peritonitis, sepsis, or haemodynamic instability, an urgent exploratory laparotomy via a midline incision is required.
See the Gastrointestinal (GI) Perforation article for more details.
Caecal Volvulus
1st line: surgical intervention (usually right hemicolectomy regardless of bowel viability) [Ref]
If a volvulus progresses to bowel perforation with diffuse peritonitis, sepsis, or haemodynamic instability, an urgent exploratory laparotomy via a midline incision is required.
See the Gastrointestinal (GI) Perforation article for more details.
Paralytic Ileus (Functional Bowel Obstruction)
Definition
Paralytic ileus is defined as functional impairment of peristalsis in the GI tract in the absence of mechanical obstruction
Aetiology
The most common precipitating factor is post-abdominal surgery (direct manipulation of bowel initiates a local inflammatory response, which impairs smooth muscle contraction) [Ref]
Other notable precipitating factors: [Ref1][Ref2]
- Electrolyte imbalance (esp. hypokalaemia and hypomagnesaemia)
- Medications that reduce bowel motility (e.g. opioids, anticholinergics, anaesthetics)
Clinical Features
Very similar to mechanical bowel obstruction:
- Abdominal pain (classically non-colicky, constantThis is due to hypoperistalsis/aperistalsis associated with paralytic ileus (the peristaltic waves in mechanical obstruction are what generate the 'colicky' intermittent episodes of pain).)
- Nausea and vomiting
- Constipation and reduced passage of gas
- Abdominal distention
On examination:
- Palpation is usually non-tender
- Tympanic percussion
- Absent bowel sounds (tinkly bowel sounds are indicative of early mechanical bowel obstruction)
Investigation and Diagnosis
Standard work-up:
- FBC
- Serum electrolyte (esp. potassium and magnesium)
- Renal function test
- Serum lactate
- Clotting profile
Imaging is NOT routinely required in typical cases
- 1st line imaging:
- Abdominal X-ray – diffuse distension of small and large bowel, without a clear transition point
- Abdominal ultrasound
- CT abdomen-pelvis is the gold standard but reserved if diagnosis remains uncertain / mechanical obstruction cannot be excluded / complications are suspected
Management
1st line: supportive care for ~48-72 hours [Ref]
- Make the patient NBM
- Correct fluid and electrolyte imbalances
- NG tube insertion if there is significant vomiting or distension
- Discontinue precipitating medications (esp. opioids and anticholinergics)
- Analgesia and antiemetics as needed
If supportive care failed, consider neostigmineacetylcholinesterase inhibitor or endoscopic decompression [Ref]