Bowel Obstruction

Intestinal obstruction is an important cause of acute abdomen, accounting for 10% of emergency surgical admissions.¹Intestinal obstruction can be classified in several different ways, most traditionally into small and large bowel obstruction. Mortality varies widely according to the cause and any associated complications, being 100% in patients with untreated strangulated obstructions.² Morbidity and mortality are particularly high in patients who are managed incorrectly or in whom the diagnosis is delayed.³

Learning Bite

Bowel obstruction carries a high morbidity and mortality if managed incorrectly.

Classification

Small / Large Bowel
Acute / Subacute / Acute-on-chronic / Chronic
Mechanical / Functional
Simple / Strangulated
Partial / Complete

Aetiology

The causes of intestinal obstruction can be broadly divided into mechanical or functional categories, and further subdivided as in the tables below:

Mechanical

Functional

• Adhesions from previous surgery are currently the leading cause of small bowel obstruction in industrialised countries (70%), followed by malignancy, inflammatory bowel disease, and hernias.²
• Malignancy and volvulus are the commonest causes of large bowel obstruction.⁴
• Functional (also referred to as paralytic) obstruction is relatively rare as a presentation to the emergency department.⁵
• In one prospective observational study of 150 consecutive adult patients admitted with acute mechanical bowel obstruction, 114 (76%) presented with small bowel and 36 (24%) with large bowel obstruction.⁶

In one prospective observational study of 150 consecutive adult patients admitted with acute mechanical bowel obstruction, 114 (76%) presented with small bowel and 36 (24%) with large bowel obstruction.

Learning Bite

Adhesions are the commonest cause of mechanical bowel obstruction.

Basic Science and Pathophysiology

The small bowel extends from the pylorus to the ileocaecal valve and ranges in length from 3 to 9 metres. The diameter of the gut lumen narrows progressively from the duodenojejunal flexure to the ileocaecal valve. The large bowel extends from the ileocaecal valve to the rectum and anus. If the ileocaecal valve remains competent in the presence of colonic obstruction, increasing pressure within the colon may result in perforation.²

Innervation of the intestine is via the parasympathetic and sympathetic nervous systems which form a myenteric plexus. This plexus controls orderly, propulsive contractions of the muscular layers of the gut wall. The sympathetic nervous system mediates the sensation of visceral pain (typically poorly localised, periumbilical and colicky in nature) experienced when the gut contracts against an obstruction.

Intestinal obstruction refers to any form of impedance to the normal passage of bowel content through the small or large bowel. As such, universal findings irrespective of aetiology can be demonstrated in the flow diagram below.

Universal findings of intestinal obstruction⁷

The more proximal the obstruction, the more the fluid loses. This is because fluids secreted by the upper gastrointestinal tract fail to reach the absorptive areas of the distal jejunum and ileum. Sequestration causes tissue injury, inflammation and capillary leak, manifesting as local oedema at the site of injury.² Such third-space losses can be large and frequently under-estimated, particularly if the obstruction is prolonged.

Learning Bite

Fluid losses are frequently under-estimated in patients with small bowel obstruction.

Adhesions are thought to result from a reduction in peritoneal plasminogen-activating factor (PAA), which in turn leads to a failure to break down the postoperative fibrinous adhesions that follow all intra-abdominal operations. Drying and abrasion of the peritoneum as well as foreign substances such as talc in surgical gloves increase the reduction in PAA, increasing the future risk of adhesion formation.⁸

Small bowel can become trapped in a pre-existing hernia, causing obstruction. Hernias can be classified as described in the table below:

HERNIA	ANATOMY	INCIDENCE
Indirect inguinal hernia	Bowel passes through inguinal canal via a congenital weakness of the internal inguinal ring	most common
Direct inguinal hernia	Hernia exits abdominal cavity directly through the deep layers of the abdominal wall	uncommon
Femoral hernia	Abdominal contents pass through femoral canal just below inguinal ligament	rare

Inguinal hernias are the most common type of hernia in both men and women, the indirect type accounting for 2/3 of cases. Almost all femoral hernias occur in women because of the wider bone structure of the female pelvis; however inguinal hernias are still more common in women than femoral hernias.⁹

Functional obstruction results from atony of the intestine and loss of normal peristalsis. Atony of the bowel can be localised to a particular segment or generalised throughout the entire bowel. Localised atony is thought to result from an abnormality in the myenteric plexus of the bowel wall, whereas more generalised atony probably results from an imbalance in autonomic nerve supply, although there is little direct evidence for this.¹⁰ Different terms are often used to describe functional obstruction of the small or large bowel: paralytic ileus and pseudo-obstruction respectively.²

History

Initial assessment of the patient should involve concomitant history taking with prompt identification of the shocked patient. Generic treatment incorporating analgesia and fluid resuscitation should be performed alongside the initial history and examination.

Classic symptoms of intestinal obstruction are colicky abdominal pain, abdominal distension, vomiting and constipation. Vomiting is a late feature with large bowel obstruction. The degree of distension depends on the level of obstruction; there may be very little distension with high small bowel obstruction in contrast to gross distension with low small bowel or large bowel obstruction.⁵ Ask specifically about any previous abdominal surgery and remember to pay particular attention to the following:¹¹

• Reproductive, contraceptive and menstrual history
• Possibility of pregnancy and its complications
• Drug history, especially favoured remedies and alcohol consumption
• Past medical history

Severe pain suggests strangulation and developing ischaemia in a closed loop of bowel.⁵

Learning Bite

Severe pain in bowel obstruction suggests complications such as ischaemia or perforation.

Examination

Begin with baseline observations looking for any physiological evidence of dehydration or shock. Adequately expose the patient to examine the abdomen meticulously, looking for surgical scars, peritonism, masses and do not forget to check the hernial orifices.¹² It is easy to miss a small femoral hernia in an obese patient. Bowel sounds may be high-pitched / tinkling or absent altogether. Careful attention should also be given to look for other causes of acute abdomen, in particular a ruptured abdominal aortic aneurysm (AAA) or ectopic pregnancy.

A rectal examination must also be performed demonstrating an empty rectum or obstructing mass, ideally only once. In the same way, repeated attempts to elicit rebound tenderness should not be made when evidence of peritonitis has already been established.¹¹

Many surgeons would advocate that rigid sigmoidoscopy should form part of a complete abdominal examination. An obstructing lesion may be visible and a sigmoid volvulus might be decompressed.³

However, rigid sigmoidoscopy is not generally performed in emergency departments in the UK.

One prospective observational study of 150 consecutive patients with acute mechanical bowel obstruction reported that absence of the passage of flatus (90%) and/or faeces (80.6%) was the most common presenting symptom, with abdominal distension (65.3%) being the most common physical finding.¹⁴ Bohner et al reported a prospective study to determine the value of plain abdominal X rays in comparison with history and examination. The six variables with highest sensitivity for a diagnosis of bowel obstruction were – a distended abdomen, increased bowel sounds, history of constipation, previous abdominal surgery, age over 50 years and vomiting.¹⁵

Learning Bite

Constipation and abdominal distension in the patient with a previous history of bowel surgery are strongly suggestive of intestinal obstruction.

Risk Stratification

With particular reference to large bowel obstruction caused by colorectal cancer, a large prospective study found that age over 65 years, higher ASA (American Society of Anaesthesiologists) grade, higher Dukes staging of cancer and greater urgency of operative intervention were associated with increased mortality.¹⁶

The differential diagnosis of the patient presenting with abdominal pain is especially wide. The most common causes in the UK population requiring admission to hospital are:³

Condition	Approximate incidence (%)
Non-specific abdominal pain Acute appendicitis Acute cholecystitis & biliary colic Peptic ulcer disease Small bowel obstruction Gynaecological disorders Acute pancreatitis Renal & ureteric colic Malignant disease Acute diverticulitis Dyspepsia Miscellaneous	35 30 10 5 5 5 2 2 2 2 1 5

General

• Blood tests FBC, U+Es, LFTs, glucose, amylase, group and save. Clotting screen if septic or on anticoagulants
• ECG
• ABG if signs of sepsis or strangulated bowel
• Urinalysis

Imaging

• CT
• Small bowel follow-through
• Water-soluble contrast enema

All patients being evaluated for small bowel obstruction should have abdominal x-rays first because of the fact that they are as sensitive as CT to differentiate obstruction from non-obstruction.^{13 17} Small bowel distension (>2.5cm in diameter) confirms the diagnosis, differentiated from the colon by the valvulae conniventes, which completely cross the bowel wall. The taeniae coli of the large bowel are incomplete across the bowel wall. The relatively central position of the small bowel and restriction in dilatation to 5cm also helps to distinguish small from large bowel on plain films.² It should be noted that the use of the abdominal x-ray in the undifferentiated patient with acute abdominal pain is not recommended in the emergency department setting,¹⁷ with several studies showing that the abdominal x-ray rarely changes the management plan and is often used as a defensive screening investigation.^{18 19 20}

Figure 2 abdominal x-ray showing small bowel obstruction

Figure 3 abdominal x-ray showing large bowel obstruction

The National Confidential Enquiry into Patient Outcome and Death (NCEPOD) for acute bowel obstruction published its report in 2020. This was developed with a wide multidisciplinary input such as NICE, RCEM, RCR, NELA, etc. The top recommendation from the report was to undertake a CT scan with intravenous contrast promptly, as the definitive method of imaging for patients presenting with suspected acute bowel obstruction. Early abdominal/pelvic CT with intravenous contrast is recommended to identify closed-loop obstruction, bowel ischaemia and bowel perforation.¹

The CT scan is the most useful tool to identify the level and cause of the obstruction, and therefore determine the management plan.^{17, 18, 19}

If there is a delay/unavailability in obtaining abdominal CT scans, plain X ray films may be used interim. Erect chest for free air and supine abdominal X rays are most commonly considered. The 3/6/9 rule for bowel imaging – upper limit of 3 cm for small bowel loop width, 6cm for colon and 9cm for cecum can be used to look for obstruction. The presence of valvulae conniventes in the small bowel which completely cross the bowel wall help to distinguish it from the taeniae coli of the large bowel which are incomplete across the bowel wall.

It should be noted that the use of the abdominal X ray in the undifferentiated patient with acute abdominal pain is not recommended in the emergency department setting with several studies showing that abdominal X ray rarely changes the management plan and is often used as a defensive screening investigation.^{20, 21, 22}

Emergency Department Ultrasound had a sensitivity of 0.91 and specificity of 0.84 for small bowel obstruction (compared to 0.02 and 0.67 respectively for Abdo X ray) and with proper training, this can be used at the bedside.^{23, 24}

Figure 4 CT abdomen showing small bowel obstruction

Figure 5 CT abdo/pelvis showing large bowel obstruction with apple core constricting lesion

A limitation of CT is its low sensitivity (<50%) in the detection of low-grade or partial small bowel obstruction. Contrast studies may be considered for patients who fail to improve after 48 hours of conservative management as a normal contrast study can rule out operative small bowel obstruction. Contrast studies are therefore rarely indicated in the initial management in the emergency department.¹²

Learning Bite

Early CT scan of the abdomen and pelvis with IV contrast helps to identify the level and cause of the obstruction, and therefore determine the management plan.

Emergency Department Management

• Insert an IV cannula taking and sending blood as mentioned above
• Start an IVI of crystalloids like Hartmanns or 0.9% NaCl
• O2 and fluid resuscitation if the patient is haemodynamically unstable
• Move the patient to an appropriate area of the department and involve an ED senior in their management
• Titrate IV analgesia (eg Morphine) with an antiemetic (eg Cyclizine)
• Insert a naso-gastric tube to decompress the stomach and proximal small bowel. Keep the patient nil by mouth (NBM)
• Insert a urinary catheter
• Consider more invasive monitoring if required for accurate fluid resuscitation (CVP and/or arterial line)
• Refer to the Surgical team early (depending on local policies, this might be to a specific upper or lower GI team for suspected small and large bowel obstruction respectively).

Systemic broad spectrum antibiotics should be considered for those patients with signs of sepsis, strangulation or peritonism at presentation because of concerns of bacterial translocation. However, there are no controlled data to support or refute this approach.²⁷ It would seem prudent however to give systemic antibiotics to patients with signs of strangulation or peritonism at presentation.

Learning Bite

Involve the surgical team early in the management of the patient with suspected intestinal obstruction.

Surgical Management

Small bowel obstruction

Patients with small bowel obstruction generally undergo an initial period of non-operative observation and continued resuscitation, unless there are signs of shock or strangulation at presentation when exploratory laparotomy is indicated. The algorithm below describes the diagnostic and therapeutic approach to patients with small bowel obstruction (SBO).²⁵

Large bowel obstruction

In general, mechanical large bowel obstruction requires operative intervention. Emergency surgery for large bowel obstruction is associated with a high morbidity and mortality. The underlying cause determines the treatment. For eg, emergency surgery for suspected or impending perforation, urgent flexible sigmoidoscopy for volvulus, colonic stunting as a bridge to surgery in malignancies or palliation. One-stage primary resection and anastomosis is the preferred choice for low-risk patients. Colonic pseudo-obstruction can be managed with pharmacological methods as well as colonoscopic decompression.¹⁹

Learning Bite

Small bowel obstruction is often treated conservatively, but large bowel obstruction generally requires operative intervention.

Key Learning Points

• Bowel obstruction carries a high morbidity and mortality if managed incorrectly
• Adhesions are the commonest cause of mechanical bowel obstruction
• Fluid losses and subsequent resuscitation are frequently under-estimated in patients with small bowel obstruction
• Severe pain in bowel obstruction suggests complications such as ischaemia or perforation
• Constipation and abdominal distension in the patient with a previous history of abdominal surgery are strongly suggestive of intestinal obstruction
• Age > 65, comorbidities, more advanced malignancy and emergent surgery are indicators of poor prognosis in patients with large bowel obstruction
• Early CT scan of the abdomen and pelvis with IV contrast helps to identify the level and cause of the intestinal obstruction, and therefore determine the management plan.
• Involve the surgical team early in the management of the patient with suspected intestinal obstruction
• Small bowel obstruction is often treated conservatively, but large bowel obstruction generally requires operative intervention

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17. Association of Surgeons of Great Britain and Ireland and the Royal College of Surgeons of England. Emergency General Surgery – Commissioning Guide (acute abdominal pain) 2014.
18. https://bestpractice.bmj.com/topics/en-gb/3000119/investigations#firstOrder
19. https://bestpractice.bmj.com/topics/en-gb/3000125/investigations#firstOrder
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22. Jackson K, Taylor D, Judkins S. Emergency department abdominal x-rays have a poor diagnostic yield and their usefulness is questionable. Emerg Med J. 2011 Sep;28(9):745-9. doi: 10.1136/emj.2010.094730. Epub 2010 Aug 15. PMID: 20713362.
23. Jang TB, Schindler D, Kaji AH. Bedside ultrasonography for the detection of small bowel obstruction in the emergency department. Emerg Med J. 2011 Aug;28(8):676-8.
24. The Pocus ATLAS Bowel
25. Ten Broek RPG, Krielen P, Di Saverio S, Coccolini F, et al. Bologna guidelines for diagnosis and management of adhesive small bowel obstruction (ASBO): 2017 update of the evidence-based guidelines from the world society of emergency surgery ASBO working group. World J Emerg Surg. 2018 Jun 19;13:24.