Gastrointestinal Pathophysiology
Small Bowel Obstruction
Small bowel obstruction (SBO) is a condition to describe interruption of the normal passage of bowel contents either due to a functional decrease in peristalsis or mechanical obstruction. Therefore this condition could be broadly described as either:
- functional/paralytic ileus
- mechanical
It is a very common acute surgical condition. You need to know this condition like the back of your hand if you plan to pursue a surgical (especially General Surgery) career!
Paralytic Ileus
Paralytic ileus, is a temporary disturbance of peristalsis in the absence of mechanical obstruction.
This can be seen in:
- The postoperative state
- Electrolyte imbalance
- Trauma
Postoperative ileus is the most common cause of paralytic ileus, which can also be caused by metabolic disturbances (e.g., hypokalemia), endocrinopathies (e.g., hypothyroidism), and certain drugs (e.g., anticholinergics).
Mechanical SBO
The three most common causes of mechanical SBO are listed below:
| Cause | % |
|---|---|
| Adhesions | 50-75 |
| External or internal hernias | 15 |
| Primary or secondary neoplasms | 15 |
Other causes of SBO include:
- Gallstone ileus
- Inflammation:
- Crohn’s disease
- Appendicitis
- Diverticulitis
- Abscess
- Congenital bands
- Radiation enteritis
- Bezoars
- Intussusception
- Small bowel volvulus
Generally mechanical SBO causes can be divided into:
Extrinsic
- Adhesions
- Hernias
- Involvement due to adjacent pathology
Mural
- Crohn’s disease
- Neoplasms
- Intussusception
Intraluminal
- Gallstone ileus
- Enteroliths
- Bezoars
Types of Mechanical SBO
On the basis of the pathophysiology of the obstructing cause, mechanical obstructions are divided into two types:
1. Simple obstruction
The bowel is occluded at one or several points along its course and, depending on the severity and duration of the process, the proximal part of the bowel becomes distended.
2. Closed loop obstruction
A loop of bowel is occluded at two adjacent points in close proximity along its course.
In closed loop obstruction, in addition to constricting two adjacent segments of bowel, the intervening mesentery, including the vascular pedicle, is compressed at the obstructing point.
The anatomical configuration of the closed loop allows the loop to rotate along its long axis, which can further aggravate the mechanical obstruction and can result in mesenteric ischaemia.
Investigations
Abdominal XRay
The initial radiological investigation is usually a plain abdominal radiograph.
Plain film examination of the abdomen usually consists of a supine abdominal film, in order to include hernial orifices.
The features of SBO as seen on plain abdominal films include:
- Dilated gas-filled loops of bowel with a diameter of >2.5 cm
- Separation of the folds in the dilated loops: the folds in the small bowel extend from wall to wall (valvulae conniventes)
- ‘String of pearls’ sign: This sign becomes visible on an erect abdominal film when there are pockets of gas trapped between adjacent folds and the remainder of the dilated bowel is fluid-filled
Other points to consider when interpreting an AXR would be valvulae conniventes, not haustra, would be present. The dilated loop would be central not peripheral (where colon would be), and no faeces is present in the loop.
Pitfalls of Abdominal XR
Gasless Abdomen
A major pitfall of plain film imaging is that when the small bowl is entirely fluid-filled it becomes inconspicuous on plain film. In this situation, ultrasound or CT can assist in the diagnosis.
Paralytic Ileus
In paralytic ileus, both large and small bowel loops may be dilated without mechanical obstruction.
Plain abdominal films are unhelpful in differentiating between mechanical SBO and paralytic ileus. The clinical and examination history is the most useful indicator in differentiating between these conditions. When in doubt, it is best to proceed to further imaging.
Intraluminal Contrast Study
Conventionally, intraluminal contrast studies were used to answer some of the questions raised by plain abdominal radiography and overcome some of its limitations. This include use of water soluble contrast e.g. gastrografin, omnipaque. The water-soluble hypertonic contrast draws fluid into the lumen of the bowel, thus reducing oedema of the intestinal wall and concurrently stimulating peristalsis.
Following contrast administration (usually orally), a repeat AXR should be performed in order to ‘follow-through’ its progress. Generally, failure of the contrast to reach the colon within 24 h would indicate a surgical exploration.
Pitfalls of Intraluminal Contrast Study
These include:
- The excessively long time taken to complete the examination
- Dilution of contrast material, leading to inadequate visualisation of the site and cause of obstruction
- Intraluminal studies fail to differentiate simple from closed loop obstruction
Computed Tomography
Computed Tomography (CT) is the modality of choice in SBO. This modality is excellent for revealing the site, level and cause of obstruction and in demonstrating closed loop obstructions and signs of threatened bowel viability.
With CT imaging, no oral preparation is necessary. In true cases of obstruction, there is sufficient fluid within the obstructed bowel, which acts as a very good negative intraluminal contrast material; however, intravenous (IV) contrast material is mandatory.
Apart from enhancing abdominal vessels, solid abdominal viscera and the bowel wall, it is also essential to evaluate the presence or absence of strangulation. Contrast (100 ml of 300 mg iodine/ml) at the rate of 3 ml per second is given using a power injector. This is why you need to check if patient has had a previous allergic reaction to iodine/contrast before putting them through the scanner!
Adhesions
Adhesions are the most common cause of mechanical SBO, occurring most commonly in the terminal ileum. Adhesions are responsible for 50-75% of SBO, of which 80% arise following surgery, 15% are due to peritonitis and the remainder are congenital or idiopathic in aetiology.
In confirmed cases of SBO, where there is failure to identify an obvious cause, adhesion can be presumed to be the cause in patients who have had prior surgery.
Hernias
External Hernia
External hernias represent 95% of hernias and occur at sites of congenital weakness or previous surgery. Example of this include inguinal or femoral hernias.
Internal Hernia
Internal hernia is an uncommon cause of SBO and involves bowel herniation through a defect of peritoneum, omentum and mesentery or through an adhesive band.
Internal hernia is a recognised cause of closed loop bowel obstruction, where the bowel is at risk of strangulation. The oedematous bowel wall with prominent mural stratification gives the ‘target sign’ when viewed end-on. The attached mesentery can become congested and be of increased density on CT.