Small intestine

The small intestine starts at the duodenojejunal flexure and ends at the caecum. The average length of the digestive tract is 450 cm. 260 cm of it belongs to the small intestine. The proximal 2/5 of the small intestine is called the jejunum, and the distal 3/5 is the ileum. The diameter of the small intestine is approximately 2.5 – 3.5 cm. The large intestine starts at the ileocecal valve and ends at the anus. On average, it is 150 cm long. The largest diameter is at the level of the cecum.

Anatomical position of small and large intestine

Available at: https://www.teresewinslow.com/digestion/#itemId=59fe74bb084665ad8a5f02e4 (Accessed: 19 November 2019).

Two types of movement occur in the gastrointestinal tract. Propulsive movements cause food to move forward along the tract at an appropriate rate to accommodate digestion and absorption. Mixing movements keep the intestinal contents thoroughly mixed at all times. Peristalsis occurs when a contractile ring appears around the gut and then moves forward. This way, any material in front of the contractile ring is moved forward. The usual stimulus for intestinal peristalsis is the distention of the gut. For example, when a large amount of food collects at any point in the gut, the stretching of the gut wall stimulates the enteric nervous system to contract the gut wall 2 to 3 cm behind this point. A contractile ring appears and initiates the movement. Other stimuli that can cause peristalsis are chemical or physical irritation of epithelial lining or strong parasympathetic nervous signals. Theoretically, peristalsis can occur in either direction, but it usually dies out rapidly in the oral direction while continuing for a considerable distance towards the anus.

Peristalsis

Available at: https://www.vectorstock.com/royalty-free-vector/digestion-and-peristalsis-esophagus-and-stomach-vector-27346462 (Accessed: 19 November 2019).

Ileus

By definition, ileus is an occlusion or paralysis of the bowel, preventing the forward passage of the intestinal content, causing their accumulation proximal to the site of the blockage. There are two known forms of ileus: mechanical and functional ileus.

In both types, the result is the accumulation of fluids and gases at elevated intraluminal pressure, microcirculatory dysfunction of the bowel wall, and disruption of the mucosal barrier. This can, in turn, lead to fluid shifts, transmigration peritonitis, and hypovolemia.

The passage of intestinal contents can be blocked either partially (incomplete ileus) or totally (complete ileus). Mechanical ileus is more common in the small bowel than the large bowel, in a ratio of 4:1. Small-bowel ileus is usually due to adhesions from prior surgery (65%) or hernia (15%), while large-bowel ileus is usually due to cancer (70%) or adhesions and stenoses after recurrent diverticulitis (up to 10%). Rarer causes of large-bowel ileus include sigmoid volvulus (5%) and hernia (2.5%).

Functional ileus is not due to a process obstructing the lumen of the bowel, but rather due to the reduced contraction of the smooth muscle of the bowel wall. It has multiple causes: postoperative ileus (after abdominal or retroperitoneal surgery or induced by intra-abdominal or retroperitoneal lesions), drug-induced ileus (consumption of opioids, neuroleptic drugs), metabolic ileus (hypokalemia or diabetes mellitus), vascular ileus (hypoperfusion of the bowel).

Clinical features of the mechanical ileus are intensified bowel sounds in the early phase, while peritoneal signs are usually absent. Nausea, vomiting, retention of stool, abdominal distention with absent bowel sounds, and abdominal pain can be found in the functional ileus.

Ultrasound examination

While the abdominal CT is the golden standard with a sensitivity of 92% and a specificity of 92%, ultrasound remains a diagnostic tool of choice in the emergency setting. An ultrasound examination in cases of ileus appears to have a sensitivity of 88% and a specificity of 96%. It produces more reliable results compared to x-ray, which is ordered very frequently, but it only has a sensitivity of 66-67% and a specificity of 50-57%. A new study from 2018 suggests even better results with the ultrasound examination: the sensitivity with a range of 94–100% and specificity of 81–100%.

Because air is the enemy of ultrasound, the examination is helpful in the case of fluid-filled bowel. A curvilinear abdominal probe is a probe of choice because of its resolution, penetration, and a large footprint, enabling us to cover more ground at once. Since the bowel can theoretically be anywhere in the abdomen, the goal is to scan the whole abdomen systematically. For example, starting at the right lower quadrant moving the probe toward the right upper quadrant and back down and then back and forth until the whole area is covered).

Several characteristic features should be looked for in this examination:

  1. dilated bowel (>2.5 cm in the small bowel and >5 cm in the large bowel),
  2. back and forth (to and fro) peristalsis – bowel content moving forward and backward,
  3. wall thickening (>3 mm),
  4. free fluid between bowel loops (Tanga sign),
  5. (collapsed or normal caliber bowel distal to the transition point).

One has to think of bowel ischemia whenever extraluminal free fluid, loss of peristalsis, bowel wall thickening, and mural gas is found. Urgent surgical evaluation is needed in cases like these.

Aperistalsis and ileus, PAME Maribor

Anatomy of small bowel

Available at: https://radiologyassistant.nl/pediatrics/normal-values-ultrasound (Accessed: 19 November 2019).

Colon transversum on ultrasound, PAME Maribor

Limitations

There is a chance of missing the dilated loops because a large area of the body needs to be covered. The air in the gut can also be a problem for ultrasound waves. Furthermore, it is hard to find a cause and transition point of obstruction with POCUS (while an intraluminal mass can suggest a cause, adhesions – the most common cause of obstruction – likely cannot be seen). Also, always remember to take the clinical findings into account and do not rely only on the ultrasound examination.

Resources

Pivec, G. et al. (2014) Kirurgija. Celje: Grafika Gracer.

Hall, J. and Guyton, A. (2016). Guyton and Hall textbook of medical physiology. Philadelphia: Elsevier.

Small bowel obstruction | Radiology Reference Article | Radiopaedia.org. Available at: https://radiopaedia.org/articles/small-bowel-obstruction (Accessed: 7 March 2020).

Mallo, R. D. et al. (2005) ‘Computed tomography diagnosis of ischemia and complete obstruction in small bowel obstruction: A systematic review’, Journal of Gastrointestinal Surgery. Elsevier Inc., 9(5), pp. 690–694. doi: 10.1016/j.gassur.2004.10.006.

Shrake, P. D. et al. (1991) ‘Radiographic Evaluation of Suspected Small Bowel Obstruction’, The American Journal of Gastroenterology. Nature Publishing Group, 86(2), pp. 175–178. doi: 10.1111/j.1572-0241.1991.tb06993.x.

Ogata, M., Mateer, J. R. and Condon, R. E. (1996) ‘Prospective evaluation of abdominal sonography for the diagnosis of bowel obstruction’, Annals of Surgery, 223(3), pp. 237–241. doi: 10.1097/00000658-199603000-00002.

Pourmand, A. et al. (2018) ‘The Accuracy of Point-of-Care Ultrasound in Detecting Small Bowel Obstruction in Emergency Department.’, Emergency medicine international, 2018, p. 3684081. doi: 10.1155/2018/3684081.

Emergency Ultrasound: Bedside Ultrasound to Diagnose Small Bowel Obstruction | MDedge Emergency Medicine (no date). Available at: https://www.mdedge.com/emergencymedicine/article/86483/imaging/emergency-ultrasound-bedside-ultrasound-diagnose-small-bowel (Accessed: 9 March 2020).