An obstruction can occur when there is no open passageway for food or digested food waste to move through the bowel, or intestine. It can occur anywhere in the small or large intestine, and there can be a partial or complete blockage.
The bowel is basically a hollow tube that transports food and digested food waste from the stomach to the back passage (anus). There are two sections of the bowel: the small bowel, also called the small intestine, which is where the nutrients in the food are digested and absorbed; and the colon and rectum form the large bowel, or large intestine, which absorbs water from the digested food, forming it into stools (faeces/poo) that are passed out of the back passage.
When an obstruction occurs, undigested food, liquids and digestive secretions accumulate above the blockage, the bowel section involved in the blockage becomes distended and the segment can collapse. The normal functions of the bowel wall are compromised and the distended section gets progressively worse. A completely blocked large bowel is a medical emergency.
About 20% of hospital admissions for acute abdominal pain are due to a bowel obstruction and the majority of these occur in the small intestine.
What are the causes of a bowel obstruction?
There are a number of causes that could be responsible for a bowel obstruction, both mechanical and non-mechanical.
Paralytic ileus refers to a non-mechanical obstruction where the rhythmic muscle contractions of the intestine, known as peristalsis, stops. The bowel becomes dilated and can no longer move the contents to the anus. It may occur when there is another medical condition such as a chest infection, acute heart attack, stroke, acute kidney failure, trauma, severe hypothyroidism, electrolyte disturbance or a complication of diabetes. Ileus can sometimes occur after some types of surgery or during the postnatal period - the 6 weeks after a woman gives birth.
Adhesions, or scar tissue, that can form after abdominal surgery and trap a section of the bowel, are one of the most common causes of a mechanical bowel obstruction. Other common types of mechanical obstruction of the small bowel include a hernia (where part of the bowel pushes through a weak area in the abdominal wall) or volvulus (where the bowel becomes twisted). A mechanical obstruction in the large bowel is most often caused by a malignant tumour (they also occur in the small bowel but only very rarely) with the risk of obstruction increasing the further along the bowel the tumour is sited. Volvulus can also occur in the large bowel, most often in the sigmoid colon.
Other possible causes of bowel obstruction include:
- Impacted stools (poo) from severe constipation
- Diseases that affect the intestinal wall such as Crohn's disease or diverticular disease
- A swallowed item
- Intussusception, where part of the intestine folds in on itself
- Congenital malformation of the bowel