Obstruction within the small intestine (small bowel obstruction or SBO) is more common than a blockage elsewhere in the gastrointestinal tract. Given its length and structure, it is more prone to a number of causes that can result in a partial or complete obstruction. Any strangulation which compromises blood flow to the bowel is considered as a medical emergency and needs immediate treatment, usually surgical intervention.
Signs and Symptoms of a Blocked Bowel
The most common signs and symptoms include :
- Abdominal distention
- Abdominal pain
- Vomiting with or without nausea
Intestinal obstruction should always be ruled out upon the presentation of these signs and symptoms. Other signs and symptoms which may vary among indidvidual cases or present early or late in the case includes :
- Increased heart rate (tachycardia)
- Signs of bleeding in the stool (melena ~ dark blood, hematochezia ~ fresh blood)
Causes of a Blocked Bowel
Most cases of small bowel obstruction are due to mechanical causes. A small portion of cases are due to impaired intestinal motility, thereby preventing the flow of chyme further down the gastrointestinal tract. This causes intestinal contents to become ‘backed up’.
The most common causes of mechanical small bowel obstruction are :
Adhesions often arise postoperatively and are closely linked to the increasing number of laparotomies conducted globally. Other causes include infection, peritonitis and endometriosis. These adhesions may form fibrous bridges which trap surrounding organs and strangulate the bowel.
Usually there is a history of surgery, with symptoms typically arising within 4 weeks after the surgery. Less frequently it may occur decades afterwards. Rare causes include congenital adhesions but this should only be considered when other causes have been excluded.
Outpouchings of the peritoneal wall may create dilated sacs with a narrow orifice (entry point). Any abdominal structure can get trapped, usually partially, within the external herniations but the loops of the small intestine are the most prone. These loops may become strangulated at the orifice thereby resulting in ischemia as explained below.
A portion of the small intestine protrudes into an adjacent portion in a telescopic manner. There is often a structural defect that allows for a weakened part to protrude in this manner, pushed through by peristaltic waves. Over time, an intestinal obstruction may arise and can even lead to an infarction as described below.
Both benign and malignant tumors of the small intestine may cause intestinal obstruction. Polyps may also cause small bowel obstruction although it is more likely to occur in the large intestine.
- Carcinoid tumors
- Kaposi’s sarcoma (always consider HIV/AIDS)
A volvulus occurs when a portion of the bowel, either large or small, rotates and the twisting causes an obstruction and leads to strangulation with ischemia. The large bowel is more likely to be affected and it may trap the small intestine within the twist. However, the twisting may occur with the small intestine itself.
Chronic Intestinal Pseudo-Obstruction
This occurs when intestinal motility is affected at a portion of the small intestine. Chyme is not propelled efficiently beyond this point and ‘backs up’ thereby causing a mechanical obstruction. The problem with intestinal motility may be due to the smooth muscle of the small bowel or the nerves supplying it. Primary idiopathic intestinal pseudo-obstruction may be a result of unknown causes (idiopathic), familial or congenital disorders of the smooth muscle and/or neurological disorders. It can also arise secondary to many conditions like Parkinsonism, hypothyroidism, scleroderma or due to drugs like opiates and tricyclic antidepressants.
Inflammatory Bowel Disease
Intestinal obstruction may arise in Crohn’s disease. Prolonged inflammation in Crohn’s disease may result in hypertrophy and fibrosis of the wall of the bowel and this can then result in strictures.
Other Causes of a Blocked Bowel
- Gallstones (gallstone ileus)
- Foreign body
Ischemic bowel disease may result in an infarction if left untreated. The infarct may affect the mucosa only (mucosal infarction), mucosa and submucosa (mural infarction) or all three layers of the bowel wall (transmural infarction). Intestinal tissue dies due to a lack of oxygen often as a result of the above causes, and this will affect motility, result in edema and could eventually form strictures over time. All of these mechanisms could result in bowel obstruction. The blockage may arise in ischemic bowel disease even if the ischemia is not due to one of the causes mentioned above, as in cytomegalovirus (CMV) infection or radiation enterocolitis.