A bowel obstruction may result from either a mechanical (physical) blockage or the loss of intestinal peristalsis (movement), a condition known as ileus. When ileus occurs, the intestinal muscles are unable to contract and as a result, food cannot move through the intestines normally.

Ileus commonly occurs for one to three days following abdominal surgery, after which normal bowel function returns. Temporary ileus does not normally cause any problems. However, ileus may also develop after surgical complications (e.g., infection, blood clot), following an injury to an intestinal artery or vein, or as a result of atherosclerosis (hardening of the arteries) that reduces the blood supply to the intestine.

Known risk factors for ileus include kidney or thoracic disease, an underactive thyroid gland and metabolic imbalances, such as low potassium or high calcium levels. The use of medications, especially opioid analgesics (narcotic pain relievers) and anticholinergic drugs, may also cause ileus.

In newborns and infants, a mechanical bowel obstruction may be caused by a birth defect, such as occurs in intussusception (when one part of the intestine folds into another) and Hirschsprung's disease (condition in which the colon lacks some nerves), or a hard mass of intestinal contents (meconium).

Common causes of mechanical intestinal obstruction in adults include:

• Adhesions. The most common cause of small bowel obstruction. These are bands of scar tissue that can bind normally separate organs together, especially the intestines. Adhesions can develop following an inflammatory infection, abdominal surgery or injury.
  
  
• Hernias. Blockage can occur when a portion of the intestine bulges through an abnormal opening (hernia), such as a weakness in the abdominal wall, and becomes trapped or pinched. The most common type of hernia resulting in obstruction is an inguinal hernia. In severe cases, it may cut off blood supply to the small intestine.
  
  
• Neoplasms (benign or cancerous tumors). These lesions can cause blockage of the small or large intestine either by pressing on the outside of the intestine and pinching it closed, or by growing within the wall of the intestine and slowly blocking its passageway.
  
  
• Foreign bodies (including bezoars). Undigestible objects can impair the digestive process and result in a bowel obstruction.
  
  
• Fecal impaction. A trapped mass of hardened feces in the colon or rectum can result in blockage. This occurs more frequently in older adults and pregnant women.
  
  
• Volvulus. An abnormal twisting of a segment of the intestine around itself. This twisting typically results in blockage of the intestine.
  
  
• Gallstones. Hard masses formed from substances in bile, these stones usually contain cholesterol and/or bilirubin (substance that results from the breakdown of red blood cells), and they can develop in the gallbladder or bile ducts. These can sometimes result in obstruction of the small intestine if they enter the organ by an abnormal inflammatory passage (fistula).  
  
  
• Peptic Ulcers. The narrowed outlet from the stomach that may develop due to ulcers can result in obstruction.
  

  

Known risk factors for a mechanical bowel obstruction include:

• Laparotomy. Patients who have undergone abdominal surgery are at increased risk of developing scar tissue (adhesions), which can cause obstruction in the small intestine.
  
  
• Inflammatory bowel disease (e.g., Crohn's disease). The chronic inflammation and irritation of tissue in the alimentary canal can result in narrowing (stenosis) of the intestines, causing them to become blocked or obstructed.
  
  
• Diverticular disease. When small pouches form in the colon and become inflamed due to infection, scars can form in the wall of the colon as it heals. This scar tissue can gradually narrow the colon, causing it to become blocked or obstructed.
  
  
• Cancer and radiation therapy. In the large intestine, obstruction may be caused by cancer. The most common cancers that cause bowel obstruction include colon, stomach and ovarian cancer. Other types of cancer, such as lung and breast cancer, can spread to the abdomen and cause obstruction. Bowel obstructions are most common during the advanced stages of cancer. Patients who undergo radiation are also at increased risk of developing a bowel obstruction due to any resulting injury.