Bowel obstructions happen when something blocks the normal movement of food, fluid, and gas through your intestines. The cause depends heavily on whether the blockage is in the small intestine or the large intestine, and whether it’s a physical blockage or a problem with how the intestinal muscles are functioning. Adhesions from prior surgery are the leading cause in the small bowel, while colorectal cancer accounts for over 60% of large bowel obstructions.
Mechanical vs. Functional Obstruction
There are two broad categories. A mechanical obstruction means something is physically blocking the intestine, either from inside (like a tumor growing into the passage) or from outside (like scar tissue pressing on it). A functional obstruction, sometimes called an ileus, means the intestine itself has stopped contracting and pushing contents along, even though nothing is physically in the way. The distinction matters because the causes, symptoms, and treatments differ significantly between the two.
Adhesions: The Most Common Cause
Scar tissue from previous abdominal or pelvic surgery is the single most frequent reason for small bowel obstruction. These fibrous bands, called adhesions, form as part of the body’s normal healing process after surgery. They can develop weeks, months, or even years later, gradually tethering loops of intestine together or to the abdominal wall. When a section of bowel gets kinked or compressed by these bands, the passage narrows or closes entirely.
Almost anyone who has had abdominal surgery carries some risk. The more extensive the surgery and the more procedures you’ve had, the higher the likelihood. Adhesion-related obstructions can recur, and some people experience repeated episodes over their lifetime.
Cancer and Tumors
In the large intestine, cancer is the dominant cause. Colorectal cancer accounts for over 60% of all large bowel obstructions. Tumors can grow slowly enough that the bowel partially compensates for months, which is why an obstruction sometimes becomes the first sign that cancer is present. The tumor either narrows the passage from within or, in advanced cases, completely blocks it.
Cancers that have spread from other organs (ovarian, stomach, or pancreatic cancers, for example) can also compress or invade the bowel from the outside. Small bowel obstructions caused by tumors are less common than adhesion-related ones, but they do occur and tend to carry a more complicated treatment picture.
Hernias
A hernia is a weak spot in the abdominal wall where tissue or a loop of intestine can push through. When that loop gets trapped and can’t slide back, it becomes incarcerated, and the bowel passage is blocked. If blood supply to the trapped segment is also cut off, the situation becomes a surgical emergency.
Inguinal hernias (in the groin) are the most common type, making up 70 to 75% of all abdominal wall hernias. Femoral hernias account for 6 to 17%, and umbilical hernias for 3 to 8.5%. Rarer types exist too: obturator hernias, which occur deep in the pelvis, represent less than 1% of abdominal wall hernias but are notorious for being difficult to diagnose because they aren’t visible or easily felt from the outside.
Inflammatory Bowel Disease
Crohn’s disease is particularly associated with bowel obstruction because of how it damages the intestinal wall. Chronic inflammation causes the tissue to thicken and scar over time, forming strictures that narrow the passage. Approximately 70% of Crohn’s patients develop stricturing or penetrating complications within 10 years of diagnosis. These strictures can be inflammatory (potentially reversible with medication) or fibrotic (permanent scarring that typically requires a procedure to open or remove the narrowed segment).
Ulcerative colitis can also contribute to obstruction, though less commonly, usually in the setting of severe inflammation or toxic megacolon rather than the chronic stricturing pattern seen in Crohn’s.
Volvulus: When the Bowel Twists
A volvulus occurs when a loop of intestine twists around itself and its supporting tissue, cutting off the passage and sometimes the blood supply. It most commonly involves the sigmoid colon (the S-shaped segment near the rectum) or the cecum (the pouch where the small and large intestine connect).
Cecal volvulus requires an unusually mobile cecum, a condition present in roughly 25% of the general population based on anatomical studies. In these individuals, the cecum and right colon aren’t firmly anchored to the back of the abdominal cavity, a variation that develops during fetal growth. The average age at diagnosis in developed countries is 50 to 65, though it appears in younger populations in parts of Asia, likely influenced by dietary and motility differences. Not everyone with a mobile cecum will ever develop a volvulus, but it’s a prerequisite.
Intussusception
Intussusception happens when one segment of the intestine telescopes into the adjacent segment, like a collapsing telescope. It’s one of the most common causes of bowel obstruction in young children, typically occurring between 6 months and 3 years of age. In children, it usually happens without an identifiable structural cause and often resolves with a non-surgical procedure that uses air or fluid pressure to push the telescoped segment back into place.
In adults, intussusception is rare and behaves quite differently. Most adult cases have a demonstrable cause, frequently a polyp, tumor, or other structural abnormality that acts as a “lead point,” pulling one section of bowel into the next during normal contractions. Because of this, adult intussusception is more likely to require surgery and to reveal an underlying condition that needs its own treatment.
Functional Obstruction (Ileus)
Sometimes the bowel simply stops moving. This is a functional obstruction, or paralytic ileus, and it produces many of the same symptoms as a physical blockage: bloating, nausea, inability to pass gas, and vomiting. But there’s no actual object or mass blocking the path.
The most common trigger is surgery itself. Handling the intestines during an abdominal operation temporarily paralyzes their normal rhythmic contractions. This postoperative ileus usually resolves on its own within a few days, but it can occasionally persist longer. Serious infections, especially those affecting the abdomen (like a ruptured appendix or peritonitis), also shut down gut motility. Electrolyte imbalances, particularly low potassium levels, interfere with the electrical signals that coordinate intestinal contractions.
Several classes of medication are known culprits. Opioid painkillers are the most well-known offenders, which creates a frustrating cycle: patients recovering from abdominal surgery often need pain relief, but the very drugs that provide it can delay the return of normal bowel function. Anticholinergic medications, some older antidepressants, and certain antipsychotic drugs also slow gut motility significantly.
Other Contributing Factors
A few less common causes round out the picture. Gallstones can occasionally erode through the gallbladder wall into the small intestine and lodge in a narrow section, creating a blockage called gallstone ileus. Large masses of undigested food, known as bezoars, can accumulate in people with diabetes-related stomach paralysis or those who’ve had certain gastric surgeries. Radiation therapy to the abdomen or pelvis, often for gynecological or prostate cancers, can cause long-term scarring and strictures similar to those seen in Crohn’s disease. Endometriosis can implant on the bowel wall and, in severe cases, cause enough narrowing to obstruct flow.
How Obstructions Are Detected
CT scanning is the primary imaging tool for diagnosing bowel obstruction. A systematic review of the evidence found that CT has a pooled sensitivity of 91% and specificity of 89% for detecting small bowel obstruction, meaning it catches the vast majority of cases and rarely gives a false alarm. CT is also reasonably good at identifying whether the blocked bowel segment has lost its blood supply (82% sensitivity for detecting reduced blood flow), which is critical because compromised blood supply turns a serious problem into an urgent one.
Equally important, CT can often pinpoint the exact location and cause of the blockage, identifying the transition point where normal, dilated bowel meets collapsed bowel downstream with 92% sensitivity. This information helps determine whether the obstruction might resolve with conservative management (rest, IV fluids, decompression through a tube) or whether surgery is needed.