Small bowel obstruction happens when something blocks the passage of food and fluid through the small intestine. The most common cause, by a wide margin, is scar tissue from previous surgery. Hernias are the second most common cause in the U.S., followed by a range of less frequent triggers including tumors, inflammatory disease, and twisting of the intestine.
Surgical Adhesions: The Leading Cause
Adhesions are bands of scar tissue that form inside the abdomen after surgery. They can kink, compress, or twist a loop of small bowel, blocking it partially or completely. The overall incidence of small bowel obstruction after abdominal surgery is about 4.6%, but the risk varies dramatically depending on the type of operation.
Surgeries involving the rectum carry the highest readmission rate for adhesion-related problems (8.8%), followed by operations on the small bowel itself (7.6%) and the colon (7.1%). Open colectomy, where part of the colon is removed through a large incision, leads to obstruction in roughly 9.5% of cases. Gynecological procedures also contribute, with ovarian surgery carrying a 7.1% risk of adhesion-related readmission. The more tissue is handled during surgery and the larger the incision, the more scar tissue tends to form.
Adhesions can cause obstruction weeks, months, or even decades after the original operation. Some people develop them after a single procedure; others undergo multiple surgeries without issue. There’s no reliable way to prevent them entirely, though minimally invasive techniques generally produce fewer adhesions than open surgery.
Hernias
A hernia occurs when a segment of intestine pushes through a weak spot in the abdominal wall. If the bowel becomes trapped or tightly pinched at the opening, it can obstruct. This is called incarceration, and if blood flow to the trapped segment gets cut off, it becomes strangulation, a surgical emergency.
Hernias can develop in the groin, around the belly button, or at the site of a previous surgical incision. Groin hernias (inguinal and femoral) are among the most common types that lead to obstruction. In parts of the world where hernia repair is less accessible, hernias actually surpass adhesions as the leading cause of small bowel obstruction.
Crohn’s Disease and Inflammatory Strictures
Crohn’s disease causes chronic inflammation in the digestive tract, most often in the small intestine. Over time, repeated cycles of inflammation and healing can thicken the bowel wall and narrow the passageway, forming what’s called a stricture. Up to 5% of Crohn’s patients already have a stricture when they’re first diagnosed, and about 15% develop one within 10 years. In children with Crohn’s, the numbers are higher: 20% at diagnosis, climbing to 40% by 10 years.
Strictures fall into two categories. Some are driven mainly by active inflammation, which means they can improve with stronger anti-inflammatory treatment. Others become predominantly fibrous, like scar tissue, and don’t respond to medication. Fibrous strictures typically require either balloon dilation (stretching the narrowed area with an inflatable device passed through an endoscope) or surgery to widen or remove the affected segment.
Tumors and Cancer
Most small bowel obstructions are caused by benign conditions, not cancer. Primary tumors of the small intestine are rare. When malignancy does cause an obstruction, it’s more often from cancers that started elsewhere and spread to the abdomen.
Ovarian cancer is the most frequent culprit, causing bowel obstruction in 5.5 to 42% of patients over the course of their illness. Colorectal cancer follows at 4.4 to 24%, and gastric cancer also contributes. Less commonly, cancers originating outside the abdomen, including breast cancer, lung cancer, and melanoma, can metastasize to the abdominal cavity and compress or invade the bowel.
Volvulus and Intussusception
A volvulus happens when a loop of intestine twists around itself, cutting off both the passage of contents and, potentially, blood supply. It can occur at any point along the bowel and often needs emergency intervention to untwist before the tissue dies.
Intussusception is a different mechanical problem: one segment of the intestine telescopes into the adjacent segment, like a collapsing antenna. It’s the most common cause of bowel obstruction in children under 3, affecting roughly 1 in 2,000 babies in the U.S. during their first year. In adults, intussusception is rare, accounting for only about 1% of bowel obstructions. When it does happen in adults, there’s usually an identifiable trigger like a tumor, polyp, or the long-term effects of Crohn’s disease.
Functional Obstruction: Paralytic Ileus
Not every obstruction involves a physical blockage. Paralytic ileus is a functional problem where the muscles of the small intestine simply stop contracting. Contents can’t move forward even though nothing is physically in the way. The symptoms, including abdominal distension, nausea, and inability to pass gas, can look identical to a mechanical obstruction.
Common triggers include abdominal surgery (the bowel temporarily “shuts down” after being handled), certain medications, and electrolyte imbalances. Opioid painkillers are a well-known cause, which creates a frustrating cycle for post-surgical patients who need pain relief. Anticholinergic medications, tricyclic antidepressants, and phenothiazines also slow gut motility. Low potassium levels (hypokalemia) can impair the electrical signals that drive intestinal contractions, producing the same effect.
What Happens Inside the Bowel During an Obstruction
Understanding why obstruction becomes dangerous helps explain the urgency doctors assign to it. When the bowel is blocked, food, fluid, and gas accumulate upstream of the blockage. This stretches the intestinal wall, which paradoxically triggers the lining to secrete even more fluid into the space, worsening the distension.
As pressure inside the bowel climbs, it first compresses the tiny lymphatic vessels in the intestinal wall, causing the wall to swell. If pressure continues rising, it overwhelms the capillary blood vessels, forcing large volumes of fluid, electrolytes, and proteins out of the bloodstream and into the intestinal space. This “third spacing” of fluid can cause severe dehydration and dangerous drops in blood pressure, even though the fluid hasn’t technically left the body. It’s simply trapped in a place where the body can’t use it.
If the blood supply to the obstructed segment becomes compromised, either from extreme distension or from twisting and strangulation, the tissue starts to die. Dead bowel can perforate, spilling intestinal bacteria into the abdominal cavity and causing life-threatening infection. This is why obstruction with signs of compromised blood flow, such as severe constant pain, fever, or rapid heart rate, is treated as a surgical emergency.
How the Cause Is Identified
CT imaging is the primary tool for confirming a small bowel obstruction and pinpointing what’s causing it. A CT scan can identify the “transition point,” the exact location where dilated, fluid-filled bowel meets collapsed bowel downstream. Radiologists correctly identify the level of obstruction in about 86% of cases. Determining the specific cause from imaging alone is harder, with accuracy around 47% in some studies, which is why a patient’s surgical history, symptoms, and physical exam remain essential pieces of the puzzle.
Someone with no prior surgeries and a painful lump in the groin likely has a hernia. A patient with a history of multiple abdominal operations and intermittent cramping almost certainly has adhesions. A person with known Crohn’s disease presenting with gradually worsening symptoms points toward stricture. The combination of imaging and clinical context is what guides treatment decisions.