Adhesions from prior abdominal surgery are the single most common cause of small bowel obstruction (SBO) in industrialized countries, responsible for roughly half to two-thirds of all cases. But the full list of causes is surprisingly long, ranging from hernias and tumors to swallowed foreign objects and even certain medications. What triggers an obstruction, how urgently it needs treatment, and whether it can be prevented all depend on which cause is at play.
Adhesions After Surgery
When abdominal tissue is cut, handled, or exposed to air during surgery, the body’s wound-healing response sometimes goes off track. Normally, the peritoneum (the thin lining of the abdominal cavity) heals cleanly. But when the fibrin that forms at the injury site is not fully broken down within about five to seven days, it hardens into bands of scar tissue that can permanently tether loops of bowel to each other or to the abdominal wall. These bands are adhesions, and they form after the vast majority of abdominal operations.
The core problem is a disruption in how the body clears fibrin. After surgery, levels of the enzyme that dissolves fibrin tend to drop while levels of an inhibitor called PAI-1 rise. Tissue oxygen levels around the surgical site also fall, which further slows the cleanup process. The result is a fibrin scaffold that persists long enough for repair cells and new blood vessels to move in, eventually creating a permanent band of connective tissue where none should exist.1Asian Journal of Surgery. Current options for the prevention of postoperative intra-abdominal adhesions These adhesions can form after any abdominal procedure, but surgeries on the colon, rectum, and gynecologic organs carry especially high rates.
Most adhesions never cause symptoms. The ones that do cause trouble typically kink or compress a segment of small bowel, creating a point where food and liquid cannot pass. The obstruction can be partial, meaning some material still gets through, or complete, meaning nothing does. An adhesive SBO can show up months or even decades after the original surgery, which is why surgeons always ask about prior operations when a patient arrives with signs of obstruction.
Hernias
A hernia is a weak spot or defect in the abdominal wall through which tissue can bulge. Inguinal hernias in the groin are the most familiar type, but hernias can develop at old surgical incision sites, at the belly button, or in rarer locations like the obturator foramen deep in the pelvis. When a loop of small bowel slides through a hernia defect and gets trapped, it becomes incarcerated, and the bowel may kink shut.
The danger escalates quickly once the blood supply to the trapped segment is squeezed off, a condition called strangulation. Research on obturator hernias found that the longer the segment of trapped bowel, the higher the risk of tissue death. In one study, each additional millimeter of incarcerated intestine on imaging was associated with a roughly 19 percent increase in the odds of necrosis.2PubMed. Increased length of incarcerated small bowel as a risk factor for intestinal necrosis in obturator hernia Unusual hernia sites can also cause SBO. In one reported case, a bone defect left after an iliac crest bone graft allowed small bowel to herniate through the hip bone, producing a complete obstruction seen on CT.3PubMed Central. Small bowel obstruction caused by an incarcerated hernia after iliac crest bone harvest
Hernias are particularly important as a cause of SBO in parts of the world where abdominal surgery is less common. In regions where fewer people have had prior operations, hernias overtake adhesions as the leading cause of bowel obstruction.
Tumors and Neoplasms
Tumors in or near the small bowel can block the lumen by growing inward, compressing the bowel from outside, or seeding the peritoneum with implants that kink the bowel. In patients who have never had abdominal surgery, tumors are a surprisingly frequent explanation for SBO. A retrospective study of patients with no operative history found that neoplasms accounted for about a third of all small bowel obstructions, and a quarter of all cases were malignant. Among patients aged 60 and older in that study, the proportion was even more striking: tumors explained over half of obstructions, with malignancies accounting for close to 40 percent.4PubMed Central. Determining the etiology of small bowel obstruction in patients without intraabdominal operative history: a retrospective study
This age pattern matters because it shifts the diagnostic calculus. In a younger person with no prior surgery who develops SBO, Crohn’s disease and adhesions from unrecognized inflammation are more likely candidates. In an older person without surgical history, malignancy should be high on the list. Primary small bowel cancers are uncommon compared to colon cancer, but metastatic disease from other organs, particularly melanoma, lung, and breast cancers, can seed the peritoneal surfaces and produce obstruction.
Crohn’s Disease and Inflammatory Strictures
Crohn’s disease has a particular tendency to affect the terminal ileum, the last stretch of small bowel before it meets the colon, and chronic inflammation there commonly leads to strictures that narrow the bowel lumen. Obstruction from these strictures remains one of the most frequent reasons people with Crohn’s disease need intervention.5PubMed Central. Crohn’s Disease Obstructions
The tricky part is that Crohn’s strictures come in two flavors. When active inflammation is the main driver of the narrowing, medications (steroids, biologics) can often open things back up. But once the stricture is dominated by fibrosis, meaning dense scar tissue has replaced the inflamed tissue, drugs have limited effect and the patient usually needs either endoscopic dilation or surgery.6PubMed Central. Crohn’s Disease Obstructions Distinguishing between inflammatory and fibrotic strictures is one of the ongoing challenges in managing Crohn’s-related SBO, and imaging alone does not always give a clear answer.
In the study of patients without prior surgery mentioned earlier, Crohn’s disease was more prevalent as a cause of SBO in people under age 60, reinforcing the point that etiology shifts with the patient’s age and history.7PubMed Central. Determining the etiology of small bowel obstruction in patients without intraabdominal operative history: a retrospective study
Intussusception
Intussusception occurs when one segment of bowel telescopes into the segment immediately downstream, like a collapsing telescope. In children, the ileocolic form (where the ileum slides into the colon) is most common and is usually idiopathic, meaning there is no identifiable structural trigger. These pediatric cases are typically treated without surgery, using air or fluid enemas to push the telescoped segment back out.
In adults the picture is different. Adult intussusception is uncommon and tends to happen in the small bowel rather than at the ileocolic junction. Most adult cases have a “lead point,” a polyp, tumor, or other lesion that the bowel’s normal contractions grab and pull downstream, dragging the surrounding wall with it. When that lead point in the small bowel is malignant, it is most often from metastatic disease such as melanoma rather than a primary intestinal cancer.8PubMed Central. Intestinal Intussusception: Etiology, Diagnosis, and Treatment This makes adult small bowel intussusception a red flag that warrants careful investigation even after the obstruction is relieved.
Volvulus
A volvulus is a twist of the bowel around its own blood supply, the mesentery. When the small bowel twists far enough, it simultaneously blocks the passage of intestinal contents and cuts off blood flow, creating a surgical emergency. Small bowel volvulus is classified as primary, where no predisposing cause is found, or secondary, where an underlying lesion triggers the twist. In the secondary category, postoperative adhesions are again the most frequent culprit.9European Journal of Medical and Health Sciences. Small Bowel Volvulus In Adult: A Review Of Literature Congenital issues such as malrotation, where the gut did not complete its normal rotation during fetal development, can also set the stage for volvulus later in life.
Gallstone Ileus
Gallstone ileus is a misleading name: it is a mechanical obstruction, not a paralytic ileus. It occurs when a large gallstone erodes through the wall of the gallbladder into an adjacent loop of bowel, creating an abnormal passage called a bilioenteric fistula. The stone then travels downstream through the small intestine until it reaches a point too narrow to pass, usually the terminal ileum. The classic imaging triad described by Rigler, consisting of bowel obstruction, air in the biliary tree, and a visible gallstone in an unusual location, points to this diagnosis.10Journal of Surgery and Surgical Specialities. Gallstone Ileus – A Rare Cause of Acute Small Bowel Obstruction in an Elderly Female, a Clinical Case Report
Gallstone ileus predominantly affects older adults, especially women, reflecting the demographics of gallstone disease in general. It accounts for only a small percentage of all SBOs, but it deserves mention because it is easily missed if the clinician is not thinking about it, and delayed diagnosis raises the stakes considerably.
Bezoars and Swallowed Objects
A bezoar is a mass of indigestible material that accumulates in the gastrointestinal tract and eventually blocks the bowel. There are several types: phytobezoars (plant fiber), trichobezoars (hair, typically in people with a compulsive hair-eating disorder), pharmacobezoars (medication residue), and lactobezoars (milk protein, seen in infants).11Journal of Surgical Case Reports. Bezoar-induced small bowel obstruction: a rare cause of a common problem Phytobezoars are the most common variety in adults and are strongly associated with foods high in insoluble fiber. A study of 35 patients with bezoar-related SBO found that the vast majority of cases occurred in autumn and winter and were linked to heavy consumption of persimmons and hawthorn. Over half of those patients had a history of gastrointestinal surgery, and many had significant dental problems that likely impaired chewing.12PubMed Central. Bezoar-induced small bowel obstruction: Clinical characteristics and diagnostic value of multi-slice spiral computed tomography
People who have had gastric surgery, especially procedures that alter the stomach’s ability to grind food, are at higher risk because larger chunks of undigested material pass into the small bowel. The ileum is the most frequent site of impaction, just as it is for gallstone ileus, because the ileum is the narrowest part of the small intestine.
Intentionally or accidentally swallowed foreign bodies, from coins in children to drug packets in body packers, can similarly lodge in the small bowel and cause obstruction, though most small foreign objects pass through without incident.
Medication-Induced Strictures
Long-term use of nonsteroidal anti-inflammatory drugs (NSAIDs) can cause a condition sometimes called diaphragm disease, in which thin, web-like strictures form inside the small bowel. These diaphragms are rings of scar tissue that narrow the lumen just enough to trap capsule endoscopes and, in more severe cases, block the passage of intestinal contents entirely.13PubMed Central. Nonsteroidal anti-inflammatory drugs induced diaphragm disease: a report of 3 cases and literature review
In one retrospective review of patients with small bowel diaphragms, long-term NSAID use was documented in the majority of cases, and the ileum was the most common location. CT scans picked up abnormalities in nearly all patients, usually showing short-segment strictures with focal wall thickening.14PubMed. Diaphragm disease of the small bowel: a retrospective review of CT findings The condition is likely underdiagnosed because the strictures are subtle and the small bowel is difficult to image directly. Clinicians are encouraged to consider diaphragm disease in any patient on chronic NSAIDs who presents with unexplained SBO symptoms.
Radiation Enteritis
Radiation therapy directed at abdominal or pelvic tumors can damage the small bowel lining, and over time this damage can produce fibrotic strictures that narrow the bowel enough to cause obstruction. The risk is highest when the small bowel lies within or adjacent to the radiation field, which commonly happens during treatment for gynecologic, colorectal, or urologic cancers. Radiation-induced strictures can appear months to years after the treatment itself, which makes the connection easy to miss if the clinical history is not carefully reviewed.15PubMed Central. Recurrent Intestinal Obstruction after Radiation Therapy: A Case Report and Review of the Literature
Endometriosis
Endometriosis, in which tissue resembling the uterine lining grows outside the uterus, can involve the bowel in roughly 3 to 37 percent of cases depending on the study and the population. When endometrial implants infiltrate the wall of the small bowel, the resulting inflammation and scarring can narrow the lumen enough to cause obstruction. One reported case involved a woman whose endometrial tissue had deeply invaded the muscle layer and submucosa of the small bowel wall, leading to acute SBO that required emergency surgery. Her symptoms, crucially, had no relationship to her menstrual cycle, which made preoperative diagnosis more difficult.16PubMed Central. Acute small bowel obstruction caused by endometriosis: a case report and review of the literature
Endometriosis-related SBO is rare enough that it is often not considered until other causes have been ruled out, but it should be on the differential for premenopausal women with unexplained bowel obstruction, especially those with a known history of the condition.
Congenital Anomalies
Some people are born with structural variants in the small bowel that can cause obstruction at any point in life. Meckel’s diverticulum is the most well-known example. It is a small pouch that forms during fetal development as a remnant of a structure that normally disappears before birth. Most people with a Meckel’s diverticulum never know it is there, but it can occasionally cause bleeding, inflammation, or bowel obstruction. Obstruction occurs when a fibrous band (the mesodiverticular band) connects the diverticulum to the abdominal wall and a loop of bowel herniates beneath it, creating a closed-loop obstruction.17PubMed Central. Small Bowel Obstruction due to Meckel’s Diverticulum: A Case Report
Other congenital causes include intestinal malrotation and congenital bands. These tend to present in childhood but can occasionally remain silent until adulthood, when they cause an unexpected SBO that puzzles clinicians unfamiliar with the anatomy.
Anticoagulants and Intramural Hematomas
A less intuitive cause of SBO is bleeding inside the bowel wall itself. When blood pools between the layers of the intestinal wall, the resulting swelling can narrow or completely block the lumen. Over 90 percent of these intramural hematomas result from blunt abdominal trauma, but among non-traumatic cases, the leading cause is excessive anticoagulation with warfarin.18PubMed Central. Extensive small bowel intramural haematoma secondary to warfarin One series of patients with spontaneous (non-traumatic) intramural hematomas found that small bowel obstruction was present in the majority of cases, with both partial and complete obstructions represented.19JAMA Surgery. Spontaneous Intramural Small-Bowel Hematoma: Clinical Presentation and Long-term Outcome
For patients on blood thinners who develop abdominal pain and signs of obstruction, this diagnosis should be considered, especially if there is no surgical history to suggest adhesions. Most anticoagulant-related hematomas can be managed without surgery if the bleeding is controlled and the anticoagulation is reversed, though close monitoring is essential.
Telling Mechanical Obstruction from Ileus
Not every bowel that stops working is mechanically blocked. Postoperative ileus, in which the bowel simply stops contracting after surgery without any physical obstruction, can look nearly identical to a true mechanical SBO on clinical exam and plain X-rays. In one study, the combination of clinical assessment and plain radiographs had a sensitivity of only 19 percent for distinguishing between postoperative ileus and complete mechanical SBO. CT, by contrast, achieved 100 percent sensitivity and specificity for making that distinction.20American Journal of Roentgenology. Distinction between postoperative ileus and mechanical small-bowel obstruction: value of CT compared with clinical and other radiographic findings
This distinction matters because the treatment is fundamentally different. Mechanical obstruction often requires surgery or procedural intervention. Ileus is treated supportively, with bowel rest and time. Treating one as if it were the other can be dangerous: unnecessary surgery for ileus adds risk, while watchful waiting for a true mechanical obstruction risks strangulation and bowel death.
Can Adhesive Obstruction Be Prevented?
Given that adhesions are the leading cause of SBO, there has been significant interest in surgical barrier products designed to physically separate healing surfaces and reduce adhesion formation. A systematic review and meta-analysis found some evidence that a hyaluronate carboxymethylcellulose barrier roughly halved the risk of reoperation for adhesive small bowel obstruction compared to no barrier.21The Lancet. Benefits and harms of adhesion barriers for abdominal surgery: a systematic review and meta-analysis That is a meaningful reduction, but it does not eliminate the problem entirely. Minimally invasive surgery (laparoscopy rather than open procedures) also reduces adhesion formation because it causes less tissue handling and less drying of peritoneal surfaces, though it does not prevent adhesions altogether. The underlying biology of wound healing described earlier, with its delicate balance between fibrin deposition and degradation, means that any abdominal surgery carries some risk of adhesion formation.22PubMed Central. Intra-abdominal adhesions: definition, origin, significance in surgical practice, and treatment options
For the individual patient, there is no reliable way to predict who will develop clinically significant adhesions after surgery. Some people undergo multiple abdominal operations and never develop SBO; others develop it after a single procedure. This unpredictability remains one of the frustrating realities of abdominal surgery and one of the reasons research into adhesion prevention continues.