Ileus
Ileus is a temporary loss of the coordinated muscular contractions that normally move contents through the intestine, occurring without any physical blockage. Contents accumulate, the bowel distends, and the resulting symptoms—abdominal distension, nausea, vomiting, and failure to pass gas or stool—closely resemble a mechanical bowel obstruction. The most common setting is the first few days after abdominal surgery, where a brief period of reduced bowel activity is expected; ileus is diagnosed when this lasts longer than anticipated. Ileus can also follow electrolyte abnormalities, opioid and anticholinergic medications, severe infection, inflammation, or critical illness. Distinguishing ileus from mechanical obstruction is the central diagnostic task, because ileus is managed supportively while mechanical obstruction may require an operation. CT of the abdomen and pelvis is the primary test used to make that distinction.
What is it?
The intestine moves its contents forward through peristalsis—rhythmic, coordinated waves of muscular contraction directed by the enteric nervous system, modulated by hormones, and influenced by the autonomic nervous system. Ileus occurs when this coordinated activity is disrupted and the bowel becomes functionally inactive, even though the passage through it remains anatomically open. It is sometimes called paralytic ileus, adynamic ileus, or functional bowel obstruction.
The distinction between ileus and mechanical obstruction is the single most important concept, because the two conditions produce similar symptoms but require fundamentally different management. In mechanical obstruction, a physical barrier—adhesions, a hernia, a tumor, a twisted segment—blocks the lumen, and surgery is often required. In ileus, no barrier exists; the bowel has simply stopped propelling contents, and the treatment is to identify and reverse the reason for that while supporting the patient until function returns. Operating on an ileus does not help and adds risk, which is why imaging that reliably distinguishes the two is so central.
Postoperative ileus is by far the most common form. A period of reduced bowel activity after abdominal surgery is normal and expected—generally understood to affect the small bowel for hours, the stomach for a day or so, and the colon for two to three days, with the colon the last to recover. This physiological phase is not a complication. The term prolonged or paralytic postoperative ileus is reserved for cases where function does not return within the expected window, delaying feeding, prolonging hospitalization, and increasing the risk of other complications.
Several mechanisms contribute to postoperative ileus, and they operate together. Surgical handling of the bowel triggers an inflammatory response in the bowel wall that directly impairs muscle activity. Sympathetic nervous system reflexes activated by incision and manipulation inhibit motility. Opioid analgesics act on receptors in the gut to slow transit—this is the same mechanism responsible for opioid-induced constipation and is one of the most modifiable contributors. Excessive intravenous fluid causes bowel wall edema that impairs contraction. Electrolyte shifts, particularly low potassium and magnesium, disrupt the electrical activity underlying muscle contraction. Immobility compounds all of these.
Ileus outside the surgical setting has a broad range of causes. Electrolyte abnormalities—especially hypokalemia, but also hypomagnesemia, hypercalcemia, and hyponatremia—are common and correctable contributors. Medications are frequently responsible, including opioids, anticholinergics, tricyclic antidepressants, antipsychotics, some antiparkinsonian drugs, calcium channel blockers, and certain chemotherapy agents. Severe infection and sepsis suppress motility, as does intra-abdominal inflammation from pancreatitis, appendicitis, diverticulitis, or peritonitis. Retroperitoneal processes such as hemorrhage or a ureteral stone can produce reflex ileus, as can spinal cord injury, vertebral fractures, and pelvic or hip fractures. Myocardial infarction, pneumonia, and critical illness of nearly any kind can be responsible. Endocrine conditions including hypothyroidism and diabetic ketoacidosis contribute in some patients.
Acute colonic pseudo-obstruction, also called Ogilvie syndrome, is a distinct and more serious variant in which massive dilation of the colon develops without mechanical blockage. It typically occurs in older hospitalized patients, often after orthopedic or cardiac surgery, trauma, or severe illness, and is thought to reflect an imbalance between sympathetic and parasympathetic input to the colon. Its importance lies in the risk of ischemia and perforation as the cecum distends, a risk that rises with increasing cecal diameter and with duration of dilation, making it the form of functional obstruction that most requires active monitoring and intervention.
Symptoms overlap substantially with mechanical obstruction, but several features help distinguish them.
Abdominal distension is prominent in both, and in ileus it is typically generalized rather than localized to one region.
Abdominal discomfort in ileus tends to be dull, diffuse, and continuous. The crampy, wave-like colicky pain that characterizes mechanical obstruction—produced by bowel contracting vigorously against a fixed barrier—is generally absent, precisely because the bowel is not contracting effectively.
Nausea and vomiting are common in both, and in ileus the vomiting is often less forceful and more a matter of overflow from a distended, inactive stomach and small bowel.
Failure to pass gas or stool occurs in both, though in ileus some passage of gas may continue since nothing physically blocks the lumen.
Bowel sounds differ in a way that is classically taught and clinically useful, though not definitive: mechanical obstruction produces high-pitched, hyperactive sounds early as the bowel strains, while ileus produces diminished or absent sounds from the outset.
The abdomen in simple ileus is typically soft and tympanitic without focal tenderness, guarding, or rigidity. Severe pain, marked tenderness, fever, rapid heart rate, or systemic illness point away from simple ileus and toward ischemia, perforation, an underlying infection, or a mechanical cause.
Diagnosis rests on the clinical context supported by imaging that excludes mechanical obstruction.
CT of the abdomen and pelvis with intravenous contrast is the primary test. In ileus, it typically demonstrates diffuse dilation involving both small bowel and colon, with gas present throughout and extending into the rectum, and—critically—no transition point where dilated bowel meets collapsed bowel. In mechanical obstruction, that transition point is the defining finding, with collapsed bowel distal to it. CT also identifies contributing intra-abdominal causes such as an abscess, pancreatitis, or an anastomotic leak in a postoperative patient, and assesses for complications including ischemia and perforation. In practice the distinction is not always clean—prolonged ileus and low-grade partial obstruction can look similar, and a small proportion of cases remain equivocal after imaging.
Abdominal X-rays show generalized gaseous distension of both small and large bowel with air-fluid levels, and gas in the rectum. They are much less specific than CT and often cannot confidently exclude mechanical obstruction, but they are inexpensive and easily repeated, which makes them useful for serial monitoring—particularly for tracking cecal diameter in suspected colonic pseudo-obstruction.
Water-soluble contrast studies can clarify equivocal cases by demonstrating unimpeded passage of contrast through to the colon, and the hyperosmolar contrast may have a mild therapeutic effect.
Blood tests identify contributing and complicating factors: electrolytes including potassium, magnesium, calcium, and sodium; kidney function; complete blood count; inflammatory markers; and, where relevant, thyroid function, glucose, and lactate. Blood tests do not diagnose ileus but frequently reveal the reason for it.
A careful medication review is an essential and often high-yield part of the assessment, since drug-induced ileus is common and readily reversible.
The differential diagnosis includes mechanical bowel obstruction (the principal alternative), acute colonic pseudo-obstruction, mesenteric ischemia, severe constipation or fecal impaction, toxic megacolon in the setting of colitis, gastroparesis, and, in the postoperative patient, anastomotic leak and intra-abdominal abscess—both of which can present initially as an unexplained prolonged ileus and should be actively considered when recovery is not progressing as expected.
Important to Know
Management of ileus is supportive and cause-directed, and the great majority of cases resolve without surgery. Care is typically coordinated by surgical or hospital medicine teams, with gastroenterology involved in prolonged or atypical cases.
Supportive care includes intravenous fluids to maintain hydration while avoiding overload, since excessive fluid causes bowel wall edema that worsens the problem. Electrolyte abnormalities are corrected, with particular attention to potassium and magnesium. Nutrition is maintained enterally where tolerated, since the presence of nutrients in the gut itself stimulates motility, with parenteral nutrition reserved for prolonged episodes where enteral feeding is not possible.
Nasogastric tube decompression is used selectively rather than routinely. It provides real relief for patients with significant vomiting or marked distension and reduces aspiration risk, but routine prophylactic use after abdominal surgery has been shown to delay rather than hasten return of bowel function and increase pulmonary complications, and is no longer recommended.
Reducing opioids is one of the most effective interventions available. Multimodal analgesia—acetaminophen, NSAIDs where not contraindicated, regional and neuraxial techniques such as epidural analgesia, and local anesthetic infiltration—provides pain control while limiting the opioid burden on the gut. Where opioids remain necessary, the lowest effective dose is used.
Other contributing medications are reviewed and reduced where clinically possible, including anticholinergics, tricyclic antidepressants, antipsychotics, and calcium channel blockers. This should always be done in coordination with the prescribing clinician rather than unilaterally.
Early mobilization is encouraged. The evidence that walking directly accelerates return of bowel function is weaker than commonly assumed, but mobilization reduces pulmonary complications, thromboembolism, and deconditioning, and remains a core part of recovery.
Enhanced recovery after surgery (ERAS) protocols have meaningfully reduced the incidence of postoperative ileus. Their components include minimally invasive surgical approaches where feasible, avoidance of routine nasogastric tubes, goal-directed rather than liberal fluid therapy, opioid-sparing multimodal analgesia, early oral intake rather than prolonged fasting, early mobilization, and chewing gum, which acts as sham feeding and has modest but real benefit in several trials.
Alvimopan, a peripherally acting mu-opioid receptor antagonist, is approved to accelerate recovery of bowel function after bowel resection with primary anastomosis in hospitalized patients. It acts on gut opioid receptors without reversing central analgesia and is used for a limited duration under a restricted program because of cardiovascular signals seen with longer-term use. Its role is specific and it is not a general treatment for ileus.
Prokinetic medications have generally disappointed in this setting. Metoclopramide and erythromycin have not shown consistent benefit for postoperative ileus, and are not routinely recommended, though they may be used in selected circumstances.
Acute colonic pseudo-obstruction is managed more actively. Initial care mirrors that of ileus, with correction of electrolytes, minimization of contributing medications, treatment of underlying illness, and serial monitoring of cecal diameter. If supportive care fails, neostigmine is an effective pharmacologic option that produces prompt decompression in most patients, given with cardiac monitoring and atropine available because of the risk of bradycardia, and avoided in patients with significant bradyarrhythmia, bronchospasm, or suspected mechanical obstruction or perforation. Colonoscopic decompression is an alternative or subsequent option. Surgery is reserved for perforation, ischemia, or failure of all other measures.
Outcome is generally favorable. Most postoperative ileus resolves within days with supportive care. The main consequences are delayed recovery, prolonged hospital stay, increased risk of hospital-acquired complications, and patient discomfort rather than lasting bowel damage. The principal exceptions are colonic pseudo-obstruction with perforation, and cases where an apparent ileus is actually masking an anastomotic leak, abscess, or mechanical obstruction—which is why an ileus that fails to improve as expected warrants reassessment and repeat imaging rather than continued observation.
Prevention in the surgical setting follows the ERAS principles above and is considerably more effective than treatment after the fact. Outside surgery, prevention centers on attention to electrolytes, judicious use of constipating medications particularly in older and hospitalized patients, prompt treatment of infection, and early mobilization.
Care is typically coordinated by the surgical or admitting medical team, with gastroenterology, critical care, and pharmacy input as needed. Imaging, laboratory, and clinical findings are interpreted alongside the patient’s symptoms, examination, recent operations, medication list, and broader clinical context rather than in isolation.
Patient education plays an important role. Understanding that a short period of sluggish bowel function after abdominal surgery is expected rather than alarming, that the goal is supportive care rather than an operation, why opioid use is being minimized, why early sips and mobilization are encouraged, and which symptoms warrant urgent reassessment all contribute to appropriate care and reduce anxiety during a recovery that can feel slower than anticipated.
Red flag symptoms include severe or worsening abdominal pain, particularly pain that becomes constant and localized; marked or rapidly increasing abdominal distension; high fever with chills; rapid heart rate or low blood pressure; abdominal tenderness with guarding or rigidity; persistent vomiting with inability to keep down fluids; signs of sepsis (severe illness, confusion, cold clammy skin); and, in a postoperative patient, drainage from the wound, or a recovery that is deteriorating rather than gradually improving. These warrant immediate evaluation, as they may indicate mechanical obstruction, bowel ischemia, perforation, anastomotic leak, intra-abdominal abscess, or sepsis rather than simple ileus.