What Causes Ileus
Ileus results from impaired intestinal motility due to neurological inhibition, inflammatory responses, pharmacological agents (especially opioids), metabolic derangements, and fluid overload—with postoperative manipulation and opioid use being the most common precipitating factors. 1
Primary Mechanisms
Postoperative Causes
The most common cause of ileus is abdominal surgery, where the severity directly correlates with the extent of intraoperative bowel manipulation 2, 1. The pathophysiology involves:
- Early neurogenic phase: Surgical manipulation activates inhibitory alpha-2 adrenergic reflexes through spinal and supraspinal pathways, causing immediate but short-lived motility suppression 3
- Prolonged inflammatory phase: Beginning 3-6 hours postoperatively and lasting several days, this involves local molecular inflammation and leukocyte recruitment into the intestinal muscularis, which impairs neuromuscular function 4, 3
- Surgical approach matters: Laparoscopic procedures cause significantly less severe ileus compared to open surgery due to reduced manipulation 1, 5
Pharmacological Causes
Opioid analgesics are the single most important modifiable pharmacological cause, directly inhibiting gastrointestinal motility through peripheral opioid receptors 1, 5. Other culprit medications include:
- Anticholinergic agents (severe dysmotility) 1
- Clozapine, baclofen, buserelin, clonidine 1
- Fludaribine, phenytoin, verapamil, vincristine 1
- Anesthetic agents may have residual inhibitory effects on enteric nerves 6
Fluid and Metabolic Derangements
Perioperative fluid overload is a major preventable cause that significantly worsens and prolongs ileus 2, 1. The mechanism involves:
- Splanchnic edema causing increased abdominal pressure and decreased mesenteric blood flow 2
- Impaired gastric motility from tissue edema and compromised microvascular perfusion 2
- Hyperchloremic acidosis from excessive 0.9% saline reducing gastric blood flow 2
Electrolyte abnormalities, particularly hypokalemia and hypomagnesemia, directly impair intestinal smooth muscle contractility 1, 7.
Neurological and Endocrine Causes
Disorders affecting autonomic innervation of the gut include:
- Diabetes mellitus (most common endocrine cause via autonomic neuropathy) 1
- Brainstem lesions, spinal cord injury, multiple sclerosis 1
- Parkinson's disease, myotonic dystrophy, porphyria 1
- Hypothyroidism causing generalized dysmotility 1
Infectious and Inflammatory Causes
- Sepsis and systemic inflammatory response syndrome trigger widespread intestinal dysmotility 1
- Peritonitis from any source 1
- Specific infections: Clostridioides difficile (can present as unexplained ileus), strongyloidiasis, herpes viruses, Chagas disease, botulism 1
Critical Clinical Pitfalls
Do not confuse paralytic ileus with mechanical obstruction—ileus causes diffuse constant distension with minimal pain and hypoactive bowel sounds, while mechanical obstruction produces intermittent crampy pain with high-pitched bowel sounds 7.
Avoid prolonged nasogastric decompression, as this paradoxically extends ileus duration rather than shortening it 5, 7. Use only for severe distention, vomiting, or aspiration risk 5.
Recognize that "normal" postoperative ileus should resolve within specific timeframes: small bowel within 24 hours, stomach within 24-48 hours, colon within 48-72 hours—persistence beyond 3 days is pathological and requires investigation 7.