Causes of Bowel Obstruction
Bowel obstruction etiology differs fundamentally between small and large bowel, with adhesions causing 55-75% of small bowel obstructions while cancer accounts for 60% of large bowel obstructions. 1
Small Bowel Obstruction Causes
The three major causes account for 90% of all small bowel obstructions 1:
Adhesions: 55-75% - This is the single most common cause of small bowel obstruction, typically occurring in patients with prior abdominal surgery (which has 85% sensitivity and 78% specificity for predicting adhesive obstruction) 1
Hernias: 15-25% - Including inguinal, femoral, umbilical, and incisional hernias. Critically, hernias are the most frequent cause of strangulation and bowel ischemia despite being less common than adhesions 2
Malignancies: 5-10% - Including primary small bowel tumors and metastatic disease 1
Other causes: 15% - Including carcinomatosis, endometriosis, inflammatory bowel disease stenosis, intussusception, ischemic stenosis, radiation stenosis, postanastomotic stenosis, gallstones, foreign bodies, and bezoars 1
Large Bowel Obstruction Causes
The distribution is markedly different from small bowel 1:
Cancer: 60% - Colorectal malignancy is the dominant cause, often presenting with rectal bleeding and unexplained weight loss 1
Volvulus: 15-20% - Particularly sigmoid and cecal volvulus, often associated with chronic constipation history 1
Diverticular disease: 10% - Including diverticular strictures, more common in patients with prior diverticulitis episodes 1
Other causes: 10% - Similar miscellaneous etiologies as small bowel 1
Non-Mechanical Causes
Adynamic ileus and colonic pseudo-obstruction represent functional rather than mechanical obstruction, caused by lack of enteric propulsion from drugs, trauma, postoperative state, or metabolic disturbances 1
Critical Clinical Distinction
When evaluating any bowel obstruction, the most important determination is identifying strangulation risk, as this carries a 10-fold increase in mortality 2. Hernias pose the highest strangulation risk - in one study, 12 of 15 strangulated cases (80%) were hernias, with femoral hernias being particularly dangerous 2. This makes hernia-related obstruction a surgical emergency requiring different management than adhesive obstruction, despite being less common overall.