Caliber thresholds: SBO: dilated loops >2.5 cm proximal to transition point with decompressed loops distally. LBO: colon >6 cm (cecum >9 cm = impending perforation / Ogilvie risk). Water-soluble contrast enema or CT colonography can characterize LBO level and etiology. Pneumoperitoneum = perforation.
SBO vs LBO
| Feature | SBO (Small Bowel) | LBO (Large Bowel) |
|---|---|---|
| Location | Central abdomen; jejunum (left upper) vs ileum (right lower) | Peripheral/frame-like distribution of dilated colon |
| Caliber threshold | >2.5 cm (small bowel); >3 cm (duodenum) | >6 cm colon; >9 cm cecum = imminent perforation risk |
| Transition point | Dilated proximal loops → decompressed distal loops; identify cause (adhesion, hernia, mass) | Transition at obstructing lesion (carcinoma, diverticular stricture, volvulus); may have competent ileocecal valve → closed loop cecal distension |
| Closed loop | C/U-shaped dilated loop; two transition points; mesenteric swirl; beak sign; high risk of strangulation | Volvulus creates closed loop (sigmoid or cecal); competent ileocecal valve in LBO creates cecal closed loop |
| Common causes | Adhesions (most common post-surgical), incarcerated hernia (internal or external), Crohn's stricture, malignancy, intussusception, gallstone ileus | Colorectal carcinoma (most common), diverticular stricture, volvulus (sigmoid > cecal), extrinsic compression, Ogilvie syndrome (pseudo-obstruction) |
Closed loop obstruction — surgical emergency: Two adjacent transition points creating an isolated loop with no decompression. Signs: C- or U-shaped dilated loop, mesenteric swirl (twisted mesentery), beak sign (tapered ends at transition), whirl sign (twisting of mesenteric vessels/fat), dilated loop with wall thickening or pneumatosis. Requires urgent surgical consultation regardless of other findings.
Abdominal Wall and Internal Hernias
| Type | Location | Contents at Risk | Imaging Clue |
|---|---|---|---|
| Inguinal — indirect | Lateral to inferior epigastric vessels; through deep inguinal ring | Small bowel, omentum, ovary (females) | Contents follow inguinal canal lateral to epigastric vessels |
| Inguinal — direct | Medial to inferior epigastrics; through Hesselbach's triangle | Small bowel, omentum | Protrudes directly through posterior inguinal canal wall |
| Femoral | Below inguinal ligament; medial to femoral vein in femoral canal | Small bowel (highest strangulation risk) | Bowel below inguinal ligament medial to femoral vessels; narrow neck |
| Umbilical | Through umbilical ring defect | Omentum, small bowel | Midline anterior defect at umbilicus; common in obesity, ascites |
| Incisional | Through prior surgical scar/fascial defect | Small bowel, omentum, colon | Fascial defect at scar; report defect size and contents |
| Paraduodenal (internal) | Left paraduodenal fossa (Landzert) or right (Waldeyer's); near Treitz | Small bowel | Clustered encapsulated small bowel in LUQ (left) or right of midline; displaced mesenteric vessels |
| Mesenteric defect (internal) | Through surgically created mesenteric defect; most common post-RYGB | Small bowel, biliopancreatic limb | Swirling mesentery; clustered loops posterior to anastomosis; Petersen's space |
Strangulation — report urgently: Bowel wall thickening · Absent bowel wall enhancement · Pneumatosis intestinalis · Mesenteric edema/hemorrhage · Portal venous gas · Free peritoneal fluid. These indicate compromised bowel viability — surgical emergency. Always assess for strangulation when bowel is within any hernia sac.
Reporting Checklist
- Level: gastric outlet / proximal SBO / distal SBO / LBO
- Transition point: location (describe relative to bowel anatomy / quadrant / adjacent structure)
- Proximal bowel caliber at transition (mm); degree of dilation
- Etiology if identifiable: hernia (type/location) / adhesion band / mass / volvulus / intussusception
- Closed loop: present / absent (C/U-shaped loop, mesenteric swirl, beak sign)
- Ischemia signs: wall thickening / pneumatosis / portal venous gas / mesenteric haziness / free fluid
- Free air: present / absent (perforation)
- Decompressed rectum: confirms complete obstruction
References
Paulson EK et al. Acute small bowel obstruction: CT evaluation. Radiology. 2011;261(3):686–98.
Radiopaedia — Small bowel obstruction