What are the differential diagnoses for small‑bowel obstruction?

Differential Diagnoses for Small Bowel Obstruction

The differential diagnosis for small bowel obstruction depends critically on whether the patient has prior abdominal surgery, with adhesions accounting for 65% of all cases, followed by hernias (10%), malignancy (5%), Crohn's disease (5%), and other causes (15%). 1

Primary Etiologic Categories

In Patients WITH Prior Abdominal Surgery

• Adhesions (65% of all SBO cases): Most common cause overall, presenting as either solitary band adhesions (65% of adhesive cases) or matted adhesions (35%) 2, 1
• Hernias (10%): Including incisional hernias at prior surgical sites 1
• Malignancy (5%): Primary small bowel tumors (NETs, lymphoma, adenocarcinoma) or metastatic disease from colon, ovary, or prostate 2, 1
• Crohn's disease (5%): Fibrostenotic strictures, particularly when predominantly inflammatory 3, 1

In Patients WITHOUT Prior Surgery (Virgin Abdomen)

In virgin abdomen cases, adhesions still account for 26-100% of operatively confirmed SBO, contradicting older teaching that malignancy is the primary cause. 2

The specific differential includes:

• Congenital adhesions (26-100% of virgin abdomen SBO): Anomalous intraperitoneal adhesions from physiological organogenesis remnants 2
• Abdominal wall hernias (10%): Inguinal, femoral, umbilical, or other external hernias 2, 1
• Internal hernias: Paraduodenal, transmesenteric, or other congenital defects 2, 4
• Malignancy (4-41%): Primary small bowel tumors or metastatic disease 2
• Meckel's diverticulum: With or without associated band 2
• Gallstone ileus: Stone eroding through gallbladder into bowel lumen 2
• Intussusception: More common in pediatric populations but can occur in adults 2, 4
• Mesenteric volvulus: Midgut volvulus or segmental small bowel volvulus 2
• Bezoar or foreign body: Phytobezoar being most common 2
• Inflammatory bowel disease: New-onset Crohn's disease or NSAID-induced strictures 2, 3
• Radiation enteropathy: In patients with prior pelvic radiation 3, 4

Less Common Causes (Applicable to Both Groups)

• Small bowel tumors: Adenocarcinoma, carcinoid, lymphoma, GIST 2, 4
• Intramural hematoma: From anticoagulation or trauma 4
• Strictures: From tuberculosis, ischemia, or anastomotic narrowing 2, 4
• NSAID diaphragm disease: Multiple thin strictures from chronic NSAID use 3
• Sclerosing encapsulating peritonitis: Rare fibrosing condition 2

Critical Diagnostic Approach

CT abdomen/pelvis with IV contrast is mandatory to differentiate these etiologies, achieving 87% sensitivity and 90% specificity for identifying the specific cause. 2, 5

CT Features That Distinguish Specific Causes:

• Adhesions: Transition zone without other identifiable cause; absence of mass or hernia 2
• Internal hernia: Cluster of small bowel segments (100% sensitive), crowding/convergence of mesenteric vessels (79%), mesenteric vessel engorgement (79%), mass effect on surrounding bowel (82%) 6
• Closed-loop obstruction: C-shaped or U-shaped dilated bowel loop with converging mesenteric vessels 5, 6
• Malignancy: Focal mass, lymphadenopathy, or peritoneal implants 2, 4
• Hernia: Bowel loops extending through abdominal wall defect or internal mesenteric defect 4

Common Pitfalls

• Do not assume adhesive etiology in virgin abdomen patients without CT confirmation—other causes (especially hernias and malignancy) must be excluded first 2, 5
• Do not overlook external hernias during physical examination—groin and abdominal wall must be carefully inspected 2
• Do not rely on plain radiographs alone—they provide only 60-70% sensitivity/specificity and cannot determine etiology 2, 5
• Negative laparotomy rates of 6-40% are reported in virgin abdomen cases, emphasizing the importance of high-quality preoperative CT imaging 2