What are the primary causes of small bowel obstruction?

Primary Causes of Small Bowel Obstruction

Adhesions are the dominant cause of small bowel obstruction, accounting for 55-75% of all cases, followed by hernias (15-25%) and malignancies (5-10%). 1

Major Etiologic Categories

Adhesions (Most Common)

• Adhesions represent the leading cause in 55-75% of cases overall and remain the most common etiology even in patients without prior abdominal surgery. 1
• In patients with previous abdominal surgery, adhesions account for 55-75% of obstructions, with matted adhesions being more common (67%) than band adhesions in this population. 1
• In virgin abdomen patients (no prior major surgery), adhesions still cause 26-100% of cases, though band adhesions predominate (65%) over matted adhesions (35%) in this subgroup. 1
• Having prior abdominal surgery has 85% sensitivity and 78% specificity for predicting adhesive small bowel obstruction. 1
• The risk is highest following colorectal, oncologic gynecological, or pediatric surgery. 1

Hernias (Second Most Common)

• External hernias (inguinal, femoral, umbilical, incisional) and internal hernias (paraduodenal, foramen of Winslow) collectively account for 15-25% of cases. 1
• Abdominal wall hernias specifically represent approximately 10% of small bowel obstruction cases. 1
• Hernias treated nonoperatively have higher recurrence rates compared to other etiologies, making surgical repair critical. 2

Malignancy (Third Most Common)

• Malignancies cause 5-10% of small bowel obstructions overall. 1
• Primary small bowel tumors include neuroendocrine tumors, lymphoma, and carcinomas. 1
• Metastatic tumors commonly originate from colon, ovary, prostate, breast, and melanoma. 1, 3
• In virgin abdomen patients, malignancy is encountered more frequently (4-41% of cases) compared to those with prior surgery, making this diagnosis particularly important to exclude in this population. 1, 4

Less Common Causes

• Crohn's disease accounts for 7% of cases and affects younger patients compared to other etiologies. 2
• Intussusception, volvulus, and gallstone ileus each represent distinct mechanical causes. 1
• Meckel's diverticulum (present in 2% of the population) causes obstruction through volvulus around a fibrous band or entrapment beneath a mesodiverticular band, particularly in young patients with concurrent GI bleeding. 1
• Bezoars, foreign bodies, radiation-induced strictures, post-anastomotic strictures, and sclerosing encapsulating peritonitis are additional rare causes. 1

Diagnostic Approach to Determine Etiology

• CT scan with IV contrast is the most accurate imaging modality for determining etiology, with 76% accuracy compared to operative findings. 1
• CT can identify transition points, closed-loop obstructions, signs of ischemia, and help exclude malignancy and other non-adhesive causes. 1, 5
• Plain radiographs are frequently ordered but cannot exclude the diagnosis and have limited utility. 6
• Ultrasound is a reliable diagnostic method, particularly useful in pregnancy (used in 83% of pregnant patients). 4, 6

Critical Clinical Pitfalls

• Do not assume adhesions are the cause simply because a patient has had prior abdominal surgery—recurrent cancer, occult hernias, and bowel ischemia must be excluded. 5
• Negative laparotomies (no identifiable cause found during surgery) occur in 6-40% of virgin abdomen cases, which is well-documented but clinically frustrating. 1
• In pregnant patients, the failure rate of non-operative treatment for adhesive small bowel obstruction is remarkably high at 94%, with fetal loss risk of 17% and maternal mortality around 2%, necessitating early surgical consultation. 4
• Previous abdominal surgery may negatively impact future immune responses in the abdomen, potentially affecting adhesion formation patterns. 7