What is a small bowel obstruction?

Definition of Small Bowel Obstruction

Small bowel obstruction (SBO) is a partial or complete mechanical blockage of the small intestine that prevents the normal passage of intestinal contents, fluid, and gas through the obstructed segment. 1, 2

Core Defining Features

Complete vs. Partial Obstruction:

• Complete obstruction involves total mechanical blockage preventing any passage of intestinal contents through the obstructed segment, characterized by regular vomiting or absolute constipation with abdominal distension, and constitutes a surgical emergency 1
• Partial obstruction allows some passage of intestinal contents on imaging studies and may cause intermittent symptoms with colicky pain that worsens after oral intake, potentially associated with vomiting 1

Clinical Presentation

Cardinal symptoms include:

• Colicky abdominal pain with colic onset due to increased motility attempting to overcome the occlusion 3
• Nausea and vomiting that occur earlier and more prominently in small bowel obstruction compared to large bowel obstruction 3
• Abdominal distension, which has a positive likelihood ratio of 16.8 and negative likelihood ratio of 0.27 for diagnosis 3
• Constipation and inability to pass gas 4

Pain characteristics vary by severity:

• Intense, untreatable pain suggests ischemia (such as small bowel volvulus) or perforation 3
• Feculent gastric aspirate is characteristic of distal small bowel obstruction 3

Pathophysiology

The obstruction results in:

• Accumulation of fluid and gas proximal to the blockage 4
• Potential progression to mucosal ischemia, necrosis, and perforation if untreated 4
• Hypovolemic shock in severe cases, or septic shock if perforation occurs 3

Etiologic Classification

Most common causes in adults:

• Adhesions (55-75% of cases): Post-surgical adhesions are the predominant cause, with prior abdominal surgery having 85% sensitivity and 78% specificity for predicting adhesive SBO 3, 1, 5
• Hernias (15-25%): Including umbilical, inguinal, and femoral hernias 3, 5
• Malignancies (5-10%): Both primary small bowel tumors and metastatic disease 3, 5
• Other causes (15%): Including carcinomatosis, endometriosis, inflammatory bowel disease stenosis (particularly Crohn's), intussusception, ischemic stenosis, radiation stenosis, gallstones, foreign bodies, and bezoars 3, 5

Critical Distinction: Mechanical vs. Functional

Mechanical obstruction must be distinguished from functional obstruction (pseudo-obstruction or adynamic ileus), which is caused by lack of enteric propulsion rather than physical blockage 1, 5. This distinction is crucial because:

• Mechanical obstruction requires aggressive resuscitation and consideration for surgical intervention 1
• Functional obstruction demands conservative management with avoidance of unnecessary surgery 1
• Prokinetic agents like metoclopramide should not be used in complete mechanical obstruction as they can cause perforation 1

Chronic Small Bowel Obstruction

Chronic SBO is defined as persistent or recurrent mechanical obstruction lasting more than 6 months, typically caused by stricturing, adhesion formation, fibrosis, or mass obstruction 1. This must be distinguished from chronic intestinal pseudo-obstruction (CIPO), which is a chronic syndrome with persistent failure of intestinal propulsion due to dysmotility rather than mechanical blockage 1.

Diagnostic Approach

Physical examination findings:

• Each hernia orifice (umbilical, inguinal, femoral) and all laparotomy/laparoscopy incision scars should be carefully examined 3
• Peritoneal signs indicate ischemia and/or perforation 3
• Signs of shock include tachycardia, tachypnea, cool extremities, mottled or cyanotic skin, slow capillary refill, and oliguria 3

Laboratory findings suggesting ischemia:

• Low serum bicarbonate levels, low arterial blood pH, high lactic acid level, marked leukocytosis, and hyperamylasemia 3