Small Bowel Obstruction for the USMLE 3
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Small Bowel Obstruction (SBO) for USMLE Step 3
Definition
- Small Bowel Obstruction (SBO): A blockage of the small intestine, either partial or complete, that prevents the passage of intestinal contents, leading to bowel distention, ischemia, and potential necrosis or perforation.
Etiology
- Adhesions: The most common cause in developed countries, typically following prior abdominal surgeries (e.g., appendectomy, cholecystectomy).
- Hernias: A common cause worldwide, where the bowel becomes trapped in a defect of the abdominal wall.
- Malignancy: Tumors may cause obstruction by intraluminal growth or extrinsic compression (e.g., metastatic ovarian or colorectal cancer).
- Inflammatory Bowel Disease (Crohn’s disease): May cause obstruction due to strictures or adhesions from chronic inflammation.
- Volvulus: Twisting of the bowel on its mesentery, leading to obstruction and possible ischemia.
- Intussusception: The telescoping of one segment of bowel into another, more common in children but seen in adults, often secondary to a tumor.
Pathophysiology
- Distention: Fluid and gas accumulate proximal to the obstruction, leading to increased pressure within the bowel.
- Ischemia: As pressure rises, blood flow to the bowel wall decreases, leading to ischemia, necrosis, and, potentially, perforation.
- Fluid Shifts: Fluid sequestration in the bowel and vomiting cause dehydration, electrolyte imbalances (e.g., hypokalemia), and hypovolemia.
- Perforation: Increased pressure and ischemia can lead to bowel wall necrosis and perforation, causing peritonitis and sepsis.
Clinical Features
- Abdominal Pain: Colicky, crampy, and intermittent pain is typical early on, with constant pain developing if ischemia occurs.
- Nausea and Vomiting: Bilious vomiting is common in proximal SBO, while feculent vomiting suggests distal SBO.
- Abdominal Distention: More prominent in distal obstructions as gas and fluid accumulate in the dilated loops of the small bowel.
- Obstipation: Absence of gas or stool passage, indicative of complete obstruction.
- Bowel Sounds: Initially hyperactive and high-pitched, but can become diminished or absent in advanced cases with bowel ischemia or necrosis.
Diagnosis
- Abdominal X-ray:
- First-line imaging; shows dilated loops of small bowel with air-fluid levels in a step-ladder pattern.
- Absence of gas in the colon or rectum suggests complete obstruction.
- CT Abdomen with Contrast:
- Gold standard for diagnosing the location, cause, and severity of the obstruction.
- Can also identify complications such as ischemia, strangulation, or perforation.
- Laboratory Tests:
- Electrolyte Imbalances: Hypokalemia and metabolic alkalosis from vomiting and dehydration.
- Leukocytosis: May suggest bowel ischemia, necrosis, or infection.
- Elevated Lactate: Indicates tissue ischemia and necrosis.
Management
Non-Surgical Management
- NPO (nothing by mouth): Bowel rest to reduce distention and prevent further vomiting.
- Nasogastric Tube (NGT): Decompression of the stomach and bowel to relieve distention, nausea, and vomiting.
- IV Fluids and Electrolyte Replacement: Correct dehydration and electrolyte imbalances caused by fluid shifts and vomiting.
- Observation: Patients with partial obstruction can often be managed conservatively with serial exams and imaging.
Surgical Management
- Indications for Surgery:
- Complete obstruction that does not resolve with conservative treatment.
- Signs of bowel ischemia, perforation, or strangulation, such as constant pain, fever, leukocytosis, and peritonitis.
- Surgical Procedures:
- Lysis of Adhesions: For adhesions causing obstruction.
- Bowel Resection: Required if there is bowel ischemia or necrosis.
- Hernia Repair: For incarcerated or strangulated hernias.
Complications
- Bowel Ischemia and Necrosis: Prolonged obstruction can lead to ischemia, requiring surgical resection to prevent perforation.
- Perforation and Peritonitis: Untreated ischemia can lead to perforation, causing peritonitis and sepsis.
- Electrolyte Imbalance: Persistent vomiting and fluid sequestration lead to hypokalemia, metabolic alkalosis, and dehydration, increasing the risk of shock.
- Sepsis: Secondary to necrosis, perforation, and bacterial translocation.
Prognosis
- Uncomplicated SBO: Partial obstructions often resolve with conservative management and have a good prognosis.
- Complicated SBO: Patients with ischemia, necrosis, or perforation have higher morbidity and mortality if surgery is delayed.
Key Points
- Small bowel obstruction (SBO) is most commonly caused by adhesions, hernias, and malignancy, with typical symptoms of colicky pain, vomiting, distention, and obstipation.
- Diagnosis is confirmed with abdominal x-ray and CT scan, which also identifies complications like ischemia or perforation.
- Conservative management involves bowel rest, nasogastric decompression, and IV fluids; surgery is required for complete obstruction or ischemia.
- Complications such as bowel ischemia, necrosis, and perforation necessitate urgent intervention, as they can lead to sepsis and multi-organ failure.