Small Bowel Obstruction for the USMLE 1
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Small Bowel Obstruction (SBO) for USMLE Step 1
Definition
- Small Bowel Obstruction (SBO): A mechanical or functional blockage of the small intestine that prevents the normal passage of intestinal contents. It can be partial or complete and may lead to bowel ischemia, necrosis, or perforation if untreated.
Etiology
- Adhesions: The most common cause of SBO in developed countries, often from prior abdominal surgery (e.g., appendectomy, cholecystectomy).
- Hernias: A frequent cause globally, involving loops of intestine trapped in abdominal wall defects.
- Tumors: Can cause SBO due to intrinsic growth or extrinsic compression, often from metastatic colorectal or ovarian cancer.
- Crohn’s Disease: Leads to SBO due to strictures or inflammation.
- Volvulus: The twisting of bowel segments leading to obstruction and compromised blood flow.
- Intussusception: Typically occurs in children but can also be seen in adults, often secondary to a tumor acting as a lead point.
Pathophysiology
- Proximal Bowel Distension: Obstruction prevents normal peristalsis, leading to the accumulation of fluid and gas in the bowel proximal to the obstruction.
- Increased Intraluminal Pressure: Progressive distention increases pressure, reducing venous return and leading to bowel wall edema and ischemia.
- Fluid Sequestration: Results in dehydration, hypovolemia, and electrolyte imbalances due to fluid loss in the bowel lumen and vomiting.
- Bowel Ischemia: Persistent obstruction can cause ischemia, necrosis, and perforation due to compromised blood supply.
Clinical Features
- Abdominal Pain: Colicky, intermittent pain, typically located in the mid-abdomen. It may become constant with ischemia or necrosis.
- Nausea and Vomiting: Bilious vomiting is common in proximal obstruction, while feculent vomiting occurs with distal obstruction.
- Abdominal Distention: More pronounced in distal obstructions, where gas and fluid accumulate over a larger bowel segment.
- Obstipation: The absence of gas or stool passage, indicating complete obstruction.
- Bowel Sounds: Hyperactive, high-pitched bowel sounds may be heard early in the course, followed by diminished or absent sounds in cases of ischemia or advanced obstruction.
Diagnosis
- Abdominal X-ray:
- Initial imaging test that shows air-fluid levels in a step-ladder pattern and dilated loops of bowel.
- Absence of gas in the colon or rectum may suggest complete obstruction.
- CT Abdomen with Contrast:
- Gold standard for diagnosing the site, severity, and cause of SBO.
- Identifies complications such as ischemia, strangulation, or perforation.
- Laboratory Tests:
- Electrolyte Imbalances: Hypokalemia, hyponatremia, and metabolic alkalosis from vomiting and dehydration.
- CBC: Leukocytosis suggests bowel ischemia, necrosis, or infection.
- Lactic Acid: Elevated levels indicate ischemia or necrosis.
Management
Non-Surgical Management
- NPO (nothing by mouth): Prevents further bowel distention by limiting oral intake.
- Nasogastric Tube (NGT): Used for decompression to relieve distention, nausea, and vomiting.
- IV Fluids: To correct dehydration and electrolyte imbalances.
- Observation: Partial obstructions may resolve with conservative measures, requiring serial abdominal exams and imaging.
Surgical Management
- Indications for Surgery:
- Complete Obstruction: Persistent symptoms or failure of conservative management.
- Strangulation or Ischemia: Suspected in patients with constant pain, fever, leukocytosis, and signs of peritonitis.
- Surgical Procedures:
- Lysis of Adhesions: The most common surgical intervention for adhesive SBO.
- Bowel Resection: Required for ischemic or necrotic bowel segments.
- Hernia Repair: To release trapped bowel in hernia-related obstructions.
Complications
- Bowel Ischemia and Necrosis: If untreated, SBO can lead to bowel wall necrosis, which requires urgent surgical intervention.
- Perforation: Leads to peritonitis and sepsis, a surgical emergency.
- Electrolyte Imbalance: Severe dehydration and electrolyte disturbances can result in shock, requiring aggressive fluid resuscitation.
Prognosis
- Uncomplicated SBO: Most partial obstructions resolve with conservative treatment, with a good prognosis.
- Complicated SBO: Patients with ischemia, necrosis, or perforation have higher morbidity and mortality if treatment is delayed.
Key Points
- Small bowel obstruction (SBO) is commonly caused by adhesions, hernias, and malignancy, presenting with abdominal pain, vomiting, distention, and obstipation.
- Initial diagnosis is made with an abdominal x-ray, while CT scan confirms the cause and site of obstruction and identifies complications.
- Conservative management with bowel rest, nasogastric decompression, and IV fluids is often sufficient for partial SBO, while surgery is required for complete obstruction or signs of bowel ischemia.
- Complications such as ischemia, necrosis, and perforation necessitate prompt surgical intervention.