Small Bowel Obstruction for the USMLE 2
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Small Bowel Obstruction (SBO) for USMLE Step 2
Definition
- Small Bowel Obstruction (SBO): A blockage of the small intestine that impairs the passage of intestinal contents, leading to bowel distention, ischemia, and potential necrosis or perforation if untreated.
Etiology
- Adhesions: The most common cause of SBO, often from previous abdominal surgery (e.g., appendectomy, hernia repair).
- Hernias: Frequently cause SBO worldwide; bowel loops can become trapped in hernial defects.
- Malignancy: Tumor growth can obstruct the small bowel, either by direct invasion or extrinsic compression.
- Crohn’s Disease: Can lead to SBO through inflammatory strictures or adhesions.
- Volvulus: Twisting of the intestine on its mesentery, leading to obstruction and potential vascular compromise.
- Intussusception: Telescoping of one segment of the bowel into another, usually seen in children but may occur in adults due to a tumor.
Pathophysiology
- Proximal Bowel Distension: Obstruction leads to accumulation of fluid and gas proximal to the blockage.
- Increased Intraluminal Pressure: Distension increases pressure within the bowel, reducing venous and lymphatic drainage, leading to bowel wall edema and ischemia.
- Fluid and Electrolyte Imbalance: Vomiting and third-spacing cause dehydration, hypovolemia, and electrolyte imbalances (e.g., hypokalemia, metabolic alkalosis).
- Bowel Ischemia: Prolonged obstruction reduces blood flow, leading to necrosis and potential perforation.
Clinical Features
- Abdominal Pain: Intermittent, crampy, and colicky pain, often localized to the mid-abdomen. Pain may become constant with ischemia or necrosis.
- Nausea and Vomiting: Bilious vomiting is seen in proximal SBO, while feculent vomiting suggests distal obstruction.
- Abdominal Distension: More prominent in distal obstructions, where gas and fluid accumulate over a longer segment of bowel.
- Obstipation: Complete absence of flatus and stool passage in complete obstruction.
- Bowel Sounds: Initially hyperactive and high-pitched ("tinkling"), progressing to absent with bowel ischemia or necrosis.
Diagnosis
- Abdominal X-ray:
- Initial imaging shows dilated loops of small bowel with air-fluid levels in a step-ladder pattern.
- Absence of gas in the colon suggests complete obstruction.
- CT Abdomen with Contrast:
- Gold standard for determining the site, cause, and severity of obstruction.
- Identifies complications like ischemia, strangulation, or perforation.
- Laboratory Tests:
- Electrolyte Imbalances: Hypokalemia, metabolic alkalosis due to vomiting.
- Leukocytosis: Suggestive of infection, ischemia, or perforation.
- Elevated Lactate: Indicates ischemia or necrosis.
Management
Non-Surgical Management
- NPO (nothing by mouth): Prevents further bowel distension.
- Nasogastric (NG) Tube Decompression: Relieves pressure by draining gastric contents, alleviating nausea and vomiting.
- IV Fluids and Electrolytes: Correct dehydration and electrolyte imbalances caused by vomiting and third-spacing.
- Observation: Partial obstructions may resolve with conservative treatment. Serial exams and imaging are used to monitor for resolution or deterioration.
Surgical Management
- Indications for Surgery:
- Complete obstruction with no resolution after conservative management.
- Signs of bowel ischemia or perforation, such as constant pain, fever, leukocytosis, or peritoneal signs.
- Surgical Procedures:
- Lysis of Adhesions: For adhesions causing obstruction.
- Resection: For ischemic or necrotic bowel.
- Hernia Repair: In cases of hernia-related obstruction.
Complications
- Bowel Ischemia and Necrosis: Prolonged obstruction reduces blood flow, leading to necrosis. This necessitates bowel resection.
- Perforation: Can result in peritonitis and septic shock.
- Electrolyte Imbalance: Dehydration and electrolyte disturbances can result in hypovolemic shock.
Prognosis
- Uncomplicated SBO: Partial obstructions often resolve with conservative measures and have a good prognosis.
- Complicated SBO: Bowel ischemia, necrosis, or perforation have higher morbidity and mortality and require emergency surgery.
Key Points
- Small bowel obstruction (SBO) is commonly caused by adhesions, hernias, and malignancy, presenting with abdominal pain, vomiting, distension, and obstipation.
- Diagnosis involves abdominal x-rays for initial assessment, with CT scans being the gold standard for identifying the site, severity, and cause of obstruction.
- Conservative treatment includes NPO status, NG tube decompression, and IV fluids, while surgery is indicated for complete obstruction, ischemia, or failure of conservative management.
- Complications include bowel ischemia, necrosis, perforation, and septic shock, all requiring prompt intervention.