Small Bowel Obstruction for the ABIM Exam

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Small Bowel Obstruction (SBO) for ABIM Exam

Definition

Small Bowel Obstruction (SBO): A partial or complete blockage of the small intestine that impedes the passage of contents. This leads to fluid accumulation, gas distention, and potentially ischemia, necrosis, or perforation if untreated.

Etiology

Adhesions: The most common cause in developed countries, often following abdominal surgery (e.g., appendectomy, bowel resection, or gynecologic surgeries).
Hernias: A major cause worldwide, occurring when bowel loops become trapped in a defect in the abdominal wall.
Malignancy: Can lead to obstruction via tumor growth within the bowel or extrinsic compression (e.g., metastatic colorectal cancer or ovarian cancer).
Inflammatory Bowel Disease (IBD): Strictures from Crohn’s disease can cause partial or complete obstruction.
Volvulus: Twisting of the bowel on its mesenteric axis, potentially leading to both obstruction and ischemia.
Intussusception: The telescoping of one part of the bowel into another, typically seen in pediatric populations but rare in adults, where it is usually associated with a tumor.
Gallstone ileus: Rare cause, where a large gallstone erodes through the gallbladder into the small intestine, leading to mechanical obstruction.

Pathophysiology

Increased Intraluminal Pressure: The obstruction causes fluid and gas to accumulate proximal to the site of blockage, leading to bowel distention.
Vascular Compromise: Severe distention increases pressure on the bowel wall, which can compromise blood flow, leading to bowel ischemia, necrosis, or perforation.
Fluid and Electrolyte Imbalances: Vomiting and sequestration of fluid in the bowel lumen cause dehydration, hypovolemia, and electrolyte imbalances (e.g., hypokalemia, metabolic alkalosis).

Clinical Features

Abdominal Pain: Crampy and intermittent, typically located around the mid-abdomen. Pain may be diffuse in more advanced cases, especially if ischemia develops.
Nausea and Vomiting: The hallmark feature, with bilious vomiting occurring earlier in proximal obstructions, and feculent vomiting in distal obstructions.
Abdominal Distention: More pronounced in distal obstructions, where gas and fluid accumulate over a larger length of bowel.
Obstipation: In complete obstructions, there is a lack of passage of both stool and gas.
High-pitched or Absent Bowel Sounds: Hyperactive bowel sounds may be heard early due to increased peristaltic activity. Late in the course, bowel sounds may diminish or disappear, indicating bowel paralysis or ischemia.

Diagnosis

Clinical Evaluation

History: Prior abdominal surgeries (adhesions), hernia history, malignancy, or symptoms suggestive of Crohn’s disease.
Physical Exam: Key findings include abdominal distention, tympany on percussion, and changes in bowel sounds.

Imaging

Initial test in most cases.
May show air-fluid levels in step-ladder pattern and dilated loops of small bowel proximal to the obstruction.
Absence of gas in the colon or rectum indicates a complete obstruction.

Gold standard for diagnosis.
Identifies the site and cause of obstruction, as well as complications such as bowel ischemia, strangulation, or perforation.
Can differentiate between partial and complete obstruction.

Useful in specific cases, such as in pediatric patients with intussusception or pregnant women to avoid radiation exposure.

May be used selectively to distinguish between partial and complete obstructions and to assess transit of contrast through the bowel.

Laboratory Tests

CBC: May reveal leukocytosis, indicating infection, ischemia, or necrosis.
Electrolytes: Hypokalemia and metabolic alkalosis due to vomiting and third-spacing of fluid.
Lactic acid: Elevated levels suggest ischemia or necrosis.

Management

Non-Surgical (Conservative) Management

NPO (nothing by mouth): Allows bowel rest and prevents further distention.
Nasogastric (NG) Tube Decompression: Relieves pressure by draining fluid and gas from the stomach and intestines. Used in most patients with significant vomiting or distention.
IV Fluids and Electrolyte Repletion: To correct dehydration, hypovolemia, and electrolyte disturbances.
Observation: Most cases of partial obstruction, especially those due to adhesions, may resolve with conservative management. Serial abdominal exams and repeat imaging are essential to monitor for resolution or progression to complete obstruction or complications.

Surgical Management

Complete obstruction: Persistent lack of bowel function, worsening symptoms, or failure of conservative management.
Strangulation or ischemia: Emergency surgery is required for bowel resection to prevent necrosis, perforation, and sepsis.
Obstruction due to hernia, malignancy, or volvulus: Requires surgical intervention to resolve the mechanical blockage.

Complications

Bowel Ischemia and Necrosis: Prolonged obstruction can lead to compromised blood flow and infarction of the bowel wall. Necrotic bowel requires resection.
Perforation: Bowel perforation can lead to peritonitis and septic shock. It is a surgical emergency.
Sepsis: Necrosis and perforation can cause bacterial translocation, leading to sepsis and multi-organ failure.
Electrolyte Imbalance: Hypovolemia, hypokalemia, and metabolic alkalosis are common in SBO due to vomiting and fluid shifts.
Short Bowel Syndrome: In cases requiring extensive bowel resection, patients may develop malabsorption due to the loss of absorptive surface area.

Prognosis

Uncomplicated SBO: Patients with partial obstructions or those managed early have good outcomes with conservative management.
Complicated SBO: Patients with bowel ischemia, necrosis, or perforation have a higher risk of morbidity and mortality, especially if surgery is delayed.
Recurrence: Adhesions remain a frequent cause of recurrent SBO, even after surgical lysis.

Prevention

Post-Surgical Adhesions: Minimally invasive surgery (laparoscopy) can reduce the risk of adhesions.
Hernia Repair: Timely repair of hernias can prevent SBO.
Management of IBD: Early and aggressive control of Crohn’s disease can prevent strictures and reduce the risk of obstruction.

Key Points

Small bowel obstruction (SBO) is commonly caused by adhesions, hernias, and malignancy. It presents with abdominal pain, vomiting, distention, and obstipation.
Imaging, particularly CT with contrast, is crucial for diagnosis and determining the cause of obstruction.
Conservative management with bowel rest, nasogastric tube decompression, and IV fluids is often sufficient for partial obstructions. Surgical intervention is required for complete obstruction or signs of bowel ischemia or perforation.
Complications of untreated SBO include bowel ischemia, necrosis, perforation, and sepsis, all of which require urgent surgical intervention.
Recurrence is common with adhesive disease, and strategies to reduce adhesion formation include the use of laparoscopy during surgeries.