Intestinal Obstruction — Comprehensive Guide

1. Definition & Classification

By Mechanism

By Completeness

• Complete obstruction — total luminal occlusion; constipation/obstipation; more urgent
• Partial obstruction — some passage of gas/stool; may wax and wane; less immediately life-threatening

By Vascular Compromise

• Simple obstruction — blood supply intact
• Strangulated obstruction — blood supply compromised → ischemia, infarction, perforation (surgical emergency)

By Level

• Small bowel obstruction (SBO) — more common; narrow lumen is easily occluded
• Large bowel obstruction (LBO) — less common; usually due to cancer, volvulus, or diverticulitis

2. Epidemiology & Etiology

Causes of Mechanical Small Bowel Obstruction in Adults

• Adhesions (postoperative) — >60% of all cases in the United States; predominance of lower abdominal procedures (gynecologic, appendectomy, colorectal resection) because bowel is more mobile in the pelvis
• Hernias — second leading cause (~10%); includes inguinal, femoral, umbilical, ventral, obturator, lumbar, and internal hernias
• Neoplastic (carcinomatosis, extrinsic tumors)
• Intraabdominal abscess

• Congenital: malrotation, duplications
• Inflammatory: Crohn disease, tuberculosis, actinomycosis, diverticulitis
• Neoplastic: primary or metastatic tumors (~5% of SBO)
• Traumatic: hematoma
• Miscellaneous: intussusception, endometriosis, radiation stricture

• Gallstone ileus, bezoar, foreign body, enterolith

Common Causes of Large Bowel Obstruction

• Colorectal cancer (~60%)
• Sigmoid or cecal volvulus
• Diverticulitis with stricture
• Hirschsprung disease (neonates/children)

3. Pathophysiology

Early Phase

• Peristalsis increases proximal to the obstruction → colicky pain
• Gas and fluid accumulate proximal to the obstruction
• Intraluminal pressure rises

Progressive Phase

• Bowel distension stimulates secretion and reduces absorption
• Fluid sequestration into the bowel lumen → dehydration, electrolyte disturbances (hyponatremia, hypokalemia, metabolic alkalosis in proximal SBO; metabolic acidosis in distal)
• Bacterial overgrowth proximal to obstruction

Strangulation

• Rising intraluminal pressure exceeds venous pressure → venous congestion → arterial compromise → ischemia → mucosal barrier failure → bacterial translocation → peritonitis, sepsis, perforation
• Cecal dilation >12–14 cm is a surgical emergency due to high rupture risk — Harrison's Principles of Internal Medicine

Closed-Loop Obstruction

• Both the afferent and efferent limbs are obstructed (e.g., sigmoid volvulus, cecal volvulus, herniated loop)
• Particularly dangerous — rapid progression to strangulation with no decompressive escape

4. Clinical Features

Symptoms

• Onset is usually insidious in malignant obstruction; acute in volvulus or strangulated hernia
• High-pitched "tinkling" bowel sounds early; absent sounds in late obstruction or ileus
• Visible peristalsis may be seen in thin patients

Signs of Strangulation (Urgent)

• Fever, tachycardia
• Localized tenderness, guarding, rigidity
• Leukocytosis
• Acidosis

5. Diagnosis

Laboratory Studies

• CBC: leukocytosis (infection/strangulation)
• BMP/electrolytes: hyponatremia, hypokalemia, elevated BUN/creatinine (dehydration)
• Lactate: elevated in mesenteric ischemia/strangulation
• ABG: metabolic alkalosis (proximal SBO with vomiting) or acidosis (ischemia)

Imaging

• First-line; multiple dilated loops of bowel with air-fluid levels (classic "step-ladder" pattern in SBO)
• "Colon cut-off sign" in LBO
• Absence of rectal gas
• A plain radiograph alone is insufficient but may demonstrate high or low intestinal obstruction and/or perforation — Grainger & Allison's Diagnostic Radiology

Erect abdominal X-ray: multiple dilated loops with air-fluid levels — hallmark of intestinal obstruction

• Defines extent, level, and nature of obstruction
• Identifies transition zone (point where bowel changes from dilated to collapsed)
• Distinguishes benign vs. malignant causes:
  • Malignant: mass at obstruction site, adenopathy, abrupt irregular transition zone, irregular bowel wall thickening
  • Benign: mesenteric vascular changes, large ascites, smooth transition zone, smooth bowel thickening
• Detects strangulation: bowel wall thickening, pneumatosis, portal venous gas, mesenteric edema
• CT enteroclysis for low-grade SBO: water-soluble contrast infused via naso-enteric tube followed by CT — Harrison's Principles of Internal Medicine

CT scan (coronal): dilated fluid-filled jejuno-ileal loops proximal to obstruction site

• Therapeutic and diagnostic role in adhesive SBO: if contrast reaches colon within 24 hours, likelihood of non-operative resolution is high

• Useful in pregnancy; can show dilated loops and absent peristalsis

6. Special Subtypes

Intussusception

Volvulus

Gallstone Ileus

Hirschsprung Disease (Congenital Aganglionic Megacolon)

• Incidence ~1:5,000 live births; more common in males
• Congenital absence of ganglion cells in the myenteric and submucosal plexuses of a distal colonic segment
• Presents in neonates with failure to pass meconium → obstructive constipation
• Risks: enterocolitis, perforation, peritonitis
• Treatment: surgical resection of aganglionic segment with anastomosis — Robbins & Kumar Basic Pathology

Paralytic (Adynamic) Ileus

• Pharmacologic: opioids (very common post-operative), anticholinergics, haloperidol, tricyclics
• Metabolic: hypokalemia (most common), hyponatremia, hypomagnesemia, uremia, diabetic coma
• Neurogenic: postoperative ileus, spinal cord injury, retroperitoneal irritation
• Infectious: pneumonia, peritonitis, generalized sepsis — Sabiston Textbook of Surgery

7. Management

Initial Resuscitation (All Patients)

1. IV access + aggressive fluid resuscitation (isotonic crystalloid)
2. Nasogastric (NG) decompression — relieves proximal distension, reduces aspiration risk, provides diagnostic information
3. Electrolyte correction — particularly potassium
4. Foley catheter — monitor urine output
5. NPO — bowel rest
6. Analgesia — do not withhold; adequate pain control
7. Serial abdominal exams — monitor for deterioration

Non-Operative Management (Adhesive SBO without strangulation)

• Appropriate for partial SBO or complete SBO without signs of strangulation
• NG tube, IV fluids, electrolyte repletion, parenteral nutrition if prolonged
• Water-soluble contrast (Gastrografin) challenge: therapeutic osmotic effect may facilitate resolution; reaching the colon within 24h predicts non-operative success

• 90% of early postoperative SBO is partial and resolves spontaneously — Sabiston Textbook of Surgery

• Trial duration: typically 24–72 hours; re-evaluate if no clinical improvement

Indications for Emergency Surgery

• Complete obstruction with no improvement on conservative management
• Signs of strangulation or peritonitis (fever, tachycardia, peritoneal signs, rising lactate)
• Closed-loop obstruction
• Cecal dilation >12–14 cm
• Perforation or pneumoperitoneum
• Volvulus that cannot be decompressed endoscopically

Surgical Options

Medical Management of Inoperable Malignant Obstruction

• Opioids — abdominal pain
• Antiemetics (dopamine antagonists, phenothiazines, butyrophenones) — nausea/vomiting
• Octreotide — inhibits GI secretion, reduces distension and colicky pain; may relieve obstructive symptoms
• Glucocorticoids — anti-inflammatory; may help resolve obstruction
• Metoclopramide — useful in partial or functional obstruction only; contraindicated in complete mechanical obstruction
• Anticholinergics — reduce secretions and colic
• Venting gastrostomy — decompression tube via PEG; allows oral intake "for pleasure" even as contents drain — Sleisenger & Fordtran's GI and Liver Disease; Harrison's Principles of Internal Medicine

8. Complications

9. Prognosis

• Simple adhesive SBO with early treatment: excellent prognosis
• Strangulated obstruction: mortality rises significantly with delayed surgery
• Malignant obstruction: poor prognosis; median survival is 3–4 months; ~25–30% of patients with cancer and obstruction have a benign cause (adhesions, drug-induced ileus) — Harrison's Principles of Internal Medicine
• Recurrence after adhesiolysis: up to 20–30% over 10 years

10. Key Differentials

Key Takeaways

• Adhesions are the #1 cause of SBO; hernias are #2
• The cardinal features are colicky pain, vomiting, distension, and obstipation
• CT with contrast is the gold-standard imaging modality
• Strangulation = surgical emergency; partial adhesive SBO = trial of conservative management
• Gastrografin challenge has both diagnostic and therapeutic utility in adhesive SBO
• Medical palliation (octreotide, antiemetics, venting gastrostomy) is the cornerstone for inoperable malignant obstruction